Effect of curing on the fracture toughness of composite materials

This thesis studies the effect of the degree of cure of a prefabricated part on the fracture toughness of co-cured composite part. A novel resin system, Airstone 780E/785H, that is used in the manufacture of wind turbine blades, is under consideration. Chemical characterization of the resin tells us its curing behavior while thermal characterization analyses the material’s response to temperature variations. From these experimental findings,an optimized heat transfer model that can identify the degree of cure and the temperature distribution at any given time during the manufacturing process is implemented. For mechanical characterization, test laminates, 3 co-cured each to different degrees of cure (α) and one control laminate that was not co-cured, are manufactured through VARTM. From these, DCB test coupons are prepared and the fracture toughness is tested in accordance with the ASTM D5528-13 standards. Samples with α ranging from 0.725 - 0.942 were realized. It was found that a decrease in α of the prefab corresponded to an improvement in fracture toughness. When α approached 1 the fracture toughness was worse than that of the control test specimen. The degree of cure is proportional to the degree of crosslinks formed in the resin system. Lower α have a relatively low degree of crosslinks formed at the interface. While at high values of α, a high level of crosslinking is achieved making it hard to form new ones. This in turn leads to the poorer fracture toughness properties of these samples. From a manufacturing perspective, use of co-curing is shown to be advantageous on the basis of process time. Curing time is reduced since we don’t have to wait for complete cure to be achieved during sample preparation. A more detailed consideration of this problem is recommended. A model that could replicate and possibly predict the influence of the degree of cure on the fracture toughness would allow the possibility of finding an optimum α for this resin system.