Zencrack provides a general 3D fatigue / time / combined fatigue and time crack growth prediction with a full mixed mode capability. Evaluation of energy release rates and stress intensity factors along crack fronts allows determination of the local crack growth vector at crack front nodes. Taken with load history and crack growth data the crack growth increment can be calculated at each crack front node.

Automatic mesh updates allow crack front advancement with re-meshing of the surrounding region. This includes growth out of the initial crack plane (i.e. non-planar mixed mode capability). Surface mapping routines ensure that moved nodes remain on any original (possibly curved) surfaces.

Zencrack uses a "load system" approach to define the loading spectrum applied to the component. Several basic load system types are available, including:

• "full cycle" system for complex load scenarios or analyses with non-linearities
• static e.g. residual stress, centrifugal load
• cyclic - constant amplitude
• cyclic - load spectrum with "blocks" of constant amplitude loading at different load levels and with different numbers of cycles.

Implementations of the Wheeler and Willenborg retardation models are available to cater for load interaction effects in load spectra.

A number of materials options are available ranging from simple Paris data to general user subroutine options. Threshold models are also included. In both cases temperature dependency can be incorporated into the materials data.

An accurate integration scheme is required for finite element based crack growth prediction since the fracture mechanics parameters are only calculated at discrete points through the growth. Zencrack incorporates a simple constant G scheme and a more sophisticated forward predictor. The latter incorporates a dG/da term into the integration to provide a more accurate solution.

A key issue in the accuracy of the solution is the maximum step size, da, between successive finite element analyses. Zencrack allows use of a fixed step size but also has an error control scheme to monitor accuracy of energy release rate values between f.e. analyses. This provides:

• cut-back or increase in step size depending upon accuracy
• a numerically stable solution and high accuracy in life prediction.