by Sheng, Jianshun; Landis, Chad M.

In this paper Mode I steady state crack growth in single crystal ferroelectric materials is investigated. Specifically, the fracture toughness enhancement due to domain switching near a steadily growing crack tip is analyzed. For this purpose, an incremental phenomenological constitutive law for single crystal ferroelectric materials is implemented within a finite element model to calculate the stress and remanent strain fields around the crack tip. Also, the ratio of the far field applied energy release rate to the crack tip energy release rate, i.e. the toughening, is calculated. The numerical computations are carried out for single crystal ferroelectric materials of tetragonal or rhombohedral structure with different switching hardening and irreversible remanent strain levels. Toughening levels for crack growth along different crystallographic directions and planes are obtained and compared. Results from numerical computations for the toughening anisotropy for both tetragonal and rhombohedral crystals are presented and discussed.

DOI: 10.1007/s10704-007-9056-7
Online Date: 3/27/2007
Print publication date: 1/1/2007
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by Bellett, Daniel; Taylor, David; Morel, Franck

The stress distribution ahead of a notch is of great practical interest when undertaking fatigue and fracture analyses. In particular it is generally the first principal stress close to the notch which is desired. For a sharp notch this can be characterized by the stress field parameter K

^N
which is referred to as the notch stress intensity factor (or N-SIF). The finite element method is a very powerful tool which is commonly used to determine K

^N
. However, unless specialized methods are used the finite element mesh must be extremely refined in the region of the notch in order to calculate an accurate value. In practical situations, the degree of mesh refinement necessary is often not possible, due to either time or computer limitations.The following describes a simple technique which can be used to accurately determine the stress distribution close to a sharp notch, by remodelling or reshaping a stress distribution that has been obtained from a finite element analysis using a coarse or inadequate mesh. A theoretical equation for defining the principal stress distribution ahead of a sharp notch, which has been developed by Atzori et al. (2005) is used to do this. It is shown that the theoretical distribution can be explicitly determined from the finite element distribution by using global equilibrium conditions.It is shown that this technique is independent of the finite element mesh size. The method is used to calculate K

^N
for seven different combinations of geometry and loading condition, using various FE mesh refinement. It is shown that the results are accurate to within 15%.

DOI: 10.1007/s10704-007-9058-5
Online Date: 3/16/2007
Print publication date: 1/1/2007
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by Oh, Chang-Kyun; Kim, Yun-Jae; Baek, Jong-Hyun; Kim, Woo-sik

The present paper proposes ductile failure criteria in terms of true fracture strain (the equivalent strain to fracture) as a function of the stress triaxiality (defined by the ratio of the hydrostatic stress to the equivalent stress) for the API X65 steel. To determine the stress-modified fracture strain, smooth and notched tensile bars with four different notch radii are tested, from which true fracture strains are determined as a function of the notch radius. Then detailed elastic–plastic, large strain finite element analyses are performed to estimate variations of stress triaxiality in the tensile bars, which leads to true fracture strains as a function of the stress triaxiality, by combining them with experimental results. Two different failure criteria are proposed, one based on local stress and strain information at the site where failure initiation is likely to take place, and the other based on averaged stress and strain information over the ligament where ductile fracture is expected. As a case study, ligament failures of API X65 pipes with a gouge are predicted and compared with experimental data.

DOI: 10.1007/s10704-006-9036-3
Online Date: 3/16/2007
Print publication date: 1/1/2007
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by Greve, Lars; Andrieux, Florence

The deformation and failure mechanisms of toughened high strength adhesives used in the automotive industry are very complex and require advanced numerical models for crashworthiness simulation. The theoretical background of two new modelling approaches for thin adhesive layers is presented: firstly, a simplified elastic damaging node-to-element tied interface model approach for convenient and efficient modelling, and secondly a detailed modelling approach for improved accuracy using an elasto-viscoplastic solid element representation of the adhesive layer. The material model parameters required for both approaches are determined by a comprehensive set of experiments, including quasi-static and dynamic adhesive coupon testing, fracture toughness testing, and quasi-static tension/shear (and combined) testing of thin adhesive layers. A more complex adhesively joined assembly of two aluminium extrusions subjected to quasi-static (QS) and dynamic loading serves as the final validation example for both modelling approaches. Good agreement of experiments and numerical predictions was observed for both modelling approaches.

DOI: 10.1007/s10704-007-9054-9
Online Date: 3/16/2007
Print publication date: 1/1/2007
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