Materials Letters (18734979)379
Expert Systems with Applications (09574174)264
Additive manufacturing (AM) has become a transformative technology in modern production, enabling complex geometric designs with minimal material waste. A significant aspect of AM, particularly in fused deposition modeling (FDM), is the need for precise prediction of mechanical properties, such as ultimate tensile strength (UTS), which is crucial for industrial applications. This study examines whether simple machine learning (ML) algorithms can accurately predict the UTS of 3D-printed polylactic acid (PLA) parts, and evaluates the effectiveness of ML techniques, especially ensemble methods, in enhancing prediction accuracy. To this end, the study compares simple ML algorithms to identify the most accurate model for predicting the UTS of 3D-printed PLA parts. Subsequently, an average ensemble technique combines four ML algorithms, namely categorical boosting (CatBoost), extreme gradient boosting (XGBoost), gradient boosting machine (GBM), and light gradient boosting machine (LGBM), to predict UTS. In this technique, the average predicted UTS values of CatBoost, XGBoost, GBM, and LGBM are taken as the final predicted UTS value. Additionally, 11 ensemble configurations of these algorithms are analyzed to determine the optimized ensemble configuration. The results show that the CatBoost algorithm, with an R2 of 94.46%, achieved the highest predictive accuracy among individual ML algorithms. Moreover, the CatBoost-XGBoost-GBM-LGBM ensemble was the most accurate configuration, achieving an R2 of 98.05% with less than 10% error in predicting 37 external data points not included in the training and testing sets. This study advances predictive modeling in AM by demonstrating that ML, particularly ensemble techniques, can reliably predict material properties, paving the way for more robust applications of AM in industry. © 2024 Elsevier Ltd
Progress in Additive Manufacturing (23639520)
Extrusion-based additive manufacturing of thermosets and short fiber-reinforced thermoset composites is a challenging task and remains, despite recent advances, unable to fully leverage the entire design freedom offered by state-of-the-art technology due to low viscosity and solidification way of ink. This study introduces an enhanced direct ink writing (DIW) technique for effectively printing thermoset resins and corresponding short glass fiber-reinforced composites, achieved without adding any rheological modifiers and using ultraviolet (UV) curing. The proposed method utilizes time-dependent rheological control to enhance the ink's properties, offering a cost-effective and experimentally simplified approach. Experimental results suggest that the raster angle had no substantial effect on the mechanical properties, or in other words, the printed specimens behave like an isotropic material. To achieve maximum tensile properties, the ink parameters, such as fiber weight fraction, mixing time, and mixing speed, were optimized using the Taguchi design of experiment. The results showed a strong correlation between predicted and observed values, confirming the efficacy of the approach. © The Author(s), under exclusive licence to Springer Nature Switzerland AG 2025.
Polymer Composites (02728397)
In the realm of material extrusion additive manufacturing, components often suffer from low thermal and mechanical characteristics when compared with counterparts produced through traditional methods like injection molding. This study assessed how the incorporation of graphite powder enhances the thermal and mechanical properties of 3D-printed acrylonitrile butadiene styrene (ABS) specimens. Through the strategic addition of graphite in varying weight percentages of 2%, 8%, 14%, and 20% into ABS, composite filaments were produced using the fused filament fabrication (FFF) technique. The results illustrated that when ABS is combined with 2 wt% graphite, it shows the best results with higher tensile and flexural strengths. With a creative approach, annealing heat treatment was applied to this formulation, bringing about significant improvements of 5.95% and 5.56% in tensile and flexural strengths, respectively, for the annealed ABS-2 wt% graphite composite. Additionally, the study found an interesting pattern. The more graphite content there is, the higher the glass transition temperature; however, the lower the degradation rate of composites. Not only does this inquiry shed light on the potential of graphite-enhanced ABS composites but also paves the way for further advancements in the field of additive manufacturing. Highlights: First-time thermo-mechanical characterization of 3D-printed ABS-graphite. Using the annealing heat treatment to improve the mechanical properties. A comprehensive study on SEM, TGA, and DMTA tests for 3D-printed ABS-graphite. © 2025 Society of Plastics Engineers.
Additive manufacturing (AM) technology of thermoset polymer composites has great potential to address the disadvantages of widely used thermoplastic resins in terms of processing, cost, modification of compound formulation, dimensional stability, and stress crack resistance. Whilst there are other AM processes for thermoset polymer composites, the two most common techniques; VAT photopolymerization and extrusion-based methods are discussed. This chapter deals with the basics of these two technologies and attempts to describe the limitations and advantages of each. In particular, the key features and challenges regarding both techniques are presented. Furthermore, common materials available for thermosetting AM systems are described in combination with the 3D printing of fiber-reinforced polymer composites. A description of the important parameters that enhance the performance of printed parts is provided. © 2024 Elsevier Inc. All rights are reserved including those for text and data mining AI training and similar technologies.
Mechanics of Advanced Materials and Structures (15210596)31(18)pp. 4295-4308
In this study, a novel micromechanics-based damage model is proposed for the damage evolution of a two-component microencapsulated-based self-healing polymer composite. In this way, a representative volume element (RVE) including an epoxy matrix with randomly distributed poly(methyl methacrylate) (PMMA) microcapsules is modeled in DigimatTM software and analyzed in Abaqus®. A new technique is developed to investigate the progressive damage by pre-inserted cohesive elements along all element boundaries of the epoxy matrix, PMMA shell, and capsule-matrix interfaces with the bilinear traction–separation law. Moreover, the impact of interface bonding strength, interface fracture energy, and PMMA microcapsules volume fraction on the load-carrying capacity of the RVEs under uniaxial tension loading was studied. The results indicated that the tensile strength of the self-healing polymer composite increased as the interfacial strength and fracture energy increased from 10 to 60 MPa and 100 to 1000 J/m2, respectively. Furthermore, the higher volume fraction of 5% PMMA microcapsules results in a lower load-carrying capacity of self-healing polymer composite with a strength of 4.9 N. A similar trend of Young’s modulus was observed for microcapsule-loaded epoxy composite compared to the pristine epoxy matrix. The micromechanical model has proper accuracy in predicting the tension behavior of self-healing composite in comparison to experimental results. Finally, two healing strategies are considered for the damaged RVE. © 2023 Taylor & Francis Group, LLC.
Materials Today Communications (23524928)41
Assessing the elastic modulus of 3D-printed polylactic acid (PLA) components is essential for understanding their stiffness and load capacity, which are crucial for predicting product performance and durability. In this study, the predictive accuracy of a Tabular Neural Network (TabNet) algorithm for determining the elastic modulus of 3D-printed PLA components via fused deposition modeling (FDM) was investigated. Utilizing a comprehensive dataset of 128 literature-sourced data points, divided into 80 % for training and 20 % for validation, the study proposed a new Taguchi-based method for efficient hyperparameter optimization of the TabNet algorithm. This optimization revealed that a configuration of 8 decision blocks, 16 attention blocks, and 5 decision steps, along with the “Adam” optimizer, a gamma of 1, learning rate of 0.1, and lambda-sparse of 0.01, yielded the highest prediction accuracy for the elastic modulus of PLA parts. The performance of the optimized TabNet model was evaluated using R-squared (R²), Mean Absolute Error (MAE), Mean Squared Error (MSE), and Root Mean Squared Error (RMSE) measures. The findings highlighted an R² of 96.855 %, an MAE of 0.158, an MSE of 0.037, and an RMSE of 0.193 in the validation dataset, demonstrating substantial predictive reliability. To further test the model's robustness, fourteen unseen data points were analyzed. The observed discrepancies between predicted and actual values were under 10 %, affirming the Taguchi-optimized TabNet algorithm's effectiveness in forecasting the elastic modulus of FDM 3D-printed PLA components. This investigation provides a significant advancement in additive manufacturing, introducing a precise and reliable method for predicting the mechanical properties of 3D-printed materials. © 2024 Elsevier Ltd
Engineering Fracture Mechanics (00137944)
Large-scale fiber bridging makes a significant impact on fatigue delamination growth (FDG) in fiber-reinforced polymer composites. As bilinear cyclic cohesive zone modeling (CCZM) is not a promising modeling tool in this case, another solution is needed. The present study aims to evaluate a new easily-calibrating trilinear CCZM framework for modeling the FDG in glass/epoxy double cantilever beam (DCB) laminates with large-scale fiber bridging. First, mode I delamination growth, characterized by a significant R-curve effect, is experimentally determined under both quasi-static and high cycle fatigue regimes. Next, trilinear forms of the Turon et al. and the Kawashita-Hallett damage models are developed to predict such fatigue delamination behavior. Results show that the accuracy and efficiency of the proposed trilinear CCZM framework are enhanced by increasing the bridging length of the composite structure. Additionally, the Kawashita-Hallett trilinear model yields the most consistent and precise predictions with the least computational time. © 2024 Elsevier Ltd
Polymer Composites (02728397)(18)
Flax fiber has emerged as a promising, eco-friendly alternative to traditional synthetic reinforcement in polymer composites. However, manufacturing biocomposites using three-dimensional (3D) printing technology is typically accompanied by significant processing challenges and weak product performance under dynamic loading conditions. This study aims to unlock the potential of 3D-printed polylactic acid (PLA) by incorporating chemically modified chopped flax fibers and thermoplastic polyurethane elastomer to improve impact strength and processability. To achieve this, we employed the fused deposition modeling (FDM) technique to prepare composite specimens for the study. The crystallization behavior, tensile and impact properties, as well as the fracture behavior of the composites were investigated. The findings suggest that our approach stands out because it not only facilitates the challenging task of 3D printing PLA with fiber additives of high weight fraction and high aspect ratio but also results in a remarkable 120% enhancement in impact strength and an around 31.2% increase in tensile elongation compared to neat PLA, without compromising the elastic modulus. Highlights: Flax fibers were modified through alkalization and silanization. Alkalization significantly enhanced printing quality. Silanization reduced fiber attrition and doubled the fiber aspect ratio. TPU particles facilitated the 3D printing of biocomposites. For the first time, the hybrid strategy doubled the impact strength of PLA. © 2024 Society of Plastics Engineers.
Materials Today Communications (23524928)39
Alloys are engineered materials aimed at enhancing mechanical properties. Extensive research has focused on identifying the optimal metal composition for alloys with superior tensile strength. This study validates the stiffness and strength values of an aluminum-copper alloy through a comparison with a molecular dynamics simulation. Subsequently, 100 data points were obtained from the simulation, and a deep neural network (DNN) with three hidden layers was employed. The DNN was trained, tested, and its structure optimized using the Taguchi design of experiment. The proposed DNN structures successfully predicted the maximum values of the stiffness and strength, which were further verified using molecular dynamics simulation. Notably, the results demonstrated the complete reliability of the Taguchi-designed DNN algorithm in this application. © 2024 Elsevier Ltd
International Journal of Advanced Manufacturing Technology (02683768)132(3-4)pp. 1827-1842
A simple and inactive structure is able to transform into a complex and active one via four-dimensional (4D) printing. Controlling bending deformation, activation time, and temperature is crucial in 4D printing. This study aimed to comprehensively evaluate and analyze the effect of different process parameters on the bending deformation of polylactic acid (PLA) shape-morphing produced by material extrusion additive manufacturing. These parameters included layup, layer thickness, printing speed, nozzle temperature, nozzle diameter, and bed temperature. Since the bending deformation is significantly affected by the specimen wall, this study has focused, for the first time, on the simultaneous influence of process parameters and presence of a wall on the deformation. Furthermore, the study examined the influence of printing parameters on activation time and activation temperature. The results indicated that increasing the pre-strain stored in the parts led to a decrease in activation time and activation temperature. Subsequently, the Taguchi design of experiment method was used to optimize the most influential parameters on the bending deformation. The difference between the optimal predicted and the experimental deformations was less than 2%. Layer thickness, layup, nozzle temperature, and printing speed were recognized as the most effective parameters for controlling deformation, respectively. © The Author(s), under exclusive licence to Springer-Verlag London Ltd., part of Springer Nature 2024.
Diamond and Related Materials (09259635)144
Graphene is a widely used nanoparticle in different industries, especially in nanocomposite applications. Prediction of its properties is of great importance for engineers. Therefore, a comprehensive study is first performed in this study to investigate the effect of temperature, strain rate, number of layers, and dimensions on the tensile properties of graphene nanosheets including elastic modulus (E), yield strength (YS), and ultimate tensile strength (UTS) using molecular dynamics simulation. Then, E, YS, and UTS are simultaneously optimized via response surface methodology. To determine the simplest and the most accurate machine learning algorithm for the prediction of tensile properties, three algorithms of Decision Tree (DT), Random Forest (RF), and Gradient Boosted Tree (GBT) are compared and the GBT algorithm is introduced as the best one. Furthermore, the architecture of each algorithm was optimized via the Taguchi design of experiment method to enhance the prediction accuracy. The DT algorithm with a maximum depth of 8 was obtained as the most accurate one. © 2024 Elsevier B.V.
Journal of Composite Materials (00219983)57(30)pp. 4675-4686
One of the most important applications of electromagnetic wave absorption is in stealth aircrafts and electromagnetic protection of avionic systems. The main limitations in the design of these structures are aerodynamics, thickness or weight, mechanical strength, manufacturing process, and reasonable cost. In this study, a novel three-layer woven fabric composite laminate (with a total thickness of about 3 mm) is proposed which each layer is reinforced by individual polyaniline, carbonyl iron, or (PANI + CI) core-shell fillers. The developed Non-dominated Sorting Genetic Algorithm II optimization algorithm suggests the stacking sequence of layers, the appropriate thickness of each layer, and the filler weight fraction in each layer to achieve a broadband absorption. Due to using both dielectric and magnetic absorbing fillers, this structure shows well-impedance matching and approximately absorbs 80% of the X-band (8-12 GHz) electromagnetic waves. The maximum reflection loss is about −14dB. Finally, the effect of the addition of absorbent particles on the mechanical properties has been investigated. Experimental results showed that the tensile modulus and strength decrease by about 21.5% and 20.6%, respectively, and the flexural modulus and strength reduce by 21.7% and 19.7%, respectively. However, the (PANI + CI) core-shell filler can be introduced as a high performance absorber filler because it suggests maximum reflection loss with low weight fraction compared to other fillers and consequently the minimum reduction in mechanical properties. © The Author(s) 2023.
The present study is an original try toward establishing a simple, inclusive, and verified fatigue design methodology tied with finite element analysis (FEA) to boost the design quality before running expensive and arduous preclinical tests. First, a reliable framework is established for material selection (Ti-6Al-4V) and life estimation (an “infinite-life” viewpoint) considering the role of microstructure, processing, stress concentration, physiological environment, and manufacturing. Second, an efficiently simplified FEA framework is introduced for analysis of both “preclinical” and “clinical” situations. Third, the extended finite element method with phantom nodes coupled with virtual crack closure technique (XFEMPN-VCCT) in Abaqus is used to investigate the three-dimensional crack propagation and life estimation in the stem (a damage tolerance viewpoint). Results are then verified and validated using several analytical solutions, numerical results, and clinical data. Infinite-life results reveal that the simple methodology proposed in this paper is an efficient tool for evaluating and improving a stem design with the least loss of time and money. Damage tolerance studies show that three-dimensional XFEMPN-VCCT suffers from mesh sensitivity, dependency on the damage extrapolation parameter, and error in calculating the strain energy release rate. Additionally, it is demonstrated that the remaining useful life of a stem with a propagating long fatigue crack might be significantly shorter than the values predicted in the literature. © 2023, Springer Nature B.V.
Engineering Failure Analysis (13506307)
Epoxy-based polymer concretes (PCs) behave in a quasi-brittle manner and suffer from low tensile strength. Adding different kinds of fillers has been widely used to improve the ductility of these materials and change the failure mechanisms. In this study, we used polyethylene terephthalate (PET) fillers from recycled bottles to enhance the tensile strength and ductility of PCs and prevent sudden brittle failure. Two different sizes of fillers (i.e., fine and coarse) are considered and the Brazilian disk configuration is used to measure the indirect tensile strength of reinforced PCs. Experimental results showed that the addition of coarse PET fillers to the PC has more significant effects on tensile strength and failure mechanism than fine PET fillers. Then, micromechanical damage analyses are conducted via finite element simulations of two-dimensional representative volume elements. The effects of void content and recycled PET fillers on the tensile strength and ductility of reinforced PCs were studied in the micromechanical simulations. Since the Brazilian disk examines the indirect tensile behavior, a relationship was proposed between the direct micromechanical tensile strength and the indirect macro mechanical tensile strength. The new procedure is experimentally validated for the prediction of the tensile strength of both reinforced and unreinforced PCs. © 2023 Elsevier Ltd
Journal of Reinforced Plastics and Composites (07316844)42(9-10)pp. 430-445
The main challenge in the design of radar-absorbing composite structures (RASs) is that there is a variety of different parameters affecting the absorbing performance. In this study, these parameters are categorized into: (1) reinforcing materials including fabric types (i.e., glass fabric, carbon fabric, and 3D fabric) and filler types (i.e., carbon black, carbonyl iron, and polyaniline); (2) geometric parameters including layer thickness and stacking sequences; and (3) manufacturing methods. Up to now, the effect of all these parameters has not been simultaneously investigated and optimized on the X-band radar-absorbing feature of composite structures. Therefore, the influence of all these parameters is first experimentally investigated using waveguide tests; then, a new multi-objective optimization algorithm based on NSGA II technique is developed to simultaneously optimize the effective parameters for high-performance RAS with maximum average reflection loss and minimum weight, while considering structural limitations. Finally, the proposed algorithm is evaluated by experimental results. © The Author(s) 2022.
Composites Part A: Applied Science and Manufacturing (1359835X)171
Additive manufacturing (AM) has a great potential to create complex parts and systems lighter and stronger compared to traditional manufacturing operations. So far, several polymeric materials including different types of thermoset polymers and recently fiber reinforced thermosetting composites have been used in different additive manufacturing processes. Printed parts have shown an enhanced performance compared to their counterparts made by conventional techniques. This review article presents the state-of-the-art in the field of polymer-based additive manufacturing processes employed for thermoset resins, their corresponding fiber reinforced composites, main process parameters, build strategies, and their effects on the mechanical behavior of printed parts. This paper enlightens the basics of material extrusion, vat photopolymerization, and hybrid AM processes. In particular, these techniques involve Direct Ink Writing (DIW), Frontal Polymerization (FP), Reactive Extrusion (RE), In-bath print and cure (IBPC) that fall under extrusion-based AM system, and Stereolithography (SLA), Digital Light Processing (DLP) falling under vat photopolymerization AM. © 2023 Elsevier Ltd
Journal of Sandwich Structures and Materials (15307972)24(2)pp. 1313-1339
A novel angle graded auxetic honeycomb (AGAH) core is designed for sandwich structures in the present study. The angle of the cells is varied through the thickness of the AGAH core using linear functions. Therefore, the thickness of the cell walls is kept constant along the gradation of the cell angle, and the length of the cell walls is changed through the core thickness as the result of angle variation. New analytical relations are proposed to predict the equivalent elastic properties of the AGAH core. The performance of the new proposed core is analytically assessed for the vibrational behavior of a sandwich plate. The governing equations are deduced adopting Hamilton’s principle under the assumption of quasi-3D exponential plate theory. Three-dimensional finite element (3D-FE) simulation is accomplished to verify the analytical results of the vibrational response of the sandwich structure. The influence of variation of the cell wall, the cell angle and cell aspect ratio of AGAH core, and geometric parameters of the sandwich structure are investigated on the vibration response of the sandwich panel. The present graded design of the auxetic honeycomb enhances the specific stiffness (i.e., stiffness to density ratio) and consequently increases the natural frequencies of sandwich structures with this type of core. © The Author(s) 2021.
Engineering Failure Analysis (13506307)
The type IV composite pressure vessels (CPVs) are used as a reliable solution for storing compressed gas but still require research and development to reach commercial advancements. The main two challenges in the design of CPVs are: a) debonding of liner and composite shell during the curing process, and b) accurate finite element modeling of the thickness and angle variation of helical layers at the dome regions. In this study, debonding of composite shell from polymeric liner and bosses due to the temperature variations during the curing process is simulated by using bilinear cohesive law. Regarding the second challenge, the Abaqus WCM plugin is used to model the actual geometry of two-liter type IV CPV's domes by defining and controlling manufacturing parameters. The UVARM subroutine is developed to apply stress analysis and predict the damage initiation using the Maximum Stress, Tsai-Wu, Tsai-Hill, Hashin, and Puck failure criteria. Also, a USDFLD subroutine is written to implement progressive failure analysis using the Puck failure criterion with sudden material degradation model. The numerical axial and radial displacements as well as burst pressure results are compared with the experimental results available in the literature. A good correlation is observed between the finite element and experimental results. Also, numerical results showed that the initial debonding gap due to curing does not affect the progressive damage of the composite shell. © 2021 Elsevier Ltd
Fatigue and Fracture of Engineering Materials and Structures (8756758X)(8)
The present study aims to evaluate and improve the remeshing-free fatigue crack growth (FCG) simulation and life estimation through two developing FEM-VCCT and XFEMPN-VCCT algorithms in Abaqus. First, new energy-based forms of the Paris, Walker, and Elber FCG models are implemented by a user subroutine, a novel systematic meshing strategy is proposed, and some challenging features of these algorithms are investigated. Despite the success of this methodology in enhancing efficiency, decreasing the run-time, and preventing potential errors, it is observed that the XFEMPN-VCCT algorithm has some fundamental errors such as a serious overestimation in FCG life prediction. Thus, a novel “adaptive VCCT” is introduced and implemented by a Python script in Abaqus in the form of a new XFEMPN-AVCCT algorithm. It is finally concluded that the adaptive VCCT can significantly enhance the accuracy of FCG simulation and life estimation. © 2022 John Wiley & Sons Ltd.
International Journal of Adhesion and Adhesives (01437496)118
Various geometric and material parameters such as adhesive thickness, adhesive overlap, adherend thickness, and composite layup may affect the strength of the joint under quasi-static loading and failure mode of bonded composite-to-composite single-lap joints (SLJs) and are investigated by previous studies. The study herein broadens these findings by looking into the effect of lamina fiber angle adjacent to the adhesive layer on the damage initiation and evolution in detail. In this regard, a composite-to-composite adhesively bonded SLJ with adherends made of E-glass/epoxy composites and [04//θ/03] (where//shows the adhesive location) layups are manufactured and tested under quasi-static tensile loading. The adhesive type is semi-flexible Araldite 2015. Experimental results show that by increasing the fiber angle from 0° to 90°, the shear stress in the adhesive layer is decreased while the peel stress is increased. In examining typical fracture interfaces for each layup configuration, a full description of failure mode assessment is obtained. In particular, the SLJ is modeled in Abaqus using cohesive elements with bilinear traction-separation law. Numerical results indicate that the bilinear cohesive law cannot model the exact load-displacement curve due to semi-flexible behavior of the epoxy adhesive, but it can predict maximum strength precisely. The failure of composite joints is significantly influenced by shear stress. © 2022
Composite Structures (02638223)286
The present paper proposed an auxetic honeycomb for sandwich structure with a novel graded design. The auxetic graded design is achieved by a variation of honeycomb cell angle through the core thickness. This variation affects the other geometric parameters and the mechanical properties. The enhanced specific bending properties of the sandwich structure are obtained by using Taguchi design of experiments (DOEs) by optimizing the cell wall thickness, cell aspect ratio, and cell angle gradation. The specimens of the DOEs are fabricated using fused deposition modeling (FDM) 3D printer. The strain fields in the core and the damage evolution under real-time flexural loading conditions are assessed by performing digital image correlation (DIC) analysis. The experimental and DIC analysis results are validated by the three-dimensional finite element analysis with consideration of elastoplastic behavior and crack growth possibility. The results indicate that the reduction of the cell wall thickness to length ratio increases the bending failure stress and specific absorbed energy by 35% and 45.8%, respectively, and the optimum cell angle gradation improves the flexural modulus to density ratio with an increase of 18.9%. © 2022 Elsevier Ltd
Additive Manufacturing (22148604)
Fused deposition modeling (FDM) is the most common technique used in the additive manufacturing process of polymers. However, there is a need for more accurate failure models for structures made by additive manufacturing, thus limiting the widespread application of this technique. This paper presents a novel conservative failure model to promote the efficient design of FDM products. The conservative model is tailored to provide underpredictions for the ultimate tensile strength (UTS) and presents a safety margin for designers. Two distinct failure modes have been widely reported for FDM parts – the layer separation mode and the layer breakage mode. Consequently, the model consists of a linear interpolation for the layer separation mode and a quadratic simplification for the layer breakage mode. Three data sets have been adopted from the literature to verify the model accuracy with minimized randomness error. The experiments were carried out for polylactic acid specimens with three layer thicknesses (i.e., 0.1 mm, 0.2 mm and 0.3 mm) and seven print orientation angles (i.e., 0°, 15°, 30°, 45°, 60°, 75° and 90°). The trends in the UTS and in-plane shear strength are analyzed and discussed with respect to different layer thicknesses. The results indicate that the failure model correctly underpredicts the UTS in 95.2% of the cases. Furthermore, the accuracy of the model was investigated, and the errors were found to be insignificant. © 2021 Elsevier B.V.
Composites Part B: Engineering (13598368)
The combination of two joining mechanisms – bonding and bolting – in a single hybrid joint may potentially result in a stronger and more durable joint than either of the separate constituents. Studies have shown that the load sharing between the adhesive and the bolt is a key parameter for such an improvement. The present work executes a parametric study on single-lap hybrid bonded/bolted composite joints with multiple bolts. Joints of five different geometric configurations are subjected to the static tensile loading. The experimental study is supported by modeling results. The study reveals a significant influence of the joint overlap length on the joint strength. The impact of the bolt positioning is less pronounced. Joint stiffness is shown to be mainly governed by the joint overlap length. The load sharing between the adhesive and the bolts is shown to be geometry-dependent, i.e., facilitated by a shorter joint overlap length and smaller bolt-edge distance. The overlap area is shown to be a dominant factor for the strength improvement over that of the load sharing. However, providing that the overlap area is kept unchanged, enhanced load sharing leads to a higher joint strength, revealing their non-linear relationship. © 2021 Elsevier Ltd
Engineering Fracture Mechanics (00137944)
In this study, virtual crack closure technique (VCCT) and extended finite element method (XFEM) are coupled to each other as XFEM-VCCT approach to simulate mode I fatigue delamination growth in composites, employing the direct cyclic method in Abaqus. Both two-dimensional plane strain and three-dimensional finite element models under force and displacement control are considered. Numerical simulation results are compared with the existed experimental test data for double cantilever beam (DCB) specimens and validated. Finally, challenges ahead of VCCT and XFEM-VCCT are discussed in detail and the appropriate method for modeling fatigue delamination growth in laminated composites under high cycle loading is suggested. It is found that simulation of the DCB fatigue delamination via the displacement control loading leads to more accurate results in comparison to the force control. VCCT was found as a suitable method for simulation of fatigue delamination growth in 2D and 3D-shell models. While XFEM-VCCT shows high accuracy and low computational time in 3D-solid finite element models. The key conclusion is that the XFEM-VCCT coupled approach is independent of time increment, whereas the time increment is more effective on the results of VCCT analysis, and it affects the run-time significantly. © 2021 Elsevier Ltd
Smart Structures and Systems (17381584)27(6)pp. 1001-1010
In this study, the effect of agitation speed as a key process parameter on the morphology and particle size of epoxy-Poly (methyl methacrylate) (PMMA) microcapsules was investigated. Thus, a new interpretation is presented to relate between the microcapsule size to rotational speed so as to predict the particle size at different agitation speeds from the initial capsule size. The PMMA shell capsules containing EC 157 epoxy and hardener as healing materials were fabricated through the internal phase separation method. The process was performed at 600 and 1000 rpm mechanical mixing rates. Scanning electron microscopy (SEM) revealed the formation of spherical microcapsules with smooth surfaces. According to static light scattering (SLS) results, the average diameter size of the epoxy/PMMA capsules at two mixing rates were 7.49 and 5.11 µm for 600 and 1000 rpm, respectively, indicating that the mean size increased as the mixing rates of the process increased. The D50, D90 and mean particle size values were the lowest for hardener/PMMA microcapsules at 1000 rpm. Moreover, the Fourier transform infrared (FTIR) spectroscopy was conducted to describe the chemical structure of epoxy and hardener PMMA capsules. To investigate the reinforcing role of microcapsules, they embedded in EPL-1012 epoxy resin with various amounts of 1 and 2.5 wt.% epoxy/PMMA capsules. The investigation also involved the effect of microcapsules on mechanical behavior as well as the reinforcement of polymer composite material. Experimental results showed that the tensile strength of the self-healing polymer composite slightly increased by 1 wt.% PMMA microcapsules prepared at 1000 rpm and then reduced with an increase in the concentration and mean size diameter of PMMA microcapsules. In addition, a similar trend of Young’s modulus was seen for pristine epoxy matrix and microcapsule-loaded epoxy composite. Copyright © 2021 Techno-Press, Ltd.
Archive of Applied Mechanics (09391533)(6)
In this study, a new closed-form solution for transverse free vibration analysis of laminated composite beams (LCBs) with arbitrary number of concentrated masses is developed. The LCB is modeled based on the Euler–Bernoulli beam theory and concentrated masses are simulated considering Dirac delta function. Obtained governing equations are, then, solved semianalytically, while the frequency equation and mode shapes are extracted for two different boundary conditions, i.e., clamped-free and simply supported. In order to verify the closed-form solution, the represented model is simplified for a beam without concentrated mass and outcomes are compared with available results in the literature. Finally, the effects of mass as well as location and number of concentrated masses on the free vibration response of the beam are investigated in detail. The results highlight that with increase in the value of point masses, the natural frequencies decrease. Also, it was revealed that the number of point masses influences on the vibration of cantilever beam more than the simply supported one. These outcomes would practically be used to minimize detrimental effects of vibrational noises, leading to increase in the structural components’ lifetime. © 2021, The Author(s), under exclusive licence to Springer-Verlag GmbH Germany, part of Springer Nature.
Composites Communications (24522139)
In this study, recycled polyethylene terephthalate (PET) bottles are shredded in different sizes and added to the polymer concrete composition to investigate the effect on fracture toughness and fracture energy as well as an idea to reduce environmental pollution. In order to measure the mode I fracture toughness, center cracked Brazilian disk (CBD) specimens are manufactured with 20 wt% epoxy resin and 80 wt% quartz aggregate in the size of 2.4–4.75 mm. Recycled PET fillers in two different sizes, i.e., fine (less than 2.4 mm) and coarse (2.4–4.75 mm) with 4 wt% are added to the polymer concrete. Experimental results show that the addition of the fine and coarse PET filler materials to the polymer concrete mixture can increase the fracture toughness relative to the control material. However, the coarse PET filler can improve much more significantly the fracture toughness and fracture energy compared to the polymer concrete containing the fine PET filler. This is due to resistance made by the coarse PET filler via two energy dissipation mechanisms: a) crack growth through the matrix/PET interface by rounding the coarse PET filler; b) PET bridging. In addition, a linear relationship between the fracture toughness and fracture energy of tested polymer concrete mixtures was observed. © 2021 Elsevier Ltd
Mechanics of Advanced Materials and Structures (15210596)28(24)pp. 2585-2594
Since lightweight and energy-absorbing materials have an effective role in occupant safety during accidents, the use of aluminum or composite tubes and their optimum designs are of great importance in crashworthiness. In this study, numerical simulation of crushing and multi-objective optimization of aluminum and composite cylinders are performed to evaluate the effects of tube thickness on the objective functions (the specific energy absorption and the peak force). Besides, the effects of annealing and tempering of ductile aluminum alloys (Al 6061) are investigated. The results show that annealing of ductile aluminum alloys yields a significant reduction in objective functions. With the same thickness of the aluminum and composite shell, the composite tube exhibits proper results in terms of both peak load and energy absorption. Finally, it seems that in the design of crash boxes, a thicker composite tube leads to more appropriate results than aluminum shell. © 2020 Taylor & Francis Group, LLC.
International Journal of Applied Mechanics (17588251)12(4)
Since lightweight and energy-absorbing materials have an effective role in occupant safety during accidents, the use of hybrid aluminum-composite tubes and their optimum designs are of great importance in the crashworthiness. In this study, finite element simulation and multi-objective optimization of a hybrid aluminum-composite tube are performed under axial crushing to investigate the effect of metal volume fraction (MVF) on the objective functions, the specific energy absorption and the peak force. Besides, the effects of annealing and tempering of ductile aluminum alloys (Al-6061) as the base metal of hybrid tubes are investigated. The optimum values of the objective functions are obtained at MVF 0.5 (the same thickness of aluminum and composite). Also, annealing of ductile aluminum alloys has a negative effect on the objective functions. As a guideline for the design of fiber metal laminates under crushing, it is suggested to use tempered Al-6061 and increase the thickness of composite material so that MVF < 0.5. © 2020 World Scientific Publishing Europe Ltd.
Steel and Composite Structures (12299367)(4)
Creating different cutout shapes in order to make doors and windows, reduce the structural weight or implement various mechanisms increases the likelihood of buckling in thin-walled structures. In this study, the effect of cutout shape and geometric imperfection (GI) is simultaneously investigated on the critical buckling load and knock-down factor (KDF) of composite cylindrical shells. The GI is modeled using single perturbation load approach (SPLA). First, in order to assess the finite element model, the critical buckling load of a composite shell without cutout obtained by SPLA is compared with the experimental results available in the literature. Then, the effect of different shapes of cutout such as circular, elliptic and square, and perturbation load imperfection (PLI) is investigated on the buckling behavior of cylindrical shells. Results show that the critical buckling load of a shell without cutout decreases by increasing the PLI, whereas increasing the PLI does not have a great impact on the critical buckling load in the presence of cutout imperfection. Increasing the cutout area reduces the effect of the PLI, which results in an increase in the KDF. Copyright © 2019 Techno-Press, Ltd.
Composites Part B: Engineering (13598368)
The effect of curvature on the delamination R-curve behavior of composite unidirectional laminates is investigated in both the experimental and numerical manners. The flat and curved double cantilever beam specimens with different radii of curvatures are manufactured and tested subjected to mode I loading. A new data reduction method is developed for the curved specimens by adopting the Timoshenko curved beam theory. Experimental R-curves indicate that curvature has no effect on the initiation toughness, while it significantly affects the steady-state toughness and fiber bridging length. Variation of the steady-state toughness and fiber bridging length vs. The different curvatures is formulated for the curved specimens with R/h > 25 (R/h: the ratio of the radius of curvature to thickness). Finally, delamination propagation is simulated in the curved double cantilever beam specimens in commercial finite element software, ABAQUS, by applying both the virtual crack closure technique and cohesive zone model. © 2019 Elsevier Ltd
International Journal Of Automotive And Mechanical Engineering (22298649)16(2)pp. 6568-6587
Crash boxes play an important role in different industries as energy absorbers to reduce damage of accidents. An ideal crash box has lower maximum force and higher energy absorption. The aim of this study is to investigate the effect of various parameters such as geometry (diameter and thickness), triggering and filling with polymeric foam on axial crash behaviour of a composite cylindrical cash box. To this end, a composite crash box is modelled in a commercial finite element software, Abaqus, utilising the Hashin failure criterion to predict damage initiation. Linking damage initiation with material degradation rules provides the capability for damage evolution prediction on the basis of fracture energy of different failure modes. A new parameter (β) is defined to study the performance of a crash box with different geometries, triggers and foam-filling. The results show that three different triggering geometries (chamfer, fillet, and tulip) decrease the maximum load about 7-33%, and improved energy absorption about 40-86% compared to the crash box without trigger. Filling a triggered crash box with polymeric foam also improves energy absorption about 20%. Applying both triggering and foam-filling simultaneously on a crash box has a complementary role to receive a better performance. © Universiti Malaysia Pahang, Malaysia.
Journal of Intelligent Material Systems and Structures (1045389X)30(18-19)pp. 2651-2669
In this study, the influence of carbon nanotubes agglomeration is investigated on the electroelastic dynamic behavior of a sandwich plate. The smart sandwich plate consists of functionally graded porous layer as the core and piezoelectric layers as the face sheets, which is subjected to the harmonic electrical loading. In order to take into account the continuum model for the silica aerogel foundation of the smart structure, the modified Vlasov’s model is applied. The porosity distribution of the core layer varies non-uniformly throughout the thickness due to the non-uniform function. The equivalent material properties of nanocomposite core layer are determined using the Eshelby–Mori–Tanaka approach, in which the influence of carbon nanotube agglomeration is considered. For modeling the electroelastic fact sheets behavior, the piezoelasticity theory is adopted. On the basis of non-polynomial shear and normal deformation theory, the governing equations of motion are inferred applying the Hamilton’s principle and the obtained equations are solved by an iterative procedure. The verification is accomplished through the available results in the literature and the influences of carbon nanotube agglomeration, different geometrical parameters, porosity index, and applied voltage are assessed on the dynamic deflection of nanocomposite sandwich plate. © The Author(s) 2019.
Journal of Mechanical Science and Technology (1738494X)33(5)pp. 2067-2074
Thin-walled aluminum tubes have been widely used in engineering structures, aerospace and transportation industries due to their excellent properties. In this paper, the effect of tempering and annealing on the crushing behavior of aluminum alloy tubes, in brittle or ductile manner, under quasi-static compression were investigated. The chemical composition, the Brinell hardness number and the tensile stress-strain curves of various types of Al alloys, i.e., Al 2024, Al 7075 and Al 6061 were obtained in both tempered and annealed state. Then, the axial compression tests were performed on the tubes by a universal testing machine at a controlled displacement rate of 5 mm/min. The crushing mode, load-displacement curve, and crashworthiness characteristics were achieved to obtain specifications of mentioned aluminum tubes. Annealing process, apart from changing the deformation mode and material strength, has often reduced energy absorption in the ductile alloy, Al 6061, and increased in brittle alloys, Al 2024-T3, T4 and Al 7075-T651. This process could also be used as a triggering mechanism to decrease the initial peak force. These experimental results give useful information regarding the material behavior of aluminum alloys to be utilized in the design process of crashworthy components. © 2019, KSME & Springer.
Theoretical and Applied Fracture Mechanics (01678442)
Virtual crack closure technique (VCCT), cohesive zone modeling (CZM)and extended finite element method (XFEM)are three well-known numerical methods frequently used for crack propagation modeling. It is often questioned by new researchers and engineers: which method is more appropriate for modeling of delamination propagation in composites? In this study, advantages, limitations, and challenges of each method are discussed with the goal of finding a suitable and cost-effective solution for modeling of delamination propagation in laminated composites. To this end, a composite double cantilever beam (DCB)specimen as a benchmark example is modeled in ABAQUS and delamination propagation is simulated using three above methods and the combination of XFEM with VCCT and CZM. Two-dimensional plain strain and three-dimensional DCB models are both considered. Finite element results are compared with experimental results available in the literature for unidirectional DCB specimens. Finally, the accuracy, convergence speed, run-time and mesh dependency of each method are discussed. The XFEM-CZM was found as a suitable method for simulation of delamination growth. © 2019
Polymer Testing (01429418)
Hybrid bolted/bonded joints are less effective when designed with strong structural adhesives as insignificant load is introduced to the bolt before the bond reaches failure. Advancement in hybrid joint design requires further knowledge on the behavior of flexible adhesives, which involve significant complexities such as large inelastic deformation. This study investigates the mechanical properties of EA9361 AERO, a representative flexible epoxy adhesive, within the context of the design of hybrid bolted/bonded joints. Tensile and shear tests were performed to obtain the tensile stress/strain relation, strength, strain-to-failure, Poisson's ratio, and adhesive's responses under pure shear state. A practical methodology for strain measurement of a thin bondline of adhesive was proposed using digital image correlation (DIC). It was concluded that the nonlinear tensile stress/strain relation of the flexible adhesive can be accurately represented with a bilinear elastic/plastic material model. The Poisson's ratio was found to significantly change throughout the strain development. © 2019 Elsevier Ltd
Advanced Composite Materials (09243046)(1)
Local buckling of intact thin-walled columns is generally performed by modeling the wall segments as long plates and by assuming that edges common to two or more plates remain straight. Thus, the buckling load can be determined by considering the wall segments as individual plates rotationally restrained by the adjacent wall segments. This technique is combined with plate theories as a new analytical method to predict the buckling load of an initially delaminated column with any arbitrary sections (open or closed). First, moments at the rotationally restrained edges of delaminated segment (web or flange) are obtained from the curvature and stiffness of the adjacent laminates. Then, the strain energy of this delaminated segment with distributed moment at edges is calculated based on the first-order shear deformation theory. Using the principal of minimum potential energy, the governing equations are obtained and solved by the Rayleigh–Ritz approximation technique. Results of the present approach are compared with three-dimensional finite-element results obtained from eigenvalue buckling analysis in ANSYS software for both box- and channel-section columns with cross-ply and angle-ply stacking sequences. Finally, the effects of delamination size and location are investigated on the buckling loads. © 2016 Japan Society for Composite Materials, Korean Society for Composite Materials and Informa UK Limited, trading as Taylor & Francis Group.
Journal of Reinforced Plastics and Composites (07316844)(16)
Laminated fiber reinforced composites are increasingly being used in various load-bearing applications including for instance aerospace and wind energy power industries. Understanding the mechanical behavior of a unidirectional lamina as the basic part of a laminated composite is of great interest from a design perspective and also for the analysis of composite structures. At the micro-level, mechanical behavior of a lamina including stiffness and strength is studied based on the mechanical properties of individual constituents. This study focuses on micromechanical approaches such as strength of materials, elasticity, semi-empirical, and numerical methods for the prediction of the stiffness and strength of a unidirectional lamina. To assess the accuracy of these models, results of each model are compared against experimental results available in the literature for unidirectional composites with different fiber volume fractions. Moreover, recent studies on micromechanical modeling of unidirectional composites are reviewed. © The Author(s) 2018.
Mechanics of Materials (01676636)
In this study, the compressive behavior of polymer concrete (PC) is investigated using micromechanics-based representative volume element (RVE) concept. The RVE is composed of silica aggregates and epoxy matrix. The aggregate and matrix are modeled as linear elastic and elasto-plastic material, respectively. The interface between aggregates and matrix is modeled by employing a bilinear traction–separation law and its parameters are computed from Mohr–Coulomb failure criterion. RVE is modeled in Digimat software and imported into ABAQUS for damage analysis. Three types of boundary conditions, i.e., Dirichlet, periodic, and mixed are considered in the RVE modeling. The influences of aggregates shape, distribution, volume fraction, and interfacial parameters on the overall compressive behavior of PC are studied under uniaxial compression loading. In order to assess the micromechanical RVE model, standard cylindrical specimens of PC are manufactured and tested under uniaxial compression. Comparison of numerical and experimental results shows that: 1) the more the interfacial strength and fracture energy increases, the more the compressive strength of PC increases; 2) the compressive behavior of PC is highly dependent on aggregate volume fraction and distribution in comparison to aggregate shape, 3) the model has appropriate accuracy in predicting the compressive behavior of PC. © 2017 Elsevier Ltd
Theoretical and Applied Fracture Mechanics (01678442)
Shape and parameters of a cohesive zone model (CZM) significantly affect the finite element modeling of delamination propagation in composites. The influence of various forms of interface laws on the delamination initiation and propagation behavior of end-notched flexure (ENF) specimens with R-curve effects is addressed. Four different traction-separation laws have been considered whose shapes are bilinear, linear-exponential, trapezoidal, and trilinear. The trilinear CZM used in this study is produced by superposing of two bilinear CZMs. All CZMs have the same values of the maximum traction and of the associated fracture energy, and are implemented in the ABAQUS/Standard. Numerical results are assessed with the experimental load-displacement responses of glass/epoxy ENF specimens available in the literature. Results show that the trapezoidal and trilinear CZMs simulate the delamination propagation in ENF specimens more accurately than others. At the same time, the superposed trilinear traction-separation law considers the length of fracture process zone and predicts both the nonlinearity point and maximum point of load-displacement curve more accurately than the trapezoidal one. © 2017 Elsevier Ltd
Theoretical and Applied Fracture Mechanics (01678442)
In this paper, the role of interface fiber angle on the bridging traction of double cantilever beam (DCB) specimens is investigated experimentally. In order to eliminate the effect of the remote ply orientation on the bridging traction during delamination initiation and propagation, DCB specimens with stacking sequences of [011/θ//012] where θ = 0, 30, 45 and 90 were considered. An experimental test set-up was established for measuring the Initial crack tip opening displacement (ICTOD) using image processing method and conducting the fracture tests. The J-integral approach was used for obtaining the bridging laws from the experimental data. The experimental results show that by increasing the interface fiber angle, the maximum bridging traction in the bridging zone increases, but the ICTOD at the end of the bridging zone is independent of the interface fiber angle. Finally, the measured bridging laws were used with cohesive elements in ABAQUS software to model the delamination propagation in DCB specimens accurately. © 2017 Elsevier Ltd
Journal of Reinforced Plastics and Composites (07316844)36(15)pp. 1116-1128
A new practical procedure is presented for delamination detection in beam-like composite structures. This technique identifies the delamination axial location and its length accurately based on the only first natural frequency. An additional simply support condition besides the boundary conditions is defined and it is moved along the beam length. When this support is located on delamination, especially its tips, the frequency reduction will be noticeable. Simulating this idea in ABAQUS finite element software shows that the location and size of delamination can be detected accurately. To verify the numerical results, some experiments are conducted and a new fixture is designed and manufactured for simulating the additional moving support along the length of the beam. Both finite element and experimental results show the ability of the proposed method in delamination detection for different boundary conditions. Also, this new simple and applicable technique can be used even for small delamination lengths by monitoring only the first natural frequency, unlike the available methods in the literature. © SAGE Publications.
International Journal of Engineering, Transactions A: Basics (17281431)30(10)pp. 1565-1572
In this paper, a relatively new method, namely variational iteration method (VIM), is developed for free vibration analysis of a Timoshenko beam with different boundary conditions. In the VIM, an appropriate Lagrange multiplier is first chosen according to order of the governing differential equation of the boundary value problem, and then an iteration process is used till the desired accuracy is achieved. Solution of VIM for natural frequencies and mode shapes of a Timoshenko beam is compared to the available exact closed-form solution and numerical results of differential quadrature method (DQM). The accuracy of VIM is approximately the same as exact solution and much better than the DQM for solving the free vibration of a Timoshenko beam. Also, convergence speed and simplicity of this method is more than the other two methods because it works with polynomial at the first iteration. Thus, VIM can be used for solving the complicate engineering problems which do not have analytical solution.
Composite Structures (02638223)
A new crack propagation modeling method via the finite element method is developed based on the strong discontinuity approach in this study. The proposed method is capable of modeling a crack within a bilinear quadrilateral element without adding extra degrees-of-freedom in order to introduce a new crack. This is done by introducing constraint equations to extrapolate the strain fields of adjacent element to the element containing a crack. Then the constraint equations are used to eliminate the degrees-of-freedom of slave nodes located at the crack faces. The errors due to no-additional degrees-of-freedom are checked by L2-norm, stresses, and strain energy release rate. The errors are compared to those of standard finite element method and are not significantly large for practical uses. To assess the proposed crack growth modeling method, delamination as an interlaminar crack in laminated composites is simulated in a double cantilever beam and compared to that of VCCT for Abaqus®. The load-opening displacement curves are in good agreement. © 2016 Elsevier Ltd.
Structural Engineering and Mechanics (12254568)(1)
Fiber metal laminates (FMLs) represent a high-performance family of hybrid materials which consist of thin metal sheets bonded together with alternating unidirectional fiber layers. In this study, the buckling behavior of a FML circular cylindrical shell under axial compression is investigated via both analytical and finite element approaches. The governing equations are derived based on the first-order shear deformation theory and solved by the Navier solution method. Also, the buckling load of a FML cylindrical shell is calculated using linear eigenvalue analysis in commercial finite element software, ABAQUS. Due to lack of experimental and analytical data for buckling behavior of FML cylindrical shells in the literature, the proposed model is simplified to the full-composite and full-metal cylindrical shells and buckling loads are compared with the available results. Afterwards, the effects of FML parameters such as metal volume fraction (MVF), composite fiber orientation, stacking sequence of layers and geometric parameters are studied on the buckling loads. Results show that the FML layup has the significant effect on the buckling loads of FML cylindrical shells in comparison to the full-composite and full-metal shells. Results of this paper hopefully provide a useful guideline for engineers to design an efficient and economical structure. Copyright © 2016 Techno-Press, Ltd.
Theoretical and Applied Fracture Mechanics (01678442)
In this study, the effect of interface fiber angle on the R-curve behavior of double cantilever beam (DCB) specimens made of E-glass/epoxy under mode I loading is investigated experimentally. For this purpose, DCB specimens with stacking sequences of [011/θ//012] and θ = 0, 30, 45, 90 are manufactured by hand lay-up method. These stacking sequences are chosen to eliminate the effect of remote ply orientation on the R-curve behavior of DCB specimens during the delamination propagation. In order to obtain the critical strain energy release rate, fracture tests are conducted on these specimens. Results show that DCB specimens with 0°//0° interface have the lowest initiation interlaminar fracture toughness and the greatest bridging zone length due to good penetration of two adjacent layers of the delamination interface. Moreover, results indicate that the interface fiber angle has significant effect on the steady-state interlaminar fracture toughness as well as the bridging zone length. © 2016 Elsevier Ltd
International Journal of Mechanical Sciences (00207403)115pp. 1-11
Delamination is a major damage mode in laminated composite structures which causes reduction in stiffness and strength and affects their vibration characteristics. This paper deals with the effects of delamination size and its thickness-wise and lengthwise location on the vibration characteristics of cross-ply laminated composite beams. Free and constrained mode models are introduced and compared in the analytical and finite element methods for the first three modes and a comprehensive discussion among these results is done. To verify the results, modal tests were carried out on the delaminated specimens. Unlike the available experimental research, the proportion of delamination size to the beam length (a/L) is relatively small (i.e., a/L=0.20, 0.10 and 0.05). Moreover, these experiments are focused on the effect of the axial location of delamination on the first three natural frequencies. All results are considered under both the clamped-free and clamped-clamped boundary conditions. Finally, some interesting relationships are presented between the frequencies reduction and their corresponding mode shapes, which can be useful for delamination detection. © 2016 Published by Elsevier Ltd.
International Journal of Advanced Structural Engineering (20083556)(4)
In this study, the analytical solution of interlaminar stresses near the free edges of a general (symmetric and unsymmetric layups) cross-ply composite laminate subjected to pure bending loading is presented based on Reddy’s layerwise theory (LWT) for the first time. First, the reduced form of displacement field is obtained for a general cross-ply composite laminate subjected to a bending moment by elasticity theory. Then, first-order shear deformation theory of plates and LWT is utilized to determine the global and local deformation parameters appearing in the displacement fields, respectively. One of the main advantages of the developed solution based on the LWT is exact prediction of interlaminar stresses at the boundary layer regions. To show the accuracy of this solution, three-dimensional elasticity bending problem of a laminated composite is solved for special set of boundary conditions as well. Finally, LWT results are presented for edge-effect problems of several symmetric and unsymmetric cross-ply laminates under the bending moment. The obtained results indicate high stress gradients of interlaminar stresses near the edges of laminates. © 2015, The Author(s).
Composite Structures (02638223)
Mode I delamination of a five harness satin weave carbon fibre composite and the corresponding toughening mechanisms are studied using a multiscale finite element model of delamination growth in a double cantilever beam (DCB) specimen. The toughening mechanisms related to the fabric structure are studied by embedding a meso-scale model of the fibre architecture in the delamination zone into a macro-scale model of a DCB specimen. The R-curves and the load-displacement curves obtained from this analysis agree with the lower bound of the experimental results. The analysis identified two major toughening mechanisms: the inter-yarn locking ahead of the delamination front causing stress relaxation and the formation of sub-surfaces. Contribution of each toughening mechanism towards total delamination toughness is quantitatively evaluated, identifying the inter-yarn locking as the main source of toughening in mode I delamination of fabric composites. Video abstract: Multimedia 1 shows the different stages of the mode I delamination growth in a 5HS composite DCB specimen and its corresponding load-displacement curve. © 2015 Elsevier Ltd.
Computational Materials Science (09270256)(PB)
In the present research, nonlinear vibration in a coupled system of Boron-Nitride nano-tube reinforced composite (BNNTRC) micro-tubes conveying viscous fluid is studied. Single-walled Boron-Nitride nano-tubes (SWBNNTs) are arranged in a longitudinal direction inside Poly-vinylidene fluoride (PVDF) matrix. Damping and shearing effects of surrounded medium are taken into account by visco-Pasternak model. Based on piezoelectric fiber reinforced composite (PFRC) theory, properties of smart coupled BNNTRC micro-tubes are obtained. To enhance the accuracy of results, strain gradient theory is developed in cylindrical shell model, and the motion equations as well as the boundary conditions are derived using Hamilton's principle. Considering slip flow regime, the effects of various parameters such as Knudsen number, volume fraction and orientation angle of fibers, temperature change, viscosity and density of fluid on stability of coupled BNNTRC micro-tubes are investigated. Results indicate that stability of smart composite system is strongly dependent on orientation angle and volume percent of BNNTs. Results of this investigation can be applied for optimum design of shell and tube heat exchangers in micro scale. © 2014 Elsevier B.V. All rights reserved.
Materials and Design (02613069)
In this paper, the effect of initial delamination length is experimentally investigated on obtaining the mode I bridging law of unidirectional E-glass/epoxy double cantilever beam (DCB) specimens manufactured by hand layup method. To this end, an experimental test set-up is established for accurate measurement of crack tip opening displacement (CTOD) using digital image processing method. DCB tests are performed for three different delamination lengths and the corresponding bridging laws are calculated using J-integral approach. Results showed that the maximum bridging stress, the shape of bridging law and energy dissipation in bridging zone are slightly affected by changing initial crack length. In other words, the measured bridging law acts independent of initial delamination length. Therefore, the obtained bridging law can be used with the cohesive elements available in the commercial finite element software to simulate the delamination propagation behavior in unidirectional DCB specimens. © 2013 Elsevier Ltd.
Computational Materials Science (09270256)
The aim of this study is to find a comprehensive viewpoint about the results of analytical and finite element methods usually used for prediction of buckling behavior, including critical buckling load and modes of failure, of thin laminated composites with different stacking sequences. To this end, a semi-analytical Rayleigh-Ritz approach is first developed to calculate the critical buckling loads of square composite laminates with SFSF (S: simply-support, F: free) boundary conditions. Then, these laminates are simulated under axially compression loading using the commercial finite element software, ABAQUS. Critical buckling loads and failure modes are predicted by both eigenvalue linear and nonlinear analysis in conjunction with three well-known failure criteria, i.e., Hashin, Tsai-Wu and Tsai-Hill criteria. To validate the analytical and numerical results, layups of [0°/90°] s, [±30°]s and [±45°]s are tested under uniaxial buckling load. Since there is no standard for buckling test of composite plates with simply-supported boundary conditions, a new test setup is designed. Results showed that nonlinear finite element analysis predicts the critical bucking loads of multidirectional laminates with a good accuracy in comparison to experiments. In addition, non-linear finite element analysis associated with the Tsai-Wu and Tsai-Hill failure criteria are more efficient in prediction of buckling modes of failure in comparison to the Hashin criterion. © 2014 Elsevier B.V. All rights reserved.
Engineering Solid Mechanics (22918752)2(4)pp. 313-320
In this paper, an analytical method is proposed for calculation of natural frequencies of a delaminated composite beam from both free and constrained mode frequencies. In previous studies, the frequencies of a delaminated composite beam were computed with assumption of occurring open or close delamination during the vibration. According to this assumption, two separated modes, i.e., “free mode” and “constrained mode”, are occurred in vibration of the delaminated beam. In fact, a delamination may breathe (open and close) during the vibration and the assumptions of the free or constrained mode models are not completely correct in the whole of the vibration period. For this reason, a new formulation is proposed for calculation of natural frequencies based on the breathing of delamination. The obtained results are compared with various theoretical and experimental results available in the literature. Thus, the effects of location and size of delamination can be investigated on the natural frequencies of delaminated beams. © 2014 Growing Science Ltd. All rights reserved.
Construction and Building Materials (09500618)
Investigating the tensile strength (σt) and mode I fracture toughness (KIc) of polymer concrete (PC) materials due to their quasi-brittle behavior is of great interest to engineers. In this paper, the mechanical durability of an optimized epoxy PC, focused on the two above properties, are experimentally investigated under three different freeze/thaw cycles. The diametrally compressed un-cracked Brazilian disc (BD) and the single edge notch bending (SENB) test configurations are used to measure the split tensile strength and fracture toughness, respectively. The thermal cycles; 25 °C to -30°C (cycle-A), 25°C to 70°C (cycle-B) and -30°C to 70°C (cycle-C) applied for 7 days to the test specimens; are chosen according to the climate of Iran in different seasons. Experimental results show the noticeable influence of thermal cycles, especially cycle-B, on both fracture toughness and tensile strength. Heat-to-cool thermal cycle-A and thawing thermal cycle-B indicate the most increase and reduction, respectively on both σt and KIc in comparison to ambient conditions. Also, it was shown that the fracture toughness and tensile strength of tested PC materials are reduced by increasing the mean temperature values of thermal cycles. © 2014 Elsevier Ltd. All rights reserved.
Composites Part B: Engineering (13598368)(1)
This paper proposes a three-linear cohesive zone model (CZM) to capture the mode I delamination initiation and propagation behavior of unidirectional DCB specimens under large-scale fiber bridging conditions (R-curve behavior). This CZM is produced by superposing two bilinear CZMs and the required parameters are obtained from the experimental R-curve of a DCB specimen only knowing the initiation fracture toughness (Gi), the fiber bridging length (l FPZ) and the steady state toughness (Gss). The proposed method does not need the measurement of the crack tip opening displacement during the experiments and, therefore, it eliminates the current difficulties of the traditional CZMs in the simulation of delamination propagation under large-scale bridging. © 2012 Elsevier Ltd. All rights reserved.
Engineering Solid Mechanics (22918752)1(1)pp. 9-20
In this paper, a differential quadrature element method (DQEM) is developed for free transverse vibration analysis of a non-uniform cantilever Timoshenko beam with multiple concentrated masses. Governing equations, compatibility and boundary conditions are formulated according to the differential quadrature rules. The compatibility conditions at the position of each concentrated mass are assumed as the continuity in the vertical displacement, rotation and bending moment and discontinuity in the transverse force due to acceleration of the concentrated mass. The effects of number, magnitude and position of the masses on the value of the natural frequencies are investigated. The accuracy, convergence and efficiency of the proposed method are confirmed by comparing the obtained numerical results with the analytical solutions of other researchers. The two main advantages of the proposed method in comparison with the exact solutions available in the literature are: 1) it is less time-consuming and subsequently moreefficient; 2) it is able to analyze the free vibration of the beams whose section varies as an arbitrary function which is difficult or sometimes impossible to solve with analytical methods. © 2013 Growing Science Ltd. All rights reserved.
Materials and Design (18734197)
It is still questionable to think of delamination resistance of a double cantilever beam (DCB) as a material property independent of the specimen size and geometry. In this research, the effects of initial crack length and DCB specimen thickness on the mode I delamination resistance curve (R-curve) behavior of different unidirectional glass/epoxy DCB specimens are experimentally investigated. It is observed that the magnitudes of initiation and propagation delamination toughness (GIc-init and GIc-prop) as well as the fiber bridging length are constant in a specific range of the initial crack length to the DCB specimen thickness ratios of 8.5
Structural Engineering and Mechanics (12254568)(6)
An experimental method was suggested for obtaining fracture toughness (KIc) and the tensile strength (σt) of chopped strand glass fiber reinforced polymer concretes (PC). Semi-circular bend (SCB) specimens subjected to three-point bending were used for conducting the experiments on the PC material. While the edge cracked SCB specimen could be used to evaluate fracture toughness, the tensile strength was obtained from the un-cracked SCB specimen. The experiments showed the practical applicability of both cracked and un-cracked SCB specimens for using as suitable techniques for measuring KIc and σt in polymer concretes. In comparison with the conventional rectangular bend beam specimen, the suggested SCB samples need significantly less material due to its smaller size. Furthermore, the average values of σt and KIc of tested PC were approximately 3.5 to 4.5 times the corresponding values obtained for conventional concrete showing the improved strength properties of PC relative to the conventional concretes.
Composite Structures (02638223)(4)
Mode I delamination toughness (G Ic) of a laminated composite depends not only on the stacking sequence or indirectly coupling parameter of Dc=D122D11D22, but also on specimen geometrical ratios a 0/b and a 0/h. In this study, a non-uniformity ratio, β=(G Imax-G Iavg)/G Iavg%, is introduced to take into account the influence of above factors on the energy release rate distribution along the width of double cantilever beam (DCB) specimens simultaneously. Results show that the G Ic of multidirectional DCB specimens with 0°//0° crack interface and β<20% can be predicted by measuring the G Ic of the unidirectional plies with an error less than 10%. Moreover, a methodology is proposed to predict the maximum delamination toughness (G Imax) of multidirectional DCB specimens from β parameter without performing any experiments. With the present method, the effect of curved thumbnail shape of energy release rate along the delamination front is considered on the available closed-form relations for G I. © 2011 Elsevier Ltd.
Polymer Testing (01429418)(1)
A novel theoretical approach is presented to calculate the mode I interlaminar fracture toughness (G Ic) of double cantilever beam (DCB) specimens with low ratio of initial crack length-to-thickness (a 0/2h). This method is based on a sixth-order beam theory, namely Reddy-Bickford beam (RB), on Winkler elastic foundation (WEF) to account for both transverse shear deformation of the beam and local effects at the delamination front (root rotation). RB with only two generalized displacements w and φ; and three boundary conditions at ends and loading points of a shear deformable beam gives more accurate results than the fourth-order Timoshenko beam theory. The accuracy of the proposed method in prediction of initiation G Ic values is evaluated together with other available models considering the experimental fracture toughness for moderately thick unidirectional E-glass/epoxy DCB specimens with small initial delamination lengths. © 2011 Elsevier B.V. All rights reserved.
Computational Materials Science (09270256)
In this investigation, a finite element formulation for Timoshenko beam element with only displacement degrees of freedom is first addressed for the laminated composite beams. The resulting continuous isoparametric quadrilateral element is simple to formulate and efficient through the convergence with coarse meshes along the crack tip. Afterwards, a finite element procedure is proposed for the simulation of mode I delamination growth in symmetric multidirectional double cantilever beam (DCB) specimens based on the fracture mechanics using the above-mentioned element. To take into account R-curve effects in DCB specimens, a variable strain energy release rate is utilized instead of constant initiation fracture toughness. The strain energy release rate is computed using virtual crack closure technique (VCCT) method. The results of the finite element simulation agree well with the experimental data available in the literature. It confirms that the proposed approach is reliable and feasible for modeling of mode I delamination growth in laminated composites with large-scale fiber bridging. © 2012 Elsevier B.V. All rights reserved.
Structural Engineering and Mechanics (12254568)(2)
The aim of this research is a comprehensive review and evaluation of beam theories resting on elastic foundations that used to model mode-I delamination in multidirectional laminated composite by DCB specimen. A compliance based approach is used to calculate critical strain energy release rate (SERR). Two well-known beam theories, i.e. Euler-Bernoulli (EB) and Timoshenko beams (TB), on Winkler and Pasternak elastic foundations (WEF and PEF) are considered. In each case, a closed-form solution is presented for compliance versus crack length, effective material properties and geometrical dimensions. Effective flexural modulus (Efx) and out-of-plane extensional stiffness (E z) are used in all models instead of transversely isotropic assumption in composite laminates. Eventually, the analytical solutions are compared with experimental results available in the literature for unidirectional ([0°]6) and antisymmetric angle-ply ([±30°]5, and [±45°]5) lay-ups. TB on WEF is a simple model that predicts more accurate results for compliance and SERR in unidirectional laminates in comparison to other models. TB on PEF, in accordance with Williams (1989) assumptions, is too stiff for unidirectional DCB specimens, whereas in angle-ply DCB specimens it gives more reliable results. That it shows the effects of transverse shear deformation and root rotation on SERR value in composite DCB specimens.
Aerospace Science and Technology (12709638)(7)
In this research, a Timoshenko beam (TB) resting on two-parametric (or Pasternak) elastic foundation (PEF) is developed for determination of strain energy release rate (SERR) of mode-I delamination in multidirectional double cantilever beam (DCB) specimens. Based upon the compliance approach, a closed-form solution is obtained for SERR versus delamination length, applied load, effective mechanical properties, and geometrical dimensions of DCB specimen. The required effective flexural modulus in this model is determined with three different methodologies, plane stress resultant method, plies arrangement method, and using longitudinal tensile and compressive moduli method. The proposed model is assessed by experimental and numerical results available in the literature for various lay-ups. A comprehensive evaluation is also carried out among various theories which are used to model mode-I interlaminar fracture toughness. Results show that the developed model predicts GI at the onset of delamination growth very well for unidirectional and multidirectional DCB specimens. However, the main advantages of this model are: 1) It gives a simple and accurate explicit closed-form solution for SERR of symmetric unidirectional and multidirectional lay-ups in comparison with the high-order shear deformation beam theories with a very lengthy and tedious procedure. 2) It models both first-order transverse shear deformation and local effects at the crack tip (root rotation) to improve the split beam solution. 3) By simplifying the Timoshenko beam on two-parametric foundation, as a generalized case, special cases like Euler-Bernoulli and Timoshenko beams on the Winkler elastic foundation (WEF) are obtainable. © 2010 Elsevier Masson SAS.
Materials Science and Engineering: A (09215093)(1)
In this study, the influence of stacking sequence on mode I delamination resistance (R-curve) behavior of E-glass/epoxy laminated composites with an initial delamination between 0°//0° interface is experimentally investigated. To this end, symmetric double cantilever beam (DCB) specimens of stacking sequences; [0°12]s, [(0°/90°)3]2s and [0°/90°/±45°/90°/0°]2s with two initial crack lengths are used. A pronounced R-curve behavior is observed on all stacking sequences due to locating delamination between two similar layers. Comparison of R-curve behavior of cross-ply and quasi-isotropic DCB specimens with unidirectional (UD) one reveals the significant effect of the non-dimensional coupling parameter, Dc=D122/D11D22, on the R-curves. Thus, three main outputs of R-curves could be summarized as; (a) the initiation delamination toughness (GIc-init) of multidirectional (MD) laminates are much lower than that of UD one, (b) stacking sequence has no effect on the fiber bridging length in DCB specimens, and (c) the greater the Dc value of a laminate, the higher the steady-state propagation toughness (GIc-prop) is. © 2011 Elsevier B.V.
Construction and Building Materials (09500618)(8)
The aim of this study is the design, fabrication and experimentally characterization of an optimized polymer concrete (PC). To this end, three factors, namely: the aggregate size, epoxy resin weight percentage, and chopped glass fiber percentage; are considered as the influencing factors on the compressive strength, bending strengths and interfacial shear strength between the PC and steel. The number of tests which are necessary to simultaneously optimize three above strengths of the PC are reduced based on the design of experiment using the orthogonal array technique or so-called Taguchi method. Comparison of the predicted strengths based on the Taguchi approach with the measured experimental results shows a good correlation between them. Afterward, the effect of three freeze/thaw thermal cycles; 25 °C to -30 °C (cycle-A), 25 °C to 70 °C (cycle-B) and -30 °C to 70 °C (cycle-C) for 7 days; on the strengths of the optimized PC is experimentally investigated. Comparison of the experimental results for the mechanical strengths measured at room temperature (RT) and above thermal cycles shows that the compressive strength of the optimally designed PC is not affected by heating and cooling cycles. On the other hand, the bending strength is more affected by exposing PC to the thermal cycle-B. The interfacial shear strength becomes affected by exposing the PC to cycles-A and -B, whereas no changes are observed on this strength by exposing to the thermal cycle-C. In general, among the three thermal cycles, cycle-B exerted the most deteriorating effect on the strengths. © 2011 Elsevier Ltd. All rights reserved.
Steel and Composite Structures (12299367)(6)
Corrosion of steel rebars in bridge decks which are faced to harsh conditions, is a common problem in construction industries due to the porosity of concrete. In this research, the behavior of one-way concrete slabs reinforced with Glass fiber reinforced polymer (GFRP) molded grating is investigated both theoretically and experimentally. In the analytical method, a closed-form solution for load-deflection behavior of a slab under four-point bending condition is developed by considering a concrete slab as an orthotropic plate and defining stiffness coefficients in principal directions. The available formulation for concrete reinforced with steel is expanded for concrete reinforced with GFRP molded grating to predict ultimate failure load. In finite element modeling, an exact nonlinear behavior of concrete along with a 3-D failure criterion for cracking and crushing are considered in order to estimate the ultimate failure load and the initial cracking load. Eight concrete slabs reinforced with steel and GFRP grating in various thicknesses are also tested to verify the results. The obtained results from the models and experiments are relatively satisfactory.
Iranian Polymer Journal (10261265)(8)
Moulded grating is a lattice of connected beams that has wide applications in various industries. In the case of structural applications, deflection control is usually expected to be the limiting factor in design rather than strength control. Thus, this research is mainly focused on an analytical solution to predict the load-deflection behaviour of a moulded grating under concentrated and uniform loads. The general differential equation of an orthotopic plate is expanded by considering a moulded grating as several beams with bending and torsional rigidities. Afterward, the developed model is validated by a finite element modelling technique as well as by the experimental data provided by Strongwell Company. Results showed that the data obtained by the proposed analytical model and those of the finite element method and experimental are in good agreement. Thus, using a developed closed form solution method in this article, the deflection of a grid with any arbitrary dimensions and meshes can be calculated properly.
