Abstract
X-ray diffraction has been widely used in measuring surface residual stresses. A drawback of the conventional d ~ sin2ψ method is the increased uncertainty arising from sin2ψ 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}_{psi }+ {d}_{-psi }}{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.
Highlights
X-ray diffraction (XRD) is a powerful analytical tool in characterising polycrystalline materials owing to its accurate measurement of lattice d-spacings
Applications of the d ~ sin2ψ method become problematic in analysing surfaces where residual shear stresses co-exist with residual normal stresses
We present a new approach of XRD residual stress measurement through a modification to the conventional d ~ sin2ψ method
Summary
X-ray diffraction (XRD) is a powerful analytical tool in characterising polycrystalline materials owing to its accurate measurement of lattice d-spacings. The conventional d ~ sin2ψ method is suitable for measuring in-plane normal stresses (i.e., zero normal stress vertical to the measured surface) if the related residual shear stresses can be ignored In such circumstances, the d-spacings measured at a series of off-axis angle ψ, dψ, are correlated to sin2ψ with small data scattering. Zauskova et al examined the three-dimensional residual normal and shear stresses by employing the d ~ sin2ψ method at three sample orientations [24] These examples suggest that the measurement of residual shear stress has drawn the attention of researchers in recent years. The conventional d ~ sin2ψ method shows drawbacks such as low precision arising from the sin2ψ splitting This method is limited to the measurement of residual normal stresses only, because of the theoretical difficulty in measuring combined normal and shear stresses. A procedure of detailed XRD experiments will be provided, followed by the measurements on a few machined steel bars
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