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This study focuses on the evaluation of several strain measuring techniques for the instrumentation of an in-plane biaxial test bench. Strain gages, electronic speckle pattern interferometry (ESPI) and digital image correlation (DIC) were used combined or separately to measure the strains in the central gauge area of a biaxially loaded cruciform composite specimen. Applicability of the technique for this specific test and accuracy of the results are important parameters in the evaluation process of the different strain measuring techniques. The results of the three techniques are compared with the results obtained with a 3D finite element (FE) model. It is shown that DIC is the most promising technique because of its full-field character and ability to cope with vibrations of the test bench. The main drawback remains its incapability to correlate across discontinuities.

Advanced composite material systems are increasingly used in almost every industrial branch. The structural components manufactured from these composite material systems are usually subjected to complex loading that leads to multi-axial stress and strain fields at critical surface locations. The current practice of using solely uniaxial test data to validate proposed material models is wholly inadequate. In order to test closer to reality, a biaxial test bench using four servo-hydraulic actuators with four load cells was developed. Besides the development of the test facility, a mixed numerical/experimental method was developed to determine the in-plane stiffness parameters from testing a single cruciform test specimen. To obtain the strength data an optimized geometry for the cruciform type specimen was designed. For the optimization procedure a full three-dimensional finite element model was used. The numerical results were validated with strain gauge, digital image correlation, and electronic speckle pattern interferometry data. The material system used for the experimental validation was glass fibre-reinforced epoxy with a lay-up [(+45°−45° 0°)4(+45°−45°)] typically used for wind turbine blades.