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.     

Shear fracture tests of concrete

Symmetrically notched beam specimens of concrete and mortar, loaded near the notches by concentrated forces that produce a concentrated shear force zone, are tested to failure. The cracks do not propagate from the notches in the direction normal to the maximum principal stress but in a direction in which shear stresses dominate. Thus, the failure is due essentially to shear fracture (Mode II). The crack propagation direction seems to be governed by maximum energy release rate. Tests of geometrically similar specimens yield maximum loads which agree with the recently established size effect law for blunt fracture, previously verified for tensile fracture (Mode I). This further implies that the energy required for crack growth increases with the crack extension from the notch. The R-curve that describes this increase is determined from the size effect. The size effect also yields the shear fracture energy, which is found to be about 25-times larger than that for Mode I and to agree with the value predicted by the crack band model. The fracture specimen is simple to use but not perfect for shear fracture because the deformation has a symmetric component with a non zero normal stress across the crack plane. Nevertheless, these disturbing effects appear to be unimportant. The results are of interest for certain types of structures subjected to blast, impact, earthquake, and concentrated loads.     

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.     

Theoretical and experimental evaluation of the Iosipescu shear test

Use of the Iosipescu shear test for measurement of shear properties of unidirectional laminae has been studied both analytically and experimentally. The intralaminar shear strength and shear stiffness of glass-reinforced polyester material have been measured using specimens with two different fibre orientations. Acoustic emission has been monitored and a fractographic study carried out. A finite element analysis has been conducted to evaluate the stress distribution within the specimen, assuming isotropic and orthotropic elastic properties of the material. There is a complicated stress distribution in the specimen, particularly in the vicinity of each notch root, depending on the elastic properties of the composite. The shear stress region in the specimen gauge section is almost uniform, though small normal compressive stresses exist. The experimental results have shown that the measured shear modulus does not depend on reinforcement orientation. However, it has been observed that different failure modes occur in each case. This results in a change in apparent shear strength of the composite with fibre orientation. Some explanations of these differences have been found in a detailed analysis of the local stresses at the roots of the notches. It is considered that the presence of tensile stresses in this area is primarily responsible for the apparent reduction in the shear strength.     

紧凑四点剪切砼Ⅱ型断裂实验研究与数值分析

有关Iosipescu四点剪切实验的争论很多,国内外的研究者提出了各种各样的结果。本文对紧凑四点剪切加载下砼Ⅱ型断裂破坏机理进行了大量的实验研究和数值分析。通过对实验中试件裂纹的扩展过程及开裂角cq等的分析,结合有限元法对裂尖应力场进行了研究。分析了加载点与裂纹面的距离对Ⅱ型断裂性能的影响。可以看出,虽然随着加载点与裂纹面距离的缩短,CKⅡ在逐渐增大,但纯剪加载下裂纹扩展并不是沿剪应力所在方向,即破坏并不是剪切破坏。     

Assessment of interfacial bonding between polymer threads and epoxy resin by transverse fibre bundle (TFB) tests

Two experimental approaches were employed to assess the fibre/matrix adhesion between polymer threads and epoxy resin by transverse fibre bundle (TFB) tests. The first approach was to measure interfacial bonding strength of the fibre/matrix interface in dog-bone-shaped tensile specimens by applying normal stress until failure, simulating the Mode I failure mode. The second approach was to determine the fibre/epoxy interfacial bonding strength in shear (simulating the Mode II failure mode) by means of a V-notched beam shear testing method, i.e. a modified Iosipescu test. In both methods, polymer threads were transversely incorporated in the middle section of the specimens. It was found that both methods were simple, reliable, and sensitive to changes in the fibre/matrix adhesion conditions, though interpretation of the test results was somewhat complex. The two experimental approaches were able to produce consistent results and can thus be adopted as alternative methods for determining the interfacial bonding properties between fibres and matrix in composite systems where conventional micro-mechanical or macro-mechanical testing methods cannot be used.     

ELASTIC-PLASTIC FINITE ELEMENT ANALYSES OF CRACKED NOTCHES IN PLATES UNDER MIXED-MODE LOADING

Plates containing inclined elliptical notches with and without cracks are analysed for five different major axis inclinations, namely, θ equal to 0, 11.25, 22.5, 33.75 and 45?. Short cracks emanating from the roots of such inclined circular and elliptical notches are analysed. Under various mixed-mode loading conditions, when cracks are present, a simple method is described for the evaluation of the Mode I and Mode II stress intensity factors, for the elastic state. Values of the J integral along three different contours are also evaluated in the elastic and elastic-plastic states, as well as the Mode I and Mode II components of the crack tip opening displacenient (CTOD), the maximum principal stresses, the maximum average equivalent plastic strains and the location of the elements in which they occur. Comparisons of the different loading conditions are presented and the implications of the effects of mixed-mode loading on fracture and fatigue crack propagation are discussed.     

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.     

Tensile fracture of soft and hard PZT

Power applications generate high stresses which can damage piezoceramic components. In this study tensile fracture of several types of PZT (hard/soft) is investigated. After validation of the specimen geometry by means of numerical simulation, samples are led to failure using a specific device. Weibull law parameters enable the characterisation of the tensile strength distribution and highlight clear differences between soft and hard ceramics. A fractographic approach emphasises the specificities of the fracture mode and the fracture origin for each type of samples.     

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.     

A STUDY OF SUPERIMPOSED FRACTURE MODES I, II AND III ON PMMA

A test apparatus has been developed to study the fracture behaviour of engineering materials subjected to superimposed tensile and shear (Mode I and II) loads using a single edge notch specimen. Stress intensity factors were calculated using finite element analysis. Test results for PMMA are reported. Results from circumferentially notched round bar specimens, subjected to combined tension and torsion (Mode I and III) loading are also reported. The Mode I/II results are consistent with the mixed mode fracture response of a wide range of brittle materials, although there is some evidence of non-linear behaviour. The fracture behaviour for superimposed Modes I and III indicates the strong influence of non-linear deformation which causes the mixed mode toughness to be dependent on the sequence and type of loading.     

Toughness of high-density polyethylene in shear fracture

This paper evaluates the validity of a new test methodology for measuring shear fracture toughness (mode II) of high density polyethylene (HDPE). The methodology adopts Iosipescu test for the shear loading, and determines the toughness based on the essential work of fracture (EWF) concept. The results show that even under the Iosipescu loading, tensile deformation (mode I) is still involved in the fracture process, possibly due to the significant work hardening that HDPE develops during the plastic deformation. The study found that the mode II fracture toughness can be determined through data analysis using double linear regression, i.e., by extrapolating specific work of fracture to zero ligament length and zero ligament thickness. The paper demonstrates that the new test methodology can be used to evaluate mode II fracture toughness of ductile polymers like HDPE in which significant work-hardening may be involved in the fracture process. The paper also provides quantitative comparison of the fracture toughness for HDPE in mode II with its mode I counterpart.     

Optimization of the adhesive joint Iosipescu specimen for pure shear test

This paper reviews some of the difficulties encountered in achieving a uniform stress distribution within the adhesive layer of bonded Iosipescu shear test specimens. The asymptotic singular stress field at the terminus of a skewed bimaterial interface, intersecting the straight free surface of the wedge, has been studied macroscopically and microscopically using the finite element method (FEM) and the finite element iterative method (FEIM). Different mechanical properties of the adhesives and adherends, and various skewed interface angles have been considered in this study. A critical skewed interface angle, _c=126°, has been found beyond which the interfacial stress singularity vanishes. This critical angle is independent of the elastic properties of the adhesives and adherends. Based on the results obtained in the present investigation, in conjunction with recently reported research on sharp notches [7, 21–23], an optimized adhesive joint Iosipescu specimen geometry is proposed. This specimen should be capable of generating a uniform shear stress state within its adhesive layer under pure shear loading conditions.     

Fatigue behaviour of clinched joints in a steel sheet

Clinch joining has been used in sheet metal work owing to its simplicity and because it facilitates the joining dissimilar metal sheets. In this study, monotonic and fatigue tests were conducted using coach‐peel and cross‐tension type specimens to evaluate the fatigue strength of clinch joints in a cold‐rolled mild steel sheet. The monotonic experimental results reveal that the coach‐peel specimen exhibits the lowest monotonic strength among the three specimen configurations. The coach‐peel and cross‐tension specimen geometries exhibit very low fatigue ratios, compared with the tensile‐shear specimen. The maximum von Mises and principal stresses at the fatigue endurance limit are much higher than the engineering tensile strengths of the steel sheet used to determine the three specimen geometries. Compared with the effective stress and maximum principal stress, the Smith–Watson–Topper fatigue parameter can be used for an appropriate prediction of the current experimental fatigue life. With regard to the coach‐peel specimen geometry, all samples exhibit pull‐out failure mode in the fatigue testing range. However, for the cross‐tension specimen geometry, mixed (pull‐out and interface) and interface failure modes occurred, depending on the number of cycles to failure.     

A new mode I fracture test for composites with translaminar reinforcements

A new test method is presented for Mode I delamination fracture toughness testing of laminated composites containing a high density of stitches or translaminar reinforcements. The test set up, which is similar to the standard Double Cantilever Beam test, induces an axial tension in the specimen in addition to the transverse forces responsible for propagation of delamination. The tensile stresses reduce the compressive stresses in the vicinity of the crack tip caused by the large bending moments required for crack propagation. The nonlinear differential equations of equilibrium of the new specimen are solved using an iterative procedure to obtain the strain energy release rate as a function of load and crack length. Experiments were conducted using carbon/epoxy specimens containing 6.2 stitches per square centimeter (40 stitches per square inch). Results include Mode I fracture toughness, crack tip bending moment, transverse deflection and slope as a function of crack length. It is found that the apparent fracture toughness of the specimens tested remains constant as the stitches break and crack propagates, and is about sixty times that of unstitched specimens.     

The dependence of the strength of zinc sulphide on temperature and environment

The dependence of the strength of zinc sulphide on temperature, environment, surface finish and specimen size has been assessed. Room-temperature fracture stresses were determined using a bursting disc geometry for a number of different surface finishes and for two different sample sizes. High and low-temperature fracture stresses in a dry nitrogen atmosphere were obtained from experiments using the Brazilian test geometry and showed that the average strength of the material remained above or equal to the room-temperature value within the range –70 to +600 °C. The Brazilian test is an indirect tensile technique which is attractive for its experimental simplicity but gives fracture stress values which are consistently below those obtained by direct tensile techniques. The data from this test were therefore compared at room temperature to results obtained from the bursting disc test on samples which had been prepared using the same techniques. The possibility of delayed failure through environmentally enhanced slow crack growth was evaluated using the double-torsion technique which revealed slow crack growth below the critical stress intensity factor.     

Influence of notch severity on the impact fracture behavior of aluminum alloy 7055

In this paper, the conjoint influence of notch severity and test temperature on the impact behavior of an Al-Zn-Mg-Cu alloy 7055 in the T7751 microstructural condition is presented and discussed. Notch angles of 45°, 75° and 90° were chosen for a standard charpy impact test specimen containing two notches. For a given angle of the notch the increase in dynamic fracture toughness, with test temperature, is most significant for the least severe of the notches, i.e. 45°. At a given test temperature, the impact toughness of the T7751 microstructure decreased with an increase in notch severity. An increase in notch severity resulted in essentially Mode I dominated fracture at all test temperatures. The influence of localized mixed-mode loading is minimal for the alloy has low dynamic toughness. The impact fracture behavior of the alloy is discussed in light of alloy microstructure, mechanisms governing fracture and the deformation field ahead of a propagating crack.     

Evaluation of in-plane ply shear properties from unidirectional plate torsion tests

Torsion tests were conducted on unidirectional carbon/epoxy laminated plates. Preliminary finite element analyses showed that the specimen geometry selected avoided pronounced geometric non-linearity and ensured that a significant volume of material would be under a high fraction of the maximum shear stress. Furthermore, the clear prevalence of in-plane shear stresses allowed the development of a simplified data analysis model. Calculated shear-stress strain curves were consistent with the results of tensile tests on angle-ply coupons, despite lower failure stresses that may have been caused by surface defects or by spurious transverse tensile stresses. Nevertheless, the unidirectional plate torsion test is worthy of further research, given the structural relevance of torsional loads and the problems of in-plane shear tests methods.     

Nonlinear Analysis of Woven Fabric-Reinforced Graphite/PMR-15 Composites under Shear-Dominated Biaxial Loads

An elastic-plastic, time-independent, macroscopic, homogenous model of an 8HS woven graphite/PMR-15 composite material has been developed that predicts the nonlinear response of the material subjected to shear-dominated biaxial loads. The model has been used to determine the response of woven composite off-axis and Iosipescu test specimens in nonlinear finite element analyses using a multilinear averaging technique. The numerically calculated response of the specimen was then compared to experimentally obtained data. It has been shown that the numerically calculated stress - strain diagrams of the off-axis specimens are very close to the experimentally obtained curves. It has also been shown that the numerically determined shear stress - strain and load-displacement curves of the woven Iosipescu specimens are close to the experimentally obtained curves up to the point of significant interlaminar damage initiation and propagation. The results obtained in this study clearly demonstrate that the nonlinear material behavior of the graphite/polyimide woven composites subjected to shear-dominated biaxial loading conditions cannot be ignored and should be considered in any stress analysis. The linear-elastic approach grossly overestimates the loads and stresses at failure of these materials in the off-axis and Iosipescu tests. It can be assumed that the same discrepancies will arise in the numerical analysis of the woven composites tested under other biaxial shear-dominated loading conditions using other biaxial test methods.