开发了一套在材料疲劳实验机上直接在线测量超声非线性系数的实验系统。利用该系统进行3组不同加载应力下的镁合金试件疲劳在线非线性超声检测,同时,利用光学显微镜对镁合金材料随着疲劳周数增大的微观结构变化进行原位观察。结果表明,在疲劳寿命的55%之前,随着疲劳周数的增加,位错滑移产生的驻留滑移带增多,微裂纹萌生扩展,超声非线性系数随之增大;在疲劳寿命的55%之后,宏观裂纹的出现使超声衰减系数增大,超声非线性系数有减小趋势。微观观察结果可以很好地验证超声实验结果。另外,高周和低周疲劳以及拉-拉和拉-压等疲劳模式的变化对实验结果没有明显的影响。 A set of experimental system was developed to directly measure the nonlinear coefficient of ultrasonic on the material fatigue testing machine. The system was used to perform fatigue online non-linearity ultrasonic testing of three groups of magnesium alloy specimens under different loading stresses. At the same time, the microstructure changes of magnesium alloy material with fatigue cycles were observed by optical microscope. The results show that before the fatigue life reaches 55%, the number of resident slip bands generated by dislocation slip increases with the increase of the number of fatigue cycles, and the crack initiation and propagation of microcracks expand and the nonlinear coefficient of ultrasonic waves increases. As the fatigue life increases, After 55%, the appearance of macrocracking increased the ultrasonic attenuation coefficient and the non-linear ultrasonic coefficient decreased. The results of microscopic observations can well verify the results of ultrasound experiments. In addition, high-cycle and low-cycle fatigue and pull-pull and pull-pressure changes in fatigue mode, the experimental results did not have a significant impact.