Abstract: Intact aluminum alloy sheet mainly presents as interatomic nonlinearity, so the nonlinear acoustic characteristics are unobvious and difficult to measure. However, when the same aluminum alloy sheet is damaged, its nonlinear acoustic characteristics are obvious, which is favorable for identifying fatigue crack in aluminum alloy sheet. The equation of nonlinear acoustic characteristics is derived when ultrasonic wave transmits in solid metal with fatigue cracks. A fatigue crack test system composed of piezoelectric transducer, non-contact laser vibration meter, arbitrary wave generator, digital oscillograph and computer is constructed. When the sheet is excited by a single frequency ultrasonic wave (center frequency 270 kHz), the nonlinear acoustics characteristics of ultrasonic wave transmitting in 2024 aluminum alloy sheet with fatigue crack is studied by using this system. The experimental results show that the high harmonics can be taken as the characteristic parameters to characterize the fatigue cracks or defects. Based on the scanning results at different points, the relationship between time domain peak magnitude, frequency magnitude and scanning position is obtained, thereby the positions of fatigue cracks can be approximately determined.

Key words: Acoustic characteristics, Aluminum alloy sheet, Fatigue crack, Nonlinearity