Load system methodology

In order to cater for the widest possible range of loading an analysis, Zencrack utilizes a "load system" methodology. This allows definition of relationships between results of analysis increments in the f.e. analysis and the applied load spectrum.

For analyses without crack growth the relationship is a simple one - Zencrack merely processes all available results data and generates output of stress intenstity factors and/or energy release rates.

The schematic to the right demonstrates the way that the load system relationships can be set out in a fatigue crack growth analysis. In this example, the overall load spectrum contains a series of constant amplitude load blocks, being defined from a sequence of three separate spectra which have two different underlying load conditions. The f.e. analysis models these two separate load conditions and produces one set of results data for each. Three load systems are defined for the Zencrack analysis - one for each of the separate spectra. The spectrum for growth is defined by the sequence in which the loadings occur and is simply constructed by an ordered list of load systems - in this case the sequence has four systems defining the total spectrum. In this type of situation, the crack growth direction may vary from one load system to the next - the growth between f.e. analyses is the overall integrated effect of this potentially zig-zag behaviour.

Within the load system approach several basic load system types are available, including static (e.g. residual stress, centrifugal load), constant amplitude cyclic and a cyclic spectrum (i.e. "blocks" of constant amplitude loading at different load levels and with different numbers of cycles).

A "combination" load system is available to combine a set of static and cyclic load systems. This allows crack growth prediction including, for example, the effects of residual stress. This is only appropriate if the individual systems can be combined by superposition. When non-linearities occur, it is necessary to adopt a different approach. The minimum and maximum conditions during the load cycle must be analysed explicitly as two steps in the f.e. analysis to define the range for crack growth calculations. This requirement may arise, for example, due to the use of crack face contact or the existance of non-linear geometry effects in an analysis with a static load component and a cyclic load.