Off-axis Tension Tests Research Articles

In-plane Shear Measurements of Textile Composites with Large Unit Cells using the Plate Twist Test

The application of the plate twist test method to 3D-woven textile composites was investigated using both numerical analyses of the test set-up as well as experimental results. Comparisons with the widely used V-notched beam shear and 10°-off-axis tension tests are introduced in an attempt to identify the true in-plane shear response. The results of this study demonstrate that with careful specimen preparation and an adequate test fixture precise in-plane shear modulus data can be obtained. Moreover, for 3D-woven textile composites with their large unit cells the plate twist test appears to be superior in revealing the “true” in-plane shear behaviour.

Determination of G12 by means of the off-axis tension test. Part II: a self-consistent approach to the application of correction factors

Abstract Several different tests and procedures have been proposed for the determination of the intralaminar shear modulus G 12 in composite materials. The off-axis tension test, the ±45° test and the Iosipescu test are the most widely used. The advantages of the off-axis test are the ease of specimen preparation and the simplicity of the test procedure. A detailed review of the most significant papers on the determination of G 12 by means of the off-axis test has been presented in Part I of the present study. In this paper, a new graphical procedure, based on the numerical results from a test with clamped end conditions, is introduced. The use of these graphs requires the use of clamping conditions in the tests. Thus, an experimental study, using Moire interferometry, has been carried out to check the satisfaction of this condition in the most commonly used methods of performing the test. It has been found that a specimen with bonded end tabs reproduces the most reliable clamped condition. Finally, the results of several 10° off-axis tests on a graphite–epoxy composite are presented as a demonstration of the applicability of the correction scheme proposed in a wide range of specimen aspect ratios.

Determination of G12 by means of the off-axis tension test.

Abstract The determination of the intralaminar shear modulus G 12 in composite materials is by no means a straight forward task. Several different tests and procedures have been proposed, with the off-axis test achieving popularity as a result of its apparent simplicity. A review of the most significant studies and proposals connected with the determination of G 12 by means of the off-axis tension test is presented in this paper. The majority of the contributions can be classified in two general approaches. The first approach contains the proposals to modify the gripping system in order to allow the ideal deformation of the specimen. Particular attention is paid to the use of rotating grips and the results obtained. Reasons why other authors did not achieve satisfactory results are explored. In the second approach a coherent correction scheme, based on an iterative procedure applied to measurements obtained from the test, is presented. Finally, the dependence of the correction factor, based on the Pagano and Halpin analysis, upon the properties of the material is discussed.

An Investigation into the Accuracy of a One-Parameter Nonlinear Model for Unidirectional Composites

The nonlinear response of unidirectional composites has often been modeled using orthotropic plasticity theories. Of these, the one-parameter model developed by Sun and Chen is the simplest and hence attractive from the point of view of applications. The range of applicability of this theory is investigated by performing a number of off-axis tension tests on laminae, shear tests on 0°/90° laminates and uniaxial tension tests on +45°/–45° laminates. The theory appears to explain the results of off-axis tension test of laminae, but is unable to accommodate the full range of stress-strain response observed in the case of laminates. This is clearly due to extensive micro-cracking that laminates with orthogonal fiber orientations are able to withstand prior to failure—a phenomenon the single parameter model is not designed to account for. Nevertheless, it appears from the limited number of results presented, that the model if properly calibrated, can be used to predict the behavior of both laminae and laminates with reasonable accuracy.

A 3-D Finite Deformation Anisotropic Visco-Plasticity Model for Fiber Composites

A 3-D finite deformation anisotropic visco-plasticity model is presented for fiber composites in total Lagrangian co-ordinates. The plastic potential function is given by a quadratic function in stresses in the local co-ordinates system of the lamina. The model is used to derive the anisotropic plastic constitutive relation of a woven composite made of S-2 glass fibers embedded in polyester resinwith approximately 60% by weight of fibers. The coefficients of the constitutive model are experimentally determined through off-axis tension tests and out-of-plane shear tests. Off-axis tension tests are carried out by varying the angle between the fiber orientation and loading direction. The measured stress-strain curves are used to derive a master effective stress–effective plastic strain curve, which is described by two power laws. Amodified Arcan fixture is used to carry out pure shear tests to determine the out-of-plane shear coefficient. Compression tests are carried out to establish the material compressive response in the plane of the lamina and along the fiber direction. The anisotropic plasticity model is integrated into the in-house finite element code FEAP98. Numerical analyses are carried out for the off-axis tension tests and compression tests. These analyses show that the model reasonably predicts the constitutive response of wovenGRPcomposites in confirmation with the experimental data. The model further incorporates strain rate and temperature dependence on the anisotropic plastic flow constitutive law. Ballistic penetration simulations are carried out using the integrated code. The velocity at the back surface of the composite target, obtained by analyses, is compared with the data measured experimentally using interferometry. Insight into the failure process is obtained through analysis of different energy dissipation mechanisms.

Correlating Off-Axis Tension Tests to Shear Modulus of Wood-Based Panels

The weakness of existing relationships correlating off-axis modulus of elasticity \iE\dθ to shear modulus \iG\D12\N for wood composite panels is demonstrated through presentation of extensive experimental data. A new relationship is proposed that performs better than existing equations found in the literature. This relationship can be manipulated to calculate the shear modulus \iG\D12\N of orthotropic specimens such as wood-based panels solely from off-axis tension (or compression) tests. The proposed relationship reduces to classical theory for the isotropic case. In all cases tested, the newly proposed relationship is a better fit to the data than the traditional orthotropic elasticity equation.

Estimation of the shear strength of wood by uniaxial-tension tests of off-axis specimens

To determine shear strength we conducted uniaxial-tension tests of off-axis specimens and examined the proper off-axis angles. Sitka spruce (Picea sitchensis Carr.) and katsura (Cercidiphyllum japonicum Sieb. and Zucc.) were used for the studies. Uniaxial tension tests of the specimens with various off-axis angles were conducted, and the shear stress at failure was obtained. Independent of the tension tests, torsion tests were conducted, and the shear strengths were obtained. Comparing the data of the uniaxial tension and torsion tests, we examined the validity of estimating shear strength by the off-axis tension test. The shear strengths obtained from the tension tests coincided well with those measured by the torsion tests when the specimen had an off-axis angle of 15°–30°. In this off-axis angle range, the tensile stress perpendicular to the grain might have a serious influence on the shear strength, and we thought that the shear strength predicted by uniaxial tension tests should be treated as an approximate value despite the simplicity of the tension test. Other test methods should be adopted to obtain the precise shear strength of wood.

Experimental Characterization of Constitutive Models for PEEK Thermoplastic Composite at Heating Stage during Forming

The deformation behavior of APC-2 PEEK matrix thermoplastic composite at elevated temperatures was characterized in order to increase the understanding of the mechanics in solid-phase forming of advanced thermoplastic composite structures. Off-axis tension tests were performed to determine the inelastic and rate-dependent material properties. An anisotropic elastic/viscoplastic constitutive model was proposed to describe the inelastic and rate-dependent material responses. The constitutive model was evaluated using off-axis and laminate specimens subjected to uniaxial tension.

Biaxial Testing of Unidirectional Carbon-Epoxy Composite Using Biaxial Iosipescu Test Fixture

In this study, the biaxial Iosipescu test fixture has been employed to determine the failure properties of a unidirectional carbon-epoxy composite subjected to combined shear and transverse tensile stresses. The range of possible biaxial stress states has been extended by varying the fiber orientations in the Iosipescu specimens. Results obtained from the shear tests conducted using the biaxial Iosipescu test fixture have been compared with independent Iosipescu tests conducted at the University of Idaho, and with the 100 off-axis tension test. It is found that the Iosipescu shear testing of specimens with a fiber angle of 450 can provide reliable estimates for the transverse tensile strength of the material. Using this approach, the biaxial failure properties have been derived in the complete range of shear-transverse tension stress space.

(±45) Degree Off-Axis Tension Test for Shear Characterization of Plain Weave Fabric Composites

Different test techniques for shear characterization are reassessed for their suitability for woven fabric composites. Considering the accuracy of results, ease of specimen fabrication, and simplicity of test technique, (±45) degree off-axis tension test is recommended for shear characterization of woven fabric composites. Calculations show that for typical E-glass/epoxy LY556 and T-300 carbon/epoxy LY556 two-dimensional orthogonal plain weave fabric laminates, the percentage shear contribution at failure is about 99 and 97–98, respectively. Hence, this test technique leads to a near pure shear condition in the test section for WF laminates.

End effects for plane deformations of an elastic anisotropic semi-infinite strip

In the linear theory of elasticity, Saint-Venant's principle is used to justify the neglect of edge effects when determining stresses in a body. For isotropic materials, the validity of this is well established. However for anisotropic and composite materials, experimental results have shown that edge effects may persist much farther into the material than for isotropic materials and as a result cannot be neglected. This paper further examines the effects of material anisotropy on the exponential decay rate for stresses in a semi-infinite elastic strip. A linearly elastic semi-infinite strip in a state of plane stress/strain subject to a self-equilibrated end load is considered first for a specially orthotropic material and then for the general anisotropic material. The problem is governed by a fourth-order elliptic partial differential equation with constant coefficients. In the former case, just a single dimensionless material parameter appears, while in the latter, only three dimensionless parameters are required. Energy methods are used to establish lower bounds on the actual stress decay rate. Both analytic and numerical estimates are obtained in terms of the elastic constants of the material and results are shown for several contemporary engineering materials. When compared with the exact stress decay rate computed numerically from the eigenvalues of a fourth-order ordinary differential equation, the results in some cases show a high degree of accuracy. In particular, for strongly orthotropic materials, an asymptotic estimate provides extremely accurate estimates for the decay rate. Results of the type obtained here have several important practical applications. For example, they provide physical insight into the mechanical testing of anisotropic and laminated composite structures (including the off-axis tension test), are useful in assessing the influence of fasteners, joints, etc. on the behavior of composite structures and allow for “tailoring” a material with specific properties to ensure that local stresses attenuate at a desired rate.

Failure Behavior of Plain Weave Fabric Laminates Under In-Plane Shear Loading

A two-dimensional (2-D) woven fabric composite strength model is presented for the prediction of in-plane shear strength of 2-D orthogonal plain weave fabric laminates under inplane shear loading. The shear strength studies were carried out for three idealized laminate configurations. The possible shift of layers with respect to each other along x-, y-, and z-directions were considered for the laminates. The shear moduli were predicted for the three idealized laminate configurations using the 2-D woven fabric composite stiffness model. A comprehensive experimental program was carried out on different material systems using 10° and ±45° off-axis tension tests and the Iosipescu test. A good correlation is observed between the predicted and the experimental results.

Equivalence and instability correlations for isotropic and anisotropic sheet plasticity

Equivalent stress-strain correlations are given for deformation and instability in thin sheet materials for initial conditions of isotropy and anisotropy. The stress states apply to off-axis tension and circular bulge tests. A good correlation is provided from the Levy-Mises isotropic theory in the case of an annealed stainless steel. A general consideration of the strain at instability shows that the greatest strain is attained from equi-biaxial tension of flat sheet. Though bulge curvature reduces this maximum strain to 77.5%, it still remains more than 100% greater than the strain at instability in a pressurised sphere and 40% greater than the tensile instability strain. For anisotropic brass the bulge test supplies the greatest instability strain (60%) of all in-plane biaxial tests. It is shown that the orthotropic theory of Hill can provide a satisfactory correlation between the flow curves from these tests. The anisotropy is characterised from the constant ratio ( r-value) between incremental plastic strains in the width and thickness directions for tensile testpieces cut from the principal orthotropic directions. These r-values re-appear within new analyses of the instability in a spherical vessel and in the hydrostatic bulging of anisotropic material.

Overall Instantaneous Viscoplastic Properties of Composites

A computer procedure was developed to analyze viscoplasticity and strain rate effects of continuous fiber reinforced composite materials. The procedure provides the overall instantaneous moduli, coefficients of thermal expansion and micromechanical stresses and strains. It was implemented with the help of a displacement formulation of the finite element method and a periodic hexagonal array microstructural model. It also can be adapted to evaluate instantaneous overall viscoplastic properties at integration points in finite element analyses of composite structures. This procedure was primarily developed to simulate the viscoplastic response of an aramid/epoxy laminate, used in the production of ARALL® laminates, in an off-axis tension test.

Characterization of Elastic-Plastic Behavior of AS4/PEEK Thermoplastic Composite for Temperature Variation

Inelastic and strength properties of AS4/PEEK composites were characterized with respect to temperature variation by using a one-parameter orthotropic plasticity model and a one-parameter failure criterion. Simple uniaxial off-axis tension tests were performed on coupon specimens of unidirectional AS4/PEEK thermoplastic composite at various temperatures. To avoid the complication caused by the extension-shear coupling effect in off-axis testing, new tabs were designed and used on the test specimens. Experimental results showed that the nonlinear behavior of constitutive relations and the strength can be characterized quite well using the one-parameter plasticity model and the failure criterion, respectively.

Mechanical Characterization of SCS-6/Ti-6-4 Metal Matrix Composite

Off-axis tension tests were performed on SCS-6/Ti-6-4 metal matrix com posite. A one-parameter plasticity model was used to characterize the elastic-plastic prop erties. In addition, a micromechanical model was developed assuming elastic fiber and elastic-plastic matrix properties. This model was employed to relate the apparent yielding with the fiber/matrix separation in the MMC. From the micromechanical model, the fiber/matrix interfacial bond strength was estimated, and with the aid of a damage model, the nonlinear off-axis stress-strain curves were accurately predicted.

Mechanical Response of Aramid/Epoxy under Tensile, Compressive and Shear Loading

The results of an extensive room temperature mechanical characterization of unidirectional aramid/epoxy carried out using three different test methods are summa rized. The three test methods employed in the above investigation were: off-axis tension and compression tests and Iosipescu shear test. The tests were performed under mono tonic and quasi-static cyclic loading conditions in order to determine the initial elastic pa rameters and characterize subsequent nonlinear response, including failure stresses, under combined loading. It is demonstrated that the initial elastic response correlates well with the predictions of the transformation theory. The nonlinear response obtained from the three test methods is essentially dissipative, exhibiting apparent permanent strain accu mulation and hysteresis loops. The extent and character of nonlinearity and dissipation is a function of the applied load with respect to the fiber direction and depends on the sign of the load. The dramatic differences observed in the nonlinear response and failure char acteristics in tension and compression are discussed in terms of different mechanisms operative at the microscale.

A New End Tab Design for Off-Axis Tension Test of Composite Materials

A new end tab fabricated using a fiberglass knit in a silicon rubber matrix was used for off-axis tension tests on unidirectional composites. It was shown experimentally that this new tab allowed shear deformation of the specimen to occur in the hydraulic grips and, thus, produce a uniform state of strain in the specimen. Consequently, the shear modulus could be determined accurately using this new tab. Furthermore, due to the absence of stress concentrations, this procedure could also be used to determine shear strength.

The improvement of end-boundary conditions for off-axis tension specimen use

Inaccuracies in using the off-axis tension test are caused by the nonuniform stresses produced by the end constraints. The purpose of this study is to show that these nonuniformities can be virtually eliminated by adjusting the amount of tab clamping and selectively locating the point about which the clamp may rotate.

A Methodology for Accurate Shear Characterization of Unidirectional Composites

A combined experimental-analytical methodology is presented for accurate determina tion of the intralaminar shear modulus G 12 of unidirectional composites using the off-axis tension test and/or the Iosipescu test. It is demonstrated that consistent values of G 12 can be obtained with the two methods provided that: (1) specimen geometry is optimized for the off-axis test, (2) correction factors are employed to account for the shear stress nonuni formity in the test section of Iosipescu specimens. The problem of measuring the shear strength with these specimens is discussed. The 45-deg off-axis tensile coupon is recom mended for determination of the shear modulus. The 0-deg Iosipescu specimen is recom mended for determining an upper lower bound on the shear strength.