An inspection technique used to assess the structural integrity of critical
components in a nuclear power plant must be able to discern surface-breaking
cracks from subsurface cracks. This work proposes an ultrasonic method
to provide that information and presents a theoretical investigation into
it. The main assumption of the model is that water carried by pressurized
pipes infiltrates and fills a surface-breaking crack, while a subsurface crack is
dry. The model simulates an inspection in which the modulation technique
is employed and the surface hosting the crack is not accessible. A ratio,
,
constructed with signals recorded in backscattering configuration during a
modulation cycle, is examined and shown to provide a clear criterion allowing
subsurface cracks to be distinguished from surface-breaking cracks when a shear
vertical wave at 45 degree incidence is employed as a probe.