High Rate Fracture Toughness Measurement of Laminated Composites

Abstract

The aim of this research is to understand and analyse the behaviour of specimens used to measure fracture toughness in laminated composites when loaded at high displacement rates. Finite element (FE) analysis is used as a tool to investigate dynamic behaviour of rapidly loaded mode I intralaminar specimens and to investigate of the data reduction strategy. Finally, using knowledge from current literature and experience gathered from the FE analysis, an experimental test procedure for the measurement of intra-laminar fracture toughness at high loading rates is designed and evaluated. To gain confidence in the dynamic FE modelling the behaviour of a mode I interlaminar double-cantilever beam specimen loaded at high rates was analysed. The findings were compared to experimental observations and a data reduction strategy from the published literature was assessed. FE modelling was then used to investigate the behaviour of an intralaminar compact tension (CT) test performed at high rates and a data reduction strategy was developed which does not require the measurement of the applied load. A CT specimen for measurement of interlaminar fracture toughness at high loading rates was also developed which can be analysed using the same data reduction strategy. Experiments were carried out at displacement rates between 0.2 mm/min and 15 m/s to determine the effects of test velocity and crack growth velocity on inter- and intralaminar fracture toughness. Results from the dynamic experimental tests were filtered to remove the noise in the data. Critical energy release rates were calculated using the filtered and unfiltered data and the results were used to assess the accuracy of the filtering procedure. Results are then presented in terms of the trend of critical energy release rate, GIc, with test velocity and average crack speed. The findings are discussed with reference to micrographs of the fracture surfaces of specimens at each test speed. The intralaminar specimens showed a possible decrease in GIc with test speed and average crack speed, but the range of values fell within the scatter in the results. The interlaminar test results suggest interlaminar GIc is constant with test. The increase in interlaminar GIc with crack speed was also within the range of the scatter in the results.