Report Residual Stress Workshop.

Materials Science Centre, University of Manchester, 14^th February 2007

Residual stresses remain within a material, component or assembly when all external forces, for example machining stresses, are removed. They add to the stresses which are applied in service and can cause unexpected failure. Residual stresses are not immediately apparent and can be difficult to measure. Residual stress is one of the most common cause of the sudden, unexpected failure of engineering materials. Understanding residual stresses is an integral part of component design and lifeing and is vital in the Engineering Industry.

A workshop covering the measurement of residual stresses, using, neutron synchrotron and laboratory X-rays was held on February 14th in conjunction with the British Society for Strain Measurement (BSSM).
The BSSM are interested in all methods of measuring strain, for example hole drilling, photo elasticity, contour measurement, optical correlation etc.

The workshop was aimed primarily at people in the Engineering Industry, who have no specialised knowledge of crystallography. Fourteen people attended, more than expected, it was a bit of a squeeze, we really had room for only ten.

The aim of the workshop was to discuss the strengths and limitations of the various methods, to give some practical experience, hints and tips.

Firstly, Prof. Philip Withers gave an introduction to the activities of the new School of Materials, which was formed by the amalgamation of three Departments, Materials Science, Corrosion & Protection and Paper Science. Phil presented a over view of the Department's Research activities and the new building program

Judith Shackleton then gave an introduction to basic crystallography and diffraction we discussed the nature of crystalline materials, Bragg's Law and diffraction geometry. We talked about the various components of laboratory X-ray diffractometers, how they work and how to get the best results.

We moved on to describe the principal method for measuring residual stress by diffraction which is the sin²Ψ method. This method is, of course, only applicable to crystalline materials, metals, ceramics, etc. The great strength to the sin²Ψ method is that it does not require calibration using a stress free standard, which is s very difficult thing to make! Its principal weakness is that it's very surface sensitive and this point was picked up later in the workshop.

Dr Jo Kelleher, from the School of Materials, discussed elastic constants. The sin²Ψ method actually measures strain by determining the change in d-spacing. Stress is not measured directly; it is calculated via Hook's Law, using the elastic constants, usually Young's modulus and Poisson's Ratio. Jo described the differences between X-ray and bulk elastic constants. Sources of elastic constants were also discussed for example, tables and literature references, as well as how they can be calculated using various models or measured.

After lunch Joao Fonseca took up the baton and introduced more exotic techniques using neutron and synchrotron radiation. These are much more energetic that laboratory X-rays and can be used to probe to a greater depth within a component. Neutrons will penetrate several centimetres into a sample, where as with laboratory X-rays it's just a few microns.

Joao described the various experimental facilities, the methods, which can be used, their strengths and weaknesses. Joao illustrated his talk with lot's of examples, for example, a strain map of the damage to a gas turbine blade which was caused by a bird strike, the stress fields around TIG welds and examples of gauge corner cracking in railway lines

Jo Kelleher took the stage again talked about sample preparation and electro polishing. This is very important, particularly when using laboratory X-rays, as the technique is very surface sensitive. If we want to look at the bulk of the material we have to remove the surface chemically. Mechanic removal will radically change the stresses, which we are tying to measure. Electro-polishing is a bit of a black art and can be little dangerous. Jo talked us through methods for polishing safely and effectively.

The final talk was given by Tony Fry of the National Physical Laboratory (NPL). Tony described the NPL's Good practice Guide which was written to provide practical advice for those who are making measurements with laboratory X-rays for the first time. Tony also described the round robin studies, which are run by the NPL (using many methods, not just X-rays) and invited everyobe to join in.

Finally, we adjourned to the X-ray laboratories to make some measurements using Manchester's two Proto residual stress diffractometers. We set the machines up to measure a fan blade and some shot peened steel. Chris Pryor (Materials Science Centre) demonstrated how to use the machine for example, sample positioning, data processing etc.

The meeting finished quite late at 16:30, it was full day!

Judith Shackleton,
Materials Science Centre,
University of Manchester.

Last updated 14-May-2007

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