Residual stress is often a critical issue for the structural integrity of engineering components and structures. In industry, one widely used technique for residual stress quantification is the hole-drilling method using a strain gauge rosette. This requires extensive surface preparation, and the hole must be drilled precisely at the centre of the rosette. Diffraction-based non-destructive techniques are either very expensive or limited to laboratory use. Optical techniques such as moiré, holographic and speckle interferometry have long been used for displacement and strain measurement. However, most interferometric methods except shearography are prone to environmental disturbances, and are therefore not robust enough to be used in an engineering environment.
Being a non-contact, full-field measurement technique, the Digital Image Correlation (DIC) method has already been identified as a promising technique for surface residual stress measurement. The conventional approach uses the following procedure:
- Images are compared to measure the displacement.
- Strain information is obtained by differentiation of the displacement.
- Residual stress is calculated based on analytical or finite element models.
This process is complex, and can only be carried out by professionals with a sound knowledge of mechanics and materials. TWI has recently developed a novel system using DIC for residual stress measurement which addresses these shortcomings.
The conventional procedure in optomechanics requires complex interpretation of the displacement field to calculate residual stress. The new procedure avoids this by using the residual stress components as direct variables during the numerical optimisation.
This has paved the way for the development of a new, compact system for residual stress measurement. Compared with the strain gauge rosette, DIC avoids the need for extensive surface preparation and precise drilling of the hole. In addition, in contrast with conventional DIC, the new system is convenient, easy to use, and does not require expertise in mechanics and materials. The principle is not limited to residual stress measurement, and can be extended to other measurement tasks with known deformation patterns. A detailed technical paper will be published in Applied Optics, and further development is continuing.
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