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Composites Science and Technology (02663538)
This study presents an innovative application of the Taguchi design of experiment method to optimize the structure of an Artificial Neural Network (ANN) model for the prediction of elastic properties of short fiber reinforced composites. The main goal is to minimize the computational effort required for hyperparameter optimization while enhancing the prediction accuracy. By utilizing a robust experimental design framework, the structure of an ANN model is optimized. This approach involves identifying a combination of hyperparameters that provides optimal predictive accuracy with the fewest algorithmic runs, thereby significantly reducing the required computational effort. The results suggested that the Taguchi-based developed ANN model with three hidden layers, 20 neurons in each hidden layer, elu activation function, Adam optimizer, and a learning rate of 0.001 can predict the anisotropic elastic properties of short fiber reinforced composites with a prediction accuracy of 97.71 %. Then, external validation of the proposed ANN model was done using experimental data, and differences of less than 10 % were obtained, indicating an appropriate predictive performance of the proposed ANN algorithm. Our findings demonstrate that the Taguchi method not only streamlines the hyperparameter tuning process but also substantially improves the algorithm's performance. These results highlight the potential of the Taguchi method as a powerful tool for optimizing machine learning algorithms, especially in scenarios where computational resources are limited. The implications of this study are far-reaching, offering insights for future research in the optimization of different algorithms for improved accuracies and computational efficiencies. © 2024 The Authors
Progress in Additive Manufacturing (23639512) (4)
Material Extrusion (MEX) is the predominant technique in additive manufacturing of polymers, with Polylactic Acid (PLA) being the most commonly utilized material. Alongside the long list of advantages, MEX faces a major pitfall due to the mechanical weakness of parts. Moreover, accurately modeling the anisotropic failure of MEX specimens remains a persistent challenge. This paper, first, reviews the few previously established tensile strength prediction models to predict the mechanical behavior of PLA and meticulously analyzes their advantages and disadvantages. By phenomenologically exploring failure modes—specifically, the Layer Separation Mode (LSM) and the Layer Breakage Mode (LBM)—this study proposes a novel bilinear model to describe failure in MEX parts and predict the ultimate tensile strength of PLA in the conditions studied. The proposed bilinear model offers the advantage of simplicity and eliminates the need for assumptions regarding the shear strength and other complex performance factors. Experimental investigations were conducted with varying layer thicknesses (0.1 mm, 0.2 mm, and 0.3 mm) and printing angles (i.e., 0°, 15°, 30°, 45°, 60°, 75°, and 90°) were carried out, and a thorough comparison between the existing and the proposed models is made to strengthen the understanding of the behavior of PLA. In addition, three methods of deriving the shear strength are investigated for the first time, and the dependence of the models on this parameter is comprehensively explored. It was found that the established bilinear model performs exceptionally well in predicting the tensile strength, and its performance does not depend on other parameters such as shear strength. © The Author(s), under exclusive licence to Springer Nature Switzerland AG 2024.
International Journal of Solids and Structures (00207683)
Safety helmets with high energy absorption are crucial for bike riders and represent a significant priority for the sports industry. This study proposes an innovative design of a mountain bike helmet with an auxetic re-entrant metastructure made out of thermoplastic polyurethane (TPU) for its liner and a thin layer of Polyethylene Terephthalate Glycol (PETG) for its outer shell. The metastructure is designed in SolidWorks software and the impact test is simulated according to the conditions of the EN 1078 standard in Abaqus software. Finite element modeling utilizes input data from the compression tests on 3D-printed TPU specimens. The Taguchi design of experiment (DOE) is used to find the optimal cell geometry of the metastructure and minimize the deceleration during impact tests. A fused deposition modeling (FDM) 3D printer manufactures the entire liner integrally. Two different impact test scenarios, flat anvil and kerbstone anvil, are performed on the manufactured helmet. A comparison of experimental and finite element results shows good accuracy of the numerical model. In addition to a customized helmet liner tailored to individual head shapes and sizes, the proposed liner provides low deceleration during impacts. © 2025 The Authors
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.
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.
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
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.
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
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
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
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
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 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.
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.
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.
