铝合金薄板疲劳裂纹的非线性声学特性

Nonlinear Acoustic Characteristics of Fatigue Cracks in Aluminum Alloy Sheet

在没有任何损伤的铝合金薄板中,主要表现为原子间的非线性,以至于非线性声学特征不明显并很难测量;然而当铝合金薄板中出现疲劳损伤时,将产生明显的非线性声学特征,这对于识别铝合金薄板中的疲劳损伤十分有利。推导超声波在含有疲劳裂纹的固体金属中传播时非线性声学特征方程。利用非接触激光测振仪、任意波形发生器、数字示波器和计算机等组建铝合金薄板疲劳裂纹试验系统,并以含有疲劳裂纹的2024铝合金薄板为研究对象,利用该系统,在单频超声波即中心频率为270kHz激励下,对超声波在2024铝合金薄板中传播的非线性声学特性进行试验研究。结果表明,高次谐波可作为表征疲劳裂纹或缺陷的特征量。根据不同点的扫描结果,获得时域峰值幅度和频率幅度与扫描位置之间的关系,由此可大致确定疲劳裂纹的位置。

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.