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In this study, void coalescence with and without a plastic prestrain history is studied using stress-controlled axisymmetric unit cell models. In addition to spherical voids, both oblate and prolate voids are considered. In the case with prestrain history a uniaxial prestrain up to 10% was applied and the material is thereafter subjected to loadings with constant stress triaxiality. It is found that the microscopic position of the maximum axial stress in void ligament can be taken as an indicator for void coalescence. In the beginning of plastic loading the maximum axial stress occurs at the edge close to the void. With the increase of plastic deformation, the position of maximum axial stress shifts from the void edge to cell boundary and coalescence starts when the position appears at the boundary. It is shown that a prestrain history significantly reduces the void coalescence strain. The prestrain effect on void coalescence depends strongly on the initial void shape. Prestrain history induces both strain hardening and void shape change. The effect of prestraininduced void shape change on coalescence strain is relatively small while the effect of prestrain-induced local hardening is significant. [ABSTRACT FROM AUTHOR]

Abstract: The present study addresses the use of CTOD and T-stress in fracture assessments of surface cracked shell structures. A new software is developed for this purpose, denoted LINKpipe. It is based on a combination of a quadrilateral assumed natural deviatoric strain thin shell finite element and an improved linespring finite element. Plasticity is accounted for using stress resultants. A power law hardening model is used for shell and linespring materials. A co-rotational formulation is employed to represent nonlinear geometry effects. With this, one can carry out nonlinear fracture mechanics assessments in structures that show instabilities due buckling (local/global), ovalisation and large rigid body motion. Many constraint-measuring parameters have been proposed, with the Q-parameter or the T-stress being the most popular ones. Solid finite element meshing for complex structures such as pipes containing semi-elliptical surface cracks in order to compute Q is at present not a feasible approach. However, shell structures are most conveniently meshed with shell finite elements, and the linespring finite element is a natural way of accounting for surface cracks. The T-stress is readily obtained from the linespring membrane force and bending moment along the surface crack. In this study we present a new approach to analyse cracked shell structures subjected to large geometric changes. By numerical examples it is shown how geometric instabilities and fracture compete as governing failure mode. [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]