The effect of a graphene nano-platelet modified interlayer on the toughness of a composite laminate
Creators: name: NAGI, CHIRNJEEV SINGH
Publisher: name: University of Surrey
Year: 2021

description: Composite materials are widely used in structural aerospace applications due to their lightweight and excellent strength to weight ratio. Typically, laminated composites are utilised which have poor interlaminar performance due to the lack of through thickness reinforcement in most cases. Interlaminar toughening of composites has been addressed in terms of matrix modifications, introducing through thickness reinforcement (3D composites), introducing interleaves in the resin-rich region between plies and in more recent years using micro- and nano-modifications to toughen the matrix.
description: The novelty of this study was to devise a low-cost manufacturing approach to incorporate nanomaterials into CFRP laminates while also making the method industrially relevant by devising an approach that could be implemented into existing CFRP lamination methods. In this study solution and spray deposition were used to modify a non-woven thermoplastic interleaf with nanomaterials, which subsequently was used to modify the interlaminar region of CFRP laminates manufactured using pre-preg CFRP. The nanomaterial used was graphene nano-platelets (GNPs) with oxygen functionality, with the oxygen groups facilitating bonding between the GNPs and the epoxy resin matrix at the interlaminar region of the CFRP. The GNP-modified CFRP laminates were tested under mode-I and mode-II loading conditions to determine the effect the deposition method and nano-modification had on the interlaminar fracture toughness.
description: The role of graphene in terms of the fracture toughness differed with respect to the testing method. In mode-I, a continuous GNP-deposition prevented fibre bridging, resulting in a reduction in the toughness value, when compared to the baseline CFRP. In mode-II, where a resin dominated failure mode occurred, the GNPs bonded to the matrix resin leading to GNP-matrix deformation on loading. The toughening imparted by GNP-matrix deformation combined with the existing CFRP toughening mechanism of shear cusps resulted in the greatest improvement in mode-II toughness when compared to the baseline CFRP.
description: The solution deposition method facilitated GNP loadings of up to 1.88 g.m-2 to be incorporated at the interlaminar region of CFRP pre-preg laminates. However, at all loadings in mode-I, the GNP-modification supressed the CFRP toughening mechanism of fibre bridging resulting in a reduced mode-I toughness. In mode-II, at a GNP loading of 0.57 g.m-2, the GNP-modified interlaminar region was sufficiently tough to redirect crack propagation to an adjacent ply. At a higher GNP loading of 0.86 g.m-2, the fracture surface showed regions of GNP-polymer matrix and other regions showed shear cusps, which resulted in an increased mode-II toughness.
description: The spray deposition method was devised to address the suppression of fibre bridging in mode-I. This method allowed for a more controlled deposition, including producing a GNP strip-pattern onto the thermoplastic interleave. The spray deposition method showed a significant improvement in the mode-I toughness, and a smaller improvement in the mode-II toughness, compared to the solution deposition specimens. The strip-pattern GNP distribution, in mode-I, also showed the re-emergence of the fibre bridging toughening mechanism.
description: Overall the spray deposition method showed the greatest possibility of a low-cost and industrially relevant method of introducing GNPs at the interlaminar region of pre-preg CFRP laminates. The ability to control the GNP-deposition by imparting a strip pattern allowed for toughening contributions from the GNP-matrix resin and CFRP material. This suggests a method for optimising fracture toughness values in both mode-I and mode-II.