STRESS MEASUREMENT BY X-RAY DIFFRACTOMETRY

摘要

The preferred instrument disclosed is a portable X-ray diffractometer for measurement of residual stress in metallic specimens, especially welds (20) in large structures, such as bridges, pipelines (21) etc. The instrument consists of a pair of position sensitive detectors (D1, D2) arranged to be mounted in fixed positions relative to the specimen. An x-ray source (F) that projects a collimated incident beam onto the area of the specimen under examination is located between the detectors so that each receives a diffraction line. The source is scanned stepwise in an arc about the specimen area, while the specimen and both the detectors remain fixed. The diffraction lines received in each detector are stored in a computer as histograms of intensity values. For each angular relationship between the incident beam and the chosen direction of strain measurement, a series of such intensity values corresponding to a given diffraction angle for successive diffraction lines is averaged and a resultant diffraction line obtained for each detector. To keep the mean strain directions constant and achieve a constant range of grain orientations for all diffraction angles, the intrument restricts the intensity values so averaged to those that fall within a virtual window. This window is moved along the channels, one or more channels per scanning step. The two resultant series of averaged values are examined to find peaks or other characteristics of the diffraction line and the angular relationships corresponding thereto. This knowledge enables determination of lattice strains in two directions, the two strains determining the stress in the specimen surface.