ESPI HOLE DRILLING
Hole-drilling is a residual stress measurement technique, where stressed material is removed by drilling a small blind hole in the area of interest.
The remaining material around the hole spontaneously finds a new stress equilibrium. This re-arrangement of stresses leads to a slight distortion of the surface near the hole. Though the displacements are small, they are measurable with ESPI and allow the calculation of the stresses that were present in the part prior to drilling.