Fatigue crack detection in aluminum alloy plate by nonlinear time reversal method

Based on the combination of time reversal method and elastic nonlinear response, the imaging of fatigue cracks in 7075-T6 aluminum alloy plate is experimentally studied by a self-developed system. This system consists of piezoelectric transducers (PZTs), arbitrary waveform generator, power amplifier, laser vibrometer, digital oscilloscope and computer. In experiments, four actuating signals (f=400kHz) are simultaneously emitted by four independent channels of arbitrary waveform generator and are applied to the PZTs array. The frequency components of time reversal refocused signals around fatigue crack are measured and analyzed. The results show that the closer is the laser distant from the crack, the larger is the peak amplitude of third harmonics. Therefore, third harmonic can be used to indicate the presence of fatigue crack. Through scanning the sample surface, the two-dimensional contour plot of the peak amplitude of third harmonics can be gained, and the position of the crack can be located by the imaging.