Interlaminar fracture behaviour of fibre reinforced powder epoxy composites
Item statusRestricted Access
Embargo end date01/07/2027
The aim of this work is to study delamination in fibre reinforced composites with applications in wind and tidal turbine blades. Powder epoxy composites are a very promising option for the manufacturing of large composite structures due to their low exotherm and viscosity, a suitability for out-of-autoclave manufacturing and their ability to perform a separate melting and curing phase allowing complicated parts such as wind turbine blades to be formed separately and cured in a one-shot process. It was shown that quasi-unidirectional carbon fibre and glass fibre reinforced powder epoxy composites could be manufactured out-of-autoclave with a low void content. However, a relatively high variation in the thickness between samples and fibre distortion, especially in the CFRP samples were measured. This shows that, although powder epoxy is a promising resin for the manufacturing of composites, an automated process such as the fabrication of powder epoxy prepregs is preferable to the hand spraying process for the fabrication of consistent composites. The powder epoxy resin was show to be very ductile with a strain to failure of nearly 10%. However, it lies within the mid-range of reported strength values for epoxy with an ultimate strength of 61 MPa and in the low range of reported elastic modulus values with a value of 1.64GPa. The tensile properties of powder epoxy composites were shown to be comparable to those of commonly used epoxy resins but the compressive strength was shown to be quite low with a 48% and 59% reduction for the GFRP and CFRP composites compared to their tensile strengths. The interlaminar fracture toughness of carbon and glass fibre composites was studied in pure mode I by performing Double Cantilever Beam (DCB) tests and at 25% mode II, 50% mode II and 75% mode II by performing Mixed Mode Bending (MMB) testing and in pure mode II by performing an End-Loaded Specimen (ELS) test. The strain energy release rate (SERR) at both crack initiation and propagation were shown to be significantly higher than both conventional and toughened epoxy composites for which published data is available. While the powder epoxy is not the only factor explaining the high fracture toughness, with the off-axis fibres, evidenced by fibre bridging, played a role, the powder epoxy composites used in this study still had a higher toughness than published data for epoxy quasi-unidirectional fabrics which also contain off-axis fibers. To understand the influence of moisture on the mechanical properties of the studied powder epoxy composites, they were hygrothermally aged in 60°C seawater. The CFRP and GFRP samples were saturated after around 4 months. Water diffusion in the composites induced plasticisation of the resin, leading to a 25°C reduction in the resin Tg. The longitudinal elastic modulus of the CFRP and GFRP were unaffected by the presence of water but reductions in the longitudinal, transverse and shear strengths of 45-50% and 15-30% were measured for the GFRP and CFRP respectively. SEM micrographs showed a degradation in the fibre/matrix interfacial strength, especially in the GFRP samples. In contrast, water absorption reduced the compressive strength properties of CFRP to a greater degree than GFRP. Hygrothermal ageing led to reductions in the interlaminar fracture toughness but the reductions were dependent on the mode ratio. The propagation GC decreased between 25% for mode I and 40% for mode II for the GFRP samples, while a 25% decrease in the mode I was observed for CFRP while the mode II Gc remained unaffected. The degradation of the fibre/matrix interface was a likely explanation for the reduction in mode I toughness for the CFRP as fibre pull-out likely occurred at a lower energy while the 90° fibres of the UD glass fabric were not able to arrest mode II crack growth as efficiently. A finite element delamination analysis was carried out using the virtual crack closing technique (VCCT). A parametric model was developed to allow for automated modelling of a variety of ply drop configurations. This was followed by a sensitivity analysis to understand the influence of the various model parameters on the predicted delamination failure load. Finally, a method was developed for the optimization of chamfered ply drop specimens with regards to the chamfered slope ratio and the vertical height at the end of the ply drop, called the toe height. It was shown that the delamination failure load increased with the chamfered slope ratio but decreased as the toe height increased. As a result of this study, the combination of chamfer slope and vertical height leading to failure in the plank occurring prior to delamination initiation were obtained, allowing for the manufacturing of chamfered ply drops where the risk of delamination is suppressed. The methodology for the characterisation of the fatigue delamination properties needed for an accurate prediction of delamination growth under fatigue loading was defined. Then a finite element predicting fatigue crack growth in chamfered ply drop specimens, based on the Paris law and VCCT models to extract SERR as a function of crack length and applied strain, was developed for a variety of geometric configurations. The comparison of predicted fatigue life with experimental results shows that the FE model captured the same trends observed during testing, but the predicted strain values leading to a given number of cycles to failure were up to 30\% different from the experimental ones. In the absence of fatigue delamination properties for the materials used in this study, the fatigue life predictions were carried out using Paris law parameters obtained from literature, giving a likely explanation for this relatively large difference.