Dynamical characterisation and damage mechanisms of E-glass/vinylester woven composites

What is it about?

Within the EUCLID project, ‘Survivability, Durability and Performance of Naval Composite Structures’, one task is to develop improved fibre composite joints for naval ship superstructures. In many practical situations, the structures are subjected to loading at very high strain rates like slamming, impact, underwater explosions or blast effect. Material and structural response vary significantly under such loading as compared to static loading. In this paper, the results from a series of Split Hopkinson Pressure Bar tests on the woven composites are presented. These tests were done in two configurations: in-plane and out-of-plan compression test. It is observed that the failure strength varies with the different loading directions. The results indicate that the stress–strain curves, maximum engineering stresses and strains evolve as strain rate changes. The woven composites have higher values of engineering stress and dynamic stiffness for in-plane than for out-of-plane compression at the same strain rate; however, the in-plane strain at maximum stress is higher than that of out-of-plane compression. During the experiments, a high speed camera was used to determine the damage mechanisms. The specimens are mainly damaged in a crushing and shear failure mode under out-of-plane loading, as for in-plane test, the failure was dominated by fibre buckling and delamination.

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Why is it important?

A SHPB was used to conduct high compressive strain rate experiments. Samples were subjected to in-plane and out-of-plane tests. The first observation can make, for in-plane and out-of-plane tests, is that materials show a strength evolution with loading direction and strain rate. Moreover, the stress–strain curves manifest significant influence of the strain rate on the composite mechanical behaviour. The maximal stress obtained during dynamic compression tests are higher for the composite subjected to out-of-plane loading. The composites have higher engineering stress and dynamic stiffness in-plane than in out-of-plane direction at same strain rate. Also, at higher strain rates a decreased modulus and engineering stress were found in both directions. Comparison between the out-of-plane and in-plane directions, strains at maximum engineering stress were greater in out-of-plane direction at high strain rate. Damage appears only for specific impact velocity on the samples: 11.5–12.3m/s for in-plane loading and 19.6–19.8m/s for out-of-plane loading. The out-of plane compression test generates a crushing layer from where the plastic deformation appears, shearing through the layers, micro- and macrocracks. The multiplication of the microscopic and macroscopic cracks involves the catastrophic failure. The damage for in-plane loading case appears by a cracking along the diagonal of the cube, fibre buckling and delamination.

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