Fatigue in composite shells of flextensional transducers can be invest
igated experimentally but in service results transposition is uncertai
n. This study can also be carried out numerically. However, a simple m
odel rarely allows to describe the fracture in fatigue of an existing
structure. The approach that is suggested here aims (starting from a m
odal analysis of a damaged structure in situ and according to results
of a numerical computation by finite elements) at identifying the type
of flaw in a qualitative and quantitative way. This paper first prese
nts the finite element used to deal with this problem. Second the appr
oach is validated on a shell example. Finally an experiment on a simpl
e G.R.P. plate is used to compare numerical and experimental results t
o validate the approach.