4.1 This test method provides a means of verifying the alignment of an X-ray diffraction instrument in order to quantify and minimize systematic experimental error in residual stress measurement. This method is suitable for application to conventional diffractometers or to X-ray diffraction instrumentation designed for residual stress measurement of either the diverging or parallel beam types.3^,4

4.2 Application of this test method requires the use of a flat stress-free specimen that diffracts X-rays within the angular range of the diffraction peak to be used for residual stress measurement. The specimen shall be sufficiently fine-grained, homogeneous, isotropic, and of sufficient depth such that incident X-rays interact with and diffract from an adequate number of individual coherent domains and or grains such that a near random grain orientation distribution is sampled. The crystals shall provide intense diffraction at all angles of tilt, ψ, which will be employed (see Note 1).

Note 1: Complete freedom from preferred orientation in the stress free specimen is, however, not critical in the application of the technique.

1.1 This test method covers the procedure for preparation and usage of a stress-free test specimen for the purpose of verifying the systematic error caused by instrument misalignment and/or sample positioning during X-ray diffraction residual stress measurement.

1.2 This test method is applicable to all X-ray diffraction instruments that measure diffracted X-rays from the crystal structure of a polycrystalline specimen and is applicable to single, double, and multiple exposure techniques. Through measurement of a high-angle back-reflection, these techniques are used to derive the interatomic spacing (d-spacing) and the macroscopic strain and to then calculate residual stress in which the θ, 2θ, and ψ rotation axes can be made to coincide (see Fig. 1).

FIG. 1 X-Ray Diffraction Residual Stress Measurement Geometry and Angles Defined

1.3 This test method describes the use of iron powder for fabrication of the stress-free test specimen that is used to verify the systematic error for residual stress measurement of ferritic or martensitic steels. This test method is easily adapted to other alloys and ceramics through use of powder having a similar diffraction angle as the material to be measured.

1.4 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety, health, and environmental practices and determine the applicability of regulatory limitations prior to use.

1.5 This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.