Evaluation of residual stress determinations conducted with laser ablation and optical displacement measurement

The presented work is related to the determination of residual stresses in ceramic coatings, where the material removal is done by laser ablation and the occurring surface deformation is measured with digital holography. Insights into the underlying assumptions and calculations to determine the depth dependent stress values from extensive displacement measurements shall be given. Moreover, the inhomogeneous elastic material properties and deviations from idealized removal geometries (in this case holes and notches) have to be considered. To cover all involved fields, i.e. thermodynamics and continuum mechanics, numerical studies for conventional and laser hole drillings were conducted and the ablation process was simulated. As the laser cannot produce ideal cylindrical or rectangular geometries, appropriate mathematical approximations are applied to choose a representative profile of the removed material. Furthermore, the calculation of stresses based on full field displacement measurements is done with an own program in several depth increments. These calculations are based on numerical calibration data for the elastically inhomogeneous compound of coating and substrate. Especially notch-like forms of material removal, where case sensitive weighting functions are applied for evaluation, can lead to reliable results. The physical experiments were done on aluminium plates with applied alumina layers. (author)