传统超声方法是根据线性声学原理检测的,这对于疲劳损伤尤其是微小疲劳裂纹的识别,存在灵敏度和精度不高的问题.针对此问题,提出了一种非线性声学和时间反转法联合识别介质中的疲劳裂纹.利用超声换能器、非接触激光测振仪、数字示波器和任意波形发生器等组建了铝合金板材微裂纹试验系统.以LY12铝合金板材为研究对象,研究了单频超声波激励下,试样缺陷处的非线性时间反转声学特征.不同时间反转窗的试验表明:三次谐波信号是表征裂纹存在的最佳时间反转窗.通过对试样裂纹处的逐点扫描获得了LY12铝合金薄板微裂纹位置和大小与三次谐波聚焦峰值幅度之间的关系,根据此研究结果,可以对LY12铝合金板材中裂纹的大小和位置做出识别.
Classical ultrasonic test methods were based on linear acoustic theory. As a result, the tests suffered the disadvantages of low sensitivity and inaccurate identification of fatigue damage, especially fatigue cracks. To overcome this limitation, a method combining nonlinear acoustics with time reversal was developed to identify fatigue cracks. A system which was composed of a piezoelectric transducer, a non-contact laser viblation meter, a digital oscillograph and an arbitrary wave generator was constructed to study cracks in LY12 aluminum alloy plate. When a plate was excited by a single frequency ultrasonic wave, nonlinear acoustics time reversal characteristics of ultrasonic waves near the cracks were studied. The experimental results showed that the third harmonic is the optimal time-reversed window and it can be used to indicate the presence of the cracks. By scanning the cracked sample point-by-point, the relationship between the position and size of the crack and the locus of peak values was obtained, and thus cracks in an LY12 aluminum alloy plate can be more accurately judged by this relationship.