Papers by Author: Uwe Wasmuth

Abstract: The determination of strain from neutron diffraction data is normally based upon the fit of a Gaussian function to a Bragg reflection. The error in the fit is assumed to be that based on ‘counting statistics’ and this error propagates through the analyses until the final stress evaluation. This relies on there being a big enough number of diffracting grains/crystallites within the gauge volume to ‘approximate’ to counting statistics. The number of grains however depends on the gauge volume size chosen and the average size of the grains (and hence diffracting grains) within the gauge volume and this should be taken into account. The aim of this work is to give an estimate of the uncertainty due to these ‘grain-size statistics’ due to grain size, gauge volume, FWHM of the Bragg reflection (for angular dispersive diffractometers), scattering angle (2), size of detector (and hence number of diffracting grains ‘seen’ on the detector), hkl multiplicity (m) and eventually texture.

Abstract: In response to the development of new materials and the application of materials and components in new technologies the direct measurement, calculation and evaluation of textures and residual stresses has gained worldwide significance in recent years. Non-destructive analysis for phase specific residual stresses and textures is only possible by means of diffraction methods. In order to cater for the development of these analytical techniques the new Materials Science Diffractometer STRESS-SPEC at FRM-II is designed to be equally applied to texture and residual stress analyses by virtue of its flexible configuration. The system compromises a highly flexible monochromator setup using three different monochromators: Ge (511), bent silicon (400) and pyrolitic graphite (PG). This range of monochromators and the possibility to vary the take-off angles from 2θM = 35º to 110º allows wavelength adjustment such that measurements can be performed around a scattering angle of 2θS ~ 90º. This is important in order to optimise neutron flux and resolution, especially for stress analysis on components, since the gauge volume element in that case is cubic and large vertical divergences due to focusing monochromators do not affect the spatial resolution. The instrument is now available for routine operation and here we will present details of recent experiments and instrument performance.