Key Engineering Materials Vol. 794

Abstract: A stress-based model is developed to describe shear ductile fracture of lightweight metals. The proposed function couples the effect of the maximum shear stress and the stress triaxiality on fracture limits of metals during plastic deformation. Effect of the maximum shear stress in the proposed fracture model is correlated with the influence of the Lode parameter on fracture limits. The proposed fracture model is applied to depict the fracture locus of AA2024-T351. The predicted fracture locus is compared with experimental results of the alloy. The comparison demonstrates that the proposed fracture model reasonably characterizes the fracture stress in various loading conditions of compression, shear and tension.

Abstract: We proposed constitutive equations for the strain rate and temperature-dependent behavior of rubber by employing the nonaffine molecular chain network theory and reptation theory. The finite element homogenization method along with the proposed constitutive equations have the capability of predicting the deformation behaviors of particle-filled rubber under changes in volume fractions, distribution patterns, and size heterogeneity of the particles without additional parameters. The only existing problem is the modest estimation of the stiffness of rubber immediately after the abrupt change in strain rate direction (ACSD) as can be seen in the cyclic deformation behavior. We restricted our attention to the generalization of our nonaffine molecular chain network theory to overcome the problems associated with ACSD. We consider the effect of the delay of deformation in surrounding chains on the elasticity modulus by introducing an amplification parameter dependent on the current chain stretch and direction of strain rate immediately after ACSD. The potential of the proposed constitutive equations is examined against the predictability of the experimentally obtained deformation exhibiting ACSD.

Abstract: The acoustic emission (AE) monitoring technique is widely used in mechanical and materials research for detection of plastic deformation, fracture initiation and crack growth. However, the quantitative dependences of the AE signal parameters on material fracture parametersare not completely understood. This paper presents recent research results on AE monitoring of the fracture behavior of X80 line pipe steel, a critically important material for the oil and gas transportation industry.Fracture of this steel was studied using tensile testing of small scale specimens coupled with AE monitoring and high speed video camera. The dependence of fracture behavior and AE parameters on loading conditions (strain rate and presence or absence of a notch) was investigated. The AE parameters were analyzed using the “Average Hit” features and “Wave Form and Power Spectrum” methodologies. The fracture surface was characterized using scanning electron microscopy and a dependence of the AE parameters on the average void size has been obtained.

Abstract: In multistage deep drawing process, the subsequent drawing force (SDF) is a key reference value, which plays a significant role on the practical die structure design. In the present study, an analytical calculation of the radial stress at the die exit as well as the SDF in cylindrical cup drawing process are proposed, considering the previous strain hardening in the first drawing, normal anisotropy, frictional coefficient and die arc radius. As a result, the calculated maximum subsequent drawing force (MSDF) is in good agreement with experimental result. Besides, the MSDF decreases as normal anisotropy value increases, while increases as frictional coefficient and strain hardening exponent increases respectively.

Abstract: In this study, the Drucker yield function and S-shaped strain hardening model are employed to describe the yielding and hardening behaviors of SUS304 respectively. An uncoupled ductile fracture criterion is calibrated and then utilized to construct the fracture locus of SUS304. To explore the effect of various notches on the prediction of ductile fracture, the constitutive models and ductile fracture criterion are then introduced into the ABAQUS/Explicit code to predict the onset of ductile fracture of various notched specimens. The comparison demonstrates that the ductile fracture criterion captures the fracture strains for all specimens with different notch radius accurately and the finite element models predict the strain distribution, strain evolution and load-stroke curves with good agreement for specimens with large notch radius compared with the experimental results.

Abstract: This paper assesses various newly developed ductile fracture criteria including modified Mohr-Coulomb (MMC), DF2012, DF2014, DF2016, Hu-Chen and Mu-Zang, which were all proposed in the last decade. The AA2024-T351 is used for the assessment by comparing the predicted fracture limits to the experimental results both in strain and stress spaces. Fracture loci are also constructed by these criteria to evaluate their characteristics. The evaluation demonstrates that the Lode parameter and stress triaxiality should be properly coupled for reasonable modeling of ductile fracture in wide loading conditions. This study also shows that the coupling of the Lode parameter can also be realized by introducing the effect of the largest shear stress in fracture criteria.

Abstract: To predict material’s formability in the hydroforming processes, the plane stress assumption would be invalid. The instability perturbation approach proposed by Hu et al. [1] is extended with the through-thickness normal stress by combining Hill’48 and Hosford’s yield criteria. The influences of through-thickness normal stress on the predicted forming limit strains in the forms of traditional Forming Limit Diagram (FLD) and equivalent plastic strain (EPS) based FLD (epFLD) are investigated. The results show that forming limit curves (FLCs) in both forms of FLD enhance with increasing through-thickness normal stress under proportional and non-proportional loadings. This new model can be utilized to study the effects of fluid pressure on the formability of orthotropic thin sheets.

Abstract: Since its foundation, the concept of forming limit diagram has been widely accepted in sheet metal forming community as a powerful tool for studying formability. There are pyramid models that were developed to estimate the forming limit curve theoretically, for example, Swift's diffuse necking criterion, Hill's localized necking criterion, Marciniak and Kuczynski model, Modified Maximum Force Criterion, etc.. Implement of these models, however, is a laborious task. To simply the task, this study presents a graphical method to estimate forming limit curve of sheet metal. Some new insights into the Modified Maximum Force Criterion, the Hora method, are discussed. The insights pertain to the use of a graphic tool to estimate limit strains at three critical forming modes in sheet metal forming that are the uniaxial tension, plane strain, and equi-biaxial tension. Connecting three points by linear lines yields to a simple graph of forming limit curve. Method validation is supported by comparing the estimated forming limit curve with experimentally measured data for several automotive sheet metals.

Abstract: Dislocation structures at crystalline scale play an important role in the scale effect of materials. The higher-order crystal plasticity, in which a dislocation information is introduced as the gradient of slip and affects the hardening behavior of slip, is a useful model to describe a scale dependency of metallic material. In this study, a large deformation finite element analysis of a bicrystalline micropillar is demonstrated to investigate the grain boundary effect on the dislocation motion. The effect of condition on the grain boundary is numerically discussed. It is suggested that the large angle grain boundary and the coherent twin boundary can be represented by boundary conditions of non-penetration and penetration of dislocation.