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

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

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

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

Abstract: A ductile fracture criterion is newly proposed to model fracture behavior of sheet metals for nucleation, growth and shear coalescence of voids during plastic deformation. In the new ductile fracture criterion, void nucleation is described as a function of the equivalent plastic strain, void growth is a function of the stress triaxiality and void coalescence is controlled by the normalized maximal shear stress. The new ductile fracture criterion is applied to construct a fracture forming limit diagram (FFLD) of a dual phase steel sheets of DP780 (1.0t). The FFLD is approximated using both the reverse engineering method and circle grid analysis (CGA) since DP780 fails with slight thickness reduction from the analysis of the fracture surface. Predicted FFLDs are compared to experimental results to validate the performance of the new criterion in the intermediate stress triaxiality between 1/3 and 2/3. The new criterion is also applied to construct the fracture locus of Al 2024-T351 (Bao and Wierzbicki, 2004) to validate the performance of the new criterion in the low and negative stress triaxiality. The fracture locus constructed by the new criterion are close to the experimental data points for all these two materials in a wide stress range from the uniaxial compression to the balanced biaxial tension. The new ductile fracture criterion is recommended to be utilized in finite element analysis to predict the onset of ductile fracture of sheet metals. [Copyright &y& Elsevier]

Highlights • Mechanisms of ductile fracture is investigated experimentally in the wide range of loading conditions from compressive upsetting to the tension of notched specimens for two lightweight metals of DP980 (t1.2) steel sheets and a bulk aluminum alloy of AA7075. • All the specimens tend to fail along the direction of the maximum shear stress in various loading states of plane strain compression, uniaxial compression, shear, uniaxial tension and plane strain tension. • Fractured voids elongate along the direction of the maximum shear stress for all specimens with the stress triaxiality ranging from negative in compression to 0.57 in the plane strain tension. • The experiments of DP980 and AA7075 reveal that ductile fracture takes place along the direction of the maximum shear stress in the wide loading conditions of compressive upsetting, shear, uniaxial tension, and plane strain tension with stress triaxiality below 0.6. • Effect of the maximum shear stress must be correctly coupled in modeling of ductile fracture in these loading conditions. Abstract Mechanisms of ductile fracture are investigated experimentally in a wide range of loading conditions from compressive upsetting to the balanced biaxial tension for two metals with high strength-to-density ratio of DP980 (t1.2) steel sheets and a bulk aluminum alloy of AA7075. Specimens are carefully designed to achieve various loading conditions from shear at low stress triaxiality to the balanced biaxial tension at high stress triaxiality for DP980, while both tensile and compressive tests are conducted for AA7075. Fractured specimen surfaces are analyzed macroscopically focusing on their relations with the maximum shear stress. It is observed that all the specimens tend to fail along the direction of the maximum shear stress in various loading states of plane strain compression, uniaxial compression, shear, uniaxial tension, plane strain tension and the balanced biaxial tension. Scanning electron microscope analyses of fracture surfaces are also conducted to explore the underlying mechanism of void coalescence since coalescence of voids is viewed as the last step of ductile fracture after nucleation and growth of voids. It is noted that fractured voids elongate along the direction of the maximum shear stress for all specimens with the stress triaxiality ranging from about −0.57 in compression to 0.67 in the balanced biaxial tension. The experiments of DP980 and AA7075 reveal that ductile fracture takes place along the direction of the maximum shear stress in the wide loading conditions of compressive upsetting, shear, uniaxial tension, plane strain tension and the balanced biaxial tension with stress triaxiality below 0.67. Thus, ductile fracture is expected to be governed by the maximum shear stress in these wide loading conditions of compression, shear and tension. It is suggested that effect of the maximum shear stress must be correctly coupled in modeling of ductile fracture in these loading conditions with uncoupled and coupled ductile fracture criteria. Graphical abstract Shear fracture takes place in wide loading conditions of tension, shear and compression. Image, graphical abstract [ABSTRACT FROM AUTHOR]

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

Highlights • Wide range of stress states is experimentally described for AISI 316L. • Detailed study of fracture surfaces produced by penetration tests. • Non-standard cracking is observed due to ferrite phase. • Uncoupled ductile fracture criteria are calibrated for austenitic stainless steel. Abstract This paper deals with a small punch test and ductile failure prediction of austenitic stainless steel 316L. Comprehensive investigation of fracture surfaces was conducted as well as examination of δ-ferrite and σ-phase present in the material in order to reveal the reason of occurrence of nonstandard type of fracture. Three phenomenological ductile fracture criteria were selected for comparative simulations regarding the prediction of crack initiation and propagation during the SPT. These criteria were calibrated by using the fracture tests of smooth cylindrical specimens, notched cylindrical specimens, notched tube specimens and cylinders with spherical recess. The whole material model including the fracture criteria with von Mises plasticity and element deletion technique for simulation of the crack initiation was implemented by using the user subroutine Vectorised User MATerial into the commercial finite element code of Abaqus. Finally, the simulation results with three ductile fracture criteria are compared and discussed based on the experimental results. Only one of the failure criteria captured the displacement at fracture correctly. [ABSTRACT FROM AUTHOR]