Acoustic
resonant inspection is based on the analysis of
the natural frequencies of a part. An impact causes the
part to vibrate in certain characteristic frequencies.
These frequencies (fr) are whole-body properties of a
given structure, which are determined by dimensions and
material properties of the part.
A simplified physical model of the resonance can
be expressed with a mass and string as fr=
√(k/m), where k is
stiffness (material properties) and m is mass
(dimensions, density). Any structure has an
infinite number of resonances, each
determined by a combination of material properties and
dimensions. The presence of structural defects results
in out-of-specification condition for material
properties or dimensions, which causes shifts in these
frequencies, changes in damping factors, or nonlinear
effects such as generation of new frequencies. By
monitoring the aforementioned changes and analyzing the
multi-variable relations between the natural
frequencies, various structural defects can be detected.
