National Physical Laboratory

Residual stress and strain measurement (XRD and hole drilling)

Residual stress and strain measurement 1 Residual stress and strain measurement 2 Residual stress and strain measurement 3 Residual stress and strain measurement 4 

The lifetime of a structural component is usually determined by the interactions between the defects within the component and the stresses to which it is exposed. These stresses are a combination of those applied in service and those which develop during manufacture and processing, namely the residual stresses. These may be beneficial and/or detrimental to the service performance. For example, they may be deliberately introduced (e.g. by shot peening) to improve fatigue performance. Whilst applied stresses can be accounted for in the design of a component, it is more difficult with residual stresses because they are difficult to predict and measure reliably. With the continuing drive to optimise material performance and minimise component weight, there is an increasing emphasis on understanding the role of residual stress.

X-Ray Diffraction (XRD)

X-Ray Diffraction is a common NDE technique that can be used to determine the levels of residual stress in the near surface layers of a component. X-rays probe a very thin surface layer of the material (typically tens of microns), and it is from this layer that the near surface residual stress is measured. The sample need to be polycrystalline materials.

NPL facilities include:

  • Siemens D5000 Diffractomer, Bruker D8 Discover
  • High temperature stage, grazing incidence, texture and reflectometry capability
  • NPL holds a stock of Cr, Cu, Mo and Mn X-ray tubes

There is currently a European standard being formulated.

For more details on the technique  see NPL Good Practice Guide No. 52: Determination of Residual stresses by X-ray Diffraction. M E Fitzpatrick, A T Fry, P Holdway, F A Kandil, J Shackleton and L Suominen.

Incremental Centre Hole Drilling

The hole drilling method is a well-established and widely accepted technique for measuring residual stress. The basic procedure involves introducing a small hole into the surface of a component, at the centre of a special strain gauge rosette and measuring the relieved strains.
The strain gauge rosette is first bonded to the surface of the component under investigation. A hole is then drilled into the component through the centre of the gauge to a depth approximately equal to half its diameter; the original stress state in the component is then calculated from the relieved strain values. Both the magnitude and directions of principal stresses are calculated.

 
The basic analysis methods are strictly only valid when the residual stress field is uniform and does not vary significantly with depth. Finite element solutions have opened new possibilities for improving the calculation of non-uniform residual stresses from incremental strain data via the Integral Method in which the contributions to the total measured strain relaxation of the stresses at all depths are considered simultaneously, and this is now the preferred analysis method.

NPL hole drilling capability enables stress profiles to be generated using incremental drilling in a series of small steps (of the order of 10-30µm) which can be useful for measuring near surface residual stress profiles and stresses introduced during machining and surface treatment.

For further advice and details see: NPL Good Practice Guide No. 53: The Measurement of Residual Stresses by the Incremental Hole Drilling Technique. P V Grant, J D Lord and P S Whitehead.

Contact

Customer Service tel: +44 20 8943 8681
E-mail: materials_enquiries@npl.co.uk

Last Updated: 30 Mar 2012
Created: 26 Jul 2007

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