In order to find out the stress imposed on machine part, strain gages are very accurate, but it is difficult to find out the distribution. Photoelasticity, stress coat etc. is adapted for this purpose, but the accuracy is insufficient.
We have developed a new method to find out the stress and its distribution accurately, especially for repeated stress.
When cyclic load is applied to the specimen plated with copper, grain-growth is started in the copper deposit resulting in the flecks on the surface. Number and size of grain are proportional to the magnitude of the stress applied. As the relation of stress and grain size are determined by the standard sample, the stress magnitude of any specimen can be measured by comparing the grain size after undergoing cyclic stress.
Therefore this is a good method to determine the fatigue life.