Fatigue cracks in machine components are subjected to stress fields induced by the external load and residual stresses resulting from the surface treatment. Stress fields in such cases are characterized by non-uniform distributions and handbook stress intensity factor solutions for such configurations are not available. The method presented below is based on the generalized weight function technique enabling the stress intensity factors to be calculated for any Mode I loading applied to arbitrary planar convex crack. The method is particularly suitable for modeling fatigue crack growth in presence of complex stress fields.