Processing dependent morphology, interfacial interaction and shear behavior of short carbon fiber reinforced PEEK

Shear properties of compression-molded discontinuous AS4 carbon fiber reinforced poly(ether ether ketone) composites are evaluated with Iosipescu tests. It is found that both shear modulus and shear strength strongly depend on the molding conditions. A comparison between shear behavior of the composites and composites morphology reveals that fusion of the composite pellets and fiber/matrix interfacial interaction on molecular scale are the two dominant factors which determine the processing dependent shear properties of the materials studied.     

Analysis of the Iosipescu shear test as applied to unidirectional carbon-fibre reinforced composites

Intralaminar shear properties of unidirectional carbon-fibre reinforced epoxy and PEEK composites were investigated using the Iosipescu shear test. The apparent shear strength and shear moduli were measured using specimens with two different fibre orientations. Finite element analysis was applied to determine the stress distribution within the Iosipescu specimen. Using numerical correction factors to account for the non-uniform shear stress distribution in the gauge-section of Iosipescu specimens, the actual shear moduli were established. The Iosipescu shear test also provided a reasonable estimate of the shear strength.     

Hygrothermal effects on the shear properties of carbon fiber/epoxy composites

The environmental factors, such as humidity and temperature, can limit the applications of composites by deteriorating the mechanical properties over a period of time. Environmental factors play an important role during the manufacture step and during composite’s life cycle. The degradation of composites due to environmental effects is mainly caused by chemical and/or physical damages in the polymer matrix, loss of adhesion at the fiber/matrix interface, and/or reduction of fiber strength and stiffness. Composite’s degradation can be measure by shear tests because shear failure is a matrix dominated property. In this work, the influence of moisture in shear properties of carbon fiber/epoxy composites (laminates [0/0]_s and [0/90]_s) have been investigated. The interlaminar shear strength (ILSS) was measured by using the short beam shear test, and Iosipescu shear strength and modulus (G ₁₂) have been determinated by using the Iosipescu test. Results for laminates [0/0]_s and [0/90]_s, after hygrothermal conditioning, exhibited a reduction of 21% and 18% on the interlaminar shear strenght, respectively, when compared to the unconditioned samples. Shear modulus follows the same trend. A reduction of 14.1 and 17.6% was found for [0/0]_s and [0/90]_s, respectively, when compared to the unconditioned samples. Microstructural observations of the fracture surfaces by optical and scanning electron microscopies showed typical damage mechanisms for laminates [0/0]_s and [0/90]_s.     

Determination of shear strength of unidirectional composite materials with the Iosipescu and 10° off-axis shear tests

The purpose of this research was to determine the shear strength of a unidirectional carbon-fibre/epoxy composite by means of the 10° off-axis and 0° Iosipescu specimens subjected to shear. Detailed non-linear finite-element computations of these two tests were conducted, taking into account the actual non-linear material behavior of the composite. The tests were compared in terms of stresses and strains at failure. It was found that the shear strength of the composite can be very accurately determined by using the two independent testing techniques only if fully non-linear finite element computations of the tests are performed. The stresses and strains at failure in the 10° off-axis specimen closely match the stresses and strains at the onset of intralaminar damage near the roots of the notches in Iosipescu specimens. Owing to the difficulties associated with the measurement of the shear strength of the composite using the Iosipescu test, and in particular, with the interpretation of the experimental data, this test was found to be almost impractical for the determination of shear strength. The test can only be used if fully non-linear finite element computations of uncracked and axially cracked Iosipescu specimens are conducted in conjunction with the continuous monitoring of intralaminar damage near the roots of the notches during testing. In addition, the shear strength results obtained from the Iosipescu specimen should be independently verified by using another method, such as the 10° off-axis test.     

Towards rapid screening of new composite matrix resins

A study to test the correlations among neat-resin and composite properties was performed, to identify possible ways of reducing the number of tests necessary to screen candidate resins for use in aerospace structures. Neat-resin and composite tests were conducted for four different resin matrices, whose neat properties varied over a wide range, allowing reasonably strong tests of correlations. Since different fiber architectures and fiber types are often of interest to a designer, tested correlations included those between composite coupons containing the same resin but with different fibers and lay-up. Composite structural properties were represented by unnotched tension, open-hole tension, unnotched compression, and open-hole compression. Correlations were sought between these and neat-resin properties, composite shear strength, and Mode I composite delamination toughness. Strong linear correlations, which are proposed to be the most useful because they discriminate best between different resin systems, were found in certain cases, but not others. Most significantly, an Iosipescu shear test for a [0°/90°] composite of one fiber type was shown to be a good predictor, for the tested resins, of open-hole compression and unnotched compression in composites with different fibers and architecture; and a fair predictor of open-hole tension. Open-hole tension strength was shown to correlate better with neat-resin fracture toughness, but neat-resin properties were otherwise inferior indicators of most composite properties. While data for further resins are needed to enrich the statistical base, some rationalizations can be found for the presence of the stronger correlations that were observed.The correlations described here suggest that reduced test matrices, containing tests for only Iosipescu composite shear and either neat-resin toughness or Mode I delamination toughness, may suffice to evaluate the likely structural performance of composites containing new candidate resins. The tests could be performed on a standard fiber type and architecture, yet be predictors for other fiber types and architecture.     

Critical Examination of the Iosipescu Shear Test as Applied to 0degrees Unidirectional Composite Materials

Several issues regarding the application of the shear and biaxial Iosipescu tests for the shear strength characterization of unidirectional composite materials are addressed in this article. First, the nonlinear effects of specimen sliding and geometric nonlinearity on the mechanical response of 0degrees standard unidirectional graphite/polyimide Iosipescu specimens with different loading conditions and loading block geometries have been investigated. Second, an attempt has been made to improve the Iosipescu shear test to eliminate normal compressive stresses in the specimen gauge section and at the same time prevent axial splitting. Finally, several Iosipescu shear and biaxial experiments have been performed to select proper specimen geometry and loading conditions for the shear strength measurements of unidirectional composites. The nonlinear effects are examined with respect to various coefficients of friction, displacements, loading angles, and fixtures (biaxial with short and modified biaxial with long loading blocks) using nonlinear finite-element techniques. It is shown that the effect of nonlinearity is small on the stresses at the center of the standard Iosipescu specimen, but significant for the stresses near the notch root up to 2 mm applied displacements. In some cases, significant differences in the stresses calculated for different coefficients of friction have been observed. All of these results are somewhat consistent for both fixtures, but with the stress components sigma x, sigma y, and sigma xy significantly lower in the standard Iosipescu specimens tested in the fixture with the long blocks. Numerical load/displacement diagrams show that specimen sliding and geometric nonlinearity have a negligible effect on reaction forces in the biaxial fixture, and a significant effect on the reaction forces in the modified biaxial fixture. Since the various combinations of the loading conditions evaluated in this study do not eliminate transverse compressive stresses in the gauge section of the standard Iosipescu specimens, a major improvement to the Iosipescu shear test has been proposed. Using an optimized specimen geometry subjected to biaxial shear/tension loading conditions, a state of almost uniform pure shear stress can be generated in 0degrees unidirectional composite Iosipescu specimens without the possibility of axial splitting along the fibers at the roots of the notches. However, it is shown in the experimental part of this study that for the optimized Iosipescu specimen, crushing at the inner loading blocks can significantly affect the shear intralaminar failure process. Only by reducing the cross-sectional area of the optimized Iosipescu specimen can the effect of crushing on the failure process be reduced without, however, highquality shear stress fields present in the gauge section at failure.     

Predicting failure in textile composites using the Binary Model with gauge-averaging

First introduced over a decade ago, the Binary Model has evolved into a computationally efficient tool for predicting the properties of textile composites. Key to the formulation is the question of what details of the textile composite and the distributions of stress, strain, temperature, etc., are necessary and sufficient to represent the physics of the problem adequately and to ensure useful engineering predictions. This paper is concerned specifically with the prediction of the ultimate strength in cases where failure follows a single substantial local damage event, such as the rupture or kinking of a tow or the creation of a shear band mediated by matrix damage, without further increase in the external load. The accuracy of predictions is assessed for some triaxially braided carbon/epoxy composites. A gauge length is introduced that is suggested by the micromechanics of the failure mechanisms. Predictions are made by reference to strains that are averaged over a volume whose sides are commensurate with this gauge, but nevertheless retain spatial variations associated with the textile architecture. Failure criteria for tow rupture and matrix shear failure are taken from a single un-notched tensile test; the calibrated model then successfully predicts the failure mechanism (matrix shear or fiber rupture) and ultimate strength in un-notched and open-hole tension tests for any orientation of the textile fabric relative to the load axis, as well as bending and simple shear tests. The successful predictions are made using strains calculated for an entirely elastic representation of the material, which is possible because of the brittle character of the stress-strain curves. Predictions are also attempted using strains computed under the assumption that the textile material is homogeneous. These predictions are significantly inferior.     

2D-C/SiC复合材料面内剪切性能统计及强度B基准值

针对2D-C/SiC复合材料进行大子样面内剪切实验,研究材料面内剪切模量和强度的分布规律及强度B基准值。运用线性回归结合假设检验的方法,确定2D-C/SiC复合材料面内剪切力学性能的分布规律及参数,对比两种不同经验失效概率得到统计结果;通过观察试样最窄净截面微CT照片及断口电镜扫描照片,解释材料面内剪切强度分散性微观机制,基于分布规律,最终计算得到2D-C/SiC复合材料面内剪切强度威布尔B基准值。结果表明:强度和模量均同时服从威布尔、正态和对数正态分布,且理论模型与实验结果吻合良好,两种经验失效概率不影响力学性能分布规律;面内剪切强度分散性与最窄净截面致密度和界面脱粘长度有关;2D-C/SiC复合材料面内剪切强度威布尔B基准值为80.41MPa。     

Shear strength of polymers and fibre composites: 1. thermoplastic and thermoset polymers

The shear strengths of eight thermoplastics and three DGEBA-based epoxies in sheet form have been tested by the punch and Iosipescu tests. The testing temperatures ranged from 20 to 120°C, and the glass transition temperatures were measured as well. The shear strengths of the epoxies were also estimated from compressive tests on short cylinders. The Iosipescu test gave very unreliable results for polymers in the rubbery state because large deformations were induced before failure, and this caused high tensile stresses to develop instead of high shear stresses. With the punch test the force often had two maxima before failure, and the discs punched out did not have straight sides, so there was also much doubt about the purity of the shear stresses developed. The two methods were often in sharp disagreement. However, they gave comparable results with epoxies tested at room temperature. Comparison with compressive tests indicated that the ratio of compressive yield strength to shear yield strength varied from 1.5 to 2.4. In view of the uncertainties in the tests, compressive testing may be a good method to obtain an approximate value for the shear strength. The Tresca criterion (i.e. divide the compressive yield strength by two to get the shear yield strength) is probably good enough in view of the uncertainties in the shear tests. It works equally well for the ultimate shear strength. A new and better test is clearly needed for the estimation of the shear strength of polymers in sheet form.     

Multiscale progressive failure analysis of textile composites

The experimental determination of stiffness and strength of textile composites is expensive and time-consuming. Experimental tests are only capable of delivering properties of a whole textile layer, because a decomposition is not possible. However, a textile layer, consisting of several fiber directions, has the drawback that it is likely to exhibit anisotropic material behavior. In the presented paper a finite element multiscale analysis is proposed that is able to predict material behavior of textile composites via virtual tests, solely from the (nonlinear) material behavior of epoxy resin and glass fibers, as well as the textile fiber architecture. With these virtual tests it is possible to make predictions for a single layer within a textile preform or for multiple textile layers at once. The nonlinear and pressure-dependent behavior of the materials covered in the multiscale analysis is modeled with novel material models developed for this purpose. In order to avoid mesh-dependent solutions in the finite-element simulations, regularization techniques are applied. The simulations are compared to experimental test results.     

基于纤维束增强树脂基复合材料测试的单向层合板层间剪切性能的预估方法

纤维束增强树脂基复合材料（FBC）及其单向层合板在标准Iosipescu剪切实验中表现出非常相似的破坏特征,然而测量得到的剪切强度却有明显差异。本文使用两种碳纤维和两种环氧树脂制备了3种FBC和单向层合板,对FBC剪切强度和单向层合板层间剪切强度进行了测量与分析。应用界面单元方法分析了纤维束与基体之间的界面应力场,发现FBC剪切试件中纤维束/基体界面附近的应力状态为拉剪耦合,而单向层合板中界面处于纯剪切应力状态,这一差异导致FBC剪切实验测量的强度低于单向层合板的剪切强度。本文基于Yamada-Sun强度理论建立了FBC剪切强度与单向层合板剪切强度之间的关系模型,应用该模型预测的单向层合板剪切强度与实测强度之间达到良好的一致性,相对偏差为10%左右。根据本文提出的方法,通过制样较简单的FBC试验能够预测和评估相应单向层合板的层间剪切性能。     

Experimental and computational composite textile reinforcement forming: A review

Preforming is an important step in the manufacturing of textile-reinforced composites with resin infusion processes. It is important to control the fiber orientation to avoid fiber misalignments and wrinkles, which would reduce the mechanical properties of the composite part. The objective of the present paper is to give an overview of the literature dedicated to the textile reinforcement forming process. Therefore, experimental tests for the determination of the basic fabric properties, the experimental characterization of the forming and the numerical approaches for the modeling of the textile forming are reviewed. A great part of the literature has been devoted to the characterization of the shear behavior since it is the most important property for textile reinforcement forming processes. The bending behavior was initially neglected in mechanical models but was found to be important for the simulation of wrinkles.     

Studies of microstructural and mechanical properties of Nylon/Glass composite Part II The effect of microstructures on mechanical and interfacial properties

This is a part of a series of studies on the influence of thermal processing on microstructures and mechanical properties of thermoplastic composites. In this paper, the effect of cooling rate during thermal moulding processes on the mechanical properties of bulk unidirectional commingled yarn GF/PA6 composites (Iosipescu shear strength, transverse flexural tensile strength and elastic modulus) has been investigated. Cooling rate from fast to slow, –60°C/min, –3°C/min and –1°C/min, were achieved at 1.5 MPa pressure. Scanning electron microscopy (SEM) was used to analyse the damaging mechanisms of the fracture surfaces of the tested samples. The different dynamic responses of the samples were observed by polarised optical microscopy (POM) during the mechanical tests. The results indicated that when the cooling rate was varied from fast to slow, the interfacial tensile and shear strength were improved associated with enhanced elastic modulus. These results may be attributed to the slow cooling achieved a high transcrystallinity between the glass fibres and PA6 matrix, and high crystallinity of phase in the PA6 matrix.     

塑木复合材与木材主要力学性质的比较研究

分析了塑木复合材与木材的抗弯强度、抗弯弹性模量、抗压强度和抗剪强度之间的差异,结果表明,塑木复合材的抗弯性能远低于鹅掌楸(Liriodendron sp.)和速生杨木(Populussp.);纵向抗压强度也低于木材的顺纹抗压强度,横向抗压强度为木材横纹抗压强度的2.95倍(I-69杨)～3.74倍(鹅掌楸),纵向抗剪强度与木材的顺纹抗剪强度与木材相当.因此可采用降低密度、改进材料结构或改进材料成型方式来增加塑木复合材料的抗弯性能和纵向抗压性能,以扩大其应用范围.     

Static Properties of Fibre Metal Laminates

In this article a brief overview of the static properties of Fibre Metal Laminates is given. Starting with the stress-strain relation, an effective calculation tool for uniaxial stress-strain curves is given. The method is valid for all Glare types. The Norris failure model is described in combination with a Metal Volume Fraction approach leading to a useful tool to predict allowable blunt notch strength. The Volume Fraction approach is also useful in the case of the shear yield strength of Fibre Metal Laminates. With the use of the Iosipescu test shear yield properties are measured.     

An orthotropic shear specimen in computerised stress analysis

The Iosipescu specimen, invented for investigation of shear strength of orthotropic materials, in its original shape did not meet expectations, producing a complex stress pattern. In the present investigation a improved shape of the specimen is presented, which depends sharply on tested material properties. Resulting relations both for the shape of the specimen (1) and calculation of shear strength (2) are given. The research has shown a strong dependence of the shape of the specimen on elastic properties of the tested material.     

A photoelastic investigation of asymmetric four point bend shear test for composite materials

The asymmetric four point bend (AFPB) test, which utilizes a notched specimen and is a modification of the Iosipescu test, is investigated for measuring the shear stress fringe value of orthotropic birefringent model materials. The influence of the notch parameters, such as the notch angle and the notch radius, is first studied in the case of an isotropic model material; a notch angle of 120°, with or without a radius at the notch tip, lowers the shear stress concentration and leads to a better stress fringe value compared to a 90° notch. In the case of a unidirectionally reinforced glass-polyester model material, while the shear stress fringe values given by the 90°, 120°, sharp and radiused notches are reasonably close to the value obtained for an off-axis tensile specimen, no conclusions about the influence of the notch parameters can be drawn due to the peculiarities of the photoelastic response of the inhomogenous orthotropic model material. However, the failure modes indicate that a notch radius and the 120° notch angle reduce the stress concentration. Comparison is made with finite element results.     

Shear strength of polymers and fibre composites: 2. carbon/epoxy pultrusions

Pultrusions were made with carbon fibres and an epoxy resin. Three different curing agents were used, so that the matrices were resins with different glass transition temperatures. The composites were tested for shear strength at different temperatures, so that the effect of the resin shear strength on composite shear strength could be observed, with a fixed fibre architecture. It was found that the composite was always much stronger than the resin both for the 0 and 90° fracture modes. The 90° fracture surfaces contained many broken fibres, and shear hackles were observed in the resin-rich regions. These suggested that shear failure (rather than tensile failure) took place in the Iosipescu test for the 90° specimens. It was concluded that the fibre architecture played a dominant role in the composite shear strength, with interphase effects being involved also.