Interlaminar toughness of fusion bonded thermoplastic composites

Thermoplastic composites are of increasing interest to the aerospace industry. The melt-processability of the thermoplastic matrix allows for fast manufacturing and assembling techniques, such as thermoforming and fusion bonding, which are also highly suitable for process automation. Fusion bonding involves heating of the interface between the parts to be bonded, application of pressure and finally cooling of the bonded parts. Even though successful commercial application of fusion bonding can already be found in the aerospace industry, a wider use requires additional developments in order to improve the predictability, reliability and robustness of fusion bonded joints. This first of all requires a better understanding of that what is perceived as ‘the load bearing capacity’, as measured by mechanical testing of fusion bonded joints.
Two mechanisms that are essential for the generation of the load bearing capacity of fusion bonded joints are (i) intimate contact development, followed by (ii) the interdiffusion of polymer chains across the interface. Although these two mechanisms are a prerequisite for the development of a bond, they are not the only mechanisms that determine the performance of a fusion bonded joint. The physical state of the bond line and the structural morphology of the interface also plays an important role. The objective of this work is to identify, to analyse and, when possible, to quantify the relation between the physical state and the structural morphology induced by the fusion bonding process, and the resulting mechanical performance of the joints. For this purpose, the most relevant variations in physical state and structural morphology, as induced by the fusion bonding process, were identified. These factors were then isolated experimentally, and their effects on the interlaminar fracture toughness of the joints were studied.