Nonlinear thermosonics and laser vibrometry for barely visible impact damage of a composite stiffener panel

Two methods have been evaluated in order to locate barely visible impact damage (BVID) in a composite stiffener panel. A nonlinear thermosonics technique and a nonlinear laser vibrometer technique were evaluated. Damaged regions were excited using a piezo shaker in both methods. Evaluation of the damaged regions was done by first determining the second and third order nonlinear harmonic response of the damaged regions. This was then used to determine the excitation frequency. By evaluating the presence of nonlinear responses in the output signal it is possible to excite the damaged structure at frequencies that give high heat generation and high displacements at the damaged regions. The results showed that both methods can be used to locate damaged regions, although it was shown that the stiffener impedes the propagation of the exciting wave and that these tests should be carried out in-between stiffeners in order to maximise the excitation and heating of damaged regions. Furthermore, both methods allowed for excitation of damaged regions over a large area.