Mapping two-dimensional state of strain using synchroton X-ray diffraction

Department of Materials Scienceand Metallurgy, University of Cambridge, Pembroke Street, Cambridge, CB2 3QZ, UK(Received February 16, 1998)(Accepted in revised form September 14, 1998)IntroductionComposite strain mapping is a procedure which is naturally addressed by techniques utilising diffrac-tion. Measurements in both the matrix and reinforcement cannot be made using conventional methods,such as strain gauging or hole drilling. Diffraction techniques most commonly used for strain mea-surements include neutron diffraction, conventional (laboratory) X-ray, and synchrotron X-ray diffrac-tion. Compared to laboratory X-ray, synchrotron radiation offers dramatic improvements in beamintensity and penetration depths. Where conventional diffraction stress measurements rely on interro-gating only a thin surface layer (,50mm) of such material as aluminium, synchrotron X-ray possessessufficient flux to allow collection of diffraction patterns from sampling volumes over 20mm thick.Important additional benefits of using such high flux beams are the improved spatial resolution, and thepossibility of using extremely short collection times. These two experimental parameters are interre-lated, and both influence the quality of data in a complex way. Additionally, acceptable data qualityvaries depending on the interpretation procedures and the requirements of accuracy and consistency. Asa consequence, suitable values of the test parameters, such as sampling volume and collection time,must be determined experimentally, using carefully selected test systems, possessing suitable geometryand subjected to controlled loading. The main content of the present paper is the description of such anexperiment.In this study, measurements were made on the ID11, BL2 materials science beamline (1) at theEuropean Synchrotron Radiation Facility at Grenoble, a diffractometer allowing controlled translationof the sample in three linear directions. The technique involved direct collection of diffracted patternsin digital form using a high resolution CCD camera. Due to the novel nature of this application, thecapabilities and limitations of the present configuration for engineering strain measurement had not yetbeen fully explored in a systematic way. The aim of the present experiment was to test the capabilitiesof both the hardware and software for making accurate and reliable strain measurements in thetransmission geometry, to investigate their limitations, and to determine the routes for further devel-opment.