Surface strain measurement: a comparison of speckle shearing interferometry and optical fibre Bragg gratings with resistance foil strain gauges

The performance of two complementary optical strain measurement techniques, speckle shearing interferometry ( shearography) and fibre Bragg grating ( FBG) sensors, is compared with that of resistance foil strain gauges ( RFSGs) and with theoretical predictions. The test object used for the surface strain measurements was a hydrostatically loaded ABS pipe. A multi-component shearography instrument, capable of full surface strain measurement, was used to determine the displacement gradient components, from which the surface strain components were calculated. Six surface mounted wavelength division multiplexed FBG sensors were used to measure the axial and the hoop strains. RFSGs located on the surface of the pipe, adjacent to the FBGs, were used for comparison. Reasonable agreement between theory and the axial and hoop strains determined by the different techniques was found. Issues associated with deploying and comparing the techniques are discussed.