V-notched Rail Shear Research Articles

Design of a Shear Test Fixture System and Optimization of a Metal Shear Specimen for Cyclic Loading

Abstract The objective of the present paper is to investigate the optimization of the existing sheet metal specimen geometry and the optimization of fixtures for fatigue shear tests. Simple static shear tests are used for the material characterization of sheet metal and composite materials. It is well known that it is difficult to achieve pure shear conditions in the specimens because of stress concentrations. Moreover, buckling must be prevented during cyclic tensile and compressive loads, which can be achieved with suitable fixtures. This paper proposes a symmetric specimen geometry for alternating in-plane loads, where the location of crack initiation is predicted and stress concentration is minimized. The design of the geometry was supported with numerical simulations. According to the final specimen geometry, unique fixtures were developed, based in part on the standard ASTM D7078, Standard Test Method for Shear Properties of Composite Materials by V-Notched Rail Shear Method, solution. The fixtures allow operation at elevated temperatures, easy specimen change, universal use on different test rigs, and possible extension to other specimen shapes. By the proposed fixture system, any type of cyclic test can be performed (i.e., alternating, pulsating, or variable). The fixtures are suitable for measuring strain with mechanical extensometers, video extensometers, or digital image correlation. The experimental shear tests on aluminum 6061 specimens showed comparable shear strain fields as calculated by the finite element method.

Fracture behaviour of fibre-reinforced composite materials subjected to shear loading: An experimental and numerical study

Compared to fibre-dominated behaviour, a response dominated by the matrix under shear loading can considerably limit the load-bearing capability and restrict the utilisation of composite materials. It is therefore practically significative to understand the shear response of composite materials. This paper presents a detailed experimental and numerical investigation on the fracture behaviour of composite laminates subjected to shear-dominated loading. Composite specimen with a lay-up of [(0/90)4/0]S are tested using the V-Notched Rail (VRS) shear test method. Subsequently, the Scanning Electron Microscope (SEM) investigation is also conducted on the fracture surfaces of the tested specimens to get insights into the damage mechanisms due to shear loading. In addition, a high-fidelity computational damage model is developed to predict the shear fracture behaviour of fibre-reinforced composites. Simulation results, including loading response, strain distribution and shear fracture, well correlated with experimental results, which demonstrates the predictive capability of the developed elastic-plastic damage model.

Behaviour of hybrid glass fibre-reinforced polymer and timber composite laminates under shear loading: Importance of fibre rotation

This paper presents a study aimed at investigating the behaviour of hybrid glass fibre reinforced polymer (GFRP)-timber (HFT) composite laminates under shear loading. A series of V-notched rail shear tests were carried out to determine the properties of HFT laminates, both in perpendicular and parallel to timber grain direction. In addition, shear tests were also conducted for pure timber and GFRP specimens. Except for pure timber specimens loaded parallel to grains, all specimens showed significant non-linear behaviour. During the testing, significant rotation of the glass fibres was observed. To further investigate the behaviour of the shear test specimens, a numerical model was developed. A numerical simulation approach that combines the progressive damage modelling of GFRP and timber with advanced fibre rotation modelling was developed. The simulation approach was implemented in finite element (FE) software program ABAQUS\\Explicit via a vectorized user-defined material model (VUMAT). FE results showed a good agreement with the test results and confirmed the significant influence of fibre rotation on the post peak behaviour of the HFT specimens under shear loading.

Comparison of the shear behavior in graphite-epoxy composites evaluated by means of biaxial test and off-axis tension test

Abstract Characterization of shear behavior in composite materials remains a not fully solved problem. In the last fifty years, many different approaches have been proposed to solve this problem (rail shear, thin-walled tube torsion, off-axis tensile, ±45° tensile, Arcan, Iosipescu, asymmetric four-point bend, plate twist, v-notched rail shear, off-axis flexural, and shear frame), although none of these approaches have achieved an unquestionable solution. For this reason, proposals of alternative methods and comparison between different experimental techniques are of interest. In the present work, the use of cruciform samples with the fiber oriented at 45° with respect to the load directions, and subjected to tension-compression (creating a pure shear stress state at the central part of the samples), is studied. The experimental results of the cruciform samples have been compared with the off-axis tests (with the fiber at 10°) for the same material (AS4/8552), finding a good agreement between the shear experimental curves, especially at the initial part of the curve, where the shear modulus is calculated. Nevertheless, the shear strength value obtained by means of the cruciform specimen has shown to be significantly lower than that obtained using the off-axis test. A Finite Element numerical model of the cruciform specimen has been developed to analyze the stress field of the samples. Numerical results have shown that there is a central area of the cruciform specimens where a pure and uniform shear stress state is developed, which is suitable for the evaluation of the shear constitutive law of the material. It has been observed that there is a (σ 22) stress concentration in the transition between the straight and curved parts of the boundary geometry of the samples, which explain some premature failures of the samples. This premature failure could be avoided with tabs extended up to the beginning of the central part of the sample.

Experimental determination of deformation homogeneity and shear states using the digital image correlation method

One of the great challenges in the pure shear and simple shear tests is to maintain the deformation state and deformation homogeneity of a specimen under large deformations. For this reason, the aim of this work is to investigate the state of shear deformation and degree of deformation homogeneity of three different shear specimens. A simple and easily replicable methodology was proposed to estimate the degree of deformation homogeneity of specimens, while a time-independent kinematic vorticity parameter was employed to identify pure shear and simple shear deformation states. All data were obtained from full-field displacements that were extracted with the two-dimensional digital image correlation (2D-DIC) method. Specimens of polytetrafluoroethylene (PTFE) were tested using the modified slotted shear test and the V-notched rail shear test, which are used to induce a simple shear state. The planar tensile test was used to produce a pure shear state in a thin sheet of rubber-like material. For small deformations, both slotted and V-notched shear tests induced a good degree of deformation homogeneity along the shear path between the notches of specimens, mainly in the central region. However, the V-notched specimens buckled at large deformation, producing a decrease in the degree of deformation homogeneity. In this case, the results were qualitatively assessed due to the errors associated with out-of-plane motions. High degree of deformation homogeneity was also observed in pure shear specimens. All tests demonstrated to be effective for inducing a state of pure shear or simple shear. Results show that the two approaches can be used as supplementary tools for determining the state of shear deformation and degree of deformation homogeneity.

Implementation of the modified V-Notched Rail Shear method for characterizing glass fibre thermoplastic composites at sub-zero and elevated temperatures

In the recent years, the thermoplastic-based composites are being increasingly used in a variety of industrial applications such as the automotive due to the unique features they exhibit, mainly their recyclability and their interesting capacity to be formed and produced in high volume rate. Among the most interesting categories utilized extensively in these sectors, the short fibre composites are an attractive solution to non-critical load carrying structures while the textiles are utilized in more critical components. Nevertheless, all of them are implemented into a varying service temperature range, from sub-zero up to higher temperatures than the ambient. Thus, the definition of their mechanical performance with respect to their material and fabrication characteristics is a crucial procedure. Among them, the shear properties are the most challenging due to the complexity of the standardized mechanical tests. Moreover, these properties are greatly dependent on the matrix quality and, in general, to the overall composite's quality. In the present work, presented is an experimental campaign implementing temperatures from −30 °C up to 80 °C that includes both short and textile thermoplastic based composites, fabricated with different processes. In addition, introduced is a modified testing apparatus based on the standardized one that aims to overcome some of the drawbacks observed so far.

Experimental and numerical investigation of the effects of porosity on the in-plane shear properties of CFRPs using the V-notched rail shear test method

Voids occurred after the manufacturing process are a common defect of composite materials. They appear to have a negative impact on the matrix-dominated material properties, the gravity of which depends upon the porosity content, the shape and the volume of pores. In the present work, the effects of porosity on the shear mechanical properties of unidirectional carbon fiber-reinforced plastic composites are evaluated via mechanical testing and numerical simulation. In this framework, the V-Notched Rail Shear test method is applied on carbon fiber reinforced plastic specimens of four porosity levels. Moreover, two finite element methodologies are utilized for simulating this particular mechanical test namely the progressive damage model (PDM) and the Virtual Crack Closing Technique (VCCT). Double cantilever beam (mode I) and end notched flexure tests (mode II) are also conducted for the development of the VCCT model. The results from the shear mechanical tests reveal a considerable drop in both the elastic properties and strength. In addition to that, for larger porosity contents, more cracks are present and crack initiation and propagation occur at a faster pace. Finally, the advantages and disadvantages of the two numerical methods are presented and assessed revealing a satisfying consistency with the results obtained by the mechanical tests.

Assessment of the shear properties of thermoplastic composites using the ±45° tension and the V-notched rail shear methods

Composite materials consisting of thermoplastic matrix are gaining the interest of both the aeronautical and the automotive industry as they comprise a series of advantages regarding their mechanical performance, their recyclability and their ability to be produced in large quantities. Nevertheless, some notable drawbacks have been noticed related to the fabrication process affecting their in-plane shear properties the characterization of which is complicated. Among the notable number of testing methods proposed throughout the years, several advantages and drawbacks were observed, mostly related to the way the load is applied, the stress uniformity and the applicability of each method to various material architectures. In the present work, the modified V-notched rail shear and the ±45° shear testing methods are applied to short and textile glass fiber reinforced thermoplastics aiming to assess the influence of both the fabrication method and the strands direction. Consecutively, the results obtained from the two different testing methods are compared revealing a relatively good agreement while, in parallel, the stress uniformity and the local failures observed on the specimens are analyzed.

Determination of the in-plane shear modulus of unidirectional carbon fiber-reinforced plastics using digital image correlation and finite-element analysis

Accurate in-plane shear properties are essential to simulate the mechanical behavior of fiber-reinforced laminated composites. Herein, a digital image correlation technique was used to analyze unidirectional carbon fiber-reinforced composites by three shear-test methods (±45° in-plane, Iosipescu, and V-notched rail shear tests), focusing on the uniformity of shear deformation during each test. The V-notched rail shear test imposed the most uniform shear deformation. This test, however, still imposed non-uniformity around the deformation regime for the in-plane shear modulus calculation, which needed to be accommodated. The non-uniformity was virtually characterized by a combination of various material properties using finite-element analysis. A universal correction factor of 0.925 resolved the non-uniformity involved in the V-notched rail shear test. This enabled the in-plane shear modulus of unidirectional carbon fiber- or glass fiber-reinforced composites to be accurately determined.

The Influence of Uneven Temperature Distributions on Local Shear and Interlaminar Fracture Toughness Properties of a Thick Composite Laminate

Abstract Because wind turbine blade length has increased over the years, so have the root and spar cap thicknesses. Temperature gradients and residual strains related to the manufacturing process of thick laminates are factors that determine the final laminate mechanical properties. The aim of this study is to investigate the manufacturing process influence on shear and fracture toughness properties for an epoxy glass fiber-reinforced polymer. Epoxy-infused laminates of 84 layers (55–60 mm) thick were manufactured. The laminate was manufactured using vacuum infusion on a single-sided mold. Thermocouples and strain gauges were embedded at different thickness positions to monitor temperature and residual strain during the curing process. The laminate was manufactured using the sublaminate technique: with the help of peel plies at different thickness positions, sublaminates were extracted and separately tested as standard coupons. Shear coupons were tested according to ASTM D7078-12, Standard Test Method for Shear Properties of Composite Materials by V-Notched Rail Shear Method, and fracture toughness double-cantilever beam coupons were tested in Mode I according to ASTM D5528-13, Standard Test Method for Mode I Interlaminar Fracture Toughness of Unidirectional Fiber-Reinforced Polymer Matrix Composites. Based on the extracted sublaminates, the relations between local curing cycles, residual strains, and the influence on the shear and fracture toughness were studied. This work reports temperature profiles and residual strain measurements that were monitored during thick laminate manufacturing. In addition, experimental data from shear and fracture toughness tests (static and fatigue) are correlated with the local residual strains and temperature measurements.

Nonlinear numerical analysis of the V-notched rail shear test specimen

V-Notched rail (VNR) shear test is a fairly recent shear test method and has been standardized as ASTM D7078. In the present research, shear properties of glass/epoxy specimens were evaluated by the VNR test method and compared with the three-rail shear test results. Shear modulus obtained by the two test methods are almost the same, however, VNR gives the shear strength around 36% greater than the three-rail test. Considering the observed nonlinearity in the experimental shear stress-strain diagram, nonlinear finite element analysis of the VNR specimen was performed by developing UMAT subroutine. Numerical analysis showed such non-uniformity in the shear stress and shear strain distribution between the notches. To take into account the non-uniformity, a correction factor for shear modulus and shear strength are introduced to correct about 8% and 4% error in the shear strength and shear modulus, respectively. The effect of some errors which are likely to occur during the test, was also studied. Finally, uncertainty analysis was employed to evaluate the quality of measurement. Uncertainty values are determined as 3.6% of the shear strength and 1.7% of the shear modulus measured value.

The role of material characterisation in the crush modelling of thermoplastic composite structures

The predictive capability of high fidelity finite element modelling, to accurately capture damage and crush behaviour of composite structures, relies on the acquisition of accurate material properties, some of which have necessitated the development of novel approaches. This paper details the measurement of interlaminar and intralaminar fracture toughness, the non-linear shear behaviour of carbon fibre (AS4)/thermoplastic Polyetherketoneketone (PEKK) composite laminates and the utilisation of these properties for the accurate computational modelling of crush. Double-cantilever-beam (DCB), four-point end-notched flexure (4ENF) and Mixed-mode bending (MMB) test configurations were used to determine the initiation and propagation fracture toughness in mode I, mode II and mixed-mode loading, respectively. Compact Tension (CT) and Compact Compression (CC) test samples were employed to determine the intralaminar longitudinal tensile and compressive fracture toughness. V-notched rail shear tests were used to measure the highly non-linear shear behaviour, associated with thermoplastic composites, and fracture toughness. Corresponding numerical models of these tests were developed for verification and yielded good correlation with the experimental response. This also confirmed the accuracy of the measured values which were then employed as input material parameters for modelling the crush behaviour of a corrugated test specimen.

The second Sandia Fracture Challenge: blind prediction of dynamic shear localization and full fracture characterization

In the context of the second Sandia Fracture Challenge, dynamic tensile experiments performed on a Ti–6Al–4V alloy with a complex fracture specimen geometry are modeled numerically. Sandia National Laboratories provided the participants with limited experimental data, comprising of uniaxial tensile test and V-notched rail shear test results. To model the material behavior up to large plastic strains, the flow stress is described with a linear combination of Swift and Voce strain hardening laws in conjunction with the inverse method. The effect of the strain rate and temperature is incorporated through the Johnson–Cook strain rate hardening and temperature softening functions. A strain rate dependent weighting function is used to compute the fraction of incremental plastic work converted to heat. The Hill’48 anisotropic yield function is adopted to capture weak deformation resistance under in-plane pure shear stress. Fracture initiation is predicted by the recently developed strain rate dependent Hosford–Coulomb fracture criterion. The calibration procedure is described in detail, and a good agreement between the blind prediction and the experiments at two different speeds is obtained for both the crack path and the force–crack opening displacement (COD) curve. A comprehensive experimental and numerical follow-up study on leftover material is conducted, and plasticity and fracture parameters are carefully re-calibrated. A more elaborate modeling approach using a non-associated flow rule is pursued, and the fracture locus of the Ti–6Al–4V is clearly identified by means of four different fracture specimens covering a wide range of stress states and strain rates. With the full characterization, a noticeable improvement in the force–COD curve is obtained. In addition, the effect of friction is studied numerically.

Evaluation of porosity effects on the mechanical properties of carbon fiber-reinforced plastic unidirectional laminates by X-ray computed tomography and mechanical testing

The effects of porosity on the matrix-dominated mechanical properties of unidirectional carbon fiber-reinforced plastic composites were evaluated using X-ray computed tomography and mechanical testing. Carbon fiber-reinforced plastic plates of four porosity levels were manufactured by implementing different curing cycles. Porosity was detected by X-ray computed tomography tests, conducted on samples taken from the plates, and quantified by analyzing the computed tomography scans using the VGStudio Max software. Four different types of mechanical tests were conducted; namely, transverse tension, V-notched rail shear, three-point bending, and short-beam shear tests. The porosity analysis showed that with increasing the porosity volume fraction, the number of pores decreases, their volume increases while their shape changes from spherical or ellipsoidal to a needle-shape. The results from mechanical tests reveal that the presence of pores reduces all matrix-dominated material properties of the UniDirectional (UD) carbon fiber-reinforced plastic material. The reduction in strength is greater than the reduction in the elastic properties. Moreover, the reduction in the in-plane shear and interlaminar properties is greater than the tensile properties of the UD carbon fiber-reinforced plastic material. Between porosity contents of similar volume fraction, the one with the few large pores proved more severe than the one with the many small pores. The large standard deviation observed for some of the tests is attributed to the non-uniform dispersion of pores.

Modified slotted shear test for a thin sheet of solid polymer under large deformations

A modified slotted shear test based on ASTM B831 is proposed and used for determining the mechanical behavior of a solid polymer at large deformations. The shear stress-strain response of a thin sheet of polytetrafluoroethylene was experimentally investigated and a V-notched rail shear test was carried out to validate the experimental results obtained with the proposed approach. Displacement and strain fields of the test specimens were estimated by the Digital Image Correlation method. The results indicate agreement between both tests for small deformations, while for large deformations significant discrepancies in shear stress-strain responses are observed. These discrepancies can be attributed to unwanted distortions of the gage section observed in the V-notched specimens. The modified slotted shear test provided a uniform shear strain distribution across the gauge section of specimens. In addition, the principal advantages of the proposed approach over V-notched rail shear test are a wide range of shear strains and absence of unwanted distortions.

Modified V-notched rail shear test fixture for shear characterisation of textile-reinforced composite materials

The V-notched rail shear test (VNRS) is recognised as a standard test method for the determination of shear modulus and shear strength of fibre-reinforced composite materials. This method is based on a double V-notched specimen with a large gauge section and an approximately uniform shear stress distribution between the two notches. The construction of the test device prescribed in standards was revised with the main focus of lines of force as well as precise and economic specimen clamping. Therefore, the modified test fixture has been equipped with guide units and the number of clamping bolts could be reduced. The standardised and modified VNRS fixtures were evaluated using a finite element method. It can be shown that the modified VNRS test fixture is well suited for robust and reliable material characterisation of shear property data.

Initial damage induced by thermal residual stress and microscopic failure analysis of carbon-fiber reinforced composite under shear loading

In order to study the mechanical properties and the progressive failure process of composite under shear loading, a representative volume element (RVE) of fiber random distribution was established, with two dominant damage mechanisms – matrix plastic deformation and interfacial debonding – included in the simulation by the extended Drucker–Prager model and cohesive zone model, respectively. Also, a temperature-dependent RVE has been set up to analyze the influence of thermal residual stress. The simulation results clearly reveal the damage process of the composites and the interactions of different damage mechanisms. It can be concluded that the in-plane shear fracture initiates as interfacial debonding and evolves as a result of interactions between interfacial debonding and matrix plastic deformation. The progressive damage process and final failure mode of in-plane shear model which are based on constitute are very consistent with the observed result under scanning electron microscopy of V-notched rail shear test. Also, a transverse shear model was established as contrast in order to comprehensively understand the mechanical properties of composite materials under shear loading, and the progressive damage process and final failure mode of composite under transverse shear loading were researched. Thermal residual stress changes the damage initiation locations and damage evolution path and causes significant decreases in the strength and fracture strain.

Estimating mechanical properties of 2D triaxially braided textile composites based on microstructure properties

Textile composites manufactured using Resin Transfer Modeling (RTM) can offer advantages in some automotive applications including reduction in weight, while being relatively simpler to fabricate than standard laminated composites used for aerospace applications. However, one of the challenges that arise with these textile composite materials is that the mechanical properties are inherently dependent on the local and final (in-situ) architecture of the textile itself as a result of the molding and curing processes. While this provides additional latitude in the composite design process it also necessitates the development of analytical models that can estimate the mechanical properties of a textile composite based on the textile architecture and the properties of the manufactured component.In this paper, an analytical model is developed and its estimations are compared against experimental in-plane engineering properties for composites with various textile architectures. Results from the model are also compared against finite element (FE) based computational results. The microstructures of the 2D triaxially braided composite (2DTBC) studied were extensively characterized. The microstructure properties thus measured were used in the analytical model to estimate the mechanical properties. Uniaxial tension and V-notched rail shear tests were conducted on 2DTBC with different textile architectures. Good agreement between the analytical, computational, and experimental results were observed and are reported here. Furthermore, computational estimations of matrix mechanical properties are limited to the linear elastic range of a representative material volume (unit cell) and coupon data. Full mechanical response of larger 2DTBC structures, albeit of prime interest, is beyond the scope of this work and could be the focus of follow up studies.

Study of localized damage in composite laminates using micro–macro approach

A robust multiscale scheme referred to as micro–macro method has been developed for the prediction of localized damage in fiber reinforced composites and implemented in a finite element framework. The micro–macro method is based on the idea of partial homogenization of a structure. In this method, the microstructural details are included in a small region of interest in the structure and the rest is modeled as a homogeneous continuum. The solution to the microstructural fields is then obtained on solving the two different domains, simultaneously. This method accurately predicts local stress fields in stress concentration regions and is computationally efficient as compared with the solution of a full scale microstructural model. This scheme has been applied to obtain localized damage at high and low stress zones of a V-notched rail shear specimen. The prominent damage mechanisms under shear loading, namely, matrix cracking and interfacial debonding, have been modeled using Mohr–Coulomb plasticity and traction separation law, respectively. The average stress at the notch has been found to be 44% higher than the average stresses away from the notch for a 90N shear load. This stress rise is a direct outcome of the geometry of the notch.

The V-Notched Rail Shear Test

A V-notched rail shear test has been developed for measuring the shear modulus and shear strength of composites and other materials. This test method, standardized as ASTM D 7078, incorporates attractive features of the existing Iosipescu and two-rail shear tests. The V-notched rail shear specimen provides a larger gage section than the standard Iosipescu shear specimen and provides enhanced loading capability compared to either existing test method. Results from finite element analysis show that the V-notched specimen design reduces shear stress concentrations near the rails and produces relatively uniform shear stress distributions in the specimen gage section for a variety of carbon/epoxy laminates. Mechanical testing results from six carbon/epoxy laminates demonstrate excellent gripping performance using the V-notched rail shear test fixture and acceptable gage section failures. Additionally, results suggest that the use of [0/90] ns cross-ply laminates is preferred over [0] n unidirectional laminates for characterizing the lamina shear properties of composite materials.