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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.

It is significant to study the relationship between the stress triaxiality and the fracture behavior for analyzing the failure process of additive manufactured titanium alloy. This paper combined the tensile test of smooth round bar specimens and notched round bar specimens with the numerical simulation to obtain the stress triaxiality distribution and fracture strain in order to study fracture behavior of additive manufactured titanium under different stress triaxiality. Fracture analysis was proceeded by using scanning electronic microscope to analyze the correlation between the stress triaxiality and the fracture behavior. Results show that the fracture strain decreases while the stress triaxiality increases and the location of the crack initiation moves from the center to the edge of the minimum cross-section while the radius of the notch decrease.

Hydrogen embrittlement (HE) in API 5L X70 steel was investigated by testing notched and unnotched uniaxial tensile specimens and single-edge tension specimen, SE(T). Apparent hydrogen diffusivity (Dapp = 1.4 x 10-10 m2/s) and solubility (Sapp = 4.9 mol H/m3) were determined by electrochemical hydrogen permeation tests. Through mathematical fitting, it was possible to separate the strong traps present at the beginning of the first permeation curve (φ = 0.43 mol H/m3). Uniaxial tensile tests showed a loss of ductility of up to 33% in the hydrogenated condition. Fracture mechanics tests exhibited a toughness decrease of 14% after exposure to hydrogen. The high resistance to HE was presented suggesting that these microalloyed steels can solubilize hydrogen in the matrix with low segregation, reducing the impact on embrittlement. A mix of ductile and quasi-cleavage fracture was observed in the hydrogenated samples with an increased stress triaxiality.

A series of tensile tests were conducted for notched plate specimens whose shapes are similar to the pressure shell to clarify the local failure characteristics for the steel nuclear containment. The specimens included the full or partial-width notched plate specimens of 10 mm thickness, and full-width notched thick plate specimens of 34 mm thickness the same as the actual pressure shell. In the full-width notched plate specimens with a small curvature notch, the cracking occurred on the notch bottom, and the crack propagated and finally ruptured. The plate specimens with a large curvature notch failed in shear suddenly without notch bottom cracking. Those failure modes were very unlike notched round-bar specimens, in which failure always starts at the notch section center. In the case of partial-width notch plate specimens, the notch bottom cracking also occurred for a small curvature notch, and the notch section failure happened before the specimen’s rupture for a large curvature notch. The limit strain diagrams were 40% to 50% lower than those of the notched round-bar specimens. The limit failure stress indexes, which correspond to the stress indications for the limit strain diagrams, could be applied to the shear failure of notched plate specimens. The indexes of the notched plate specimens were approximately 10% lower than those of the round-bar specimens. The crack initiation points for small curvature notch specimens were concentrated in the range of stress triaxiality 0.6 and equivalent plastic strain 0.5 to 0.8. In the full-width notched thick plate specimens, notch bottom cracking occurred for all specimens, and the cracking conditions were almost the same as the full and partial notched plate specimens. However, the maximum nominal stress exceeded 2/3 of the design tensile strength, and it was concluded that local failure occurs at more than twice the design pressure.

Complete understanding of the local stress triaxiality and stress concentration is essential to ensuring structural safety of several structures. A combination of mechanical tests with numerical simulations can be used to obtain this information. One way to study stress triaxiality is by modifying the standard tensile test geometry (ASTM E8) with a notch. Based on previous results from the literature, five notches were chosen: 10, 5, 3, 2, and 1 mm. These geometries were tested, and the results were numerically reproduced using the Abaqus/Explicit 2020 software. The models used were a non-linear model with the Gurson-Tvergaard-Needleman damage model to reproduce the failure. The numerical analyses allowed the assessment of the von Mises stress and stress triaxiality near the notch to compare with the standard smooth specimen. Two instants were considered as crack propagation onset; the instant of the maximum von Mises stress in the element at the center of the specimen, where the failure process begins; and the moment of maximum stress in the true stress x true strain curve. For the von Mises stress analysis, the difference between the curves was small. The stress triaxiality is a better variable to visualize the influences of the notch. When the strain is equal to a 0.07 (instant of the maximum force for the standard specimens), for the smaller notches (1 and 2 mm), there is a region where the effective plastic strain is zero. Consequently, the stress triaxiality is larger in this region than in the center. For the crack propagation onset instant, the plastic strain occurs along the whole transversal section. In this instant, the maximum value of stress triaxiality occurs in the center for all specimens. These results demonstrate that the stress triaxiality changes as the strain increases, i.e., varies with time.

As a flexible sheet metal forming process, incremental sheet forming (ISF) has been extended to fabricate lightweight sheet metal components, and increasing the formability is one of the key topics. In the present work, a novel ISF process integrating an auxiliary sheet (ISF-AS) is proposed for the improvement in formability, and investigated by using commercially pure titanium (CP-Ti) as the target sheet (TS). The experimental results demonstrate that the fracture strain obtained by ISF-AS process is increased by 50.6% compared with conventional ISF (CISF), and the geometric accuracy by ISF-AS is less than that by CISF. To explore the deformation mechanism during ISF-AS process, an analytical model of stress triaxiality based on membrane analysis has been developed, which demonstrates that ISF-AS can further increase the hydrostatic compression stress and reduce the stress triaxiality of TS compared with CISF. The fracture morphologies indicate that the dimples by CISF and ISF-AS processes are both elongated along the circumferential direction, but there are more tiny dimples with uniform distribution on the fractured surface of ISF-AS. In addition, orange peels on the lower surface of the part can be obviously restricted by ISF-AS with the improved surface quality. [ABSTRACT FROM AUTHOR]

Tensile deformation behavior of the heat affected zone (HAZ) of Mod. 9Cr-1Mo weldment has been characterized. Different microstructural regions (coarse grain, fine grain and inter-critical) of the HAZ have been experimentally simulated using weld simulator. Flat tensile specimens, with desired microstructural region in the gage portion, were extracted from simulated bars prior and after post weld heat treatment (PWHT). The tensile tests have been carried out at ambient temperature. Parallelly, the local deformation characteristics of the HAZ have also been assessed using automated ball indentation tests to assess material specific parameters for constitutive model. Further, FEM analyses have been carried out to assess the effect of stress triaxiality on deformation behavior across the HAZ interfaces. The effect of triaxiality on the spread of plastic zone has been discussed. [ABSTRACT FROM AUTHOR]

To study the deformation and fracture mechanism of 6061 aluminum alloy extruded pipe after secondary heat treatment under different stress triaxiality, a Johnson–Cook failure model was developed. Through the FEM method and SEM, the fracture mechanism of different types of aluminum alloy tensile specimens was analyzed. The research results show that the Johnson–Cook failure model could better simulate the tensile deformation of 6061 aluminum alloy specimens of different types, the parameters of the Johnson–Cook failure model were finally obtained D1 = 0.29, D2 = 1.356, and D3 = −2.567. With the increase of the stress triaxiality, the fracture strain showed a decreasing trend as a whole, and the fracture mechanism changed from a shear type to a hole aggregation type. The stress triaxiality gradually decreased with the increase of the notch radius/angles of the aluminum alloy notch specimen, and the stress triaxiality at the center of the notch was higher than the stress triaxiality at the root of the notch. [ABSTRACT FROM AUTHOR]

The determination of the mechanical response of engineering materials subjected to high loading rates plays an important role in determining their performance and application. The high strain-rate tensile response of metals is usually investigated by means of the split-Hopkinson tension bar (SHTB) apparatus. The interpretation of the obtained results is, however, subjected to analogous stress and strain uniformity challenges present during quasi-static tensile experiments. Beyond the onset of necking, strains cease to be uniform along the gauge length and localize around the necking zone. Consequently, the nominal strain rate underestimates the effective strain rate experienced by the material. The analysis of the effective strain rate and stress state beyond the onset of necking has received considerable attention in the literature. Several research efforts have focused on the optimization of the geometry of specimens to be employed for the characterization of the dynamic tensile response using the SHTB. The present work investigates, systematically, the effects of strain history and adiabatic heating on the stress state during dynamic loading. A series of monotonic and various strain history experiments were conducted and analyzed. The diameter evolution, effective strain rate, and temperature histories were measured for all conducted experiments. Numerical simulations were carried out to examine the stress state during strain localization and to accurately reproduce engineering and local thermos-mechanical variables. The effectiveness of existing postnecking corrections for high-rate experiments is assessed. A modified postnecking correlation taking into account the effects of adiabatically induced thermal softening is proposed. [ABSTRACT FROM AUTHOR]

In this paper, smooth and notched bar tensile tests of austenitic stainless steel 304 are performed, covering four different multi-axial stress states and six different strain rate conditions, to investigate the effect of the stress triaxiality and strain rate on fracture strain. Test data show that the measured true fracture strain tends to decrease with increasing stress triaxiality and strain rate. The test data are then quantified using the Johnson-Cook (J-C) fracture strain model incorporating combined effects of the stress triaxiality and strain rate. The determined J-C model can predict true fracture strain overall conservatively with the difference less than 20%. The conservatism in the strain-based acceptance criteria in ASME B&PV Code, Section III, Appendix FF is also discussed.

• Investigating a coupled damage-plasticity model to characterize the failure behavior of CuZn30 brass. • Conducting various tests to explore the failure characteristics influenced by stress triaxiality and Lode angle. • The simplified approach used to implement the Ganjiani and Lou ductile failure models in numerical simulations. • Simulating the observed softening (necking) phenomenon across all conducted tests. • Verifying fracture strains obtained by numerical methods with experiments through image processing. The investigation of ductile failure in CuZn30 brass under various stress triaxialities and Lode angles is the main subject of this paper. To accomplish this, six different specimen geometries were fabricated, including dog-bone, notched, two types of in-plane shear, torsion, deep drawing, upsetting and central-hole specimens. Only dog-bone, notched, two shears specimens were tested in three orientations: rolling, diagonal, and transverse to explore potential anisotropic properties in plastic deformation. A fully coupled ductile damage-plasticity model, incorporating the effects of stress triaxiality and Lode angle, along with a non-linear damage evolution law, is used to predict material failure. The modified Ganjiani and Lou ductile failure models were employed as failure onset criteria. Despite variations in their mathematical formulations, both models were considered due to their comparable predictions. Experimental results were validated through a combination of experimental and numerical simulations using ABAQUS with VUSDFLD and VUHARD codes. The outcomes of the simulation demonstrated good concordance with the results obtained through experiment, particularly in terms of force and the softening (necking) zone for all specimen types. Both failure models produced nearly identical failure strain values and accurately predicted damage initiation locations. The fracture strains obtained from numerical simulations were also experimentally validated using images of the ruptured cross-sections, processed with Microstructural Image Processing (MIP) software. For the dog-bone, notched, shear-60°, torsion, and central-hole specimens, failure occurs at the maximum force, where the force–displacement curve's slope reaches zero. However, for the shear-45°, deep-drawing, and upsetting specimens, failure is identified at the inflection point, where the curve changes from concave upward to concave downward. [ABSTRACT FROM AUTHOR]

Different dimensions of tensile test specimen can be used for qualification of welding procedure according to the existing standard. However, they are found to be dissimilar in tensile properties. In this study, the tensile properties of TIG welded 2219-T8 aluminum alloy joints were tested with different tensile specimen sizes. The welding residual stress releasing, specimen size effect, as well as fracture criteria were taken into account by using a sequential “welding - cutting - tensile” finite element simulating method. Results indicated that the degree of welding residual stress releasing decreased with the increasing of specimen width. For specimens with 250 mm width, over 80% of the residual stresses were kept. Tensile testing result showed that the tensile strength and elongation of joint decreased when the specimen width increased. The differences in tensile properties were proved to result from the size distinctions, rather than residual stress releasing, because the stress triaxialities were different in the specimens with different sizes. This phenomenon was precisely simulated by using the Johnson–Cook fracture criterion. Eventually, the constitutive equations of Johnson–Cook model were established based on the experimental tensile strengths to describe the tensile behaviors of 2219 aluminum alloy joints.

• The influence of stress state on fracture initiation is studied for Q960 UHSS. • Stress-state coupled damage variable is proposed for undergoing large deformation. • The cold bendability of Q960 UHSS is assessed with H-C fracture initiation locus. Ultra-high-strength structural steels are gaining popularity in civil engineering due to their exceptional strength-to-weight ratio. However, their inherent lower ductility compared to normal strength structural steel warrants particular attention and further investigation for safety assessment. In this study, a total of 24 specimens made of Q960 ultra-high-strength steel (UHSS), including 21 tensile specimens with various stress states and 3 three-point bending specimens, were tested to assess its mechanical behavior undergoing large deformation. Subsequently, the impact of stress state on plasticity, damage evolution and fracture initiation was analyzed and characterized. The Bai-Wierzbicki yield surface with a deviatoric associated flow rule was identified to characterize the complex plasticity behavior of Q960 UHSS. With respect to damage evolution, the stress-state dependent fracture energy G f was proposed in this study to describe the damage softening behavior. The uncoupled Hosford-Coulomb locus with a non-linear weight function was utilized to predict the fracture initiation behavior. Finally, the utilized stress-state dependent constitutive models for Q960 UHSS, with the calibrated parameters and user-defined material subroutine VUMAT, were successfully verified through the three-point bending test with good agreement. Moreover, based on a parametric analysis of radius-to-thickness ratio, the bendability design regarding the minimum radius-to-thickness ratio of Q960 UHSS was proposed. [ABSTRACT FROM AUTHOR]

In this paper, fracture toughness (K J) was measured for high strength rail steels and AL2024-T351 via chamfered cylindrical flat-end indentation. The indentation loading focused on applying the J-integral approach to curves of load versus indentation depth up to the crack initiation point based on a modified limit load via multiple indenter sizes. To promote single indenter size for practical use, virtual indenter sizes were proposed based on geometrical similarities, where the stress intensity factors according to J-integral approach were extrapolated to minimize the contribution of the plastic component of J-integral (J P). However, when the indentation method for K J is applied to high strength rail steels, a consideration for the modification of the J-integral approach is suggested with the inclusion of stress triaxiality effect to accommodate the pressure sensitivity experienced in compression-based testing for some materials. The K J values were seen to agree well with fracture toughness from conventional testing (K IC) for all materials in the study showing a relative difference below 5% except the JP rail steel, which showed only a relative difference of 16%. This is based on the condition that pressure sensitivity effect is present for the rail steels and not for AL2024-T351. The study also compares different indenter sizes, which show similar pressure and normalized depth profiles consequently offering the potential for a non-destructive means to measure mechanical properties and fracture toughness via micro-sized indenters. This opens an opportunity for further studies in material characterization capabilities across a wide range of industries in the future. [ABSTRACT FROM AUTHOR]

• An innovative compressive fracture characterization method is proposed using bilinear strain paths: pre-tension and compression. • A damage index approach in strain-space is utilized for the calculation of the compressive fracture under linear and bilinear paths. • A ductile fracture model capable of predicting the cut-off region is selected for the ductile fracture loci of the bilinear paths and implemented into the numerical simulation with different pre-tensile strain levels. • The verification of the proposed characterization method is performed by comparing experimental data and simulation results for fractured specimen shapes and load-displacement curves. This study proposes the compressive fracture characterization method using bilinear strain paths: pre-tension and compression. Compressive ductile fracture exhibits extremely large strain, which has been regarded as being difficult to be measured. Large deformation under compressive loading makes the shape of a specimen barreled and changes the stress triaxiality rapidly. Due to these complicated and large strains, compressive fracture strain can be considered to be within the so-called cut-off region where no fracture occurs. In order to enable compression tests to be easier, an approach that can lower the range of fracture strain is needed. Uniaxial tensile deformation is a strain path that induces the growth of voids inside ductile materials and leads to ductility reduction. Ductile materials subjected to pre-tensile loading before compressive loading can show the premature compressive fracture. A ductile fracture model capable of predicting the cut-off region is selected for ductile fracture loci of the bilinear strain paths and implemented into the numerical simulation with different pre-tensile strain levels. The verification of the proposed characterization method is performed by comparing experimental data and simulation results for fractured specimen shapes and load-displacement curves. [ABSTRACT FROM AUTHOR]

• By altering the loading stress states, a transition from shear to tensile failure in MGs is achieved. • The critical stress triaxiality that regulates the failure transition is established by determining the failure criterion under both failure modes. • The shear, tensile, and transition failure zones are distinguished by a nearly elliptical yield surface of MGs. • The microstructural evolution of SRO in MGs is analyzed to determine the failure mechanism when MGs experience a variety of failure modes. As a novel and highly promising metal in the future application of weapons equipment and aerospace fields, metallic glasses (MGs) demonstrate intricate failure modes that encompass both the brittle and plastic characteristics when subjected to varying loading conditions. In this work, a set of Cu 50 Zr 50 models subjected to a combined pure shear and equi-triaxial tension loading are simulated via molecular dynamics to investigate the impact of the stress state on the complex failure modes of MGs. The characteristic and critical moments when failure occurs are established under both the shear-band-induced shear failure and the micro-void-induced tensile fracture. The stress triaxiality is applied as a pivotal stress parameter that governs the transition from the shear failure mode to the tensile failure mode. The critical stress triaxiality of Cu 50 Zr 50 MG is approximately in the range of (2.0, 3.0) when both the shear and tension failures simultaneously occur, resulting in the largest failure strain at various stress states. We subsequently obtain a nearly elliptical yield surface of the Cu 50 Zr 50 MGs, in which the shear failure zone, tensile failure zone, and transition zone are clearly distinguished. The microstructural evolution of MGs during the failure transition is analyzed from the perspective of the specific short-range order. In contrast to the tensile deformation, icosahedral (quasi-icosahedral) clusters demonstrate a high level of shear resistance and remain stable in the shear-dominant deformations, which is confirmed as the structural origin of the stress state impacting the failure transition. [Display omitted] [ABSTRACT FROM AUTHOR]

The phase field model is a promising method for simulating fatigue crack growth (FCG) behavior. However, the conventional phase field (PF) model may not adequately account for constraint effects, where fracture toughness is affected by geometries. Therefore, stress triaxiality is incorporated into the PF model by modifying the fracture energy release rates to consider constraint effects. The model successfully simulates the FCG behavior of different geometries, such as CT, SENB, and MT specimens, as well as the mixed-mode FCG behavior of CTS specimens and other complex geometries. All simulations agree well with experiments, proving that our model is capable to capture the constraint effects in FCG behavior. These findings indicate that stress triaxiality is important to capture the constraint effects. [ABSTRACT FROM AUTHOR]

The local embrittlement of the coarse-grained heat-affected zone (CGHAZ) and intercritically reheated CGHAZ (IC-CGHAZ) in X65 pipeline steel was investigated using an in situ crack-tip opening displacement test in air and H 2 S, combined with microstructure-based simulation. The IC-CGHAZ exhibited significantly lower cracking resistance, with the fracture toughness reduced by 50 % compared to CGHAZ in both environments (0.160 mm in air and 0.017 mm in H₂S for IC-CGHAZ, compared to 0.307 mm in air and 0.034 mm in H₂S for CGHAZ). The martensite/austenite (M/A) constituents showed exceptionally higher intrinsic hardness than the ferrite matrix. The continuous distribution of M/As along the prior austenite grain boundaries resulted in local hardening and the formation of heterogeneous hard zones in IC-CGHAZ. Such microstructure heterogeneity resulted in the unique partitioning of plastic strain localization and stress triaxiality that promotes premature fracture, particularly with the coupled effect of hydrogen. This study provides a novel perspective on the fracture mechanisms and the typical fracture morphology of IC-CGHAZs in pipeline steel under the coupled effect of microstructure heterogeneity and hydrogen. • Fracture toughness of IC-CGHAZ was reduced by 50 % compared to CGHAZ in air and H 2 S. • Increased microstructure heterogeneity led to significant toughness degradation. • Formation of heterogeneous hard zones caused increased microstructure heterogeneity. • Partitioning of plastic strain and stress triaxiality influences fracture behavior. • Hydrogen dominated the cleavage fracture related to microstructure heterogeneity. [ABSTRACT FROM AUTHOR]

[Display omitted] • The effect of He bubble on the mechanical properties of materials is strongly related to the stress triaxiality and Lode parameter. • The underlying mechanism of He bubble fragmentation at low stress triaxiality, mediated by dislocation slip and surface diffusion, is clearly revealed. • T - L phase maps to illustrate regions where He bubble fragmentation, coalescence and neither of the two are possible. • The heuristic applications of coalescence onset criteria for He bubble at the nanoscale are explored. The He bubble is of utmost importance for understanding the dynamics and evaluating the performance of irradiated metals. This work systematically investigates the effect of the stress triaxiality and Lode parameter on the evolution of He bubble in Al via molecular dynamic simulations. Numerical results show that implanting He atoms into the cavity reduces the yield strength but boosts the ductility of the material, with this effect becoming more pronounced as both the stress triaxiality and Lode parameter decrease. One important discovery is the He bubble fragmentation under low stress triaxiality, and the underlying mechanism mediated by dislocation slip and surface diffusion is clearly revealed. Conversely, the He bubble tends to coalesce under high stress triaxiality, and the coalescence strain increases with the increasing He concentration. Additionally, the heuristic applications of coalescence onset criteria for He bubble are explored. The extended Thomason criterion, considering the hardening effect, provides qualitatively acceptable predictions. [ABSTRACT FROM AUTHOR]

In this study, cubic coupons of AlSi10Mg alloy were printed using the laser powder bed fusion (LPBF) technique. The effect of heating/reheating cycles on solute trapping and partitioning of alloying elements was investigated using atom probe tomography and transmission electron microscopy. Nano-hardness analysis and uniaxial tensile tests equipped with digital image correlation were employed to investigate the mechanical properties and Poisson's ratio. X-ray micro-computed tomography was utilized to detect strain localization sites along the building direction. Also, the uniaxial tensile test was simulated using finite element analysis to verify the experimental data and predict stress triaxiality. The results showed that the solute trapping and partitioning during the LPBF process results in remarkable changes in phases, their size and morphology, Poisson's ratio, strengthening factor, and consequently mechanical properties. While the tensile sample from top part of the LPBF coupon mostly shows porosity due to floating and entrapment of gases during layer-by-layer fusion/solidification, the sample from bottom part is exposed to sub-surface microcracking induced by residual stresses. The hardness, elastic, and shear moduli, Peierls stress, and cumulative strain energy of the top-part sample are higher than those of the bottom-part sample even though electron backscatter diffraction analyses report similar grain size and texture. Besides, by distancing from the build plate, the Poisson's ratio decreases. Simulation results of both samples indicate that the middle of the gauge is a high-potential area of failure initiation, where the bottom-part sample shows higher stress localization. • Undeniable role of solute partitioning on strengthening gradient in AM AlSi10Mg. • Variations in prevailing factors of strengthening through building direction of LPBF AlSi10Mg. • A transition in nature of structural defects due to residual stresses and thermocapillary forces. • A transition in type, size, morphology, and volume fraction of precipitates at constant grain size. • Variations in stress triaxiality of tensile samples taken along with building direction. [ABSTRACT FROM AUTHOR]

• Four commonly used shear test specimens were designed and optimized by FEA. • The advantages and disadvantages of each specimen were clarified to provide guidance for shear tests. • The plastic and failure characteristics of different specimens were compared and analyzed. • The underlying causes for the different characteristics and performances were also analyzed. In this paper, the dynamic shear response and failure of Ti-6Al-4V using four different test specimen geometries, viz. Hat-Shaped Specimen (HSS), Flat Hat-Shaped Specimen (FHSS), Chip Hat-Shaped Specimen (CHSS) and Double Shear Specimen (DSS), are critically examined and compared. Through a combination of experiments (using the standard Split-Hopkinson Pressure Bar system), finite-element simulations and metallographic examinations of their fracture morphology, the dynamic shear characteristics (strain hardening, strain rate strengthening effect and failure strain) of Ti-6Al-4V obtained using the different specimen geometries are critically examined, compared and analyzed. It will be shown that differences in the stress/strain uniformity, the plastic deformation zone, and the stress state induced by the different specimen geometries lead to discrepancies in the measured shear response and failure that were observed. The shear stress–strain curve obtained using the DSS will be shown to be more precise than the other specimen geometries. [ABSTRACT FROM AUTHOR]

Abstract A ductile fracture model is implemented to an elastoplastic constitutive model of steel material for large‐scale finite element analysis of steel frames. The stress modified critical strain model is extended to simulate the structural response after initiation of ductile fracture. The yield stress, Young's modulus, as well as the stress are reduced using the fracture variable. Positive definiteness of the material tangent stiffness matrix is always maintained, and the unbalanced loads are carried over to the succeeding step to analyze the responses in the range of degrading strength using an implicit finite element analysis. It is shown using a notched rod model and a double notched plate that the proposed model can simulate steep stiffness degradation due to strain localization after ductile fracture. Applicability to a large‐scale finite element analysis is investigated using a component frame of moment frame subjected to cyclic forced deformation.

The influence of temperature and strain rate on the deformation behavior of twin-roll casting AZ31 alloy has been examined as a function of stress state using notched and un-notched tensile specimens. Failure behavior during the physical tension was carried out using FEM simulations with different fracture criteria. Using a combined experimental-FEM investigation, the applicability of each fracture criterion was verified to serve the prediction for the crack initiation under multiaxial stress state. Results show that the critical fracture strain presents a clear exponential relationship with stress triaxiality and strain rate, and a linear relationship with temperature. Through the quantitative analysis of the relationship, a mathematical model can be well established. After comparison and verification, the fracture model can accurately predict the forming limit during the plastic deformation of twin-roll casted AZ31 magnesium alloy under complex stress state, and the classical Cockcroft & Latham criterion is the most suitable criterion for predicting the related crack initiation.

For more then half a centre just the same approach to the simulation of the ductile crack formation was developed independently by the scientific communities of foreign and native researchers. The importance at these studies drastically increased. A set of the characteristics, according to which it is recommendly to perform the detail comparison of the existing fracture models is developed. The examples of the analysis of a number of the most popular models by means of obtaining and study their analytical expressions regarding the conditions of the plane state are given. The generalized relations of the know models and a number of separate relations are obtained.

Titanium alloys has high fatigue resistance, high corrosion resistance, high temperature resistance, and other excellent properties, and have been widely used in deep-sea equipment and aviation industries. In this paper, the fracture mechanism and failure strain of TA31 titanium alloy, which has been widely used in deep-sea equipment, were studied experimentally and numerically in different stress states. Considering the pressure sensitivity, the Modified Johnson-Cook (MJC) model and the Bonora damage model were used to study the fracture behavior. In order to obtain the parameters of models, four types of specimens under different stress triaxiality were conducted, and a hybrid experimental-numerical approach was employed in this paper. Then, the coupled constitutive elastic–plastic-damage model was developed and implemented in ABAQUS explicit finite element analysis (FEA) code. Finally, to validate the suggested model, FEA simulation was carried out and compared with the experimental results. The comparison revealed that the Bonora model with constant parameters was not enough to predict the failure strain. The damage parameters were sensitive to the stress triaxiality. In addition, the fracture morphology was observed by scanning electron microscope (SEM), which revealed the micro-mechanism of failure for TA31 titanium alloy. It is concluded that a higher stress triaxiality and shear mechanism lead to lower plastic deformation, and will inhibit the void growth on the damage evolution.

The aim of the article is to examine the workability of sintered powder material of aluminum alloy (Alumix 321) through severe plastic deformations under the conditions of the equal channel angular rolling (ECAR) process. Accordingly, the stress–strain analysis of the ECAR was carried out through a computer simulation using the finite element method (FEM) by Deform 3D software. Additionally, the formability of the ALUMIX 321 was investigated using the diametrical compression (DC) test, which was measured and analyzed by digital image correlation and finite element simulation. The relationship between failure mode and stress state in the ECAR process and the DC test was quantified using stress triaxiality and Lode angle parameter. It is concluded that the sintered powder material during the ECAR processing failure by a shearing fracture because in the fracture location the stress conditions were close to the pure shear (η and θ ¯ ≈ 0). Moreover, the DC test revealed the potential role as the method of calibration of the fracture locus for stress conditions between the pure shear and the axial symmetry compression. [ABSTRACT FROM AUTHOR]

An efficient assessment of the mechanical properties of ductile iron (DI) under dynamic loading is usually missing. To address this gap, a plasticity model for DI grade EN-GJS-500-14 was suggested. Fracture mechanics (FM) and Charpy impact (CI) tests, high-speed and impact tensile tests as well as tensile tests at variable temperature and stress states were carried out on DIs with silicon contents from 2.10 to 3.64 wt-%. Toughness-related results indicate a difference in the transition temperatures of 100–140 K between CI and FM tests. The reason for this is assumed to be a pronounced strain rate sensitivity, which could be confirmed in the study. The plasticity model was able to predict toughness properties in CI tests. [ABSTRACT FROM AUTHOR]

This paper provides a numerical assessment of the ductile fracture of the AA6101 and AISI 4340 alloys using the Lemaitre and Gurson models. The simulations were carried out to verify the evolution of the accumulated plastic strain at fracture, assuming the high range of the triaxiality ratio, applying monotonic tensile loads on smooth and notched cylindrical specimens. The numerical strategy was structured for the Lemaitre and Gurson models, from establishing a non-linear system of equations using an implicit integration algorithm and solving the non-linear equation system using the Newton-Raphson method. When assessing the numerical and experimental results, it was observed that the cumulative plastic strain at the fracture decreases with the increasing levels of the triaxiality ratio for both alloys and models. On the one hand, the Lemaitre model was more optimistic than the experimental results. On the other hand, Gurson's model proved to be more conservative in its prediction of ductile fracture. Regarding determining the fracture onset, in general, both models showed good predictive capacity. However, the numerical results for the aluminum alloy presented more in agreement with experimental data than for the AISI 4340 alloy. In the end, assuming the determination of the level of displacement at fracture, the Gurson's model has the best performance. In this sense, for a high level of triaxiality ratio, the Gurson porous material can be recommended to describe the mechanical behavior of the material at fracture. [ABSTRACT FROM AUTHOR]

Material characterization at moderate strain rates is an important factor for improving the adequacy and accuracy of analysis of structures operating under extreme conditions. In this paper, the deformation and fracture of Ti-5Al-2.5Sn alloys were studied utilizing the punch test at strain rates up to several hundred per second. Loading velocities from 0.0003 to 15 m/s were realized during the spherical body penetration through a thin titanium plate. To describe the plastic flow and fracture of the Ti-5Al-2.5Sn alloy at strain rates ranging from 0.001 to 103 s−1, a micromechanical damage model was coupled with a viscoplastic constitutive model based on the dislocation dynamics. Numerical simulations of the punch test at 15 and 2 m/s were carried out to validate used constitutive relations. It was verified that the simulated fracture shape and deflections were similar to experimental ones. It was found that dynamic punch test is suitable for validation of damage kinetics under complex stress states. [ABSTRACT FROM AUTHOR]

Copyright of Journal of Zhengzhou University: Engineering Science is the property of Editorial Office of Journal of Zhengzhou University: Engineering Science and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

This paper aims to investigate the influence of stress triaxiality on the fracture behaviour of Ti6Al4V fabricated by Electron Beam Melting (EBM). Here, specimens with seven configurations were manufactured and tested to obtain a wide range of stress triaxialities. A combined approach using Digital Image Correlation (DIC) and finite element (FE) modelling was used to evaluate the current stress triaxiality levels of the various specimens. The material fracture envelope was defined with the triaxiality in a range from -0.28 to 0.71, noting that the fracture was strongly dependent on the stress triaxiality. The characterisations were then carried out by testing an ad-hoc specimen to evaluate failure criteria at low and high stress triaxialities. It was shown that the FE model using the failure criterion based on triaxiality offers more accurate predictions of the material's failure response than that based on the effective plastic strain. The modelling approach based on anisotropic elasto-plasticity contributes to better predictions of the alloy's response. Thus, the failure models based on the stress triaxiality are highly recommended for producing accurate numerical predictions of the fracture response of Ti6Al4V-EBM. [ABSTRACT FROM AUTHOR]

Copyright of Acta Materiae Compositae Sinica is the property of Acta Materiea Compositae Sinica Editorial Department and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

In this paper, the fracture behavior of ML15 cold heading steel was investigated based on the ductile fracture theory. Smooth and notched bar, flat grooved and tensile–shear mixed flat plate specimens were designed, and tensile tests were carried out to examine the fracture mechanism of ML15. The micromechanical characteristics of the fracture surfaces of different specimens were studied by performing scanning electron microscopy. The results showed that the specimens under different stress states showed different micro fracture morphologies. The Rice–Tracey model and Bai–Wierzbicki model were calibrated using test results. Based on the calibrated fracture locus, a finite element model is developed and compared to the test results, which confirms the feasibility of the calibrated fracture locus for metal failure analysis. [ABSTRACT FROM AUTHOR]

Large deformation and fracture processes of materials are often involved in the finite element simulation of high-speed collision. Based on the Johnson-Cook fracture failure model, TC4 titanium alloy samples with different notch sizes were designed, and the fracture strain and stress triaxiality of the samples were studied by combining test and finite element simulation. Speckle was produced during the experiment. The fracture strain of the sample under quasi-static tensile condition was obtained by using two-dimensional digital image correlation（DIC） technique at room temperature. The software Abaqus was used to simulate the quasi-static tensile process and obtain the average stress triaxiality of each sample. The parameters of Johnson-Cook fracture failure model were obtained by fitting the fracture strain with average stress triaxiality. The parameters of the fracture failure model were brought into the Pamcrash software for simulation, the fracture morphology and load-displacement curves of the test and Pamcrash simulation showed good consistency, which verified the correctness of the parameters of the failure model.

In order to accelerate the popularization of high‐strength (HS) steel, the fracture property of Chinese Q690 HS steel was investigated through experiments, numerical simulation, and model establishment in this paper. A three‐dimensional fracture dataset of Q690 steel based on "stress triaxiality‐Lode angle parameter‐fracture strain" was obtained by a hybrid method of experiment and simulation, illustrating that the ductility of Q690 steel under plane strain is lower than that under axisymmetric tension. Eight existing uncoupled fracture models were calibrated, and the Xue‐W model shows reasonable applicability on Q690 steel. To improve the accuracy of fracture prediction, a stress triaxiality and Lode angle parameter dependent uncoupled ductile fracture model was newly proposed, and a simplified version that can be conveniently calibrated without specially designed specimen was provided for engineering application. The proposed model and the simplified version were verified, respectively, and both can achieve satisfactory prediction on the fracture behavior of Q690 HS steel. Highlights: A 3‐D fracture dataset of Q690 steel was obtained by experiment and numerical simulation.Xue‐W model shows reasonable applicability on Q690 steel among 8 existing fracture models.A high‐accuracy uncoupled fracture model was proposed based on complex stress states.A simplified model that can be conveniently calibrated was established for engineering. [ABSTRACT FROM AUTHOR]

In order to understand the mechanism of conoidal fracture damage caused by a high-speed fragment-simulating projectile in titanium alloy layer of a composite armor plate composed of titanium- and aluminum-alloy layers, the ballistic interaction process was successfully simulated based on the Tuler–Butcher and GISSMO coupling failure model. The simulated conoidal fracture morphology was in good agreement with the three-dimensional industrial-computed-tomography image. Further, three main damage zones (zones I, II, and III) were identified besides the crater area, which are located respectively near the crater area, at the back of the target plate, and directly below the crater area. Under the high-speed-impact conditions, in zone II, cracks began to form at the end of the period of crack formation in zone I, but crack formation in zone III started before the end of crack formation in zone II. Further, the damage mechanism differed for different stress states. The microcracks in zone I were formed both by void connection and shear deformation. In the formation of zone I, the stress triaxiality ranged from −2.0 to −1.0, and the shear failure mechanism played a dominant role. The microcracks in zone II showed the combined features of shear deformation and void connection, and during the formation process, the stress triaxiality was between 0 and 0.5 with a mixed failure mode. Further, the microcracks in zone III showed obvious characteristics of void connection caused by local melting. During the zone III formation, the triaxiality was 1.0–1.9, and the ductile fracture mechanism was dominant, which also reflects the phenomenon of spallation.

It is the fact that the complicity of structural details and loading conditions makes a great challenge of metal structure design.One of the most important aspects is to numerically predict the metal yield and failure behavior under complex stress state in a high accuracy.However,experimental results have revealed that the traditional von Mises criterion can not accurately predict yield behavior of metal under complex stress states.In order to improve the accuracy,a yield criterion modified by stress state parameters of triaxiality and Lode angle is proposed in this paper.Then modified yield criterion is implemented into ABAQUS/explicit with VUMAT user’s subroutine and applied to numerical analysis of 2024-T 4 coupons.Numerical and experimental results show that the proposed yield criterion can improve the prediction accuracy of metal yield under complex stress state than traditional von Mises criterion.

Stress state is a primary factor controlling the ductile fracture behavior of steels, which is typically represented as the combination of the stress triaxiality and Lode angle parameter. The cleavage fracture properties of pipeline steels at low temperatures are usually assessed under plane strain conditions, such as using the fracture mechanics experiments. In this study, the cleavage fracture properties of a X70 steel at liquid nitrogen temperature (−196 °C) are characterized over a broad range of stress states. A comprehensive experimental program is carried out by performing tensile tests using various flat specimens of different geometries immersed in liquid nitrogen, including shear, central hole, notched dog bone, and side grooved plane strain. Pronounced plasticity occurs prior to the final fracture within the tested range of stress states at the very low temperature. Anisotropy effects are considered by conducting tensile tests of fracture specimens along the rolling, diagonal and transverse directions. Finite element simulations of corresponding experiments are performed using an evolving quadratic plasticity model to extract the local stress state variables to establish the fracture criteria, which are formulated based on the critical values of plastic strain and maximum principal stress. The fracture strain of the investigated material at liquid nitrogen temperature is affected by the stress triaxiality, Lode angle parameter and loading direction.

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.

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.