you are here:   / Resources / Automated Methodology for Modeling Crack Extension in Finite Element Models - Paper

Automated Methodology for Modeling Crack Extension in Finite Element Models - Paper

This paper, written by Jim Kosloski, Mike Bak and Tom Meyer of CAE Associates was presented at the NAFEMS World Congress in 2011.

The durability of aging aerospace components is compromised by the existence and continued growth of cracks due to cyclic loading conditions. Fracture mechanics provides accepted analytical and numerical approaches for predicting the remaining fatigue life for a known existing crack in a known cyclic loading environment. Fracture mechanics can be used to predict how the cracks will grow as a function of cyclic loading, the path they will take, and at which point the crack will propagate to failure.

In this paper we present an automated method for modelling the extension of a crack in a finite element analysis.  Typical procedures for calculating crack growth life involve determining the stress intensity factor (K) at the crack tip as a function of crack length and applied loading.  This data can then be used with an initial flaw size, the material fracture toughness, and Paris’ law constants to determine how many cycles it will take to grow a crack to failure.  To obtain the stress intensity factor as a function of crack length, the finite element model must be updated to include the extended crack.  The typical procedure for extending a crack involves manually modifying the underlying geometry using CAD tools, re-meshing the model, and reapplying boundary conditions and loads.  This procedure is tedious and time consuming. An automated method for extending a crack in a finite element model has been developed using User Programmable Features (UPF) in the ANSYS® finite element code.  In this method the existing mesh of the model is morphed so that the edge of an element lies along the predicted crack growth direction.  This element is then separated from its neighbouring element, thus incrementally extending the crack by one element length.  The model is reanalyzed and a new stress intensity factor is calculated.  The procedure is automatically repeated, extending the crack by one element length and recalculating K until the desired crack length is obtained. Upon completion of a series of such analyses of the structure with increasingly large cracks, the fracture life-time can be obtained by using material crack growth data.