Measurement of residual stress using laser-generated ultrasound

Abstract

Most manufacturing processes result in residual stress. The level of residual stress can be high and could affect the structural integrity of the component. Without knowledge of the residual stress field, assessments are often overly conservative. There are many techniques available for residual stress measurement. Mechanical methods, such as centre-hole drilling, have been extensively used but they require material removal and are therefore semi-destructive. X-ray and neutron diffraction methods are non-destructive but are expensive and cannot be carried out in-situ and so require the removal of components. Laser-generated ultrasound is advantageous in that it is non-contact, non-destructive and can be used in most materials. Variations in the velocity of the ultrasonic waves can be related to the residual stress state. Finite element modelling has been used to determine the capability and sensitivity of the technique for residual stress measurement. Both uniform tensile residual stress fields of different magnitudes and tensile residual stress fields that vary in magnitude through the thickness of a steel block have been investigated. A clear correlation between the magnitude of the residual stress and the surface acoustic wave behaviour is presented. In addition, the possibility of depth profiling has been demonstrated for depths below the surface of up to 0.3 mm for the particular case studied. Techniques for extending this capability to greater depths are also discussed.