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.     

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.     

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.     

Numerical simulations of fracture-toughness improvement using short shaped head ductile fibers

Fibers can be shaped so as to anchor inside the matrix and resist pullout at a crack face, thus improving the fracture-toughness of the composites. This anchoring ability enables a greatly improved utilization of the plastic potential of ductile fibers, increasing fracture-toughness while maintaining stiffness. The purpose of this paper is to explore this property of shaped head fibers for composites with weak fiber–matrix bonding. Because of the difficulty in estimating the fracture-toughness contribution of shaped head fibers analytically or experimentally, we use a FEM based numerical scheme to investigate stress profiles induced during pullout of two chosen shaped head families. Annealed copper fiber with a large residual plastic potential and an elastic epoxy matrix have been used as representative materials. Using the computed strain energy distribution in the matrix as a measure of fracture-toughness contribution, we find that flat-head fibers out-perform ball-head fibers in minimizing failure potential. We have further discovered that within each shape family there exist optimal shapes. The optimal shape for the flat-head family is also computed for the example material system.     

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.     

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.     

Optimum shapes of scarf repairs

Adhesively bonded scarf repairs are the preferred method for repairing composite structures, limited mainly by the amount of material removal associated with scarfing. In addition to the high strength restoration, scarf repairs also enable recovery of the original external surface as required by aerodynamic and/or external mould line considerations. However, scarf repairs almost inevitably result in the removal of undamaged material to make way for the scarf insert. This can be a particularly significant issue for thick structures, because the scarf length can vary between 20 and 100 times the thicknesses of the parent structure. In this investigation, an optimisation method has been developed for determining the optimum repair shapes for a given biaxial loading condition. The optimum scarf shape is determined by numerically solving the resulting non-linear differential equation governing the scarf angle. The optimum and near-optimum shapes are presented and discussed with respect to computational modelling using the finite element method.     

Interface profile optimization for planar stress wave attenuation in bi-layered plates

Stress waves scatter upon entering a new medium. This occurs due to the reflection and transmission of the waves, which depends on the impedance mismatch between the two materials and the angle of incidence. For a bi-layered structure with finite dimensions and constant impedance ratio, the scattering and intensity of the stress waves may be varied by changing the interface profile between the two layers. In this paper, a methodology is proposed for optimizing the interface profile between the layers of a finite bi-layered plate for the objective of planar stress wave attenuation. The bi-layered plates are subjected at one end to highly impulsive loadings with various durations, and the geometry of the internal interface is optimized for the purpose of minimizing the amplitude of the maximum reaction force at the opposite fixed end. The optimization methodology is based on a genetic algorithm, which is coupled with a finite element method for analyzing the wave propagation behavior of the plates. It is observed that the interface profile and the amount of stress wave attenuation depend on the duration of the applied impulsive loading, with higher amounts of attenuation obtained when the wavelength associated with the impulsive load is small compared to the dimensions of the bi-layered plates.     

Experimental assessment of dual-scale resin flow-deformation in composites processing

In this paper we are concerned with the assessment of sub-models within a two-phase continuum mechanical FE framework for process modeling of composites manufacturing. In particular, the framework considers the inclusion of two deformation dependent models describing resin flow related to: (1) meso-scale wetting and compaction of individual plies and (2) overall preform deformation and macroscopic Darcian flow. Using micro-mechanical modeling, we model the physics of these sub-processes in relation to the recently developed Out-Of-Autoclave (OOA) prepergs. The models are placed in context with a compression–relaxation experiment, employed to study the preform deformations considered separated from other sub-processes. Finally, calibrations and model validations are carried out against the relaxation experiment to relate the FE framework to the mechanical response of the preform. Therefore, using the above experiment, parameter values out of the literature and those estimated from micrographs gave a fair agreement between the simulation and experiments.     

On the local elastic–plastic behaviour of the interface in titanium/silicon carbide composites

The purpose of this study is to conduct a high-resolution nonlinear finite element analysis of the elastic–plastic behaviour of titanium/silicon carbide composites subject to transverse loading. This class of metal matrix composites is designed for the next generation of supersonic jet engines and deserves careful assessment of its behaviour under thermo mechanical loads. Three aspects of the work are accordingly examined. The first is concerned with the development of a representative unit cell capable of accurately describing the local elastic–plastic behaviour of the interface in metal matrix composites under thermal and mechanical loads. The second is concerned with the determination of the influence of mismatch in the mechanical properties between the inhomogeneity and the matrix upon the induced stress fields and the plastic zone development and its growth. The third is concerned with unloading and the role played by the interface upon residual stresses. It is found that the maximum interfacial stress in the matrix appears in the case involving cooling from the relieving temperature with subsequent applied compressive loading. It is also found that the mismatch in mechanical properties between the matrix and the inhomogeneity introduces significant changes in the stress distribution in the matrix. Specifically, it is observed that the maximum radial and tangential stresses in the matrix take place at the interface. The plastic deformation of the matrix leads to a relaxation of these stresses and assists in developing a more uniform interfacial stress distribution. However, the matrix stresses and the resulting equivalent plastic strains still reach their maximum values at that interface. The results show similarities in the patterns of the interfacial stress distribution and plastic zone development for all ranges of fibre volume fractions and loading levels examined. However, they also show marked differences in both the magnitude and patterns of matrix stress distribution between the adjacent inhomogeneities as a result of interaction effects between the fibres.     

Can carbon nanotubes grown on fibers fundamentally change stress distribution in a composite?

In carbon fiber reinforced polymer composites the onset of damage occurs at the fiber/matrix interface, where stress concentrations are the highest due to the property mismatch of the two materials. This article reports results of a modelling study indicating that carbon nanotubes (CNTs) grown on fibers are effective in suppressing stress concentrations at the fiber/matrix interface. In the case of high density CNT forests, they can even fundamentally change a profile of the interfacial stress. The study is performed using a novel two-scale finite element model of a nano-engineered composite based on the embedded regions technique.     

Analysis of a modified microbond test for the measurement of interfacial shear strength of an aqueous-based adhesive and a polyamide fibre

The microbond test was used to measure the interfacial shear strength (IFSS) between a polyamide fibre and a water-based polyurethane. Due to the low viscosity of the water based polyurethane, the droplet could not be formed using a conventional preparation method. A disc shaped microbond droplet forms when an aqueous-based colloidal polymer adhesive was deposited onto the pin hole in a mounting card with a vertical polyamide fibre in the middle after drying and curing at an elevated temperature. Since the droplets were formed with a disc shape, which differs from the conventional ellipsoid, a finite element analysis of the stress distribution at the interface for these contrastingly shaped droplets were calculated and compared. The stress analysis showed that the interfacial shear stress profiles were well matched for both providing confidence that the disc-shaped droplets could be used for interfacial analysis. The microbond test using a disc-shaped droplet was used to study the influences of silane treatment and plasma treatment on the interfacial shear strength between a polyamide fibre and an aqueous deposited polyurethane. The interfacial shear strength of the fibre after plasma treatment was 10.3 MPa, much higher than that of the control and the silanised fibres, 5.2 MPa and 5.4 MPa respectively. The results showed that the microbond test could be used to investigate the interfacial properties of the polyamide fibre and water-based polymer adhesive.     

Numerical optimization of strengthening disturbed regions of dapped-end beams using NSM and EBR CFRP

This paper presents a parametric investigation, based on non-linear finite element modeling, to identify the most effective configuration of carbon fiber-reinforced polymers (CFRP) for strengthening reinforced concrete (RC) dapped-end beams. Following a field application and laboratory tests, it focuses on effects of 24 externally bonded (EBR) and near surface mounted reinforcement (NSMR) configurations on yield strain in steel and the capacity and failure mode of dapped-end beams. The investigated parameters were the mechanical properties of the CFRP, the strengthening procedure and the inclination of the fibers with respect to the longitudinal axis. Two failure scenarios were considered: rupture and debonding of the FRP. The results indicate that high-strength NSM FRPs can considerably increase the capacity of dapped-end beams and the yielding strains in reinforcement can be substantially reduced by using high modulus fibers.     

Analysis of the stress distribution of crimped pultruded composite rods subjected to traction

The stress state of crimped pultruded composite rods subjected to traction has been investigated analytically using the linear theory of elasticity of anisotropic body and the superposition principle. The theoretical solution is able to reproduce the finite element analysis results and clarify the relation between the stress state and the boundary stresses. It can be appreciated from the theoretical solution that a longitudinal compressive stress at the edge of the crimping zone is generated by the boundary shear stress induced by the flow of metal end-fitting. Thus it can be deduced that the stress concentration at the edge of crimping zone could be mitigated through appropriately increasing the extent of the flow of the metal end-fitting away from the middle of the crimping zone. Our research shows that a radial tensile stress existing at the edge of the crimping zone is corresponding to the area of the rod that axial splitting is taken place. Comparison between analytical and numerical results shows the analytical results are in good agreement with the numerical ones except for stress distribution at the edge of the loading zone. The detailed study on stress state at the edge of the crimping zone provides better understanding of the failure mechanism, the improvement possibilities on the crimping technique and the monitoring of the structural health of the composite rod.     

Fabrication of reduced graphene oxide nanosheets reinforced Sn–Bi nanocomposites by electro-chemical deposition

Novel Sn–Bi/reduced graphene oxide nanosheets (RGOS) nanocomposites were fabricated using electro-chemical deposition process at room temperature. The RGOS dispersed and distributed in Sn–Bi alloy. Nano-sized Sn–Bi grain clusters grew on the surface of RGOS and the Sn–Bi grains became finer after the incorporation of RGOS. The mechanical properties including micro-hardness and shear strength were enhanced by the incorporation of RGOS. The enhance mechanism of RGOS in Sn–Bi/RGOS nanocomposites was proposed by finite element method simulation. The graphene with extra-thin thickness and huge aspect ratio lead to an effective stress transfer and stress concentration occurs near the edge of graphene. The Sn–Bi/RGOS nanocomposites are promising in application as TIM of high power electronics, integrated circuits.     

Composite structure composed of overlapped fibre bundles,thermoplastic resin and metal plate for secondary forming process

A simple shape of a composite is preferable in mass production, while a curved or stretched shape is sometimes preferable for final products. High formability would enable the composite to deform into a preferable shape by secondary forming. In this paper, a structure for a composite is proposed to enhance formability. The composite is composed of reinforcing fibre bundles, thermoplastic resin as the matrix and metal plates. The reinforcing fibre bundles are discontinuous, and are intentionally overlapped in the longitudinal direction. The resin including fibre bundles is sandwiched between the metal plates. As the thermoplastic resin is melted at an adequate temperature, heating would enhance the mobility of thermoplastic resin resulting in high formability at the secondary forming. If the overlapped length is adequately designed, the composite would still maintain high strength after the secondary formation. The validity of the concept was checked by finite element analyses and experiments.     

Development of viscoelastic/rate-sensitive-plastic constitutive law for fiber-reinforced composites and its applications Part II: Numerical formulation and verification

The viscoelastic/rate-sensitive plastic constitutive law to describe the non-linear, anisotropic/asymmetric and time/rate-dependent mechanical behavior of fiber-reinforced (sheet) composites were developed as discussed in Part I along with experimental procedures to obtain the material parameters for the woven fabric composite. Here, numerical formulations were developed. For verification purposes, finite element simulation results based on the proposed constitutive law were compared with experiments for the time-dependent springback in rate-dependent three point bending tests.     

Fourier series expansion in non-orthogonal coordinate system for the homogenization of linear viscoelastic periodic composites

The Eshelby method and the Fourier series are used in order to determine the linear elastic and viscoelastic properties of composites with periodically distributed inclusions in a non-orthogonal coordinate system. The relaxation moduli provided by the proposed method are compared with the moduli obtained via FEM for a unidirectional composite. This comparison gives good results. The procedure results very useful for periodic composites with hexagonal symmetry, such as some transversely isotropic composites.     

Mechanical characterization of rubberized concrete using an image-processing/XFEM coupled procedure

A coupled (two-step) numerical procedure to characterize the mechanical behaviour of Rubberized Concrete (RuC) is proposed and validated in this paper. In particular, the splitting tensile strength test is described in detail. In the first step, MATLAB Image Processing is used to obtain the model geometry and the RuC heterogeneous configuration (distribution of rubber particles within the concrete matrix). In the second step, the Extended Finite Element Method (XFEM) included in ABAQUS software is used to simulate the inelastic behaviour of the concrete matrix and allow the nucleation and development of cracks, as well as the damage evolution and ultimate strength of the RuC specimen cross-section. Additionally, a set of experimental results on mechanical behaviour of RuC is presented. This shows that RuC has both lower strength and stiffness but higher ductility (less brittle behaviour) than normal concrete (NC). Finally, a good agreement between the two-step procedure results and the experimental results (in terms of indirect tensile strength, stiffness and failure mode) is observed.