Materials Engineering - Rsidual Stress
 

The Contour Method

The Contour Method for measuring residual stress was first published in 2000. It involves cutting a body into two parts, measuring the deformed shape (contour) of the cut surface, and using a virtual model to infer the residual stress present prior to cutting. The current method can be readily applied to determine the direct component of the stress tensor acting normal to the cut in simple components (such as welded plates).  However, the accuracy and reliability of the method is currently undefined.

Our research aims to develop and apply the Contour Method to engineering components having complex geometry, and find new ways for obtaining more than one component of the stress tensor from measurements. In addition, we are undertaking fundamental studies to determine how to restrain specimens during sectioning, how to avoid plasticity and artefacts arising from cutting, how best to measure the cut surface and how to automate analysis of the results in a reliable way.

Diffraction

Analysis of the diffraction spectra produced by the interaction of X-rays or neutrons with crystallographic materials provides a powerful non-destructive means of measuring the residual stress in engineering components at multiple length-scales. We are intensive users of large-scale neutron and synchrotron facilities throughout the world as well as in-house laboratory X-rays.

Our group led the design of one of the worlds first dedicated neutron strain scanning instruments based at the ISIS neutron facility near Oxford in the UK and we continue to have a particularly close relationship with this facility. We have developed neutron diffraction simulation software, ScannSS, that enables experiments to be optimised “off-line” and allows high precision positioning of samples and measurement locations. This software is now installed at many neutron strain-scanning facilities worldwide.

Our current research is focussed on technique development, instrument development and challenging engineering applications.

  •  We are developing novel experimental techniques to guide neutron diffraction strain measurements, such as the use of neutron tomography imaging data, in collaboration with the ISIS Facility.
  • We are collaborating with ISIS staff on developing new instrument paradigms such as the proposed joint imaging and diffraction instrument (IMAT).
  • We are researching how to improve the reliability of neutron diffraction residual stress measurements in complex welds, for example in dissimilar metal welds found in nuclear power plant.

Structural Integrity

Residual stresses often have a large impact on the operating performance of engineering structures. In order to assess the life and integrity of welded components it is essential to have a good knowledge and understanding of what residual stresses are present in the structure at start of life and how they change during service operation.

Our research is addressing how to characterise the true state of residual stress in welded components. We are examining the significance of residual stress length-scale in structural integrity and how to develop reliable profiles for use in engineering standards based on limited information from measurements and numerical simulation.