This paper presents a practical approach for estimating residual fatigue life of structures with partial thickness cracks under arbitrary Mode I cyclic elastic stress fields. Residual life is defined herein as the number of cycles required to grow the crack from specified initial dimensions to some final configuration that results in static failure. A powerful influence function theory is modified for application to three-dimensional stress analysis of planar crack problems with arbitrary crack front shapes. The resulting procedure allows stress intensity factor solutions and residual fatigue life estimates for any Mode I stress field applied to the crack geometry.

Exact stress intensity factor solutions are presented for the buried ellipse under arbitrary Mode I stress fields. A procedure is outlined to build useful, accurate stress intensity factor algorithms for complex surface crack geometries from a few three-dimensional numerical stress analyses.