Translaminar fracture toughness of CFRP: from the toughness of individual plies to the toughness of the laminate
Creators: name: De Faria Teixeira, Rita
Publisher: name: Imperial College London
Year: 2014

description: The translaminar fracture toughness of fibre reinforced polymers (FRP) is important for characterising the failure resistance of composite structures. Measuring the translaminar fracture toughness for any possible layup is not feasible. Therefore, it is of interest to relate the translaminar toughness of a laminate to that of its plies. Numerous studies have measured the translaminar fracture toughness of composite laminates and of individual plies. However, any attempts to relate the two have so far been very limited, and restricted to initiation values. This work presents experimental and analytical research on Compact Tension (CT) tests on several T800s/M21 carbon-epoxy laminates with different combinations of 0° , ±45 and 90° plies, and with various ply thicknesses. Post-mortem techniques, such as X-ray, optical and scanning electron microscopy, were used to determine the damage extent in each specimen. Acoustic emission (AE) was also used to sequence the occurrence of the failure mechanisms. Failure mechanisms found in the multidirectional laminates included a combination of the failure mechanisms found on bidirectional laminates ( /90° ) made of its constituent plies. Ply splitting, fibre bridging and fibre pull-out were the main features characterizing the fracture surfaces. Assuming that the damage can be represented as a single crack, the resistance curve (R-curve) for each layup was extracted from these tests. From each laminate R-curve, three distinct fracture toughness values were obtained for each layup: non-linearity onset, initiation and propagation. The R-curves were used to define a trilinear cohesive law for each layup, and the specimens were then successfully simulated using a cohesive approach in a Finite Element (FE) model. On the one hand, there was good agreement supporting the representation of translaminar damage as a cohesive crack. On the other hand, damage was considerably diffuse when the laminate included substantial ply-blocking, thus suggesting that a single equivalent crack may, in some cases, neglect some important aspects of translaminar damage (as well as delamination). Four analytical predictive models were used to predict the translaminar toughness of the laminates from that of the constituent plies. The assumption of translaminar fracture toughness additivity by means of a rule of mixtures correlated best with the experimental results. The experimental results for a mode I crack propagation in a 45° ply were shown to corroborate a simple analytical model which relates the critical energy release rate of a 45° ply to those of 0 and 90 plies. Thickness size effects were investigated by using different 0 ply thicknesses, by means of 0° ply-blocks and using two grades of the same material system. Since it was found that excessive ply-blocking can lead to significantly diffuse damage, a second study with thin-ply TR50s/K51 carbon-epoxy system was conducted, leading to the first translaminar fracture characterisation of a 0° CFRP ply for a range of thicknesses from 0.03 mm to 0.12 mm. The toughness of the 0° plies was confirmed to be significantly dependent on the thickness, even for ranges of thicknesses where delamination does not play a significant role.