Forensic Engineering Investigation and Analysis of Crack Formation in Acetal Resin Nuts Used for Water Supply Lines

Abstract

Non-destructive and destructive techniques were used to document and model crack formation in acetal resin nuts used for household toilet water supply lines. The morphologies of creep rupture and overload failure were documented using optical microscopy and scanning electron microscopy (SEM). The forensic investigation stemmed from litigation between a homeowner, plumbing contractor, and parts supplier related to a failure at a single residence. Fortuitously, 447 fittings in service for approximately two years under similar conditions were available for examination. In many properly installed fittings, cracks had initiated at the notch created by the sharp thread root radius, as predicted by engineering mechanics and finite element analysis. This study found that the cracks had propagated via plastic creep.