Most manufacturing processes result in residual stresses which can be at a level high enough to affect the structural integrity of the component.
Without knowledge of the residual stress field, assessments are often overly conservative. There are many measurement techniques available but there are disadvantages to many of those in current use.
Mechanical methods, such as centre-hole drilling, have been used extensively but they require material removal and are therefore destructive. X-ray and neutron diffraction methods are non-destructive but expensive.
TWI has recently investigated a new technique based on laser generated ultrasound. It is advantageous in that it is non-contact, non-destructive and can be used in most materials. Variations in the velocity of the ultrasonic wavescan 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 by looking at both uniform tensile residual stress fieldsof different magnitudes and tensile residual stress fields that vary in magnitude through the thickness of a steel plate.
The modelling results show a clear correlation between the magnitude of the residual stress and the surface acoustic wave behaviour.
Laser generated ultrasound is believed to be a viable technique for residual stress measurement with the advantages of being non-destructive, applicable to most materials and relatively inexpensive to apply. The technique has thepotential to be more readily used and therefore removes the need for over-conservative assumptions for residual stress in fitness for service and engineering critical assessments.
For more information on this technique, contact nmo@twi.co.uk
