Extrapolation of Strain and Stress From Holographically Measured Surface Displacement

For isotropic elastic materials, following Hooke’s law, the strain and stress field below the surface is uniquely determined by the knowledge of the displacement of the surface itself. From the holographically measured surface displacement u and the boundary conditions for the surface-stresses one can determine all 9 components of the vector-gradient grad u, which describes the strains as well as the tilt and the rotation of a volume element adjacent to the surface. It is shown that strain and stress can be uniquely calculated in a cone-shaped zone below the observed part of the surface by stepwise linear extrapolation. The depth of this cone depends on the density of the sample points and on the accuracy of the displacement measurement on the surface, as well as on the required accuracy of the extrapolated strain and stress values. The suggested extrapolation method has been tested numerically for a thick-walled cylindrical tube under internal pressure using simulated input data. The limitations and the accuracy are discussed.