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Abstract This paper is concerned with modeling of anisotropic fracture forming limit diagram considering non - directionality of the equi - biaxial fracture strain. A new anisotropic ductile fracture criterion is developed based on the Lou–Huh ductile fracture criterion (Lou et al., 2012). In an attempt to predict the forming severity of advanced high - strength steel (AHSS) sheets, the proposed fracture criterion is converted into a Fracture Forming Limit Diagram (FFLD) and anisotropic fracture locus considering the sheet metal orientation. Tensile tests of the DP980 steel sheet with the thickness of 1.2 mm are conducted using various specimen geometries including pure shear, dog - bone, and flat grooved specimens. With Digital Image Correlation (DIC) method, equivalent plastic strain distribution on the specimen surface is computed until surface crack initiates. The fracture predictability of the proposed fracture criterion is evaluated with the experimental results which cover a wide range of stress states in various loading directions. By comparing fracture strains obtained from the experiments with the ones predicted from the proposed fracture criterion, it is clearly confirmed that the fracture criterion proposed is capable of predicting the equivalent plastic strain at the onset of fracture with great accuracy over a wide range of stress states while keeping non-directionality of the equi - biaxial fracture strain. [ABSTRACT FROM AUTHOR]

This paper deals with modeling of rate-dependent hardening behaviors of the TWIP980 steel sheet in tension and compression. Rate-dependent hardening curves of the TWIP980 steel sheet have been acquired in tension and compression with an advanced experimental technique. The hardening curves of the TWIP980 steel sheet have been approximated at each strain rate by using a rate-independent isotropic/kinematic hardening model. With the material parameters acquired at each strain rate, their dependencies on the strain rate are observed and fitted by functions of the strain rate. A rate-dependent isotropic/kinematic hardening model has been proposed with incorporation of the rate-independent isotropic/kinematic hardening model and rate-dependent functions for the material parameters. [ABSTRACT FROM AUTHOR]

This paper is concerned with modeling of fracture-based forming limit criteria for anisotropic materials in sheet metal forming to predict the sudden fracture in complicated forming processes. The Lou–Huh ductile fracture criterion is modified using the Hill's 48 anisotropic yield function instead of the von Mises isotropic yield function to take account of the influence of anisotropy on the equivalent plastic strain at the onset of fracture. For the derivation of an anisotropic ductile fracture criterion, the principal stresses ( σ 1 , σ 2 , σ 3 ) are expressed in terms of the stress triaxiality, the Lode parameter, and the equivalent stress ( η H , L P , σ ¯ H ) based on the Hill's 48 yield function. Three different kinds of fracture-based forming limit criteria are suggested and investigated with an assumption that the stress state is under the plane stress condition with proportional loading. To determine the parameters of the model proposed, the two-dimensional digital image correlation (2D-DIC) method is utilized to measure the strain histories on the surface of three different types of specimens during deformation and the measurement results are investigated to identify the anisotropy effect on the equivalent plastic strain at the onset of fracture. This paper also discusses about a scaling method for a strain-based fracture forming limit criterion in order to capture the onset of fracture using a single forming limit curve for an anisotropic material. From the comparison of various forming limit criteria suggested, it is concluded that a polar effective plastic strain-based (PEPS) fracture forming limit diagram (FFLD) is suitable for prediction of the sudden fracture in AHSS sheets in complicated sheet metal forming processes on the basis of its path independence and simplicity of measuring strains in real forming processes. [ABSTRACT FROM AUTHOR]

This paper introduces the effect of the strain rate on the ductile fracture of DP980 1.2t steel sheet. Tensile tests of DP980 sheet are performed at a range of strain rates from 0.001 s to 100 s with three different shapes of specimens: the diagonally notched specimen for the in-plane shear test; the dog bone specimen for the uniaxial tension test; and grooved specimen for the plane strain tension test. To trace the strain on the surface of the specimen, 2-D digital image correlation (DIC) technique is adopted to encompass the remarkable transition of the fracture characteristics including the fracture strain, loading path and strain rate sensitivity. The negative stain rate sensitivity is observed on the equivalent strain to fracture which corresponds to the experimental results at low strain rate ranging from 0.001 s to 0.1 s. The transition of thermal condition from isothermal to adiabatic comes into play to increase the equivalent strain to fracture at the intermediate strain rate from 0.01 s to 1 s. The fracture loci incorporating with the Lou-Huh ductile fracture criterion are developed to identify the strain rate effect in the wide regime of triaxiality. [ABSTRACT FROM AUTHOR]

This paper investigates the tensile properties of sheet metals at intermediate strain rates in relation to the pre-strain history induced by the fabrication process. Sheet metals such as SPCEN(DDQ), EZNCD(DQ), SPCC(CQ), SPRC340R, and SPRC440E were selected with the pre-strain of 2, 5, and 10% by tensile elongation with static tensile test machine, which could be equivalent to the plastic strain induced by a sheet metal forming process. The pre-strained specimens were elongated at the strain rate ranged from 1 to 100 s−1. The experimental results reveal that the stress-strain curves are noticeably influenced by the pre-strain at the intermediate strain rate for low yield stress steels such as SPCEN, EZNCD, SPCC, and SPRC340R. It is also noticed that the flow stress does not trace the given stress-strain curve when a sheet metal is pre-strained and the yield stress as well as the ultimate tensile strength increases. This behavior is against the old conjecture that the flow stress follows the stress-strain curve even with the pre-strain. This paper suggests a new flow stress model for pre-strained sheet metals. The flow stress model consists of two terms: the first term is the ratio of the yield stress of a pre-strained specimen to the equivalent flow stress obtained from the test without pre-strain; the second term is the general empirical flow stress model without the pre-strain effect. [ABSTRACT FROM AUTHOR]

This article is concerned with the effect of the strain rate on the strain hardening behavior of polymer-bonded explosives at a wide range of strain rates ranging from 0.0001 s–1 to 3870 s−1. Inert polymer-bonded explosive simulants are prepared as specialized particulate composites to acquire analogous mechanical characteristics to polymer-bonded explosives for safety reasons. Uniaxial compressive tests were conducted from quasi-static states to intermediate strain rates ranging from 0.0001 s−1 to 100 s−1 with cylindrical specimens using a dynamic material testing machine (INSTRON 8801) and a high-speed material testing machine. An experimental method was developed for uniaxial compressive tests at intermediate strain rates ranging from 10 s−1 to 100 s−1. Split Hopkinson pressure bar tests were performed at high strain rates ranging from 1250 s−1 to 3870 s−1. Deformation behavior was investigated using captured images from a high-speed camera. The strain hardening behavior of polymer-bonded explosive simulants was formulated as a function of the strain rate with the proposed rate-dependent hardening model based on the DSGZ model. The model is capable of representing the complicated strain rate effects on the strain hardening behavior for rate-sensitive materials with a second-order exponentially-increasing function of the strain rate sensitivity. The rate-dependent hardening model of polymer-bonded explosives can be readily applied to prediction of deformation modes of polymer-bonded explosives in a warhead that undergoes severe dynamic loads. [ABSTRACT FROM AUTHOR]

This paper proposes a novel pseudo-elastic model for polymer-bonded explosive considering the Mullins effect for isotropic, incompressible, hyperelastic, particle-filled materials. Polymer-bonded explosive, an energetic material in which small explosive crystals are bonded in a polymer matrix, is known to exhibit highly nonlinear behaviors of deformation such as the Mullins effect of stress softening, hysteresis, residual strain, and frequency-dependent responses. The Ogden-Roxburgh model is modified for the unloading state to describe the Mullins effect accurately, which is the most important unloading behavior of polymer-bonded explosive. Uniaxial compressive loading and unloading tests at quasi-static states were undertaken to obtain the mechanical properties of the polymer-bonded explosive simulants. The pseudo-elastic model by Ogden and Roxburgh is subsequently modified for consistency with the test results of the polymer-bonded explosive simulants for the case in which the Mullins effect is dominant. The predictions from the new model exhibit good agreement with the experimental data, demonstrating that the model properly describes the Mullins effect and the loading-unloading behavior of deformation. [ABSTRACT FROM AUTHOR]

Highlights: [•] Fracture strain of DP980 is measured by a hybrid experimental–numerical method. [•] Fracture behavior is DP980 is modeled by a new ductile fracture model. [•] The new fracture model is applied to predict fracture strokes with good agreement. [Copyright &y& Elsevier]

Abstract: This paper is concerned with the extension of a shear-controlled ductile fracture criterion for accurate prediction of fracture forming limit diagrams (FFLD) in sheet metal forming processes. A shear-controlled ductile fracture criterion is extended to a general three-dimensional stress space with dependence on the stress triaxiality and the Lode parameter. The underlying mechanisms of Lode parameter dependence of ductile fracture are first correlated to the effect of the maximum shear stress on shear-coalescence of voids. The effect of the stress triaxiality and the Lode parameter on the equivalent plastic strain to fracture is investigated in the space of . For the purpose of comparison, the Mohr–Coulomb criterion is also transformed into the space of using the technique of the Mohr’s circles. Both criteria are applied to construct fracture loci of Al 2024-T351. Fracture loci constructed are compared to experimental data points to validate the performance of two criteria. The comparison demonstrates that fracture loci constructed by two criteria are close to experimental results except for two data points in the high stress triaxiality. The big difference between two criteria is that a cut-off value for the stress triaxiality is extremely small for the Mohr–Coulomb criterion while the new ductile fracture criterion endows a constant cut-off value of −1/3 which is reasonable for ductile materials. Due to this limitation of the Mohr–Coulomb criterion, the new criterion is more suitable to model ductile fracture in metal forming processes. [Copyright &y& Elsevier]

This article is concerned with the tensile characteristics of polymers at various strain rates. Tensile characteristics at the high strain rate are important in the prediction of deformation modes of polymeric components which undergo high-speed deformation during car crashes. Uniaxial tensile tests at strain rates ranging from 0.001 to 100 s−1 were conducted using a tensile testing machine (INSTRON 5583) and developed high-speed material testing machine. Since the deformation of polymers is accompanied by localized deformation and a conventional extensometry method is no longer valid for strain measurement, a force equilibrium grid method (FEGM) is newly proposed to acquire accurate true stress–true strain curves of polymers. FEGM is utilized with images of the non-uniform distribution of deformation at each elongation stage captured by a high-speed camera in combination with a grid method. The relation between true stress and true strain at the desired strain rate and increment was determined by collecting the load data from the equilibrium and deformation images. [ABSTRACT FROM AUTHOR]

Abstract: The high strain rate properties of steel have been studied for accurate crashworthiness of vehicles. The mechanical properties at high strain rates show different characteristics from those at the quasi-static state. In order to seek for the physical phenomenon of the strain rate hardening, a microscopic investigation has been conducted for the texture evolution and dislocation behavior of steel. Tensile tests were performed at strain rates ranged from 0.001/s to 100/s, which is the common range in practical vehicle crashes to investigate the microstructural variation and dislocation behavior. According to experimental results of the EBSD and TEM experiment, the texture evolution does not develop remarkably with the variation in strain rates, while dislocation structure and behavior investigated by TEM experiments show significant difference with the variation in strain rates. The change in the dislocation structure, behavior, and density with the variation of strain rates clearly explains the mechanism of the strain rate hardening showing that round half-loop dislocations are replaced by straight and crossing dislocations and then by cell structured dislocations. [Copyright &y& Elsevier]

Abstract: This paper investigates the energy absorption characteristics of longitudinally grooved square tubes under axial compression by using explicit nonlinear finite element code LS-DYNA. The grooves are fabricated by stamping and the distributions of the effective plastic strain and the thickness variation from the stamping process are considered in the following crash analyses. From the simulation results, we find that when grooves are introduced on the sidewalls, the specific energy absorption of conventional tubes can be increased by up to 82.7% and the peak force can be reduced by up to 22.3%. The influences of several parameters, including the width of the tube, the length of the groove and the number of the grooves, are analyzed and the features of the deformation modes of grooved tubes are described. The introduction of groove is found to be an effective way to improve the crashworthiness of thin-walled structures. [Copyright &y& Elsevier]

This paper introduces the jig set for the crash test and the crash test results of shear bolts which are designed to fail at train crash conditions. The tension and shear bolts are attached to Light Collision Safety Devices(LCSD) as a mechanical fuse when tension and shear bolts reach their failure load designed. The kinetic energy due to the crash is absorbed by the secondary energy absorbing device after LCSD are detached from the main body by the fracture of shear bolts. A single shear bolt was designed to fail at the load of 250 kN. The jig set designed to convert a compressive loading to a shear loading was installed to the high speed crash tester for dynamic shear tests. Two strain gauges were attached at the parallel section of the jig set to measure the load responses acting on the shear bolts. Crash tests were performed with a carrier whose mass was 250 kg and the initial speed of the carrier was 9 m/sec. From the quasi-static and dynamic experiments as well as the numerical analysis, the capacity of the shear bolts were accurately predicted for the crashworthiness design. [ABSTRACT FROM AUTHOR]

Simulations of single-wall carbon nanotube(SWCNT)s having a different chiral vector under axial compression were carried out based on molecular dynamics to investigate the effect of the helicity on the buckling behavior. Calculation was performed at room temperature for (8,8) armchair, (14,0) zigzag and (6,10) chiral single-wall carbon nanotubes. The Tersoff potential was used as the interatomic potential since it describes the C-C bonds in carbon nanotubes reliably. A conjugate gradient (CG) method was used to obtain the equilibrium configuration. Compressive force was applied at the top of a nanotube by moving the top-most atoms downward with the constant velocity of 10m/s. The buckling load, the critical strain, and the Young's modulus were calculated from the result of MD simulation. A zigzag carbon nanotube has the largest Young's modulus and buckling load, while a chiral carbon nonotube has the lowest values. [ABSTRACT FROM AUTHOR]

This paper proposes a numerical algorithm of a time-discontinuous variational integrator based on the Hamiltonian in order to obtain more accurate results in the analysis of elasto-plastic stress wave. The algorithm proposed adopts both a time-discontinuous variational integrator and space-continuous Hamiltonian so as to capture discontinuities of stress waves. The algorithm also adopts the limited kinetic energy to enhance the stability of the numerical algorithm so as to solve the discontinuities such as elastic unloading and internal reflection in plastic deformation. Finite element analysis of one dimensional elasto-plastic stress waves is carried out in order to demonstrate the accuracy of the algorithm proposed. [ABSTRACT FROM AUTHOR]

This paper deals with the crash energy absorption and the local buckling characteristics of the expansion tube during the tube expanding processes. In order to improve energy absorption capacity of expansion tubes, local buckling characteristics of an expansion tube must be considered. The local buckling load and the absorbed energy during the expanding process were calculated for various types of tubes and punch shapes with finite element analysis. The energy absorption capacity of the expansion tube is influenced by the tube and the punch shape. The material properties of tubes are also important parameter for energy absorption. During the expanding process, local buckling occurs in some cases, which causes significant decreasing the absorbed energy of the expansion tube. Therefore, it is important to predict the local buckling load accurately to improve the energy absorption capacity of the expansion tube. Local buckling takes place relatively easily at the large punch angle and expansion ratio. Local buckling load is also influenced by both the tube radius and the thickness. In prediction of the local buckling load, modified Plantema equation was used for strain hardening and strain rate hardening. The modified Plantema equation shows a good agreement with the numerical result. [ABSTRACT FROM AUTHOR]

Abstract: This paper introduces an extended concept of limit analysis to deal with the dynamic equilibrium condition considering the inertia and strain-rate effect for dynamic behavior of structures. The conventional limit analysis method has been applied to only static collapse analysis of structures without consideration of dynamic effects in the structural behavior. A dynamic formulation for the limit analysis has been derived for incremental analysis dealing with time integration, strain and stress evaluation, strain hardening, strain-rate hardening and thermal softening. The time dependent term in the governing equation is integrated with the WBZ-α method. The dynamic material behavior is described by the Johnson–Cook model in order to consider strain-rate hardening and thermal softening as well as strain hardening. Simulations have been carried out for impact analysis of a Taylor bar and an S-rail and their numerical results are compared with elasto-plastic explicit analysis results by LS-DYNA3D. Comparison demonstrates that the dynamic finite element limit analysis can predict the crashworthiness of structural members effectively with less effort and computing time than the commercial code compared. The crashworthiness of a structure with the rate-dependent constitutive model is also compared to that with the quasi-static constitutive relation in order to investigate the dynamic effect on deformation of structures. [Copyright &y& Elsevier]

Highlights: [•] The cut-off value for stress triaxiality is discussed based on experimental observation. [•] A shear-controlled ductile fracture criterion is proposed with a changeable cut-off value for stress triaxiality. [•] A cut-off value function for stress triaxiality is suggested as the zero maximum principal stress with a constant offset. [•] The proposed criterion accurately predicts ductile fracture behavior of 2024-T351 from compression to tension. [ABSTRACT FROM AUTHOR]

Abstract: To address the need for new smart hybrid materials, this paper presents graphene based nano composites which have piezoresistivity for strain sensor. Graphene/epoxy composites cast in a mold were fabricated to a composite strain sensor. The strain response of graphene/epoxy composites showed fairly symmetrical and reversible behavior, and the gauge factor obtained was about 11.4 within the range of 1000 micro-strain. The graphene/epoxy composite strain sensor showed much higher strain sensitivity than both carbon nanotubes (CNTs) composite strain sensors and the strain gauge made of high-quality graphene films. A contact area & resistance model of graphene was introduced to explain the higher piezoresistivity of graphene composites compared to CNTs composites. [Copyright &y& Elsevier]