Keywords: finite element modelling, fracture mechanics, crack growth, crack control, stable tearing, fatigue failure, thin wall structure.

Great progress in design, analysis, and manufacturing-construction technology has often resulted in lighter and thinner structures. In the design of ductile engineering structures, limit load design has been considered as a pragmatic method to get the upper-bound on the load carrying ability of a structure. In this assessment full yielding over a cross-section is assumed, usually from the assumption of a virgin structure free of any defects. However it is well known that there exist cracks, either as inherent micro or macro cracks due to the material processing or manufacturing process, or cracks induced in the service life of the structure usually due to the dynamic nature of the loading, some obvious forms are due to wind forces, transportation, earthquakes. Fatigue consideration is needed to evaluate the integrity of the structure, which can be carried out by a simple and practical use of the S-N curve or a more sophisticated use of fracture mechanics concept of stress intensity factor combined with a fatigue crack growth rate (FCGC) characteristic curve of the material.

There is an area that has not been well explored is how to assess the soundness of a structure when the loading is a combination of fatigue loading with intermittent over-loadings which do not yet reach the limit load. Over-loadings may cause considerable yet stable crack growth, this so called "stable tearing" has been well recognized in aerospace engineering, where the presence of controlled cracks and overloading in the service life of a structure is accepted. There has been a promising approach to relate the characteristic resistance R curve of the material to the G curves, which show the relationship between strain energy release rate and the crack extension. These curves can be experimentally obtained in controlled laboratory testing of standard specimens containing simple cracks only.

This paper looks at the alternative of obtaining G curves for a general structure containing arbitrary cracks by finite element modeling and by virtual crack extension method, taking into account the non-linear plasticity of deformation. Singular elements were used to model the area near the crack front and the parametric design language of ANSYS 5.7 was used to generate G curves automatically. This FEM simulation of G curves would not only avoid tedious and expensive testing but also be applicable to a general structure having arbitrary cracks.

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