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ABSTRACT Being aimed at the strain path–related ductile fracture characteristic, this research develops a two‐component stress‐based DF2016 fracture model with 10 parameters through combining two single‐component corresponding fracture model with the addition form. This model possesses high flexibility to describe the ductile fracture behavior in various stress states. Stress‐based fracture‐related variables of the specimens with 10 different structures for WE43 alloy are captured, and the hardening behavior is with some strength differential effect. These fracture stresses have a strong sensitiveness to stress triaxiality and Lode parameter. Fracture loci of WE43 alloy are constructed by the proposed model with the smaller prediction error of 0.25683 than the stress‐based DF2016 fracture model (0.527). The reliability and high flexibility of the developed model are further uncovered through the description of ductile fracture behavior of AA2024‐T351 alloy. This research provides a valuable tool for predicting fracture failure of metals in engineering applications. [ABSTRACT FROM AUTHOR]

To investigate the microstructural changes that occur in stainless steel (SS) 304 during single point incremental forming (SPIF), experiments and finite element (FE) simulations were conducted for a truncated square pyramid geometry. Results from material characterization experiments for four stress states, i.e., uniaxial tension, equibiaxial tension, shear, and uniaxial compression, were combined to construct a material model based on the constituent phases and transformation kinetics. The material model was implemented into numerical analyses, where a two-step FE approach was utilized to predict martensite transformation in SPIF with increased computational efficiency. Validation experiments showed good agreement with the martensite transformation predictions from the FE simulations. The four locations along the pyramid wall revealed varying martensite volume fractions because of the differing stress states of bending, stretching, and shear that the blank is subjected to during SPIF, which can affect the microstructure. The stress state can be defined in terms of the stress triaxiality and Lode angle parameter. The FE results indicate that stress triaxiality impacted the martensitic transformation kinetics in SS304 more than the Lode angle parameter for SPIF for this particular material and geometry. Thus, distinct stress states in incremental forming can affect the martensitic transformation locally and, when used strategically, achieve functionally graded materials. This is pertinent to industrial applications requiring custom components, e.g., trauma fixation hardware for medical applications. [Display omitted] • SPIF pyramids exhibit heterogeneous microstructures affected by stress states. • A two-step FEA efficiently predicts the phase transformation of SS304 during SPIF. • SS304 models for constituent phases and transformation kinetics are identified. • Stress triaxiality impacted transformation kinetics more than Lode angle parameter. • Controlling microstructure can create functionally graded materials through SPIF. [ABSTRACT FROM AUTHOR]

Abstract Constrained groove pressing (CGP) process is one of the most efficient and novel methods of severe plastic deformation to manufacture ultra-fine sheet metal. The present research work is related to the study of CGP process of 2024-T3 Aluminum alloy sheet. The empirical and numerical simulation of CGPed- specimens have been examined at both room and elevated temperatures. In order to simulate the constrained groove pressing process in Abaqus software, the Johnson–Cook material model has been employed. The geometrical variables of the CGP process include the teeth number, teeth angle, and sheet thickness, and the criterion for the superiority of the numerical test is the maximum reduction of two equivalent strain and equivalent force factors, simultaneously. To determine the weight’s coefficients of the target factors and select the superior test, the Shannon’s Entropy and Simple Additive Weighting (SAW) methods have been used, respectively. After validating the numerical findings, the variation of damage parameter, stress triaxiality and the equivalent plastic strain distribution in the superior practical condition have been evaluated in detail. Based on the Entropy-SAW hybrid technique, the weight’s coefficients of equivalent strain and equivalent force target factors were computed to, in turn, 0.38 and 0.62. Also, the results revealed that the CGP process could not be performed at room temperature based on high damage evolution. But, at elevated temperature, the damage parameter (D) doesn't exceed 0.26. Also, the maximum stress triaxiality and equivalent plastic strain (PEEQ) at this process temperature reached to 0.5 and 0.1, respectively.

Structural steel (E250 grade) is used in several engineering applications involving loadings from quasi-static to high strain rates (blast discs, explosion vents, etc.), which introduce large deformation, strain and strain rate hardening, thermal softening, and damage to the material. The material's dynamic constitutive behaviour can be aptly modelled by a visco-plasticity-based Johnson–Cook (J–C) strength model and damage initiation and complete failure by the J–C's damage model. In the latter, damage initiation is modelled through continuum damage mechanics and propagation by the fracture mechanics. This paper focuses on the determination of 10 different J–C's dynamic constitutive and damage model parameters for E250 structural steel by conducting several experiments involving tensile tests at different strain rates (0.0003–1.0 s-1), stress triaxialities (0.33–0.95), temperatures (30–800 °C), and SHPB experiments (at 3000 and 8000 s-1). It explains the processes and step-by-step procedures for extracting the model parameters from the experimental results. A different approach is followed in arriving at fracture strain for extracting damage model parameters to suit fracture mechanic-based damage evolution available in the existing FEA codes. The constitutive and damage model parameters thus determined are validated through numerical simulations and comparison with three independent experiments viz. i) experiment of a plain tensile specimen, ii) tensile experiment of a notched specimen, and iii) hydrostatic burst experiment of a flat burst disc. The responses and failure patterns from numerical simulations agreed very well in all three experiments, thereby validating the determined model parameters. The determined model parameters can be utilised directly in the commercially available nonlinear explicit FEA codes. [ABSTRACT FROM AUTHOR]

The mechanical properties of local materials subjected to various stress triaxialities were investigated via self-designed small punch tests and corresponding simulations, which were tailored to the geometry and notch forms of the samples. The finite element model was developed on the basis of the actual test method. After verifying the accuracy of the simulation, the stress, strain, and void volume fraction distributions of the Ti6Al4V titanium alloy under different stress states were compared and analyzed. The results indicate that the mechanical properties of the local material significantly differ during downward pressing depending on the geometric shape. A three-dimensional tensile stress state was observed in the center area, where the void volume fraction was greater than the fracture void volume fraction. The fracture morphology of the samples further confirmed the presence of different stress states. Specifically, the fracture morphology of the globular head samples (with or without U-shaped notches) predominantly featured dimples. Modifying the specimen's geometry effectively increased stress triaxiality, facilitating the determination of the material's constitutive relationship under varying stress states. [ABSTRACT FROM AUTHOR]

We developed a pseudo-anisotropic damage model of randomly oriented strands (ROS) reinforced thermoplastic composites for reproducing flexural deformation and damage behavior by finite element analysis (FEA). The damage model applied stress triaxiality to capture the differences in the damage initiations among the tensile, compressive, and shear stress states. The damage model also included equivalent damage propagation function depending on the interlaminar shear property of ROS. The developed FE model was focused on the characteristics that ROS composites show macroscopic in-plane isotropic and out-of-plane anisotropic mechanical properties because of the laminated structure of ROS. Furthermore, all of the material parameters in the FE model were identified by performing simple deformation tests. As a result, the flexural numerical simulation could reproduce the experimental flexural deformation and damage behaviors of ROS plates with different thickness despite a complex stress state including in-plane and out-of-plane stresses. The developed FE model has potential to evaluate the safety of ROS-manufactured structural components. The evaluation of safety by FEA constructed in this study can improve the applicability of ROS composites. [ABSTRACT FROM AUTHOR]

The ability of wood–plastic composites (WPCs) to withstand various loads and resist plastic failure is attracting more and more interest due to the global increase in demand for WPCs by over 6 million tons per year. Among the most important and innovative research methods are those based on fracture mechanics—their results enable material designers to optimize the structures of these hybrid polymer composites at the nano, micro and macro levels, and they allow engineers to more accurately evaluate and select functional, sustainable, long-lasting and safe product designs. In this study, standard single-edge notched bending (SENB) tests were used to analyze the fracture toughness of two different extruded WPCs along the longitudinal (L) and transverse (T) directions of extrusion. In addition to their resistance to crack propagation, critical fracture criteria, initial contact stiffness, fracture parameters and fracture surfaces, the mechanical properties of these composites were also investigated. The results showed that WPC-A coded composites withstood higher loads until failure in both directions compared to WPC-B. Despite the larger data spread, both types of composites were more resistant to crack propagation in the T direction. Mode II of crack propagation was clearly visible, while mode III was not as pronounced. The experimental results and the numerical finite element (FE) model developed up to 58% of the maximum load correlated well, and the obtained deformation curves were best approximated using cubic equations (R2 > 0.99). The shear stress zone and its location, as well as the distribution of the equivalent stresses, had a major influence on crack propagation in the fracture process zone (FZP). [ABSTRACT FROM AUTHOR]

The Modified Mohr–Coulomb criterion has been demonstrated to exhibit high accuracy in the prediction of fracture in high-strength steels. Taking DP780 as the research object, the undetermined parameters of the Modified Mohr–Coulomb criterion at different temperatures were calibrated by tensile and shear tests combined with simulation. The relationships between the parameters and temperature were investigated. Finally, the relationship between criterion parameters and temperature was verified using the stretch-bending tests of U-shape parts. The fracture of automotive parts can be accurately predicted by simulation during warm stamping, thereby guiding actual production. [ABSTRACT FROM AUTHOR]

This study investigates the deformation responses and fracture behaviors of AA6061 under different stress triaxiality. Specimens of specific shapes and sizes are designed to achieve different stress triaxialities. Mechanical experiments and scanning electron microscopy analyses are performed to capture the mechanical behaviors and failure features of each specimen. Results show that thicker specimens deform more evenly in thickness than thinner specimens but exhibit slightly more brittleness, while the opposite is true for the notch radius of the specimens. Therefore, these two factors can be applied to the design of specimens for specific testing requirements. The failure mechanism is sensitive to the geometrical factors of the specimen. In addition, the distribution of failure features on the fracture surface is found to play a dominant role in the change in mechanism. [ABSTRACT FROM AUTHOR]

Estimates of physical and mechanical characteristics of materials at high strain rates play a key role in enhancing the accuracy of prediction of the stress–strain state of structures operating in extreme conditions. This article presents the results of a combined experimental–numerical study on the mechanical response of a thin-sheet rolled Ti-5Al-2.5Sn alloy to dynamic penetration. A specimen of a titanium alloy plate underwent punching with a hemispherical indenter at loading rates of 10, 5, 1, and 0.5 m/s. The evolution of the rear surface of specimens and crack configuration during deformation were observed by means of high-speed photography. Numerical simulations were performed to evaluate stress distribution in a titanium plate under specified loading conditions. To describe the constitutive behavior and fracture of the Ti-5Al-2.5Sn alloy at moderate strain rates, a physical-based viscoplastic material model and damage nucleation and growth relations were adopted in the computational model. The results of simulations confirm a biaxial stress state in the center of specimens prior to fracture initiation. The crack shapes and plate deflections obtained in the calculations are similar to those observed in experiments during dynamic punching. [ABSTRACT FROM AUTHOR]

Springback, buckling, and cracking are typical problems that take place during the stamping of titanium alloy sheets. Accuracy of material response characterization at moderate strain rates is an important factor for improving the adequacy of predictions of structures subjected to forming processes. In this paper, the deformation and fracture of alpha titanium alloys were studied using numerical simulations and the punch test at strain rates up to several hundred per second. Loading velocities from 0.5 to 10 m/s were realized during the spherical body penetration through a thin titanium plate. A viscoplastic constitutive model and damage model are used to describe the mechanical behavior of alpha titanium alloys at strain rates ranging from 10 to 103 s−1. Estimates of the residual stress of alpha titanium alloys are obtained using inverse simulation technique. It was found that amplitudes of residual stresses in CP-Ti after high speed punching are consistent with the increment of the yield stressas a result of strain hardening. Therefore, a comprehensive understanding of plastic deformation localization is essential for analyzing residual stresses in titanium alloys deformed at moderate strain rates. [ABSTRACT FROM AUTHOR]

The survivability of artillery projectile 155-mm Extended Range Full Bore Boat Tail (ERFB BT) inside gun barrel during gun launch mainly depends upon the mechanical properties of the shell body, boat tail and driving band materials. Any abrupt change in geometry of any one of these components can impair the structural integrity of the projectile, thereby resulting in malfunctioning of the projectile. Investigating the effect of abrupt change in geometry of the components is of much significance for assessing structural integrity. To investigate the effect of abrupt change in geometry of the components on structural failure, the notch tensile tests are performed to quantify the data on notch strength ratio (NSR), stress triaxiality and true fracture strain of the materials. To identify the mode of failure, the fractographic examination of the fracture surfaces is made. NSR is found more than unity; stress triaxiality determined by applying Bridgman's analysis reveals notch strengthening behavior of the materials. The fractographic examination shows that the shell body and boat tail materials fail in the combined ductile and brittle mode, whereas the driving band material fails by ductile mode. The outcome of the investigation confirms the load-carrying capacity and structural integrity of the projectile. [ABSTRACT FROM AUTHOR]

Grain boundaries (GBs) serve not only as strong barriers to dislocation motion, but also as important carriers to accommodate plastic deformation in crystalline solids. During deformation, the inherent excess volume associated with loose atomic packing in GBs brings about a microscopic degree of freedom that can initiate GB plasticity, which is beyond the classic geometric description of GBs. However, identification of this atomistic process has long remained elusive due to its transient nature. Here, we use Au polycrystals to unveil a general and inherent route to initiating GB plasticity via a transient topological transition process triggered by the excess volume. This route underscores the general impact of a microscopic degree of freedom which is governed by a stress-triaxiality-based criterion. Our findings provide a missing perspective for developing a more comprehensive understanding of the role of GBs in plastic deformation. [ABSTRACT FROM AUTHOR]

This study introduces a modified DF2016 criterion to model a ductile fracture of sheet metals from shear to equibiaxial tension. The DF2016 criterion is modified so that a material constant is equal to the fracture strain at equibiaxial tension, which can be easily measured by the bulging experiments. To evaluate the performance of the modified DF2016 criterion, experiments are conducted for QP980 with five different specimens with stress states from shear to equibiaxial tension. The plasticity of the steel is characterized by the Swift–Voce hardening law and the pDrucker function, which is calibrated with the inverse engineering approach. A fracture strain is measured by the XTOP digital image correlation system for all the specimens, including the bulging test. The modified DF2016 criterion is also calibrated with the inverse engineering approach. The predicted force–stroke curves are compared with experimental results to evaluate the performance of the modified DF2016 criterion on the fracture prediction from shear to equibiaxial tension. The comparison shows that the modified DF2016 criterion can model the onset of the ductile fracture with high accuracy in wide stress states from shear to plane strain tension. Moreover, the calibration of the modified DF2016 criterion is comparatively easier than the original DF2016 criterion. [ABSTRACT FROM AUTHOR]

In order to investigate the fracture behavior of the ZK60 Mg alloy rolling under different stress triaxiality, the different nominal stress triaxiality samples were well-designed, and the in-situ DIC tensile tests were carried out. The finite element method (FEM) model was validated by experimental results and the stress triaxiality evolution of samples was determined by modeling. The fracture strain of the ZK60 Mg alloy was non-monotonic with the nominal stress triaxiality increase and the best deformation performance of the central hole sample was indicated via the experimental and modeling results. The low stress triaxiality gradient of the central hole sample led to better deformation performance in terms of the macroscopic mechanical response. The transformation of the fracture mechanism from shear transgranular fracture to intergranular fracture and then to transgranular fracture during the stress triaxiality increased from 0 to 2/3 and was revealed via scanning electron microscope (SEM) characterization and FEM modeling. To further confirm the above fracture mechanism, X-ray computed tomography (XCT) was carried out to characterize the micro damage volume fraction and morphology. Due to the transgranular fracture induced by the high stress triaxiality at the initial fracture position of the central hole sample, the micro damage was smaller, delaying the fracture failure and achieving higher strain.

Accurately predicting the forming limit (FL) of sheet metals between equi-biaxial tension (EBT) and uniaxial tension (UT) is a research focus in sheet metal forming field. However, up to now, there is still a lack of an uncoupled ductile fracture criterion (DFC) which not only can predict the FL of sheet metals between EBT and UT accurately but also can be easily extended to other DFCs for bulk and sheet metals, preventing embedding only one uncoupled DFC into finite element analysis (FEA) software to satisfy the application requirements of fracture initiation prediction within different stress state ranges. The aim of this paper is to address the issue. The problems encountered in recently developed DFCs are firstly revealed and then a new uncoupled DFC is presented according to the analyses of void evolution of sheet metals. To fully understand the proposed DFC, parametric studies are conducted. In addition, the proposed DFC and four recently developed DFCs are employed to forecast the FL strains of three different sheet metals and the extensibility of the proposed DFC is studied. Research results show that the proposed DFC can not only forecast the FL of various sheet metals between EBT and UT reasonably but can also be easily extended to a unified uncoupled DFC for bulk and sheet metals, enabling the implantation of one DFC into FEA software to meet the application requirements within different stress state ranges. Moreover, the capability of the presented DFC to forecast FL in actual forming process is also verified. [ABSTRACT FROM AUTHOR]

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ABSTRACTImproved strength-ductility synergy is not uncommon among metallic materials with gradient structures of grains and twins. In this work, we report that the texture gradient innately created in Al-Mn strips by twin-roll casting is also capable of increasing the material’s ductility without sacrificing its strength. Analyses based on crystal plasticity theory demonstrated that the underlying mechanism of this texture-gradient-induced ductility enhancement lies in the orientation-dependent slip and grain rotation, which generate backstress, alter the local stress triaxiality, and delay the fracture process. These results advocate the integration of texture gradient design into the fabrication of high-performance gradient materials.

The aim of this article is to investigate numerically and experimentally the effect of stress triaxiality and anisotropy on the fracture strain for metals. As the aluminum has always been one of the most widely used metals in the industry. This metal has unique properties such as lightness and high resistance to corrosion, hence, 1100 aluminum alloy is used in this article. This metal is malleable and also has high workability so it is suitable for applications including shaping. To achieve the properties of the sheet, it is first necessary that all samples were made according to the ASTM-E8 standard and the specimens were designed to achieve some different stress triaxiality. This samples includes the standard-, notched- and shear-specimens. All samples are loaded in tension state. In order to investigate the anisotropy, the specimens are prepared in rolling direction, 45 and 90 degrees to rolling direction for each samples. The uniaxial tensile test was performed on the specimens until the onset of failure. For measuring the fracture strain experimentally, a new method with lower costs than others have been proposed. For standard and notched specimens, the strain measuring is based on the difference between cross-section areas for shear-specimen, the changes in the notched radius has been proposed for strain measuring criterion. Also, in order to calculate the stress triaxiality in the fracture zone, all experiment tests are simulated in Abaqus. The equivalent plastic strain and stress triaxiality of the elements in fracture zone are reported. The average value of these elements are compared to the corresponding experimental data. At the end comparing the results obtained from experimental and simulations shows that the failure strain is calculated with great accuracy. For more explanation, the maximum error is found to be 12.8% for notched-specimen. Furthermore, the non-linear effect of stress triaxiality on the fracture strain are well shown.

The Modified Mohr–Coulomb criterion has been demonstrated to exhibit high accuracy in the prediction of fracture in high-strength steels. Taking DP780 as the research object, the undetermined parameters of the Modified Mohr–Coulomb criterion at different temperatures were calibrated by tensile and shear tests combined with simulation. The relationships between the parameters and temperature were investigated. Finally, the relationship between criterion parameters and temperature was verified using the stretch-bending tests of U-shape parts. The fracture of automotive parts can be accurately predicted by simulation during warm stamping, thereby guiding actual production.

This work establishes a computational framework for the quantification of the effect of uncertainty of material model parameters on extremal stress triaxiality and Lode angle values in plastically deformed devices, whereby stress triaxiality and Lode angle are accepted as key indicators for damage initiation in metal forming processes. Attention is paid to components, the material response of which can be represented as elasto-plastic with proportional hardening as a prototype model, whereby the finite element method is used as a simulation approach generally suitable for complex geometries and loading conditions. Uncertainty about material parameters is characterized resorting to probability theory. The effects of material parameter uncertainty on stress triaxiality and Lode angle are quantified by means of a variance-based global sensitivity analysis. Such sensitivity analysis is most useful for apportioning the variance of the stress triaxiality and Lode angle to the uncertainty on material properties. The practical implementation of this sensitivity analysis is carried out resorting to a Gaussian process regression, Bayesian probabilistic integration and active learning in order to decrease the associated numerical costs. An example illustrates the proposed framework, revealing that parameters governing plasticity affect stress triaxiality and Lode angle the most.

Seven axisymmetric notched specimens extracted from stainless steel of yield strength 450MPa are tested under monotonic loading. The numerical simulation of all seven specimens is modeled in ABAQUS CAE, and the inbuilt constitutive model is used to capture the isotropic hardening behavior. The hardening parameters are calibrated until the numerical model simulates the exact behavior of load versus elongation obtained through experiments. The calibrated numerical model is used to determine the three fracture‐predicting parameters, namely stress triaxiality (T), equivalent plastic strain (PEEQ), and Lode angle (L). These three parameters are required in developing the fracture locus of 450 MPa Indian stainless steel. In addition, the study employs seven existing fracture prediction models for fracture initiation in all seven study samples. Using the determined values of T, PEEQ, and L at the fracture initiation point from the calibrated numerical simulation, the co‐efficient of all the seven selected models is calibrated. With the mean calibrated co‐efficient of all the specimens, the balanced error for all the models is calculated. With the help of the calculated balanced error, the efficiency of all the selected models in predicting the fracture initiation point is compared. Finally, the efficient model for 450 MPa Indian stainless steel is identified in this study. [ABSTRACT FROM AUTHOR]

This contribution proposes a study of the mechanical behavior from damage to fracture of normalized and annealed samples of AISI 4340 steel. Besides, a numerical study in a security device of offshore industry is conducted. Experimental tests were initially performed on smooth and notched cylindrical specimens subjected to monotonic tension as well as on rectangular specimens subjected to pure shear and a combination of shear and tension loads. Such experimental tests were selected to observe different combinations of the stress triaxiality and the normalized third invariant concerning the stress state. The two types of heat treatments were considered to achieve different levels of ductility and verify their influence on the fracture mode of the alloy. The approach based on the second invariant of the deviatoric stress tensor (J 2 ), with nonlinear isotropic hardening, is assumed to describe the mechanical behavior of the material in axisymmetric and three-dimensional numerical simulations. Finite elements simulations were carried out to analyze the performance of a simplified version of a mechanism of a blow-out preventer-BOP valve. The experimental and numerical curves of force versus displacement were compared, highlighting the effect of the stress triaxiality, the third invariant and calibration condition on the behavior of the material. The evolution of the equivalent plastic strain was plotted, and the fracture onset compared with its maximum value. The results show that the AISI 4340 alloy is highly dependent on the stress triaxiality and third invariant. Moreover, it can be concluded that different calibration conditions for the isotropic hardening curve can result in different levels of forces for the correct performance of the BOP in a critical situation. [ABSTRACT FROM AUTHOR]

This paper studies the hydrogen-assisted microdamage (HAMD) in fully-pearlitic steel. A detailed analysis is provided of the HAMD region in axisymmetric round-notched samples of high-strength eutectoid pearlitic steel under hydrogen embrittlement environmental conditions. The microscopic appearance and evolution of the hydrogen affected region is analyzed from the initiation (sub-critical) to the fracture (critical) situations. The use of very distinct notched samples and their associated stress distributions in the vicinity of the notch tip allows for a study of the key role of the triaxial stress state on hydrogen diffusion and micro-cracking (or micro-damage). The microscopic appearance of the hydrogen-affected zone (the so-called tearing topography surface) resembles micro-damage, micro-cracking or micro-tearing at a micro- or nano-scale due to hydrogen degradation, thus affecting the notch tensile strength and producing hydrogen embrittlement. A micromechanical model is proposed to explain these hydrogen effects on the material on the basis of the lamellar micro- and nano-structure of the pearlitic steel. [ABSTRACT FROM AUTHOR]

Hot-stamped steel is currently the most widely used lightweight material in automobiles, and accurately predicting its failure risk during the simulation is a bottleneck problem in the automobile industry. In this study, the fracture failure behavior of the hot-stamped PHS2000 steel manufactured by Ben Gang Group (Benxi, China) is investigated by experiments and simulation. Static tension and high-speed tension tests are conducted to obtain the elastic-plastic stress-strain relations, and a Swift + Hockett–Sherby model is proposed to describe the hardening behavior under static and high-speed loads. Tests under five kinds of stress states, namely static shear, static tensile shear, notched static tension, center-hole static tension, and static punching, are conducted to obtain the ultimate fracture strains under different stress states for establishing a failure model. The finite element method (FEM) is used to inversely achieve the fracture parameters of the material, and the GISSMO model in LS-Dyna is adopted to describe the fracture characteristics of the material. A fracture card is further established for simulation analysis by combining fracture characteristics with high-speed tension curves and simultaneously loading size effect curves of meshes. Finally, the card is applied in the simulation of the three-point bending test. High-precision results of fracture simulation matching the experimental results are obtained. This research proves that the proposed fracture card is accurate and can be widely used in the simulation of fracture behaviors of the hot-stamped PHS2000 steel. [ABSTRACT FROM AUTHOR]

Hydrogen can degrade the mechanical properties of steel components, which is commonly referred to as "hydrogen embrittlement" (HE). Quantifying the effect of HE on the structural integrity of components and structures remains challenging. The authors investigated an X70 pipeline steel through uncharged and hydrogen-charged (notched) tensile tests. This paper presents a combination of experimental results and numerical simulations using a micro-mechanics-inspired damage model. Four specimen geometries and three hydrogen concentrations (including uncharged) were targeted, which allowed for the construction of a fracture locus that depended on the stress triaxiality and hydrogen concentration. The multi-physical finite element model includes hydrogen diffusion and damage on the basis of the complete Gurson model. Hydrogen-Assisted degradation was implemented through an acceleration of the void nucleation process, as supported by experimental observations. The damage parameters were determined through inverse analysis, and the numerical results were in good agreement with the experimental data. The presented model couples micro-mechanical with macro-mechanical results and makes it possible to evaluate the damage evolution during hydrogen-charged mechanical tests. In particular, the well-known ductility loss due to hydrogen was captured well in the form of embrittlement indices for the different geometries and hydrogen concentrations. The limitations of the damage model regarding the stress state are discussed in this paper. [ABSTRACT FROM AUTHOR]

Existing experimental data demonstrate a significant stress-state sensitivity of the properties for a wide class of solids, including metals, rocks, plastics, and other structural materials. Conventional parameters used to describe the material stress state are the stress triaxiality and the Lode angle. However, only dependence of the fracture criteria on the type of stress state is often considered. In the present paper, a version of constitutive relations is proposed which allow to describe the stress-state dependence of the material properties for the entire deformation process and thus to provide a more accurate approach to the estimation of the stress–strain state of solids. The proposed approach is exemplified with the solutions of various spherically symmetry problems for physically nonlinear solids. It can be noticed that the stress-state sensitivity taken into account may result in essential deviations of the solutions obtained from the predictions of the classical models. Although the constitutive relations are essentially nonlinear, in some particular cases, an analytic solution of the problem can be obtained. The approach discussed in this paper provides a relatively simple but realistic continuum model able to describe complicated deformation features of materials. [ABSTRACT FROM AUTHOR]

The fracture behavior of rhombic dodecahedron (DOD) and body-centered cubic (BCC) 3D printed lattice structure considering surface roughness was investigated. Tensile tests on flat specimens with different surface roughness were performed, which indicates surface quality has an important influence on the fracture elongation. Smoothed round and round notched tensile specimens were also tested to calibrate damage constitutive model parameters. Based on the statistical data of the diameter variations of the struts and a progressive damage material model, FEM models were built to simulate the fracture behaviors of lattice structures. The simulation results are consistent with the experimental ones and damage evolution characteristics of the fractured struts are discussed. It's proved that surface roughness is a key factor to affect the strength of the lattice structure and fracture behavior. [ABSTRACT FROM AUTHOR]

Improved strength-ductility synergy is not uncommon among metallic materials with gradient structures of grains and twins. In this work, we report that the texture gradient innately created in Al-Mn strips by twin-roll casting is also capable of increasing the material's ductility without sacrificing its strength. Analyses based on crystal plasticity theory demonstrated that the underlying mechanism of this texture-gradient-induced ductility enhancement lies in the orientation-dependent slip and grain rotation, which generate backstress, alter the local stress triaxiality, and delay the fracture process. These results advocate the integration of texture gradient design into the fabrication of high-performance gradient materials. This work demonstrates for the first time that gradient texture could ductilize Al strips. It provides a new pathway — texture gradient design — to the fabrication of advanced structural materials. [ABSTRACT FROM AUTHOR]

Purpose: The fracture mechanism of S-07 steel was investigated by observing the fracture surface of the specimens with scanning electron microscope (SEM). Furthermore, the overall elastic–plastic behaviors and the stress state evolution during the loading procedure of all specimens were simulated by FE analysis to obtain the local strain at crack nucleated location and the average triaxiality of each type of specimen. Design/methodology/approach: Three types of tests under various stress states were performed to study the ductile fracture characteristics of S-07 high strength steel in quasi-static condition. Findings: Under tensile and torsion loading conditions, S-07 steel exhibits two distinctive rupture mechanisms: the growth and internal necking of voids governs the rupture mechanism in tension dominated loading mode, while the change of void shape and internal shearing in the ligaments between voids dominants for shear conditions. Originality/value: The failure criterion for S-07 steel considering the influence of the triaxial stress state was established. [ABSTRACT FROM AUTHOR]

This paper investigates the failure strain as a dependence of the stress triaxiality and the Lode angle parameter for polyurethane rigid foams (PUR) of two densities (100 and 300 kg/m 3) . Tests were carried out in tension for various configurations, resulting in different states of stress triaxiality at various Lode angles in the critical areas. The failure strain was determined for each setup using finite element analysis, as the tests were replicated with numerical models. The displacement at failure recorded in the experiments was imposed for the models, determining the failure strain as a function of stress triaxiality and the Lode angle parameter. The results were validated through the analysis of the failure of sandwich structures with aluminium faces and PUR cores. [ABSTRACT FROM AUTHOR]

Tensile tests were conducted for notched round-bar specimens to acquire the limit conditions of local failure, which is ductile failure after large local deformation for three types of steel, SGV480, SPV490 and SGV410, for nuclear containment structures. The notch deformation was monitored using a profile projector. The notch’s cross-section radius and the curvature radius of the notched profile were continuously measured by image analysis in the tensile test. From those measurements, the stress and strain at the notch’s cross-section center were determined by Bridgman’s approximate solution and the elastoplastic FEM analysis. The true stress-true strain curve for the elastoplastic FEM was identified a priori by an optimization program that searches for the good curve in the exponential form applicable to large strain regions. No cracks were observed at the notch bottom surface in all specimens until the final rupture. The failure at the notch’s cross-section center was detected from the inflection point of the load-displacement curve. The limit strain diagrams were compared with those of ASME Sec. VIII-2 and its conservatism were discussed. Further, small-size notched round-bar specimens were sampled from the weld joints of SGV480 and SPV490 to determine the limit strain diagrams and true stress-true strain curves of the base metal, HAZ, and weld metal. The limit strain diagrams at each material of the weld joints were as high as or higher than the base metal, except for the weld metal of SPV490. The authors discussed the optical micrographs for fracture surfaces of specimens, the fracture morphology of weld joints, and the modified expression of a limit failure stress index regarded as the stress indication of the limit strain diagram in the high-stress triaxiality region.

Estimates of physical and mechanical characteristics of materials at high strain rates play a key role in enhancing the accuracy of prediction of the stress–strain state of structures operating in extreme conditions. This article presents the results of a combined experimental–numerical study on the mechanical response of a thin-sheet rolled Ti-5Al-2.5Sn alloy to dynamic penetration. A specimen of a titanium alloy plate underwent punching with a hemispherical indenter at loading rates of 10, 5, 1, and 0.5 m/s. The evolution of the rear surface of specimens and crack configuration during deformation were observed by means of high-speed photography. Numerical simulations were performed to evaluate stress distribution in a titanium plate under specified loading conditions. To describe the constitutive behavior and fracture of the Ti-5Al-2.5Sn alloy at moderate strain rates, a physical-based viscoplastic material model and damage nucleation and growth relations were adopted in the computational model. The results of simulations confirm a biaxial stress state in the center of specimens prior to fracture initiation. The crack shapes and plate deflections obtained in the calculations are similar to those observed in experiments during dynamic punching.