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Abstract: In this study the effect of micro-voids on the superelastic–plastic behavior of shape memory alloys is investigated. A new constitutive model for porous shape memory alloys, based on the Gurson–Tvergaard–Needleman formulation, is proposed. The model is able to reproduce both forward and reverse stress induced phase transformation, as well as plastic deformation. In addition, the model accounts for the presence of micro-voids and void-growth through a void volume fraction. A one dimensional implementation has been conducted, and results from the new constitutive model is compared with finite element unit-cell analyses. The model does well in reproducing results from unit-cells with void volume fractions . However, some discrepancy is found for void volume fractions that can be attributed to the highly inhomogeneous stress field in the unit-cells which the new model does not account for. The main results show that even for relatively small micro-voids the stress at which transformation and plastic yielding is initiated is lowered. Also, the presence of micro-voids leads to a narrowing of the stress–strain hysteresis which affects the amount of energy dissipated during a superelastic cycle. [Copyright &y& Elsevier]

Abstract: Residual stresses play a crucial role in structural integrity assessment. In this study, a large cracked cylinder with a weld in the center is applied to investigate the effect of residual stresses on the crack-tip constraint. A modified boundary layer model with a remote displacement-controlled elastic K-field and T-stress under small scale yielding has been used to simulate the problem. A two-dimensional tensile residual stress field due to the weld is introduced into the model by the so-called eigenstrain method. It has been shown that the residual stresses can significantly elevate the crack-tip constraint and thus increase the probability for cleavage fracture. The constraint parameter R introduced by the authors can be used to rank the crack-tip constraint induced by the bi-axial residual stresses. The R value decreases with the increase in the applied J-integral. The residual stress-induced constraint is also coupled with the T-stress. The R value becomes smaller with larger T-stress. [Copyright &y& Elsevier]

Abstract: This study focuses on the stress intensity factors for free edges in multi-layered structural components. The effects of elastic constants of various material combinations on the weak singularity at free edges are analyzed. Using the H-integral approach, the effects of elastic mismatch parameters, the bond area and the thickness of the thin metal layer on the stress intensity factor are quantified and the results are compared with detailed finite element solutions. A good agreement between numerical predictions obtained from the H-integral and the detailed FE results is achieved, showing the applicability of this approach. Similar to a crack problem, the singular elastic field dominates in an annular region adjacent to the notch tip. The relationship between the valid range of the K-dominated field and the thin-film thickness is then demonstrated. Furthermore, the competition of crack initiation between the free edge interface (180° opening angle) and a 90° notch interface in a generic specimen is investigated, in order to find out which is the prevailing failure mode. Comparison between isotropic Si and anisotropic Si substrate is also illustrated. Anisotropy of the Si substrate has a significant influence on the stress intensity factor when combined with an Au or Al metal layer but not with a Cu layer. Additionally, standardized numerical formulae of the dimensionless stress intensity factor have been derived to guide the engineering application. [Copyright &y& Elsevier]