Research On Ultrasonic Nonlinearity Of Metallic Materials Under Tensile Deformation

Because of the nonlinear property of materials, second and higher harmonics will be produced when harmonic waves propagate through materials. This phenomenon is called ultrasonic nonlinearity, and is characterized by the well defined acoustic nonlin-earity parameter (NLP). Loading such as uniaxial tension, fatigue, creep deformation etc, will induce the increase of NLPs. Through measuring NLPs, damage accumulation of materials can be detected, and then degradation of materials can be evaluated.In this thesis, ultrasonic nonlinearity of metallic materials under tensile deforma-tion were studied experimentally and theoretically. In the experimental study, NLPs of AZ31 magnesium alloy and T3 copper under uniaxial tension were measured. On-line and off-line NLP-stress curves of AZ31, and on-line NLP-strain curves of T3 were ob-tained. Experimental results show that both on-line and off-line NLP-stress curves of AZ31 are quite similar; no obvious change can be observed for NLPs in the elastic stage, while NLPs increase obviously in the plastic stage. The only difference is, that when the stress is close to the yielding limit in the elastic stage, the on-line NLP in-creases slightly, but the off-line NLP does not change obviously. The on-line NLP of T3 copper exhibits no obvious change in the elastic stage, but increases slightly in the plastic stage. It increases rapidly in the local deformation stage.In theoretical study, elastic-plastic model based on theory of finite deformation and nonlinear constitutive relations were employed to evaluate ultrasonic nonlinearity quantitively. NLP-stress or NLP-stress curves were obtained. Theoretical results show that the on-line NLPs increase slightly in the elastic stage and increase rapidly in the plastic stage, while that the off-line NLPs exhibit almost no change in the elastic stage, but increase obviously in the plastic stage.The comparison of the theoretical and experimental results shows that the off-line theoretical results and experimental results correspond well, while that the on-line theoretical results have differences with the experimental results.