Analysis of fatigue crack growth under spectrum loading – The UniGrow fatigue crack growth model

• FCG is modelled based on the analysis of actual crack tip stresses and strains.
• The fatigue driving force was derived using Smith–Watson–Topper damage parameter.
• The model was validated using FCG data obtained from specimens made of Al 2324 T7.
• The method can be used to predict the fatigue life of notched and welded components.

The fatigue process near notches and cracks is governed by highly concentrated strains and stresses in the notch/crack tip region. Therefore, the fatigue crack growth can be subsequently considered as a process of successive crack increments resulting from material damage occurring in this region.The assumption mentioned above was used in order to model the fatigue crack growth based on the analysis of elastic–plastic crack tip stresses and strains. The fatigue crack growth was predicted by simulating the stress–strain response in the material volume adjacent to the crack tip and estimating the accumulated fatigue damage in a manner similar to fatigue analyses of stationary notches. The fatigue crack growth was regarded as a process of successive crack re-initiations in the crack tip region. The fatigue crack growth driving force was derived on the basis of the local stresses and strains at the crack tip using the Smith–Watson–Topper (SWT) fatigue damage parameter: D = σmax⋅Δε/2. It was found that the fatigue crack growth was controlled by a two parameter driving force derived in the form of power law expression Δκ = ΔK1−pKpmax. The two parameter driving force enabled to predict the effect of the mean stress on the fatigue crack growth rate including the influence of the applied compressive stress and tensile overloads, the overload effect and the effect of the crack tip internal (residual) stress resulted from the reversed cyclic plasticity.Experimental fatigue crack growth data set obtained for the aluminum alloy (2324 T7) was used for the verification of the methodology. The method can also be used to predict fatigue lives of notched and welded components where both stages of the fatigue process called as the fatigue crack initiation and propagation are important.