Surface roughness can have a considerable impact on residual stress measurements using X-ray diffraction (XRD) because it increases scattering and peak widening, reducing measurement accuracy. Rough surfaces provide diffuse reflections and a reduced signal-to-noise ratio, making it difficult to precisely determine diffraction peak placements. This can lead to mistakes in residual stress calculations, as the precision of these measurements is strongly dependent on the quality and clarity of the diffraction data. To counteract these impacts, surface preparation or improvements are sometimes required.

Surface roughness does indeed affect RS measurements, but not at all for reasons Abhishek describes. His (incorrect) reasoning would just lead to increased random errors, but in fact surface roughness will introduce systematic errors.

Cullity includes this statement: "Surface roughness must be strictly avoided, because the high points in a rough surface are not stressed in the same way as the bulk of the material and yet they contribute most to the diffraction pattern, especially the one made at inclined incidence, as indicated in Fig.

17-11." Study the figure. Roughness will seriously mess up your data.

Surface roughness on AM parts can affect residual stress measurements by X-ray diffraction. Surface roughness can cause irregularities in the contact of the X-ray beam with the surface, which can lead to changes in the diffraction signals. This effect can reduce the accuracy and repeatability of the measurements. The high surface roughness, especially on AM parts, causes the X-ray beams to hit the recesses and protrusions at different angles. This can cause the signal to be weakened or scattered, which can create erroneous stress measurements. Polishing the surface mechanically or chemically reduces the surface roughness, resulting in a more uniform diffraction signal. However, this option may not be suitable if the surface is not to be machined before the measurement.