Static & cyclic loading

The combination of static and cyclic loading on a component can arise from a number of situations, e.g.:

• cyclic loading imposed on a component with a residual stress distribution
• test specimen with one static and one cyclic load component, e.g. a shaft with constant tension with cyclic torsion

The load system methodology in Zencrack allows analysis of this type of situation in two ways: the superposition or combination methods.

Superposition method

This method requires that the individual loads can be analysed separately and their effect combined by superposition. The finite element analysis contains one step for the static load and one step for the maximum cyclic load. This generates one set of stress intensity factors for the static load and maximum stress intensity factors for the cyclic load. Zencrack calculates the deltaK range based on the spectrum definition and combines the instantaneous static and cyclic stress intensity factors during the crack growth integration process. The overall effect on the growth analysis is via a modification to the stress ratio, R, with deltaK being independent of the static load. This change to R gives a change in da/dn for a given value of deltaK. For this method to produce meaningful results the crack growth data must be a function of R otherwise the static load has no effect on the analysis.

This method can be used for constant amplitude loading or a general spectrum. It requires that the finite element analysis is linear elastic.

Combination method

The combination method differs slightly in that the finite element analysis contains one step that exactly represent all the loads at the minimum cyclic condition and one step for all the loads at the maximum condition. The effective stress ratio and deltaK values can then be calculated directly from the stress intensity factors for the minimum and maximum conditions.

This method is only applicable for constant amplitude loading and cannot be used for a general spectrum. It does, however, allow the use of non-linearities in the finite element analysis.