Improved measurement of low residual stresses by speckle correlation interferometry and local heat treating

The results presented in this paper clearly demonstrate that the dynamic range of this measurement technique can be improved substantially over the earlier experiments. It is just as clear that a more systematic study must be performed to quantify these improvements and to generate usable calibrations. These results are also encouraging in the sense that this technique may now be appropriate for other materials with high thermal diffusivities. Previous attempts to measure residual stresses by laser annealing and electronic speckle pattern interferometry have been successful for moderate to high stress levels. The method uses an infrared laser for relieving stress in a small spot. A dab on temperature indicating paint is applied to the spot and a specklegram of the spot and the surrounding area is captured. The paint is then heated with a laser until it melts. The heat is transferred from the paint into the metal resulting in a small amount of localized stress relief as the yield stress of the material drops below the stress levels surrounding the spot. Once the spot and area around it have cooled a second speckle-gram is captured and the images are processed to determine the in-plane strain. The amount of stressmore » relief depends on the melting temperature of the paint since yield stress is a function of temperature. The measurement of local stress relief by heating is subject to limitations that result from thermal expansion competing with the reduction in yield stress of the spot at the elevated temperature. That is, as the spot is heated it tends to temporarily reduce the stress in the region surrounding the spot as it expands into this surrounding region. This limits the amount of stress relief that can occur. This can be overcome to some extent by using higher temperature paints, which in turn lowers the yield stress in the heated spot. At some point, however, the thermal expansion overtakes the surrounding stress field and can even drive it into compression. Furthermore, for tension levels on the order of eighty percent or less of the yield stress, the sub-micrometer deformations result in less than a single fringe. The strains indicated by such sub-fringes are comparable to noise levels that occur from air turbulence, environmental thermal variations and so forth. Thus, for both fundamental and practical reasons the technique was modified to increase the fringe count at lower stress levels. The authors have successfully performed two separate experiments to raise the fringe count. One method was simply to start observing the fringes (or strains) immediately after annealing. Not only can several fringes be obtained in this way but a clear relationship has been observed with the stress levels. The other approach was to cool an area surrounding the region of interest and then observe the net strain after thermal equilibrium is reestablished. Both methods have shown the ability to handle lower tension levels than were measurable by the earlier procedure.« less