Evaluation of the Compressive Residual Stress Relaxation Behavior by in situ X-ray Stress Measurement

To clarify the relaxation behavior of compressive residual stress during the first push and pull loading cycle, an in situ X-ray stress measurement method was formulated, in which a fine particle peening-treated hourglass-shaped specimen was fixed on an axial-loading fatigue testing machine, and the surface stress of the specimen—which is the sum of applied stress and residual stress—was directly measured via X-ray diffraction without removing the specimen from the testing machine. A noticeable relaxation in compressive residual stress occurred under the first compressive loading process, and slight relaxation was observed then onward. During the first compressive loading, the surface stress decreased almost linearly as the applied compressive stress increased; however, when the stress exceeded a certain threshold value, the relation between the applied stress and the surface stress deviated from the linear relation. This threshold value is important with regard to compressive residual stress relaxation. Furthermore, the relaxation behavior during the first compressive loading process can be explained by a master diagram that shows the relationship between the applied stress and the stress measured via X-ray diffraction. The diagram consistently shows that with an increase in the applied compressive stress, there is an increase in the amount of relaxed residual stress.