The Cruciform Crack Problem and Sigmoidal Transformations

A sigmoidal transformation is an S-shaped one-to-one mapping of a compact interval onto itself. Various sigmoidal transformations are discussed, some of which are used to find approximate solutions of an integral equation, with a fixed singularity, arising from a problem in elasticity. In particular it is shown that, in this context, a sigmoidal transformation due to Sidi gives the best results.