Crack

A crack is a disruption in the normal continuum of a metal structure or component. Cracks normally (but not always) start or initiate at the metal surface. A surface crack may be visible to the unaided eye but can be more easily observed using non-destructive testing/inspection methods (NDT). For iron, steel and magnetic materials, magnetic particle inspection (MPI) is an easy and reliable inspection method. Another NDT method is x-radiography. Surface inspection and x-radiography are limited in that they do not readily indicate the depth and therefore the extent and criticality the crack, i.e., is this indication only a surface crack, or does it extend through 3/4 of the wall or component thickness? Other NDT techniques and destructive testing are used to fully assess the extent, magnitude and criticality of a crack.