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Residual stresses are an inescapable consequence of manufacturing and fabrication processes, with magnitudes that are often a high proportion of the yield or proof strength. Despite this, their incorporation into life prediction is primarily handled through sweeping assumptions or conservative application of statistics. This can lead to highly conservative fatigue design methodologies or unforeseen failures under dynamic loading. The push from the desire for higher levels of materials performance, coupled with the pull of more sophisticated techniques for residual stress measurement, favours a reassessment of the accuracy of assumptions made about residual stresses and the modification during fatigue cycling. A viewpoint is also emerging that the fatigue performance of welded joints might be optimised through careful process control, coupled with understanding of the relative positions of, and interaction between, residual stress peaks, weld defects, hardness and microstructure. This paper will present information regarding the residual stress profiles in aluminium and steel welds obtained via synchrotron and neutron diffraction at the ESRF/ILL in Grenoble. Specimens were then subjected to specific cases of fatigue loading and the residual stress field was again measured. Difficulties associated with determining the strain-free lattice spacing will be mentioned, and the import of these data for life prediction modelling will be considered.

ABSTRACT--We describe non-contact scanning with a confocal laser probe to measure surface contours for application to residual stress measurement. (In the recently introduced contour method, a part is cut in two with a flat cut, and the part deforms by relaxation of the residual stresses. A crosssectional map of residual stresses is then determined from measurement of the contours of the cut surfaces.) The contour method using laser scanning is validated by comparing measurements on a ferritic steel (BS 4360 grade 50D) weldment with neutron diffraction measurements on an identical specimen. Compared to lower resolution touch probe techniques, laser surface-contouring allows more accurate measurement of residual stresses and/or measurement of smaller parts or parts with lower stress levels. Furthermore, to take full advantage of improved spatial resolution of the laser measurements, a method to smooth the surface contour data using bivariate splines is developed. In contrast to previous methods, the spline method objectively selects the amount of smoothing and estimates the uncertainties in the calculated residual stress map.

The through-thickness residual stress distributions within three large rectilinear aluminium alloy 7449 forgings have been determined using neutron diffraction. The results from two neutron diffraction instruments, ENGIN-X at ISIS, UK and SALSA at ILL, France are reported. Both instruments indicate large magnitude (>250 MPa) tensile residual stresses in the core of the as quenched forging balanced by surface regions stressed in compression (>-200MPa). The other two forgings had been stress relieved by cold compression and had significantly lower residual stress than the as-quenched forging. Increasing the amount of cold compression from 2½% to 4% was found to cause an insignificant difference in the final residual stress distribution. The neutron diffraction results are also compared to measurements made by the new incremental deep hole drilling technique and show good correlation.

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