Numerical and analytical modeling of aligned short fiber composites including imperfect interfaces

Finite element calculations were used to bound the modulus of aligned, short-fiber composites with randomly arranged fibers, including high fiber to matrix modulus ratios and high fiber aspect ratios. The bounds were narrow for low modulus ratio, but far apart for high ratio. These numerical experiments were used to evaluate prior numerical and analytical methods for modeling short-fiber composites. Prior numerical methods based on periodic boundary conditions were revealed as acceptable for low modulus ratio, but degenerate to lower bound modulus at high ratio. Numerical experiments were also compared to an Eshelby analysis and to an new, enhanced shear lag model. Both models could predict modulus for low modulus ratio, but also degenerated to lower bound modulus at high ratio. The new shear lag model accounts for stress transfer on fiber ends and includes imperfect interface effects; it was confirmed as accurate by comparison to finite element calculations.     

Numerical study of the effects of reinforcement/matrix interphase on stress-strain behavior of YAl2 particle reinforced MgLiAl composites

For the potential influence produced by the reinforcement/matrix interphase in particle reinforced metal matrix composites (PMMCs), a unit cell model with transition interphase was proposed. Uniaxial tensile loading was simulated and the stress/strain behavior was predicted. The results show that a transition interphase with both appropriate strength and thickness could affect the failure mode, reduce the stress concentration, and enhance the maximum strain value of the composite.     

Predicting microdroplet force response using a multiscale modeling approach

An axisymmetric microscale finite element model of a microdroplet test specimen is developed where the structural response of the fiber–droplet interface is accounted for by surface-based cohesive behavior. In this study, the interface cohesive response is estimated using a nanoscale interface finite element model that explicitly includes the effects of fiber surface topography and the interphase region. The interphase behavior in the nanoscale interface model is calibrated using indirect experimental data. Once calibrated, the fiber surface topography in the nanoscale interface model is modified in order to estimate the parameters defining the surface-based cohesive behavior of similar fiber–matrix systems with different fiber topography. The effect of altering the fiber topography on the force response of the microdroplet test can then be predicted by the microdroplet FE model. Comparing the simulation results with experimental data from the literature shows that this multiscale modeling approach gives accurate predictions for the interfacial shear stress.     

Fracture analysis of epoxy/SWCNT nanocomposite based on global–local finite element model

A global–local multiscale finite element method (FEM) is proposed to study the interaction of nanotubes and matrix at the nanoscale near a crack tip. A 3D FE model of a representative volume element (RVE) in crack tip is built. The effects of the length and chirality of single walled carbon nanotube (SWCNT) in a polymer matrix on the fracture behavior were studied in the presence of van der Waals (vdW) interaction as inter-phase region. Detailed results show that with increasing the weight percentage of SWCNT, fracture toughness improves. Three situations of nanotube directions with respect to crack are considered. Results show that bridging condition has minimum stress intensity factor. In addition, it can be seen that the crack resistance improves by increasing the length and chirality for all kinds of nanotubes. Finally, epoxy/SWCNT 10 wt.% has lower stress intensity factor compared to epoxy/halloysite 10 wt.% in similar loading state.     

Investigation into three-layered HMA mixtures

Hot-mix asphalt (HMA) mixtures consist of three phases: aggregate, asphalt binder (mastic) and air voids, of which the first two (aggregate and asphalt binder) provide the structure that withstands various kinds of loading.

Due to the nature of high inhomogeneity between aggregate and asphalt binder, significant stress and strain concentration occurs at the interface between the two phases, which causes adverse effect to HMA mixtures and potentially contributes to pavement distresses/failure.

This paper presents a novel idea to mitigate the stress and strain concentration by introducing an intermediate layer between aggregate and asphalt binder in HMA mixture. Microstructural analyses of layered system indicated that the three-layered composite HMA mixture would greatly improve the performance of asphalt mixture. The composite mixture showed more than 10% reduction in internal stress and strain and consequently its performance could be potentially improved. To validate the theoretical analyses, a laboratory experiment was conducted to compare the performance of a conventional mixture to that of a conceptual three-layered composite HMA mixture, which was formed by incorporating a stiff natural asphalt (gilsonite) as the intermediate layer. The results of the limited laboratory experiment confirmed the findings from the theoretical analyses.     

Evaluation of interfacial strength in CF/epoxies using FEM and in-situ experiments

A combined experimental and numerical study has been carried out in order to study the mechanism of initial failure in transversely loaded CF/epoxy composites. Two composites with a high and a low temperature-curing matrix were investigated. Three point bending experiments on macroscopic composite specimen with special laminate lay-ups were carried out in a scanning electron microscope (SEM). The in-situ experiments allow observing the onset of microscopic composite failure under transverse loading and measurement of the macroscopic applied load at onset of failure. The experimental results show that interfacial failure was the dominating failure mechanism for both materials. For the same carbon fiber with the same treatment the interfacial failure was adhesive (weak interface) or cohesive (strong interface), depending on the matrix system. The interfacial stresses at initiation of failure were determined successfully by a non-linear micro/macro FE-analysis and compared with experimental results obtained from micro composite test. The results show that the interfacial normal strength (INS) governs failure under transverse loads.     

Interphase effect on the macroscopic elastic properties of non-bonded single-walled carbon nanotube composites

Compared to the small diameter of a carbon nanotube (CNT), the thickness of the CNT–matrix interphase in a CNT–composite is considerable. Hence, the interphase property can significantly influence the macroscopic properties of the composite. This paper applies an effective multi-scale method to explore such an interphase effect on the properties of nano-composites reinforced by single-walled CNTs. The method integrates the van der Waals (vdW) gap interphase, the dense interphase, and the randomly distributed wavy CNTs in a matrix to realize an accurate prediction of macroscopic properties with a nanoscopic resolution, by using a conventional finite element code commercially available. The study concluded that with the same volume fraction, increasing CNT waviness and diameter reduces the composite Young's modulus, and that ignoring either the vdW gap interphase or the dense interphase can lead to an erroneous characterization, and that both interphases can be ignored in some circumstances.     

The finite element discretized symplectic method for interface cracks

The method of symplectic series discretized by finite element is introduced for the stress analysis of structures having cracks at the interface of dissimilar materials. The crack is modeled by the conventional finite elements dividing into two regions: near and far fields. The unknowns in the far field are as usual. In the near field, a Hamiltonian system is established for applying the method of separable variables and the solutions are expanded in exact symplectic eigenfunctions. By performing a transformation from the large amount of finite element unknowns to a small set of coefficients of the symplectic expansion, the stress intensity factors, the displacements and stresses in the singular region are obtained simultaneously without any post-processing. The numerical results are obtained for various cracks lying at the bi-material interface, and are found to be in good agreement with the reference solutions for the interface crack problems. Some practical examples are also given.     

A numerical investigation into the size effect in the transverse crack tension test for mode II delamination

In experimental studies, a size effect has been measured for the fracture energy in the transverse crack tension test. This paper presents a numerical investigation into the cause of this size effect. A finite element model has been developed that includes delamination, friction and shear nonlinearity. After calibration of the model, the size effect was reproduced well. It is shown that shear nonlinearity and friction separately contribute to the measured size effect and that significant amplification of the size effect takes place because of their interaction. As a consequence of their interaction, the unstable crack growth that was observed for the thicker specimens in the experiments is also present in the model results.     

Mode III interlaminar fracture of carbon/epoxy laminates using a four-point bending plate test

A new four-point bending plate (4PBP) test was used for characterising the mode III interlaminar fracture of carbon/epoxy laminates. The specimen has a cross-ply lay-up and two edge delaminations whose propagation becomes visible at the edges. Although the test setup is very simple, determination of the mode III critical strain energy release rate G_IIIc requires finite element analyses (FEA). The virtual crack closure technique with an assumed initiation region was first proposed for computing G_IIIc. This scheme was subsequently validated by crack growth simulations with a cohesive zone model. The results showed an average G_IIIc = 1550 J/m², which is significantly higher than the G_IIIc = 850–1100 J/m² and G_IIc = 800 J/m² measured in previous studies.     

Interfacial damage in biopolymer composites reinforced using hemp fibres: Finite element simulation and experimental investigation

The present study aims at considering the effect of interfacial damage on the mechanical performance of a starchy composite reinforced using hemp fibres. Mechanical behaviour is approached experimentally using tensile testing coupled to digital image acquisition. Thermomoulded samples with single fibres are designed to allow sample testing perpendicularly to the direction of fibre alignment. Experimental evidence of localised damage is then highlighted in the elasticity stage. Finite element computation is attempted to explain the observed damage using an adequate mechanical model that considers weak adhesion between phases and dynamic evolution of damage. Predicted results show that the FE model is able to reproduce the observed behaviour suggesting that local damage evolution is a serious mechanism affecting the performance of the studied composite.     

Numerical simulations to support the normalization data reduction technique

The normalization data reduction (NDR) technique is an analytical methodology for characterizing the upper shelf fracture toughness of steels in the ductile regime, both in terms of critical toughness (J_Ic) and resistance to ductile crack extension (J-R curve). It represents an alternative to the more commonly used multi-specimen or single-specimen (unloading compliance and potential drop) techniques.Finite element analyses of a growing crack are executed to evaluate the performance of the technique. This approach has the advantage to remove large uncertainties entailing experimental results. Results demonstrate the precision of the method.     

Numerical investigation to prevent crack jumping in Double Cantilever Beam tests of multidirectional composite laminates

During the experimental characterization of the mode I interlaminar fracture toughness of multidirectional composite laminates, the crack tends to migrate from the propagation plane (crack jumping) or to grow asymmetrically, invalidating the tests.The aim of this study is to check the feasibility of defining the stacking sequence of Double Cantilever Beam (DCB) specimens so that these undesired effects do not occur, leading to meaningful onset and propagation data from the tests. Accordingly, a finite element model using cohesive elements for interlaminar delamination and an intralaminar ply failure criterion are exploited here to thoroughly investigate the effect of specimen stiffness and thermal residual stresses on crack jumping and asymmetric crack growth occurring in multidirectional DCB specimens.The results show that the higher the arm bending stiffness, the lower the tendency to crack jumping and the better the crack front symmetry. This analysis raises the prospect of defining a test campaign leading to meaningful fracture toughness results (onset and propagation data) in multidirectional laminates.     

Numerical investigation on the loading-delamination-unloading behavior of adhesive joints

An elastic-plastic interface model at finite deformations is utilized to investigate the irreversible delamination behavior of adhesive joints subjected to loading-delamination-unloading. The interface model accounts for the irreversible delamination in the fracture process zone induced by the localized plastic deformation and damage. The interfacial parameters in the cohesive model are obtained by fitting the available experimental data. Results suggest that the cohesive model can capture the irreversible delamination failure behavior observed in adhesively bonded joints during a loading-unloading cycle. The overall nonlinear response is dominated by the cohesive strength and initial damage displacement jump. Further, we also investigate the effect of the ductile mechanisms for the bulk layer on the competition between the plastic deformation of the bulk layer and the delamination of the interface. It is observed that the degradation of unloading stiffness is attributed to the inelastic behavior of the interface.     

XFEM simulation of delamination in composite laminates

In this paper, the extended finite element method (XFEM) is extended to simulate delamination problems in composite laminates. A crack-leading model is proposed and implemented in the ABAQUS® to discriminate different delamination morphologies, i.e., the 0°/0° interface in unidirectional laminates and the 0°/90° interface in multidirectional laminates, which accounts for both interlaminar and intralaminar crack propagation. Three typical delamination problems were simulated and verified. The results of single delamination in unidirectional laminates under pure mode I, mode II, and mixed mode I/II correspond well with the analytical solutions. The results of multiple delaminations in unidirectional laminates are in good agreement with experimental data. Finally, using a recently proposed test that characterizes the interaction of delamination and matrix cracks in cross-ply laminates, the present numerical results of the delamination migration caused by the coupled failure mechanisms are consistent with experimental observations.     

Strengthening slabs using externally-bonded strip composites: Analysis of concrete covers on the strengthening

This study pertains to the experimental and theoretical behaviour of slabs strengthened by fibre reinforced polymer (FRP). The experimental results show that FRP significantly increases punching failure stress, resulting in a reduction of slab rotation around the loading column. The theoretical investigation presents a finite element model for the bending of strengthened slabs. The developed model considers the concrete as a 3D multi-layered non-linear material and explicitly takes into account the steel reinforcement and the FRP strips. The proposed model is then used to analyse the effects of a concrete cover on the reinforcement and repairs. In the analysed cases, the results show that an average reduction in the concrete shear modulus, between steel rod and FRP, of more than 30% leads to significant reductions of stress and slab stiffness.     

A constitutive model for self-reinforced ductile polymer composites

Self-reinforced polymer composites are gaining increasing interest due to their higher ductility compared to traditional glass and carbon fibre composites. Here we consider a class of PET composites comprising woven PET fibres in a PET matrix. While there is a significant literature on the development of these materials and their mechanical properties, little progress has been reported on constitutive models for these composites. Here we report the development of an anisotropic visco-plastic constitutive model for PET composites that captures the measured anisotropy, tension/compression asymmetry and ductility. This model is implemented in a commercial finite element package and shown to capture the measured response of PET composite plates and beams in different orientations to a high degree of accuracy.     

Resistance to delamination of 3D woven textile composites evaluated using End Notch Flexure (ENF) tests: Cohesive zone based computational results

The flexural response of 3D woven textile composite panels containing an edge crack is evaluated using the End Notch Flexure (ENF) test. In doing so, the effectiveness of 3D reinforcement in increasing and/or eliminating delamination is demonstrated. A finite element model of the ENF configuration using the Discrete Cohesive Zone Model (DCZM) was used to evaluate the deformation response and fracture properties corresponding to the experimental results presented in Pankow et al. (2011) [1]. A modified trapezoidal traction law was used in the DCZM to computationally evaluate the ENF test results. Good agreement between experimental results and predictions are reported, up to the point at which the crack reaches under the loading roller and damage begins to occur locally under the roller.     

Stress transfer analysis of unidirectional composites with randomly distributed fibers using finite element method

A three-dimensional representative volume element (RVE) of unidirectional composites with both randomly distributed fibers and periodic geometry was generated using DIGIMAT-FE software. Finite element analysis of the stress transfer mechanisms around a fiber break in the RVE was performed via ABAQUS/Standard. The influences of distance to the broken fiber, fiber/matrix stiffness ratio and fiber volume fraction on the stress transfer process of intact fibers were discussed for the case of perfect fiber/matrix adhesion. The study shows that the nearest fibers and the second nearest fibers share the stress released from the broken fiber. The stress transfer coefficient and the ineffective stress transfer length of the nearest fibers was found to increase with the increasing distance to the broken fiber and the stiffness ratio, while decrease with the increasing fiber volume fraction. However, the trends in the two stress transfer parameters of the second nearest fibers are slightly different from those of the nearest fibers due to the random distribution of other intact fibers.