When a CFRP structure undergoes impact, the amount of damage that is externally visible is typically small, when compared to the total extent of damage within the material. To ensure safety, the CFRP structure must therefore be able to sustain the ultimate design load even in the presence of significant damage.
This often makes residual strength, rather than unnotched strength, the driving design requirement. Improving the residual strength will reduce the amount of material needed to compensate for potential impact damages, and therefore save weight.
Our reinforcement concept consists of creating thin-ply reinforcement units. These units consist of two thin-ply pre-preg layers, which are interlocked using a 'tab-and-slit' geometry. The reinforcement units are then placed within a composite laminate at any location that is to be reinfoced.
The performance of the reinforcement concept was investigated through mode I and II fracture toughness testing, and through compression after impact (CAI) testing.
Mode I fracture toughness was measured with a DCB test, and the 4-point ENF test was used to determine mode II fracture toughness.
The graphs show the fracture toughness results for both the baseline and the reinforced specimens.
In mode I, a 77.6% increase of propagation fracture toughness was achieved.
In mode II, the reinforcements did not have any effect, most likely because the delamination was able to bypass the reinforcing tabs. Further experiments are currently underway to investigate this hypothesis, as well as the effect of tab orientation on delamination path.
The graphs show the effect of the reinforcement on delamination area post impact, and on compression after impact strength. An 11.4% reduction of delamination area was achieved. The average CAI strength increased by 5.1%, but this was not statistically significant (2 parameter K-S test, p=0.21).
The lack of significant improvement of CAI strength is thought to be due to fibre damage, causing a change in the critical failure modes during the compression test.
We gratefully acknowledge funding from the EPSRC under grant EP/M002500/1.
S. Pimenta acknowledges the Royal Academy of Engineering for her Research Fellowship on Multiscale discontinuous composites for large scale and sustainable structural applications (2015-2019).