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In this research, ductile damage development and martensitic strain-induced phase transformation in plastic behavior of AISI 304 austenitic stainless steels at cryogenic temperatures are investigated. Nonlinear behavior of hardening and damage evolution could be observed in two-phase material as the result of strain-induced phase transformation. A simplified constitutive model for monotonic loadings, combining the effects of phase transformation and isotropic damage evolution has been introduced. Numerical analysis via implementing the model by means of a user subroutine UMAT in Abaqus/Standard is carried out. In addition, experiments including loading-unloading tensile test and X-ray diffraction test at cryogenic temperature 77[Formula: see text]K have been conducted to identify the parameters of the model and to compare with numerical results.

Enhancing the stiffness and crash characteristics of the vehicle body structure is among the most important concerns of auto manufacturers. Typically, enhancing these attributes leads to an increase in the vehicle’s mass, and as a result, a suitable optimization scheme is needed. In crash scenarios, collapsing of a section causes the load path to lose its load-carrying capability and degrades the crash safety of the vehicle. As a result, the sections should be designed accordingly, so that their collapse capacities be as high as possible. Although there are some research works that deal with the analysis and optimization of thin-walled members for stiffness or crash objectives, utilization of optimization methods for the design of vehicle sections against collapsing is still absent in the literature. In this regard, the present paper proposes an optimization framework that takes into account the collapse capacity and stiffness of the sections and optimizes their weight. Analytic formulations are derived to calculate the collapse capacity of sections under pure axial force and bending moment. The formulations are verified via finite element (FE) simulations. It is shown that the average absolute difference between the formulation results and explicit FE solver is less than 12%. Considering the highly nonlinear nature of the problem, this is a reasonably accurate approximation. Moreover, a combined collapse capacity criterion (CCCC), as a handy design guide for engineers, is also proposed for the first time to take into account the combination of axial and bending load cases. It is based on the FE analyses of numerous automotive sections in different load cases. To facilitate the optimization process, a new software interface named “JSec Design” is developed and introduced that is much faster than existing FE-based optimizers. Finally, as a case study, an automotive A-pillar section is designed under a combination of axial force and bending moment.

In this paper, a fully-involved anisotropic failure model including the effect of stress triaxiality and Lode angle proposed by Ganjiani [Ganjiani, M., 2020. A damage model for predicting ductile fracture with considering the dependency on stress triaxiality and Lode angle. European Journal of Mechanics-A/Solids, 104048] is extended to predict the fracture phenomena in the materials. For considering the anisotropy effects, Hill's 48 yield function is used. The proposed model is applied to construct the fracture loci and the corresponding Fracture Forming Limit Diagram (FFLD) to validate the performance of the model. The fully-involved anisotropic means that in constructing FFLD, the anisotropy is involved on both the stress triaxiality

The strain-induced martensitic transformation greatly affects the plastic behavior of the metastable austenitic stainless steels. The martensitic transformation continuously changes the initially homogeneous material into a strongly heterogeneous bi-phase one. In addition to the hardening behavior, this phenomenon would influence the damage growth and load-carrying capacity of the material during the plastic deformation. In this study, plastic behavior of the material AISI 304 including the hardening and damage growth, has been examined at low temperature; where a high rate of martensitic transformation affects the microstructure strongly. Experimental analysis and microscopic observations have been performed for evaluating the martensite content and damage growth. In addition, based on the continuum damage mechanics, a simplified damage evolution model has been proposed to capture the effect of phase transformation on the damage growth rate explicitly. The results show that the damage initiates with a considerable rate in early stages of transformation, however at higher levels of transformation, damage growth rate decreases until a sudden fracture. The presented model properly predicts the observed damage behavior.

In this research, we investigated the crack behavior in the Mode I fracture in a plane strain state, considering the size effects and under the couple-stress theory using finite element method (FEM). First, using the eigenfunction expansion technique, we provided the stress, couple-stress, displacement, strain, and curvature fields at the crack-tip. Then, using the FEM, fundamental parameters that determine the stress and couple-stress fields at the crack-tip (i.e., stress intensity factor and couple-stress intensity factor) were obtained. In the formulation of FEM, we used a mixed variational method, in which displacement and rotation are considered as independent variables, and their kinematic constraints are applied using Lagrange multipliers. Based on this formulation, it was possible to implement the classical quarter-point elements for the crack-tip. The results show that under the framework of the couple-stress theory, energy release rate can still be considered as the fundamental measure and determinant parameter for the crack behavior, but the stress intensity factor alone cannot appropriately describe the crack behavior.