中文摘要：

有效检测材料的非线性系数β是非线性超声评价材料力学性能及早期疲劳损伤等的前提和关键,针对当前的有限幅值法仅适用于有限孔径探头近场测量的现状,本论文研究了一种不受检测距离影响的测量方法。为抑制实际检测过程中声能的损失和声场扩散对测量结果的影响,对基波和二次谐波检测值进行衍射和衰减修正,其中利用多元高斯声束精确计算二次谐波衍射系数,在此基础上计算非线性系数β以消除其与理想的平面波推导结果间的差异,提高不同距离下测量值的精度。针对水的非线性系数β进行了仿真分析和实验验证,结果均显示本文方法相比于传统有限幅值法具有明显的精度优势,且表明该方法测量材料的β不受检测距离的影响,为放宽非线性超声检测的应用条件提供了理论依据。

英文摘要：

It is significant to determine nonlinear parameter β using nonlinear ultrasonic test for the evaluation of material characteristics, such as mechanical properties and early fatigue damages. A measurement method of β independent of the measuring distance is proposed in order to improve the performance that the present finite amplitude method only works in the near-field region of finite-size transducers. Attenuation and diffraction corrections for measured fundamental and second harmonic wave displacements are taken into account to reduce the impact on sound energy losing and field spreading, in which a more accurate diffraction correction for second harmonic wave using multi-Gaussian beam is conducted. Determining nonlinear parameter β with considerations of diffraction and attenuation can reduce the difference between the measured value and theoretical one and improve the accuracy in different measuring distances. Determination of β in water is simulated in theory and verified with experiment, and the results both show the developed method has the advantage of high precision over other methods. It is also proved that the proposed method is independent of the measuring distance and provides the theoretical basis for application of nonlinear ultrasonic test in less strict conditions.