Improved holographic blind-hole method for measuring residual stresses

In this paper, the relations between the out-of-plane displacements and residual stresses relieved from hole drilling are established by using finite element calculations. A simple and effective holographic blind-hole method for measuring residual stresses is described. In this method, the optical set-up is arranged so that only the out-of-plane displacements have contributions to the fringe pattern. An axi-symmetric fringe pattern can then be observed. By using this fringe pattern, two relative out-of-plane displacements are sufficient to determine the principal stresses; and the directions of the principal stresses can be directly indicated. The accuracy of the method is demonstrated by using it to measure a known residual stress field in an aluminum bar. Good agreement between the theory and inferred values is obtained.