On the loci of exactness for truncated Williams crack-tip stress expansions

Williams asymptotic expansions are widely used to represent mechanical fields at the vicinity of crack-tips in plane elastic media. For practical applications, series solutions have to be truncated and it is believed that a better accuracy can be achieved by retaining more terms in the summations. The influence of the truncation on the accuracy can be quantified comparing truncated closed-form Williams series solutions available for some fracture configurations to their corresponding complex exact counterparts. The computation of 2D absolute error fields reveals astonishing patterns in which appear points with numerically zero error implying the existence of loci where truncated series can provide exact results. These loci of exactness gather on curves emanating from the crack-tips and pointing towards the outside of series convergence disks. An analytical investigation of this phenomenon allows to relate the number and tangency angle at the crack-tip of these curves to the number and values of the zeros of Williams series angular eigenfunctions. Beyond its analytical interest in the understanding of Williams series framework, this property of exactness for truncated series can also help to improve the accuracy of experimental and computational techniques based on Williams series.