Implementation of a Non-Local Critical-Plane Fatigue Analysis Program with Applications to Cylinder Heads and Blocks

In automotive engines, the cylinder head and cylinder block are geometrically complex in that they have a large number of three-dimensional blends and fillets. In operation these components will experience a large number of stress concentrations with inherently large stress gradients. In an effort to provide a more accurate prediction of the propensity of a stress concentration in a cylinder head or cylinder block to develop a fatigue crack, herein a methodology is presented for adjusting the fatigue strength of the given material based upon the relative ‘size’ of the stress concentration. For a given linear-elastic stress history, a normal-stress based critical-plane method is used to first determine the predicted plane for crack initiation and growth. On this plane, the χ% stressed-area is defined as the material area that experiences χ% of the peak normal stress. The fatigue strength is then adjusted for this stress concentration using a power-law scaling function derived from fractal theory. Mean stress effects are taken into account using Goodman's linear correction. A Fortran90 program was written to implement these concepts given a finite-element analysis model and results.