Papers by Author: Gabrielle Hilson

Abstract: In this paper the challenges associated with the determination of within section macrostresses in the non-metallic materials porous reactor core graphites, glasses and thermally grown oxides, will be considered, with respect to the length-scale over which such measurements are required. Examples are briefly presented to demonstrate the capability of the methods selected, which include deep hole drilling and photoluminescence and Raman spectroscopy. These techniques span the length-scale from micro-metres to tens of millimetres. The measured values will be discussed with respect to the confidence with which these techniques may be applied and hence benefits for life/integrity evaluation.

Abstract: Residual macro-stresses have to be evaluated using trial samples that comprise a CMSX4 superalloy coated with either a RT22 or a CN91 bond-coat. The samples were exposed in air to a matrix of temperatures, in the range 850°C to 1050°C, and times extending upto 4000hrs to produce thermally grown oxide. This oxide is essentially Al2O3 which allows stresses to be measured by photoluminescence spectroscopy. In addition, Raman spectroscopy and X-ray diffraction have been used to characterise the oxides formed. The results are discussed with respect to confidence in the measurements, changes in stress with temperature and exposure time and the potential for photoluminescence spectroscopy to be used to measure stresses for service components.

Abstract: Raman spectroscopy has been used by various workers to provide a measure of the stresses within the oxides grown on metal substrates at high temperatures. In this paper, we consider thermally grown oxides produced on a Type 316 austenitic stainless steel and an iron 3% silicon ferritic steel. The oxides were grown in air at temperatures of 950oC and 650oC respectively over a range of times. These oxides have been characterised by producing cross-sections using focused ion beam milling. The variation of the Raman spectra wave number (He, Ne laser; λ = 633nm) for the oxides produced on the polycrystalline austenitic stainless steel and the ferritic steel were measured as a function of oxide thickness. This shift in wave number was a function of stress. For a fixed oxide thickness on the stainless steel substrate the specimen has been subject to a bending force. A back face strain gauge fixed to the metal substrate provided a measure of the applied strain. The peak wave number varied with applied strain. The results are discussed with respect to the potential to characterise the stresses produced in thermally grown oxides and as a tool to monitor applied stress.