Relationship between static bending and compressive behaviour of particle-reinforced cement composites

Innovative particle-reinforced materials made of alumina particles and cement-based matrix were designed, manufactured and tested to evaluate the potential use of ceramic aggregates in concretes. These particle-reinforced composites were tested in three-point bending and uniaxial compression conditions to determine the influence of the shape and size of the ceramic inclusions, and the addition of silica fume on the mechanical properties. A specific methodology combining post-mortem observations with a statistical analysis of tensile failure stresses (average strength and Weibull modulus) was conducted to deduce the origin of failure for each cement-based composite (porosity or ceramic particles/matrix decohesion). A remarkable correlation is observed between bending failure stress level and the average strength measured under uniaxial compression loading. As main conclusion, addition of alumina particles in a mortar appears to strengthen or to weaken the composite depending on whether silica fume is used in the cementitious matrix.     

The response of a multi-directional composite laminate to through-thickness loading

The through-thickness mechanical response of a carbon fibre/epoxy laminated composite of lay-up [0/45/−45]_ns is measured at low rates of strain. Uniaxial tension and compression experiments are carried out on dogbone specimens cut from a thick laminate along different directions, and failure mechanisms are observed via optical and electron microscopy. The effect of direct and shear stresses at the ply interfaces on the onset of failure is measured, and a failure envelope is constructed. The compressive response of specimens of different shape is investigated. Composite beams of different volume and aspect ratios are tested to failure in three-point bending and these tests reveal a strong dependence of the apparent out-of-plane tensile strength of the composite on the beam volume; this effect is modelled by Weibull theory.     

Progressive damage and nonlinear analysis of pultruded composite structures

This study combines a simple damage modeling approach with micromechanical models for the progressive damage analysis of pultruded composite materials and structures. Two micromodels are used to generate the nonlinear effective response of a pultruded composite system made up from two alternating layers reinforced with roving and continuous filaments mat (CFM). The layers have E-glass fiber and vinylester matrix constituents. The proposed constitutive and damage framework is integrated within a finite element (FE) code for a general nonlinear analysis of pultruded composite structures using layered shell or plate elements. The micromechanical models are implemented at the through-thickness Gaussian integration points of the pultruded cross-section. A layer-wise damage analysis approach is proposed. The Tsai–Wu failure criterion is calibrated separately for the CFM and roving layers using ultimate stress values from off-axis pultruded coupons under uniaxial loading. Once a failure is detected in one of the layers, the micromodel of that layer is no longer used. Instead, an elastic degrading material model is activated for the failed layer to simulate the post-ultimate response. Damage variables for in-plane modes of failure are considered in the effective anisotropic strain energy density of the layer. The degraded secant stiffness is used in the FE analysis. Examples of progressive damage analysis are carried out for notched plates under compression and tension, and a single-bolted connection under tension. Good agreement is shown when comparing the experimental results and the FE models that incorporate the combined micromechanical and damage models.     

Compression creep rupture behavior of a glass/vinyl ester composite laminate subject to fire loading conditions

The growing use of polymer matrix composites in civil infrastructure, marine and military applications provides the impetus for developing mechanical models to describe their response under combined mechanical and fire loading. A viscoelastic stress analysis using classical lamination theory is conducted on an E-glass/vinyl ester composite. The model includes a characterization of the non-linear thermo-viscoelasticity and its inclusion into a compression strength failure criterion for the prediction of laminate failure under combined compressive load and temperature profile simulating fire exposure. By accounting for the viscoelastic non-linearity at T_g, the proposed model yields good predictions for lifetimes of the studied composite ([0/+45/90/−45/0]_S).     

Compressive behaviour of CFRP laminates repaired with adhesively bonded external patches

Composite patches can be used to reinforce and repair both cracked composite and metallic aircraft structures. The repair of a composite structure with a composite patch may use mechanical fastening, which often introduces undesirable stress concentrations or adhesive bonding, external or flush patches. To ensure a reliable and durable bond, various parameters such as the quality of surface preparation and the design of the composite patch (size, shape, stiffness) are very important. This paper describes the testing of bonded external patch repaired CFRP laminates loaded in compression. It is found that the critical failure mechanism is fibre microbuckling in the 0° plies accompanied by matrix cracking and delamination, triggered by failures at the adhesive/adherend interface. A three-dimensional finite element analysis is performed to estimate the stress field in the repaired region. The calculated stresses are then used with the maximum stress and average stress failure criteria to predict damage initiation, mode and location. Carefully designed external patch repairs can recover more than 80% of the undamaged compressive strength.     

Design optimization of composites using genetic algorithms and failure mechanism based failure criterion

In this paper, minimum weight design of composite laminates is presented using the failure mechanism based (FMB), maximum stress and Tsai–Wu failure criteria. The objective is to demonstrate the effectiveness of the newly proposed FMB failure criterion (FMBFC) in composite design. The FMBFC considers different failure mechanisms such as fiber breaks, matrix cracks, fiber compressive failure, and matrix crushing which are relevant for different loading conditions. A genetic algorithm is used for the optimization study. The Tsai–Wu failure criterion over predicts the weight of the laminate by up to 86% in the third quadrant of the failure envelope compared to FMB and maximum stress failure criteria, when the laminate is subjected to compressive–compressive loading. It is found that the FMB and maximum stress failure criteria give comparable weight estimates. The FMBFC can be considered for use in the strength design of composite structures.     

Compressive behaviour of thin-skin stiffened composite panels with a stress raiser

The in-plane compressive behaviour of thin-skin stiffened composite panels with a stress concentrator in the form of an open hole or low velocity impact damage is examined analytically. Drop weight impact in laminated polymer composites causes matrix cracking, delaminations and fibre breakage, which together can seriously degrade the laminate compressive strength. Experimental studies, using ultrasonic C-scan images and X-ray shadow radiography, indicated that the overall damage resembles a hole. Under uniaxial compression loading, 0° fibre microbuckling surrounded by delamination grows laterally (like a crack) from the impact site as the applied load is increased. These local buckled regions continued to propagate, first in discrete increments and then rapidly at failure load. The damage pattern is very similar to that observed in laminated plates with open holes loaded in compression. Because of this resemblance, a fracture mechanics model, developed initially to predict notched compressive strength, was applied to estimate the compression-after-impact (CAI) strength of a stiffened panel; in the analysis the impact damage is replaced with an equivalent open hole. Also, the maximum stress failure criterion is employed to estimate the residual compressive strength of the panel. The unnotched compressive strength of the composite laminate required in the analysis is obtained from a three-dimensional stability theory of deformable bodies. The influence of the stiffener on the compressive strength of the thin-skin panel is examined and included in the analysis. A good agreement between experimental measurements and predicted values for the critical failure load is obtained.     

A micromechanical approach for the prediction of the time-dependent failure of high temperature polymer matrix composites

In the present study, a novel micromechanical approach is introduced to study the time-dependent failure of unidirectional polymer matrix composites. The main advantage of the present micromechanical model lies in its ability to give closed-form solutions for the effective nonlinear response of unidirectional composites and to predict the material response to any combination of shear and normal loading. The creep failure criterion is expressed in terms of the creep failure functions of the viscoelastic matrix material. The micromechanical model is also used to calculate these creep failure functions from the knowledge of the creep behavior of the composite material in only transverse and shear loadings, thus eliminating the need for any further experimentation. The composite material used in this study is T300/934, which is suitable for service at high temperatures in aerospace applications. The use of micromechanics can give a more accurate insight into the failure mechanisms of the composite materials in particular at high temperatures where the general behavior of the polymer matrix composite is governed by matrix viscoelasticity and the time-dependent failure of the matrix is a localized phenomenon. The obtained creep failure stresses are found to be in reasonable agreement with the experimental data.     

Micromechanical deformations in PP/lignocellulosic filler composites: Effect of matrix properties

Polypropylene composites were prepared from three different PP matrices, a homopolymer, a random and a heterophase copolymer, and corn cob to study the effect of matrix characteristics on deformation and failure. The components were homogenized in an internal mixer and compression molded to 1 mm thick plates. Mechanical properties were characterized by tensile testing, while micromechanical deformations by acoustic emission measurements and fractography. The results proved that the dominating micromechanical deformation process may change with matrix properties. Yield stress determined from the stress vs. strain traces may cover widely differing processes. Debonding is the dominating process when the adhesion of the components is poor, while matrix yielding and/or filler fracture dominate when adhesion is improved by the introduction of a functionalized polymer. The dominating deformation mechanism is determined by component properties and adhesion. Interfacial adhesion, matrix yield stress and the inherent strength of the reinforcement can be limiting factors in the improvement of composite strength. The properties of polymer composites reinforced with lignocellulosic fillers are determined by micromechanical deformation processes, but they are independent of the mechanism of these processes.     

Effect of thermal residual stresses on the matrix failure under transverse compression at micromechanical level - A numerical and experimental study

The influence at micromechanical scale of thermal residual stresses, originating in the cooling down associated to the curing process of fibrous composites, on inter-fibre failure under transverse compression is studied. In particular, the effect of these stresses on the appearance of the first debonds is discussed analytically; later steps of the damage mechanism are analysed by means of a single fibre model, making use of the Boundary Element Method. The results are evaluated applying Interfacial Fracture Mechanics concepts. The conclusions obtained show, at least in the case of dilute fibre packing, the effect of thermal residual stresses on the appearance and initiation of growth to be negligible, and the morphology of the damage not to be significantly affected in comparison with the case in which these stresses are not considered. Experimental tests are carried out, the results agreeing with the conclusions derived from the numerical analysis.     

On the transition from shear-driven fibre compressive failure to fibre kinking in notched CFRP laminates under longitudinal compression

This paper investigates longitudinal compressive failure in notched unidirectional and cross-ply carbon/epoxy specimens. Dedicated test jigs were developed to observe the failure processes at the microscale. In situ and post-mortem fractography reveals two types of failure mechanisms: (i) shear-driven fibre compressive failure and (ii) kink-band formation. The sequence of events leading to failure and the reasons for shear-driven fibre compressive failure or kink-band formation are investigated and discussed. Those findings are discussed further in a separate paper (Gutkin et al., accepted for publication) [1] where an FE micromechanical model is used to investigate numerically the failure mechanisms found in longitudinal compression.     

Effects of thermal exposure on mechanical behavior of carbon fiber composite pyramidal truss core sandwich panel

An experimental study was performed to investigate the effect of high temperature exposure on mechanical properties of carbon fiber composite sandwich panel with pyramidal truss core. For this purpose, sandwich panels were exposed to different temperatures for different times. Then sandwich panels were tested under out-of-plane compression till failure after thermal exposure. Our results indicated that both the thermal exposure temperature and time were the important factors affecting the failure of sandwich panels. Severe reductions in residual compressive modulus and strength were observed when sandwich panels were exposed to 300 °C for 6 h. The effect of high temperature exposure on failure mode of sandwich panel was revealed as well. Delamination and low fiber to matrix adhesion caused by the degradation of the matrix properties were found for the specimens exposed to 300 °C. The modulus and strength of sandwich panels at different thermal exposure temperatures and times were predicted with proposed method and compared with measured results. Experimental results showed that the predicted values were close to experimental values.     

Diametral compression of pultruded composite rods

Diametral compression tests were performed on pultruded composite rods comprised of unidirectional glass or carbon fibers in a common matrix. During compression tests, acoustic emission (AE) activity was recorded and images were acquired from the sample for analysis by digital image correlation (DIC). In both composite systems, localized tensile strain developed in the transverse plane under the load platens prior to failure, producing non-linearity in the load–displacement curve and AE signals. In situ SEM diametral compression tests revealed the development of matrix microcracking and debonding in regions of localized strain, perpendicular to the tensile strain direction (parallel to the load axis). Comparison of linear finite element simulations and experimental results showed a deviation from linear elastic behavior in the load displacement curve. The apparent transverse modulus, in plane shear modulus, and transverse tensile strength of the GF rod was greater than that of the CF rod, and fracture surfaces indicated greater fiber/matrix adhesion in the GF system compared to the CF system. A mixed mode fracture surface showed that two failure modes were active – matrix tensile failure and matrix compression failure by shear near the loading edge.     

Effect of thermal residual stresses on matrix failure under transverse tension at micromechanical level: A numerical and experimental analysis

In the present work the influence at micromechanical scale of thermal residual stresses, originated in the cooling down associated to the curing process of fibrous composites, on inter-fibre failure under transverse tension is studied. In particular, the effect of the presence of thermal residual stresses on the appearance of the first debonds is discussed analytically, whereas later steps of the mechanism of damage, i.e. the growth of interface cracks and their kinking towards the matrix, are analysed by means of a single fibre model and making use of the Boundary Element Method (BEM). The results are evaluated applying Interfacial Fracture Mechanics concepts. The conclusions obtained predict, at least in the case of dilute fibre packing, a protective effect of thermal residual stresses against failure initiation, the morphology of the damage not being significantly affected in comparison with the case in which these stresses are not considered. Experimental tests are carried out, the results agreeing with the conclusions of the numerical analysis.     

Thermodynamically consistent coupled viscoplastic damage model for perforation and penetration in metal matrix composite materials

Accurate modeling and efficient analysis of the metal matrix composite materials failure mechanism during high velocity impact conditions is still the ultimate goal for many researchers. The objective is to develop a micromechanical constitutive model that can effectively simulate the high impact damage problem of the metal matrix composite materials. Therefore in this paper, a multiscale micromechanical constitutive model that couples the anisotropic damage mechanism with the viscoplastic deformation is presented here as a solution to this situation. This coupled viscoplastic damage model is formulated based on thermodynamic laws. Nonlinear continuum mechanics is used for this heterogeneous media that assesses a strong coupling between viscoplasticity and anisotropic damage. It includes the strong directional effect of the fiber on the evolution of the back stress and the development of the viscoplastic strain in the material behavior for high velocity impact damage related problems.     

Compressive strength of unidirectional and crossply carbon fibre/PEEK composites

The commonly accepted production methods of composite systems generally result in departure of the plies properties from transverse isotropy due to stresses acting during fibre—matrix bond formation. This anisotropy coupled with the composite structure affects compressive loading; the ultimate stresses as well as the direction, in- or out-of-plane, of kink propagation. A unidirectional and a crossply carbon fibre/PEEK composites were compression tested at ambient and elevated temperature as well as exposed to various chemical environments. Significant disruptions in fibre—matrix interface in the crossply composite were indicated. The compression tests showed that failure occurred through in-plane and out-of-plane fibre bucking and kinking in the unidirectional and crossply composites, respectively. Failure of the longitudinal plies in the crossply laminate occurred at significantly higher compression stress than for the unidirectional composite. Compressive failure mechanisms in unidirectional and multi-directional laminates are considered.     

Failure by buckling mode of the pore-strut for isotropic three-dimensional reticulated porous metal foams under different compressive loads

Compression is one of the most basic loading modes for engineering materials, and the failure of lost stability is possibly resulted from buckling for the bar under compression. The pore-strut within porous metal foams under compression may be similar to the compression bar, so the strut is possible to buckle when the porous body is under compressive loads. With the analytical property model of the simplified structure, the pore-strut buckling behavior is analyzed for isotropic three-dimensional reticulated porous metal foams, and the failure modes resulting from this buckling are investigated for these materials under compressive loadings. These loading modes cover all of three loading conditions, including uniaxial compression, biaxial compression and triaxial compression. The treating ways of the pore-strut are relative to three slenderness-ratios, including three conditions of thin-long bar, middle-long bar and stocky bar. Based on these works, the mathematical relationships between nominal main stresses and porosity are found for this buckling failure of these materials under compression. Through the relevant expression, the relevant strength criterion and the relevant loading condition resulting in the strut buckling are further achieved for these porous metal foams under compression.     

Postbuckling analysis of laminated plates made of discontinuous metal matrix composites

The moderately large deflection response of laminated plates that are made of discontinuous metal matrix composites and are subjected to compressive in-plane loading, is addressed. Whisker-, particulate-, and plateletreinforced structures are considered. At each loading increment, a micromechanical analysis is performed to provide the overall instantaneous elastic-viscoplastic behaviour of the metal matrix composite. This is followed by a structural analysis which yields the response of a geometrically imperfect composite plate to applied external loading.

Results are presented for simply supported unidirectional and antisymmetric angle-ply SiC/Al plates, subjected to uniaxial compression, for two types of in-plane boundary conditions at unloaded edges. The effects of length-to-thickness ratio, fiber orientation angle, and temperature are illustrated. For all the cases considered, the corresponding behavior of continuous fiber SiC/Al plate with the same volume fraction is shown. Comparisons with the results, obtained by neglecting the viscoplastic effects of the metallic matrix, are presented.     

Triple-scale failure estimation for a composite-reinforced structure based on integrated modeling approaches. Part 2: Meso- and macroscopic scale analysis

In this article, the meso- and macroscopic failure features are discussed considering the identical composite component as in the foregoing article. In the meso-scale failure analysis, the risk of plastic instability of the composite tube was estimated considering the shakedown boundary as a failure criterion. The meso-scale stresses of the composite tube were computed using micromechanical homogenization and compared with the shakedown boundary of the composite obtained from the direct shakedown analysis. The stress states were close to the shakedown boundary indicating no critical danger of plastic failure. In the macro-scale failure analysis, the mechanical influence of the local composite integration was investigated with regard to the brittle failure risk of the neutron-embrittled component. To this end, a probabilistic failure analysis code was applied which was based on the fracture mechanics and the weakest-link failure theory. Various fracture criteria were considered. It was found that the failure risk of the tungsten block was strongly reduced by the composite reinforcement of the tube due to the intensification of compressive stress fields.