疲劳金属材料非线性声学特性的实验研究

EXPERIMENTAL INVESTIGATION ON ACOUSTIC NONLINEARITY OF FATIGUED METALS

非线性声学方法在材料特性的无损检测中 ,特别是在微结构的变化及金属疲劳的检测中有很大的应用前途 报道了一种用接触换能器法测量材料的超声非线性系数的方法 ,并用此方法从实验上测量了不同疲劳程度的LY - 12铝合金棒的超声非线性系数 .此外还测量了材料的声衰减 ,并从理论上考虑了声衰减对样品超声非线性系数测量的影响 .实验结果表明非线性声学参量比线性声学参量对材料的疲劳程度更为敏感 .

Nonlinear acoustics plays an important role in characterization of materials, in particular, detection of changes in the microstructure, material hardness, heat treatment of metal, component of alloy and the degree of metal fatigued. When a sinusoidal ultrasonic wave is launched to a nonlinear medium, the fundamental wave will be distorted as it propagates, so second and high harmonic wave will be generated. The second harmonic amplitude is dependent on a so-called ultrasonic nonlinearity parameter which is a characteristic of the material. Previous experiments observed that inhomogeneity in materials, for example dislocation, crack and voids etc. can greatly affect the nonlinear characteristic of material, and increase the second harmonic generation. The purpose of this project is to study the change of ultrasonic nonlinearity parameter while the materials are fatigued, and to explore the possibility on the NDE of metal fatigue by nonlinear methods. To calculate the nonlinearity parameter, at a minimum, we must absolutely measure the amplitude of fundamental wave and second harmonic wave. In order to do so, in the study of acoustic second harmonic generation in solids a capacitive transducer is often used as a detector because it is able to measure absolute displacement amplitude with good sensitivity, wide dynamic range. But it requires sample preparation including parallel surfaces, optical flatness, and clear surrounding. Due to the experimental difficulties encountered in using capacitive detector, this technique has not gained popularity outside the laboratory setting. Presently in this paper, we adopt a pulse-through method based on contact transducers. The detailed mode of this method to measure the nonlinearity parameter of the samples is described. Ultrasonic nonlinearity parameters of LY-12 aluminum samples with different fatigue degrees are measured experimentally. And the marked difference in nonlinearity parameter between fatigued and unfatigued aluminium alloy samples is observed. The acoustic attenuation of the samples under testing is also measured. Its influence on the measurements of the nonlinearity parameter is considered theoretically. The experimental results show that the nonlinear acoustic parameter of the material i.e. ultrasonic nonlinearity parameter is more sensitive to fatigue degree of samples than the linear parameter, i.e. attenuation.