Interlaminar toughness of fusion bonded thermoplastic composites
Thermoplastic composites are of increasing interest to the aerospace industry. Compared to metals, they provide a higher specific strength, specific stiffness, and an increased design freedom, as composite materials have the ability to tailor mechanical performance to a particular application. Moreover, 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, if 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. Three aerospace grade materials were used in this study, namely, unidirectional (UD) carbon fibre reinforced polyether-ether-ketone (carbon/PEEK), a 5 harness satin (5HS) woven carbon/PEEK, and a UD carbon fibre reinforced polyphenylene-sulfide (carbon/PPS).
Regarding the physical state of the bond line, two factors were identified to play a fundamental role in the interlaminar fracture toughness of the joint. The first comprises contamination of the substrates to be bonded from the release media used during consolidation of the substrates. The presence of contaminants was found to block proper development of the toughness of the bond. The second factor that was found to play an important role is the degree of crystallinity obtained at the bond line as a function of the thermal history. For PPS, a semicrystalline polymer, a low degree of crystallinity at the bond line was found to produce bonds with higher interlaminar toughness, resulting from the greater plasticity of the polymer.
Concerning the structural morphology, two factors were investigated. Firstly, the effect of the thickness of the matrix-rich bond line on the toughness was studied. An additional polymer layer is sometimes added at the bond line to ensure good wetting between the two surfaces to be bonded. As a result, depending on the nature of the process and the process parameters, a matrix-rich bond line at the interface may be generated. Such a matrix-rich bond line was found to increase the interlaminar toughness of the bond, suggesting more plastic energy dissipation before fracture occurs. This can be due to a larger volume of matrix involved in the fracture process, possibly in combination with a higher strain to failure caused by a smaller constraining effect of the fibres. Depending on the processing time of the fusion bonding technique, migration of fibres towards the matrix-rich bond line may occur. This migration was found to have no significant effect on the interlaminar fracture toughness of the bond compared to the effect of the matrix-rich bond line itself. Secondly, the direction of crack propagation with respect to the fibre direction and weave architecture in woven fabric reinforced laminates was found to affect the interlaminar fracture toughness of the joint. The presence of fibres perpendicular to the direction of crack propagation were found to generate a more tortuous crack path, leading to a tougher interface.
In the last part of this work, the results are discussed in a broader perspective by looking at the mechanisms that govern the interlaminar fracture toughness of fusion bonded joints. The knowledge gained is translated into practical guidelines for industry to manufacture fusion bonded joints with high interlaminar fracture.
