Abstract:

Previous research [X. Hou and C. Darvennes, J. Acoust. Soc. Am. 100, 2568(A) (1996)] has shown that fatigue cycles can affect the nonlinearity parameter of laminated composites. In this research a thin laminated composite plate is put under fatigue until it fails. The fatigue test is stopped at regular intervals, when a C-scan picture is taken and the material nonlinearity is measured. To determine the nonlinearity parameter, the growth of a second harmonic is measured in the material. A monochromatic ultrasonic signal is sent into the sample via a contact transducer placed on the top surface. The growth of the second harmonic is recorded, with a second transducer placed on the bottom face, as the amplitude of the input signal is gradually increased. The nonlinearity parameter is then plotted as a function of the number of fatigue cycles. The C-scan images provide a visual clue about changes in the material structure that can be correlated to changes in nonlinearity. Hopefully this method will provide a means of predicting failure in advance. [Work supported by TTU Research Planning Grant program.]