Strain measurement

The measurement of strain is a key element in material characterisation and structural testing. Besides the ubiquitous resistive strain gauge (RSG), optical measurement techniques become more and more familiar. Shearography has become popular as a tool for whole-field non-destructive testing. The strength of the technique is that strains and other displacement derivatives of an object surface can be visualised. Shearography is less sensitive to object vibrations compared to other interferometric techniques and can therefore be applied in an industrial environment. Complete knowledge of a planar strain state requires the measurement of three strain directions (RSG rosette) or the measurement of the derivatives in two orthogonal directions of the surface displacement components.

EMPAs shearographic strain sensor

Within the Brite EuRAM project Multi-Wavelength Shearography (MuWaS) and funded in part by the Ministry of Education and Science (BBW) a compact multi-wavelengths shearographic camera for the determination of 2D strain distributions has been developed by EMPA section Electronics/Metrology. This apparatus (Fig.1) is based on 3 diode lasers emitting at wavelengths between 810 and 850 nm. The sensor head houses three synchronised B/W-cameras, each of which grabs the speckle field of a single laser only. Three images can be recorded simultaneously for one shearing direction. Illuminating the object from different directions allows to measure different components of the displacement derivatives with interferometric resolution well below 1 m m. An LCD device enables to switch electronically between two directions of image shear and allows the determination of all six displacement derivatives accessible to shearography. The optical configuration has been carefully analyzed in order to receive calibrated data.

Tensile testing

Results of in-plane strain measurements with the multi-wavelength shearographic camera during tensile testing of an aluminium bar are presented in Fig.2. Good agreement was achieved between the shearographic data and that of a strain gage applied for comparison. This project demonstrates that multi-wavelength shearography is an effective tool for quantitative measurements of 2D strain distributions and can be applied in testing environments.