Analysis of Multiaxial Near-Surface Residual Stress Fields by Energy- and Angle-Dispersive X-ray Diffraction: Semi- Versus Nondestructive Techniques

Abstract

Abstract Under laboratory conditions while applying angle-dispersive X-ray diffraction, the information depth in steel is usually restricted to less than 10 μm. Access to residual stresses induced by mechanical, chemical, or both types of surface treatment in deeper regions requires either the application of the layer removal method or an analysis with highly penetrating X rays and synchrotron radiation, respectively. Successive layer removal yields the actual residual stress depth profiles σij(z) up to any depth below the surface, but this is time consuming and semidestructive. High-energy X-ray diffraction performed in the energy-dispersive mode avoids these drawbacks, but it provides only the Laplace stresses, σij(τ), which have to be transformed back into the real space in order to obtain the actual stress-depth profile σij(z). Using an example of a uniaxially ground steel specimen, it is shown that the layer removal method and the energy-dispersive diffraction method for X-ray stress analysis when performed in reflection geometry yield comparable results in the case of the in-plane stress components σ11 and σ22. However, significant differences were observed concerning the out-of-plane stresses, which can be attributed to the boundary conditions that the σi3 components must satisfy at the free surface. It is demonstrated that stress redistribution at the newly generated surfaces after successive layer removal leads to an underestimation of the shear stresses σ13. The analysis of the out-of-plane normal stress component σ33 is shown to require nondestructive measurements.