Pure Shear Tests Research Articles

Fatigue fracture of a highly stretchable acrylic elastomer

Dielectric elastomer has been extensively explored in various applications as soft active material. In most applications, dielectric elastomer is subjected to cyclic loading-unloading condition. As a result, a small initial defect in a dielectric elastomer may finally grow to a critical size to induce catastrophic rupture. In this article, we carried out an experimental study of the crack growth in an acrylic dielectric elastomer under cyclic loading-unloading. Pure-shear test specimens were used to measure the relationship between crack growth rate and energy release rate. Such relationship can be simply fit to a power-law. We further used the measured power-law to successfully predict the fatigue lifetime of the acrylic elastomer with an edge crack and subject to simple extension cyclic loading-unloading test.

Simplified phenomenological model of the nonlinear behavior of FRPs under combined stress states

Nonlinear material behavior of FRPs under shear loading is widely observed and investigated. In case of combined stress states under tension and shear, an interaction between the macroscopic shear stress–strain curve evolution and the applied tension has been observed and described by several publications in the past. In the present work, the available experimental data with combined stress states are evaluated and a specific threshold shear stress is found, above which nonlinear material behavior occurs for all stress states. Further, a new simplified phenomenological model is derived to model the nonlinear behavior of FRPs when the threshold shear stress is exceeded. This simplified model only needs the threshold shear stress and one evolution parameter, both derived from a pure shear test, to model nonlinear behavior for all combined stress states. A comparison with the available experimental results and with the predictions of the WWFE-III participants for WWFE-III test case 1 shows a very good agreement.

Study of fracture behavior for anisotropic 7050-T7451 high-strength aluminum alloy plate

The anisotropic ductile fracture behavior is significant to forming process of sheet metal. A modified elliptical fracture criterion incorporating Hill48′s criterion was applied to characterize the ductile fracture behavior of anisotropic plate. Then the fracture loci and surfaces of anisotropic AA7050-T7451 plate in the different stress spaces were studied to analyze the effect of plasticity anisotropy on fracture loci. The law of forming capability for anisotropic AA7050-T7451 plate with different stress triaxiality and Lode parameter was revealed. In order to validate the accuracy of the proposed ductile fracture model, the analyses of failure predictions for uniaxial tensile and pure shear tests at 0°, 45°, and 90° with respect to the rolling direction had been carried out by experiments and finite element method (FEM). The failure predictive results from FEM are in good agreement with the experimental and theoretical ones. It turns out that the fracture loci of at Le=−1 are sensitive to the material orientations. Meanwhile, the formability of AA7050-T7451 along 90° direction on tensile strain states is very poor.

Rupture of swollen styrene butadiene rubber

Elastomers have recently been explored to make swellable packers in the oil industry, which have many advantages over traditional cement packers. In the applications, the elastomers absorb solvent and swell against the confinement, which can seal zones in the borehole. In addition, swollen elastomers are usually subjected to a large pressure difference, which can cause fracture of the elastomer. In this article, we conduct experimental studies of the rupture behavior of an oil-swellable elastomer, styrene butadiene rubber (SBR), swollen in hexadecane. This combination of elastomer and swelling agent can be considered representative of oilfield applications. Pure-shear tests are used to measure the stretch at rupture and fracture energy of swollen SBR with different swelling ratios. It is found that swelling can significantly reduce both the stretch at rupture and the fracture energy of the swollen elastomer. Using the measured fracture energy, we have also successfully predicted the stretches at rupture of SBR for a simple extension test with different volume swelling ratios.

Experimental investigation and numerical simulation of unidirectional carbon fiber composite under multi-axial loadings

Abstract Unidirectional carbon fiber composites were tested under multi-axial loading conditions including tensile/compression/shear loadings along and perpendicular to fiber directions. Compression dominated tests showed brittle fracture mode like local kicking/buckling, while tension dominated tests showed fracture mode like delamination and fiber breakage. Pure shear tests with displacement control showed material hardening and softening before total failure. Tsai-Wu failure criterion with Mises-Hencky interaction term was used as the yield criterion, and an associated flow rule (AFR) was used to describe the plastic flow of this material. The modified Tsai-Wu failure criterion was able to predict the failure of the composites under different loading conditions. A new material post-failure softening rule was proposed, where a new parameter ψ was introduced to represent different loading conditions of the composite. This model was implemented to ABAQUS/Explicit as a user material subroutine (VUMAT). Numerical simulations using finite element method well duplicated the elasticity and plasticity of this material. Failure features like delamination was simulated using cohesive surface feature. This model showed good capacity in predicting both failure initiations and fracture modes for the unidirectional fiber composites. This model has good application potentials to large-scale engineering problems.

Identifiability of material parameters in solid mechanics

Material parameter identification using constitutive models of elasticity, viscoelasticity, rate-independent plasticity and viscoplasticity has a long history with regard to homogeneous and inhomogeneous deformations. For example, uniaxial tensile tests, pure shear tests, torsion experiments of thin-walled tubes or biaxial tensile tests are used to obtain the material parameters by solving the inverse problem. Frequently, the parameters are determined by numerical optimization tools. In this paper, we investigate some very basic single- and two-layered examples regarding identifiability, because these tests are the basis for more complex geometrical and physical nonlinear problems. These simple examples are the uniaxial tensile/compression case, biaxial tensile tests of a cruciform specimen, torsion of a thin-walled tube, a thick-walled tube under internal pressure and the indentation test. For the thick-walled tube under internal and external pressure with an axial pre-strain with several layers, an analytical solution is provided directly suitable for programming. The aim is to get an understanding whether some problems lead to non-identifiable parameters.

Velocity mode transition of dynamic crack propagation in hyperviscoelastic materials: A continuum model study

Experiments of crack propagation in rubbers have shown that a discontinuous jump of crack propagation velocity can occur as energy release rate increases, which is known as the “mode transition” phenomenon. Although it is believed that the mode transition is strongly related to the mechanical properties, the nature of the mode transition had not been revealed. In this study, dynamic crack propagation on an elastomer was investigated using the finite element method (FEM) with a hyperviscoelastic material model. A series of pure shear test was carried out numerically with FEM simulations and crack velocities were measured under various values of tensile strain. As a result, our FEM simulations successfully reproduced the mode transition. The success of realising the mode transition phenomenon by a simple FEM model, which was achieved for the first time ever, helped to explain that the phenomenon occurs owing to a characteristic non-monotonic temporal development of principal stress near the crack tip.

Rupture of Polydomain and Monodomain Liquid Crystal Elastomer

Liquid crystal elastomer (LCE) has been recently explored extensively to make diverse active structures and devices. Depending on synthetic process, LCE can be made either polydomain or monodomain when ambient temperature is below isotropic clearing temperature. In the applications, LCE may be subjected to large mechanical stretch or force and break apart. The capability of predicting the rupture of LCE under different loading conditions is crucial for the applications. However, according to our knowledge, there is no report on fracture energy measurement of LCE. In this paper, we measured fracture energy of both monodomain and polydomain LCEs using pure shear test. We found that polydomain LCE has significantly higher fracture energy than monodomain one. We suggest that the increase of fracture energy in polydomain LCE is attributed to stretch-induced polydomain-to-monodomain transition near the crack tip of the material.

Experiments and Constitutive Model for Deep and Superficial Fascia. Digital Image Correlation and Finite Element Validation

AbstractFascia is a highly organized collagenous tissue that is intimately connected with muscles. The mechanical properties of fascia strongly affect muscular actions and development of pathologies. The objective of this paper is to determine the mechanical properties of the deep and superficial fascia by uniaxial and pure shear tests and to propose and address the feasibility of a material hyperelastic constitutive model using combined Digital Image Correlation strain measurements and Finite Element (FE) computations. Experiments on deep and superficial fascia samples from the hindlimbs of six adult sheep along or perpendicular to collagen fibres and FE simulations of each experiment were compared. An anisotropic strain energy function was proposed to reproduce the mechanical behaviour of deep and superficial fascia and the material parameters were fitted using an optimization method and included in the FE simulations.The mechanical response of the deep and superficial fascia shows typical behaviour of soft connective tissues. It is shown that the samples in longitudinal direction are stiffer than the samples in transversal direction for both kinds of tissues. When considering the deep and superficial samples, it is evident a greater stiffness of deep samples for both longitudinal and transversal directions with respect to superficial samples. Overall good predictions were obtained with the model proposed which present relatively low ε values, ε = 0.1189 and ε = 0.0722 for deep and superficial fascia respectively. The experimental and FE results were compared. The simulated curves slightly underestimated or overestimated the load for uniaxial and pure shear tests, respectively, but very accurately captured the stiffness and the overall response. These results demonstrate that the proposed model and the fitted material properties applied have a good capability of reproducing the experimental conditions.

극한지용 고장력강의 평균 응력 삼축비 및 평균 정규 로드 파라메터를 고려한 3차원 파단 변형률 평면 개발: 제2부 파단 변형률 평면의 정식화

An extended study was conducted on the fracture criterion by <xref>Choung et al. (2011</xref>; <xref>2012)</xref> and <xref>Choung and Nam (2013)</xref>, and the results are presented in two parts. The theoretical background of the fracture and the results of new experimental studies were reported in Part I, and three-dimensional fracture surface formulations and verifications are reported in Part II. How the corrected true stress can be processed from the extrapolated true stress is first introduced. Numerical simulations using the corrected true stress were conducted for pure shear, shear-tension, and pure compression tests. The numerical results perfectly coincided with test results, except for the pure shear simulations, where volume locking appeared to prevent a load reduction. The average stress triaxialities, average normalized lode parameters, and equivalent plastic strain at fracture initiation were extracted from numerical simulations to formulate a new three-dimensional fracture strain surface. A series of extra tests with asymmetric notch specimens was performed to check the validity of the newly developed fracture strain surface. Then, a new user-subroutine was developed to calculate and transfer the two fracture parameters to commercial finite element code. Simulation results based on the user-subroutine were in good agreement with the test results.

Nanocomposite hydrogels: Fracture toughness and energy dissipation mechanisms

In this study, fracture toughness of nanocomposite hydrogels is quantified, and active mechanisms for dissipation of energy of nanocomposite hydrogels are ascertained. Poly(N,N-dimethylacrylamide) nanocomposite hydrogels are prepared by in situ free radical polymerization with the incorporation of Laponite, a hectorite synthetic clay. Transmission electron microscopy proves exfoliation of clay platelets that serve as multifunctional crosslinkers in the created physical network. Extraordinary high fracture energies of up to 6800 J m−2 are determined by the pure shear test approach, which shows that these soft and stretchable hydrogels are insensitive to notches. In contrast to single- and double-network hydrogels, dynamic mechanic analysis and stress relaxation experiments clarify that significant viscoelastic dissipation occurs during deformation of nanocomposite hydrogels. Similar to double-network hydrogels, crack tip blunting and plastic deformation also contribute to the observed massive fracture energies. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2015, 53, 1763–1773

J‐Integral from Full Field Kinematic Data for Natural Rubber Compounds

AbstractThe application of the digital image correlation technique to the determination of the J‐integral at fracture initiation for carbon black‐filled natural rubber compounds is discussed. Three different compounds with varying carbon black content were tested, using two different test configurations: pure shear and biaxial tensile test. Digital image correlation was used to measure the displacement field around the crack tip in the tested specimens. From the displacement field, which was interpolated using a finite element scheme, the stresses were evaluated by using Ogden's hyperelastic model, and the J‐integral could be calculated. The results compare well with both theoretical and finite element results.

OS0119-314 純粋せん断試験によるhcp金属のすべり系のCRSSの評価

o estimate CRSSs of slip systems in hcp crystals, shear tests of magnesium single crystals were carried out at room temperature. In the shear test, basal slip occurred within shear deformation area, and CRSS of the basal slip was estimated as 0.35MPa. However, specimens for prismatic slip deformed with {101^^-2} twin. The test result shows that CRSS of prismatic slip in magnesium will be more than 90MPa. To estimate CRSS of the prismatic slip precisely, alternate specimens for shear test should be necessary.

Evaluation of Critical Resolved Shear Stresses for Various Plastic Deformation Mechanisms To Understand Mechanical Properties of Magnesium-Yttrium Alloys

ABSTRACTIn order to understand enhanced mechanical properties of magnesium-yttrium (Mg-Y) alloys, applied stresses which were required to operate independent plastic deformation mechanisms on various stress axes were evaluated. Moreover, for this analysis, mechanical tests including newly-established testing method “pure-shear test” were conducted to evaluate Critical Resolved Shear Stresses (CRSSes) for various plastic deformation mechanisms of Mg-Y solid solution alloy single crystals with various Y concentration. Relatively higher solid solution strengthening of dominant plastic deformation mechanisms such as basal slip and extension twin at room temperature, results in increase in the activation of non-basal slip system. By a simple analysis based on von-Mises criterion with experimental CRSS values, it is revealed that enhanced mechanical properties of Mg-Y alloys might be attributed to the decrease of difference in the activity of plastic deformation mechanisms by Y addition.

FRACTURE PHENOMENOLOGY AND TOUGHNESS OF FILLED NATURAL RUBBER COMPOUNDS VIA THE PURE SHEAR TEST SPECIMEN

ABSTRACT Material anisotropy induced by strain in filled vulcanized rubbers strongly affects fracture toughness. The influence of carbon black content on fracture phenomenology and fracture toughness was investigated by performing video-recorded tests adopting a suitable grooved notched pure shear test specimen. In such a way, it was possible to analyze the so-called “knotty tearing” deformation mechanism occurring at the crack tip: sideways cracks perpendicular to the notch plane develop before the onset and propagation of a forward crack parallel to the notch plane. The J-integral fracture mechanics approach was adopted and digital image correlation analysis was performed to measure the strain at the crack tip. The presence of carbon black modifies the maximum chain extensibility and strain-induced crystallizability of the rubber matrix in the compound. The formation of sideways cracks occurred in all filled compounds and resulted in a link to the maximum chain extensibility. Nevertheless, toughness enhancement was observed only when strain-induced crystallization took place at the crack tip before the onset of the forward crack.

Crack Blunting and Advancing Behaviors of Tough and Self-healing Polyampholyte Hydrogel

Recently, we have reported that polyampholytes, synthesized from free radical copolymerization of anionic monomer and cationic monomer, form physical hydrogels of high toughness and self-healing. The random distribution of the opposite charges forms ionic bonds of a wide distribution of strength. The strong bonds serve as permanent cross-links, imparting elasticity, whereas the weak bonds serves as reversible sacrificial bonds by breaking and reforming to dissipate energy. In this work, we focus on the rupture behaviors of the polyampholyte physical hydrogel, P(NaSS-co-MPTC), copolymerized from sodium p-styrenesulfonate (NaSS) and 3-(methacryloylamino)propyltrimethylammonium chloride (MPTC). Tensile test and pure shear test were performed at various stretch rates in the viscoelastic responses region of the material. Tensile test showed yielding, strain softening, and strain hardening, revealing the dually cross-linked feature of the gel. Pure shear test showed crack blunting at the notched tip and a large...

Multiscale modeling and prediction of bonded joint failure by using an adhesive process zone model

In this work, a three-dimensional multiscale adhesive process zone model (APZM) is used to simulate dynamic failure process of bonded joint structures. APZM combines the ideas of the cohesive zone model (CZM), the Cauchy–Born rule, and the so-called virtual internal bond (VIB) theory to predict failures of bonded joints that involves adhesive failure, cohesive failure, and mixture of both. The proposed APZM has some unique advantages to describe mechanical behaviors of bonded joints under general loading conditions. First, in contrast to conventional FEM model, APZM model can simulate dynamic fracture process of structure without remeshing. Second, APZM replaces the ad-hoc traction–separation laws of interface used in CZM with full three-dimensional interphase model, which is based on the VIB theory. Consequently, APZM can naturally capture both mixed failure modes as well as mixture failure mode under complex loading conditions. Third, an empirical elastic energy density function for macromolecular polymers was constructed, and its material parameters are obtained by calibrating the model with material properties measured from experiments such as under pure tension and shear tests. In order to validate APZM model and to predict stress field in adhesive layer accurately, a series of numerical simulations of bonded joints under shear, tension and mixed loading conditions have been conducted. The numerical results indicate that APZM can accurately predict the failure modes as well as the dynamics fracture process of bonded joints.

Hyperelasticity with rate-independent microsphere hysteresis model for rubberlike materials

The mechanical behavior of elastomers strongly differs from one to another. Among these differences, hysteresis upon cyclic load can take place, and can be either rate-dependent or rate-independent. In the present paper, a microsphere model taking into account rate-independent hysteresis is proposed and applied to model filled silicone rubbers behavior. The hysteresis model is based on a combination of monodimensional constitutive equations distributed in space. The behavior of each direction is described by a collection of parallel spring slider elements. The sliders are Coulomb dampers with non-zero break-free force in tension. This model is tested on a filled silicone rubber by the way of uniaxial tensile and pure shear tests. The mechanical response of the material is well predicted for such tests. Finally, the constitutive equations are implemented in the finite element software ABAQUS. Calculation results highlight good performances of the proposed model.

Sandia Fracture Challenge: blind prediction and full calibration to enhance fracture predictability

The Impact and Crashworthiness Lab at Massachusetts Institute of Technology participated in the Sandia Fracture Challenge and predicted the crack initiation and propagation path during a tensile test of a compact tension specimen with three holes (B, C, and D), using a very limited number of material properties, including uniaxial tensile tests of a dog-bone specimen. The maximum shear stress and modified Mohr–Coulomb fracture models were used. The predicted crack path of A–C–E coincided with two out of thirteen experiments performed by Sandia National Laboratories, and the maximum load, as well as the load level at the first and second crack initiation, was accurately captured. However, the crack-tip opening displacements (CODs) corresponding to the initiation of the two cracks were overestimated by 12 and 24 %, respectively. After the challenge ended, we received the leftover material from Sandia and did full plasticity and fracture calibration by conducting extra fracture tests, including tensile tests, on a specimen with two symmetric round notches, a specimen with a central hole, and a butterfly specimen with double curvature. In addition, pure shear tests were carried out on a butterfly specimen. Newly identified fracture parameters again predicted the A–C–E crack path, but the force–COD response could be reproduced almost perfectly. Detailed calibration procedures and validation are discussed. Furthermore, in order to investigate the influence of the machining quality on the results, a pre-damage value was introduced to the first layer of finite elements around the starter notch, A, and the three holes, B, C, and D. This accelerated shear localization between holes A and D (and between D and C as well) and changed the crack path to A–D–C–E. Parametric study on the pre-damage value showed that there exist two competing crack paths, and the corresponding force–COD curve is influenced by the pre-damage value. The effect of mesh size and boundary conditions are also discussed.

Study on the Application of Shear Modified GTN Model to Small Punch Test

As an important non-standard test technology, the testing result of small punch test(SPT) is largely influenced by the experiment condition. In the research of SPT, an appropriate mechanical constitutive model of material is much needed. The shear modified GTN model proposed by NAHSHON is implemented into ABAQUS via user defined material subroutine VUMAT to study the material fracture behavior at various stress triaxialities. The undamaged elastic-plastic mechanical properties of the cold rolled silicon steel are obtained by the tensile test. The damage evolution parameters of void nucleation are determined by experiment data of tensile test combined with a large number of simulations. The shear modified parameter that characterizes the contribution of void distortion to damage evolution is identified by the comparison of the results of pure shear test simulations and the experiment data. The shear modified GTN model is used to simulate the small punch test. The results show that, as the void distortion is taken into account in damage evolution, the prediction results of the shear modified GTN model is much more consistent with experiment data than the original GTN model. It comes to the conclusion that the shear modified GTN model can reveal the failure mechanism of material better in intrinsic, and is a better choice to apply in the study of small punch test.