  A finite element method simulation of a carbon fibre reinforced polymer block
is used to analyse the nonlinearities arising from a contacting delamination
gap inside the material. The ultrasonic signal is amplified and nonlinearities
are analysed by delayed Time Reversal -- Nonlinear Elastic Wave Spectroscopy
signal processing method. This signal processing method allows to focus the
wave energy onto the receiving transducer and to modify the focused wave shape,
allowing to use several different methods, including pulse inversion, for
detecting the nonlinear signature of the damage. It is found that the small
crack with contacting acoustic nonlinearity produces a noticeable nonlinear
signature when using pulse inversion signal processing, and even higher
signature with delayed time reversal, without requiring any baseline
information from an undamaged medium.
