螺栓轴向应力的非线性超声检测技术研究

在工作过程中，螺栓常受到轴向应力的作用，作为最常见的紧固件之一，螺栓的服役质量对于安全生产至关重要。非线性超声技术对材料内部微结构的变化敏感，在检测螺栓轴向应力方面有极大的潜力。根据超声波在各向同性材料内部的传播规律，分析了非线性系数与基波幅值以及高次谐波幅值之间的关系，搭建了非线性超声检测系统。以45钢和304不锈钢材质的螺栓为研究对象，对不同轴向应力下的螺栓进行非线性超声检测，得到非线性系数与螺栓轴向应力之间的关系曲线。结果表明，螺栓的超声非线性系数随轴向应力的增大而单调增加，当轴向应力超过一定值时，非线性系数显著增大，说明非线性系数对应力变化敏感，可以用来表征螺栓的轴向应力。通过对相对非线性系数进行归一化和多项式拟合，得到螺栓轴向应力与归一化相对非线性系数之间的函数关系，然后采用非线性超声和线性体波检测两种方法对电子拉伸试验机施加的螺栓轴向应力进行检测验证试验，200 MPa以上的轴向应力采用非线性超声检测误差在6%以下，明显优于线性超声体波检测，研究结果为服役状况下螺栓的轴向应力检测提供了有效支撑。

Research on Nonlinear Ultrasonic Testing Technology of Bolt Axial Stress

Bolts are often subjected to axial stress in the working process. As one of the most common fasteners, the service quality of bolts is critical to safe production. Nonlinear ultrasonic technology is sensitive to the changes in the internal microstructure of materials, which has great potential in detecting the axial stress of the bolt. According to the propagation law of ultrasonic waves in isotropic materials, the relationship between the nonlinear coefficient and the amplitude of the fundamental wave, the amplitude of the higher harmonic is analyzed. A nonlinear ultrasonic testing system is built. Taking 45-steel and 304 stainless steel bolts as the research object, the nonlinear ultrasonic testing of bolts under different axial stresses is carried out, and the relationship curve between nonlinear coefficient and bolt axial stress is obtained. The results show that the ultrasonic nonlinear coefficient of the bolt increases monotonically with the increase of the axial stress. When the axial stress exceeds a certain value, the nonlinear coefficient increases significantly, which proves that the nonlinear coefficient is sensitive to stress changes and can be used to characterize the axial stress of the bolt. Through normalization and polynomial fitting of the relative nonlinear coefficients, the functional relationship between the bolt axial stress and the normalized relative nonlinear coefficients is obtained. Then two methods of nonlinear ultrasonic and body wave detection are used to test and verify the axial stress of the bolt applied by the electronic tensile testing machine. The error of the axial stress above 200 MPa tested by nonlinear ultrasonic is less than 6%, which is significantly better than linear ultrasonic body wave testing. The research results provide effective support for the axial stress detection of bolts under service conditions.