Finite Element Fatigue Without Crack Meshing, with Application to Casting

Casting and welding processes introduce defects which can grow to failure under fatigue loading. It is necessary to analyze such defects to estimate fatigue lifetimes or strength loss from uninspectable flaws. An accurate analysis should include stress gradients, singular crack stress fields, and multiple flaw interactions. It must also be easily applicable to irregularly shaped mechanical components. However, it is not feasible to mesh defects due to the tedium involved, and the need to evaluate several “what if” damage scenarios.

The finite element alternating method addresses these issues conveniently and accurately. It models defects by means of analytical crack stress functions, which are superimposed on systematic uncracked finite element meshes to achieve true stress fields. Functions are available for embedded or surface elliptic flaws, and other defect shapes. Since defects are not meshed, they are easily positioned by specifying three geometry points: crack center, and two crack front locations. It is simple to introduce multiple flaws, relocate flaws, or propagate fatigue cracks without the burden of remeshing. When flaws are near boundaries, the basic stress functions are corrected for residual effects in a concise boundary/crack face iterative equilibrium procedure. A nuclear reactor coolant pipe test and large permanent mold thermal fatigue analysis demonstrate the method.