Non-Destructive and Partially Destructive Determination of Residual Stress States with Steep Subsurface Gradients

X-ray residual stress analysis is a well established tool for the determination of nearsurface residual stress states. If a homogeneous, macroscopically isotropic polycrystalline material is investigated using the well established standard methods of X-ray residual stress analysis like the sin21C,-method [l], the DSlle-Hauk-method [2], or the q-integral method [3] which are summarized and systematically discussed in [4], generally a homogeneous macro residual stress state is assumed in the surface layer sampled by the X-rays which is in most cases between 1 and 100 pm thick. However, in an increasing number of technically important cases of X-ray residual stress analyses, the assumption of homogeneous residual stress states is not valid any more. It has been shown that residual stress gradients of more than 1 GPa/pm can occur at and near surfaces of machined ceramics. In thin multilayer coatings of crystallographically similar materials, neither the residual stress state nor the stress-free lattice parameters can be assumed to be identical in the individual layers. The necessity of obtaining a suitable depth resolution of X-ray stress analyses in materials regions smaller than the penetration depth of the X-rays is therefore evident. This paper describes approaches which allow the determination of residual stress states and stress-free lattice parameters including their gradients in near-surface materials layers. Depending on the actual problem and the ratio of the penetration depth of the X-rays and the thickness of the surface layers to be sampled, non-destructive and partially destructive methods are applied. Advantages and limits of the different procedures are pointed out. Introduction and Principal Problem X-ray stress analysis is based on the diffraction of monochromatic X-rays of wavelength X on lattice planes {hkE} of suitably oriented crystallites in a polycrystalline material. The plane spacing D of the lattice planes can be derived from the measured Bragg angles rj of the diffraction profiles The shift (0 0,) of the using Bragg’s law