X-ray measurement of triaxial residual stress on machined surfaces by the cosα method using a two-dimensional detector

In recent years, the cosα method has attracted engineers as a new method of X-ray stress measurement using the whole Debye–Scherrer (D–S) ring recorded on a two-dimensional detector. The principle of the cosα method was first proposed by Taira, Tanaka & Yamasaki [J. Soc. Mater. Sci. Jpn, (1978), 27, 251–256] for in-plane biaxial stress analysis and later extended by Sasaki and co-workers [Sasaki & Hirose (1995). Trans. Jpn Soc. Mech. Eng. Part A, 61, 2288–2295; Sasaki, Takahashi, Sasaki & Kobayashi (2009). Trans. Jpn Soc. Mech. Eng. Part A, 75, 219–227] to the triaxial state of stress. The method proposed by Sasaki and co-workers utilizes several D–S rings taken at different incident angles of X-rays in order to determine triaxial stresses. In the present paper, the cosα method was applied to measure triaxial residual stresses of uni-directionally machined surfaces of a carbon steel made by grinding, milling and planing. A recommended procedure for experimental measurements of in-plane normal and shear stresses and out-of-plane shear stress is proposed, together with a new method for determination of the out-of-plane normal stress. The tilt angle of X-ray incidence for stress determination is recommended to be larger than 35°, where the stress constant is low and the stress sensitivity is high. Normal incidence is recommended for the determination of out-of-plane shear stresses. The out-of-plane shear stress along the cutting direction was characteristic of uni-directionally machined surfaces and increased with cutting severity in the order of grinding, milling and planing. The in-plane normal stress was compressive for ground and milled surfaces, and the magnitude of compression was larger in the direction perpendicular to the cutting direction. On the basis of the stress values measured under different tilt angles, it is suggested that the magnitude of in-plane normal residual stress increases near the surface. The out-of-plane normal stress determined by the new method indicated a small compression.