The paper will report analytical work relevant to the use of Finite Element Analysis when estimating fatigue crack initiation and early growth. Individual elements may then experience orthogonal random inputs, and principal axes of stress rotate. Pitioset has recently reported ways of estimating the probability distribution of the angle of this rotation when the inputs are specified in spectral terms. The distribution may vary between elements, and one question is what features affect the life estimate. Nieslony has reported tests showing that in geometries typical of real structures the location and orientation of early cracks is affected by the relative phase between the two inputs. The paper included some analysis. The work to be reported in the proposed paper extends this analysis, taking account of the Pitoiset work. It also considers another paper by Nieslony which reported successful life estimation for combined axial/torsion without special attention to phase.

A variety of damage models are still used for this form of computation. The effect of the commonest ones on this particular aspect is being investigated.