Application of various strain measuring techniques on a biaxially loaded cruciform composite specimen

At the Vrije Universiteit Brussel, a biaxial test device has been developed. The test consists of applying in-plane loads to a cruciform specimen. The geometry and dimensions of the specimen have been analysed and determined previously in order to obtain biaxial failure. Since the calculation of the stresses in the cruciform specimen is not straightforward due to the ill-defined load bearing area, measurement of strains is of primary importance to evaluate and analyse the biaxial test increases. In the present paper, various strain measuring techniques are evaluated for use on the cruciform specimen. Since strain gages only obtain local strains, full field measuring techniques have to be used in order to assess the overall strain distribution. Strain results obtained by the Digital Image Correlation Technique (DICT) and Electronic Speckle Pattern Interferometry (ESPI) are compared with strain gage results and Finite Element calculations. The results of the various techniques showed a good agreement. The uniformity of the strains in the biaxial!), loaded zone can be confirmed. Nevertheless, near the edges of the milled central region the results do not agree. This can be explained by the presence of delaminations at the surface, which cause discontinuities in the speckle pattern. The considered techniques take these discontinuities as large deformations and thus find large strain peaks near these cracks. At these places, strain gages can be used to obtain the correct strains.