激光超声检测表面硬度分析

Laser-based Ultrasound System for Determination of Suface-hardness

和传统的压电技术相比，激光超声系统具有不需要耦合剂、非接触、以及能对曲面和复杂形状的几何形体进行检测等优点近年来，在无损检测领域内得到了广泛的应用文中介绍了利用激光超声来确定表面硬度的方法表面波的传播具有纵波和横波的特性，实验结果表明，表面波波速随硬度的增加而降低轴类零件在交变载荷下工作时，其表面要比心部承受更高的应力，表面硬化是为了增加其强度、硬度、抗疲劳强度硬化层的特性通常由其硬度和深度来表征对表面波来说，可以通过改变其波长的方法来改变它的贯穿深度，从而测取表面硬度的深度函数表面波波速的精确测量是无损检测表面硬度和深度的前提，其精度要求表面波相速的变化小于２％由于激光超声具有很高的时间和空间分辨率，能对表面波声速进行精确的测量通过对硬度分别为５８Ｒｃ和２５Ｒｃ的样品的实验，表面波速度变化不超过０．０７％（５８Ｒｃ）和０．１４％（２５Ｒｃ）通过分析测定硬度和表面波波速两个独立实验的结果，得出了两者之间的线性关系虽然两者的关系的确切联系尚未清楚。

Compared with conventional contact piezoelectric techniques,LBU system which is noncontact does not need couplants.Also it has the ability to inspect parts with curved surfaces and complex geometry. This article presents laser-based ultrasound system for determination of surface-hardness. There are both attributes of longitudinal and shear waves in the dispersion of saw.The results of experiments show that the saw phase velocity decreases with the hardness of material increased. The strain beared by the surface parts are larger than that of the center parts when the shaft is working under alternative load. Case-hardening process is to increase the fatigue and wear resistance of steel components. The properties of the case is presented by its hardness and depth. By changing the length of SAW,the depth of piercing changed. The surface hardness which is a function of the case depth is obtained. Accurate measurement of saw velocity dispersion is a key prerequisite for the nondestructive determination of surface hardness and case depth,high accuracy is required to resolve variations of less then 2% in the saw velocity. Since it provides spatially and temporally broadband generation and detection, laser-based ultrasound inspection and measurement system can do it accurately. Experiments of 58Rc and 25Rc samples show that SAW velocity varies by less than 0.07% for 58Rc sample and 0.14% for the 25Rc sample.By two independent experiments of measuring the hardness and the saw phase velocity the linear relationship between them was derived. Even when the correlation has not been definitely known, it is possible to measure the hardness/SAW velocity dependence experimentally and use it for nondestructive case-depth measurement.