The simplest form of fatigue load is a constant amplitude cycle in which the load level repeats from a fixed minimum to a fixed maximum level. This is easily modelled using a constant amplitude load system definition.

In real structures the loading is often random in nature and the load level can be recorded as a varying random function over time. This type of random loading spectrum must be converted to a counted cyclic spectrum before it can be used in a fatigue analysis. Zencrack is supplied with a rainflow cycle counting pre-processor which generates counted spectrum input ready for use in a Zencrack fatigue crack growth analysis with a spectrum load system definition.

Both the rainflow pre-processor and the spectrum capability in Zencrack are able to handle spectra containing millions of points.

Spectrum data is defined in a combination of spectrum and sub-spectrum files. Each spectrum file references one or more sub-spectrum files. This simplifies the management of large amounts of spectrum data. In aerospace applications this also allows definition of different spectrum files for different types of aircraft manoeuvre. The overall spectrum for integration can then be defined as a sequence of individual spectrum files.

Once an analysis is able to cope with spectrum loading, the question of load interaction effects arises. Retardation caused by an overload can have a significant effect on the calculated life. A number of empirical models are available to try to account for this effect. Zencrack incorporates the basic and generalised Willenborg retardation models and the Wheeler model. Acceleration effects may optionally be included when using one of these retardation options. Some forms of crack closure modelling can be used in an analysis by defining an effective deltaK in a user subroutine.