Abstract
Laser speckles can be used to measure surface strains without contact because they deform with a specific relation to the deformation of the surface illuminated by a laser beam. If it can be applied to the measurement of dynamic strains at high temperature as encountered in welding processes, it is hoped to help clarify various problems such as high-temperauture cracking. However, several problems such as disturbances due to the intense emission from the welding arc, oxidation of measuring surface, and fluctuation of the surrounding atmosphere due to convection must be overcome.
In this study, several preliminary experiments were carried out in order to examine the applicability of the laser speckle method to welding:
1) Effects of several measurement parameters on the accuracy of the strain measurement.
2) Measurement of dynamic strains on the rear surface in TIG welding of stainless and mild steel.
It was found that a larger laser spot on the measuring surface gives better accuracy and stability in the measurement for the cost of spatial resolution. In the dynamic measurement, it was difficult to obtain reliable strain values when the strain rate was higher than 1×10-3/sec. With strain rates below this value, reasonable agreements were obtained between the strain values measured by the laser speckle method and those by strain gauges. In the welding process, such a high strain rate occurs during the period right after the ignition of arc or when the arc is passing over the measuring point. It is concluded that it is necessary to speed up the sampling rate of the current speckle recording apparatus in order to widen the applicability of the method. Since some other noise factors such as out-of-plane displacement are also most vigorous during such periods, it will become necessary to take those factors into account too.
