Fracture Interface Elements For Static And Fatigue Analysis

Mabson, G.E., Deobald, L.R., Dopker, B., Hoyt, D.M., Baylor, J.S., Graesser, D.L.

16th International Conference On Composite Materials (ICCM 16), 2007.

Fracture interface elements have been developed that enable the practical application of the virtual crack closure technique within finite element models along predetermined interfaces. These elements are especially useful if non linear behavior occurs in the model, or if crack propagation predictions are desired. This paper presents the development and application of these elements to both static and fatigue analysis.

Delamination can be simulated by releasing coincident nodes and making use of the well established virtual crack closure technique (VCCT) [1]. The crack tip energy release rates can be determined from finite element models with arbitrary (but non-zero) loading. These crack tip energy release rates can be easily scaled to the critical energy release rate if the model is linear. The appropriate nodes, along a predetermined crack plane, can be released or model geometry updated and the crack can be successfully propagated for single 1-D cracks with single crack tips. The fracture mechanics scaling procedure normally occurs outside of the finite element code. If multiple crack tips exist, multiple computer runs are required to propagate the cracks. Each of these runs corresponds to a different crack length. This technique requires tedious postprocessing of multiple finite element solutions. If non-linear behavior(s) exists in the model, difficulties arise in determining the load at which the crack tip energy release rate equals its critical value. Therefore, many consider this method only generally applicable to linear problems with single crack tips. This approach is not practical for addressing problems containing, multiple crack tips on one dimensional (1-D) cracks (either two tips on a single crack, or multiple cracks with multiple crack tips), or multiple finite area cracks. A description of the implementation of this technique is given in [2]. A set of finite elements have been developed that take advantage of the linear elastic fracture mechanics theory and combines it with the ease of use of interface based finite element methods [3 through 9]. These elements allow the initiation of propagation and the propagation of interlaminar cracks, disbonds or delaminations to be simulated in layered materials using fracture mechanics procedures without many separate analyses. One must only define the critical energy release rate(s), which is mesh independent. Unlike decohesive type interface element formulations, mesh dependent maximum stress levels do not need to be identified. The fracture elements are based on the mixed-mode modified virtual crack closure technique, utilize common mixed-mode delamination growth, fatigue crack onset and fatigue crack growth criteria.