结构中微裂纹与超声波的混频非线性作用数值仿真研究

针对结构中微裂纹检测难题,本文对结构中微裂纹与超声波的混频非线性作用进行了数值仿真研究。基于经典非线性理论,得到了两列超声纵波相互作用产生混频效应的理论条件。通过有限元仿真,研究了两列纵波与微裂纹相互作用产生混频的条件,并分析了界面处静应力、摩擦系数和裂纹方向对混频效应的影响。研究发现,超声波与微裂纹相互作用产生混频非线性效应的发生条件仍符合经典非线性理论下的混频产生条件。裂纹界面处施加的静应力对差频横波幅值有明显影响;当施加静应力与无裂纹模型得到的最大应力值接近时,混频非线性效应最强;裂纹界面的摩擦系数对超声波的混频非线性效应影响较小;透射差频横波传播方向与经典非线性理论预测的理论差频分量方向基本一致,且几乎不受裂纹方向变化的影响,而反射差频横波的传播方向随裂纹方向的改变而有所不同。本文研究工作为微裂纹检出及方向识别做了有益探索。

Numerical simulation of the nonlinear effects of micro-cracks and ultrasonic waves in the structure

Aiming at the detection of micro-cracks in structures,the nonlinear mixing effect of micro-cracks and ultrasonic waves in structures is studied from a finite element model.Based on the classical nonlinear theory,the theoretical conditions for mixing effect of two longitudinal ultrasonic waves are obtained.The conditions of mixing generated by the interaction between two longitudinal waves and micro-cracks were studied through the finite element simulation.The effects of static stresses,friction coefficient and crack direction on the mixing effect were analyzed.It is found that the mixing nonlinear condition caused by the interaction between ultrasonic waves and micro-cracks agrees with the condition of mixing generation under classical nonlinear theory.The static stress at the crack interface has a significant effect on the amplitude of the differential frequency shear wave.The nonlinear mixing effect becomes strongest when the static stress is close to the maximum stress obtained by the non-crack model.The friction coefficient of the crack interface has little influence on the nonlinear mixing effect of the ultrasonic wave.The direction of shear wave in the simulation results is basically the same as the theoretical mixing frequency components predicted by the classical nonlinear theory and hardly affected by the variation of the crack direction.The direction of propagation of the mixing frequency components shear waves varies with the direction of the crack.The research work in this paper is a useful exploration for the detection of micro-cracks and direction recognition.