Material degradation is usually preceded by some kind of nonlinear mechanical behavior. When a single sinusoidal ultrasonic wave is launched to a nonlinear medium , the wave will be distorted as it propagates , so higher harmonics will be generated. But the ultrasonic nonlinearity parameter is very small and to be confused easily from measurement system nonlinearity. The project researched the measurement method of nonlinearlity parameters, developed a robust experimental procedure. Experimental system was optimized by the selection of instruments and design of test fixture. There is linear relationship between amplitudes of the second-harmonic waves and the squared fundamental waves as a function of increasing input ,roltage amplitude , the system is reIiable. Using this method,ultrasonic nonlinearity parameteR of a group of LY12 aluminum samples monotonic loaded were measured. The experimental results show that there is a significant increase in ultrasonic nonlinearity parameter linked to applied stress level;ultrasonic nonlinearity parameter can characterize the mechanical performance degradation degree of LY12 aluminum.%材料早期力学性能退化总是伴随着某种形式的材料非线性力学行为,从而引起超声波传播的非线性,即高频谐波的产生.查超声非线性系数非常小,易被测量系统的非线性干扰淹没.通过实验仪器的选择和夹具的设计,改进了一套非线性超声测试实验系统.在相同条件下测得同一试件在不同输入电压下的二次谐波幅值和基波幅值平方近似线性关系,表明实验系统是可靠的.利用该系统进行了一组LY12铝合金拉伸试件非线性超声检测实验.在塑性阶段,随着拉伸应力的增大超声非线性系数显著增加.实验结果表明,超声非线性系数可以表征金属材料的力学性能退化程度.
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