Papers by Author: Rainer Schneider

Abstract: In recent years the use of a special Bayesian approach on averaging ‘round-robin’ residual stress data has been implemented. This averaging approach is useful in that it copes with the situation where systematic errors have occurred in one or more of the measurements and thus diminishes the influence of these particular ‘wrong value’ outlier data points. The analyses not only take into account the measurand value, but also the uncertainties associated with each measurand. It should deal with data that may contain individual members with uncertainties larger than the stated error and assumes that the quoted error bar is only a lower bound on the uncertainty. This work shows what could happen when there is a ‘strong mismatch’ in uncertainties when averaging over a limited amount of data. It has been observed that in a case where there are few data points (for example 5 or less), a strong bias can occur towards data points with a relatively small quoted uncertainty compared to other data points with larger quoted uncertainties. A ‘mismatch’ in uncertainty quotation can arise when averaging very good data with poorer data or when averaging with data obtained from other measurement techniques. This effect is demonstrated in this work by using fictitious data and also based on the example of real measurement data obtained by neutron diffraction.

Abstract: Bending of metal plates with high-energy laser beams presents a flexible materials forming technique where bending results from the establishment of a steep temperature gradient through the material thickness. This inevitably leads to non-uniform thermal expansion/contraction and subsequently residual stresses. Non destructive residual strain mapping with neutron diffraction through the 8mm thickness of a series WA 300 grade structural steel plate samples, focused on the region straddling the centerline of the heating bead location, shows the presence of large residual stress fields. Directly below the laser track the longitudinal strains are tensile and dominant, normal strains compressive and transverse strains slightly tensile. The magnitudes of the strains decrease outside the width of the laser beam footprint. The first laser pass induces throughthickness strains close to yield, whereafter their magnitudes decrease with increased number of laser beam passes. A comprehensive mapping of the longitudinal stresses as function of the number of laser passes is given.

Abstract: This paper considers the measurement of residual stresses induced by mechanical loading in a weld Type 347 stainless steel. The work is based in part on an ongoing Round Robin collaborative effort by the Versailles Agreement on Materials and Standards, Technical Working Area 31, (VAMAS TWA 31) working on ‘Crack Growth of Components Containing Residual Stresses’. The specific objective of the work at Imperial College London and HMI, Berlin is to examine how residual stresses and prior straining and subsequent relaxation at high temperature contribute to creep crack initiation and growth for steels relevant to power plant applications. Tensile residual stresses have been introduced in the weld by pre-compression and neutron diffraction measurements have been carried out before and after stress relaxation at 650 oC. Significant relaxation of the residual stresses has been observed, in agreement with earlier work on a stainless steel. Preliminary results suggest that the strains local to the crack drop by over 60% after 1000 h relaxation at 650 oC for the weld steel. The results have been compared with finite element studies of elastic-plastic pre-compression and stress relaxation due to creep.

Abstract: Recent progress in engineering includes the development of new materials and innovations in their processing and treatments. Material technologies, like the study of metals, alloys, ceramics and composites, especially non-destructive analyses of residual stresses profiles and textures, have gained an increasing importance. The dedicated residual stress diffractometers E3 and E7 at BENSC, HMI, Berlin are already equipped with new two-dimensional position sensitive detectors. An upgrade of the monochromator system is planned for 2006 which includes perfectly bent silicon crystals in order to optimise both intensity and angular resolution yielding a large gain of the diffractometer efficiency for strain measurements. A range of equipment for sample positioning is available, such as a closed Eulerian cradle for samples with weights of up to 5 kg, a second cradle for heavy samples (up to 50 kg) with the ability to tilt the samples up to 90° and a translation table carrying samples of up to 300 kg and 1000 mm in diameter. Gauge volumes can be adjusted by a new computer controlled variable slitsystem in a range from 1x1x1 mm³ up to several mm³. In-situ residual stress analysis can be performed within industrial components during mechanical or thermal loading (up to 2000 K). Rapid data visualization as well as evaluation is performed by the specially designed software. The powder diffraction pattern is calculated by summation over the scattering angle dependent Debye- Scherrer lines on the two-dimensional 400*400 mm² planar area detector. A large amount of beam time is exclusively used for industrial research. Among the components that were investigated are crankshafts, impellers, pistons, cylinder heads, turbine blades and welds. Both instruments are similarly designed, where E3 is set up for higher flux and therefore penetration depths and E7 is designed for higher angular resolution.

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.

Abstract: A large distance between the reactor core and the monochromator together with a take-off angle of 90° leads to low flux but high angular resolution of the diffractometer around 90° scattering angle. The sample-table consists of two goniometers for heavy loads. An x-y-z table for heavy loads can be mounted, as well as different eulerian cradles, a mirror furnace or a tensile rig. The detector is a 20x20cm2 position sensitive delay-line detector made by EMBL. A sophisticated primary and secondary slit system together with the recently installed high-resolution camera system allows the application of the high precision instrument alignment and calibration system recently developed at HMI. Here some special features of the instrument are presented and the consequences are outlined.