A modified X-ray diffraction method to measure residual normal and shear stresses of machined surfaces

X-ray diffraction has been widely used in measuring surface residual stresses. A drawback of the conventional d~sin2 method is the increased uncertainty arising from sin2 splitting when a significant residual shear stress co-exists with a residual normal stress. In particular, the conventional method can only be applied to measure the residual normal stress while leaving the residual shear stress unknown. In this paper, we propose a new approach to make simultaneous measurement of both residual normal and shear stresses. Theoretical development of the new approach is described in detail, which includes two linear regressions, \frac{d_Ψ+\ d_{-Ψ}}{2}~sin2Ψ and {dΨ-d-Ψ}~sin(2Ψ), to determine the residual normal and shear stresses separately. Several samples were employed to demonstrate the new method, including turning-machined and grinding-machined cylindrical bars of a high strength steel as well as a flat sample of magnetron sputtered TiN coating. The machined samples were determined to have residual compressive normal stresses at both the axial and hoop directions as well as various scales of residual shear stresses. The TiN coating showed a high scale of residual compressive (normal) stress whereas the measured residual shear stress was extremely low. The new method showed significantly increased precision as compared to the conventional d~sin2Ψ method.