Development of image-based inertial impact tests for characterising the high-strain-rate interlaminar properties of composites

Reliable measurement of interlaminar properties at high strain rates (∼ 103 s−1) is very challenging with existing test methods due to inertia generated by dynamic loading. With the emergence of ultra-high-speed cameras has come the opportunity to use full-field measurements to quantify the high-strain-rate behaviour of materials and directly measure constitutive properties. This project explored the design and experimental validation of two image-based inertial impact (IBII) tests to measure the interlaminar elastic modulus, shear moduli, and tensile failure stress of fibre-reinforced polymer composite materials at high strain rates. A new set of special optimised virtual fields were developed as part of this work for the direct identification of the elastic and shear moduli from full-field maps of strain and acceleration. Synthetic image deformation routines were used to rigorously quantify the error introduced on the identification of stiffness by experimental factors (contrast and noise), and post-processing parameters (temporal and spatial smoothing). The potential of the IBII tension/compression tests was demonstrated by successfully identifying the interlaminar elastic modulus and tensile failure stress at 3-5×103 s−1. Back-to-back, surface measurements made with synchronised ultra-highspeed cameras were used to analyse the assumption that the IBII tension/compression test is 2D. It was found that out-of-plane loading has a relatively small effect on stiffness identification (bias of 4% on average), but has a large effect on failure stress estimates (bias of up to 30% on average). Finally, the IBII shear test developed in this work demonstrated that the interlaminar shear modulus can be characterised with remarkable consistency at 1.5×103 s−1. Overall, this project clearly demonstrates that the IBII method is an excellent tool for interlaminar material property identification at high strain rates. In the future, camera technology will improve increasing the efficacy of the IBII methods developed in this work