Understanding joint formation in thermoset composite welding (TCW) joints

Understanding joint formation in thermoset composite welding (TCW) joints

The increasing prominence of composite materials in the aerospace industry has reached a point at which primary structures are now manufactured using these materials. Until recently a reliable method of joining composite structures without the use of mechanical or adhesive fastening had not been identified. The unique technology known as Thermoset Composite Welding (TCW) is a potential solution; patented by the Cooperative Research Centre for Advanced Composite Structures (CRC-ACS). TCW allows for the welding of thermoplastic materials to join aerospace grade composite materials, consisting of a carbon-epoxy composition. TCW incorporates a thin semi-crystalline thermoplastic surface film, Polyvinylidene Fluoride (PVDF), as the first ply in a standard layup process for prepreg composites. Once the part is cured, a surface is formed that can be welded to another like component.

This PhD thesis is aimed at forming a greater understanding of the welding stage of TCW, providing knowledge previously unexplored for this new technology; this includes squeeze flow of PVDF polymer melts during welding, transport and removal of trapped air within the welded interface, and healing of the mated thermoplastic surfaces.

This PhD thesis contains a detailed literature review in Chapter 2 presenting the current knowledge of squeeze flow of viscous fluids, void transport within composite materials and welded interfaces, and healing of semi-crystalline thermoplastic interfaces. The literature review provided initial guidance for the numerical squeeze flow analysis and the experimental healing analysis of PVDF interfaces. The constituent material properties integral to the understanding of the welding stage of TCW are presented in Chapter 3; containing known properties from literature and properties measured experimentally. Chapter 3 includes a Differential Scanning Calorimetry (DSC) investigation and rheometric study for PVDF at elevated temperatures, and mechanical testing to determine the stiffness properties of Hexcel HexPly® M21. The DSC investigation identified the temperature at which a fully-amorphous structure is obtained during heating and the temperature at which re-crystallisation begins during cooling due to the semi-crystalline nature of the polymer; the results provide an indication of the welding time for a fully-amorphous polymer at the thermoplastic interface. The rheometric investigation indicated that PVDF polymer melts at elevated temperature (between 165-195°C) are shear-thinning. The tensile and flexural stiffness properties of Hexcel HexPly® lamina and laminates respectively were found at room temperature and at 185°C. These tests indicated that matrix dominated properties were severely reduced at elevated temperature, with fibre dominated properties unchanged. Chapter 4 presents a numerical investigation using FEA simulations to model squeeze flow during the welding stage of TCW. Of most importance was determining the effect of a number of practical considerations unable to be solved using analytical methods. These considerations included: non-Newtonian material properties, spew fillet formation, initial PVDF geometry variations, and elastic adherents. The effects of each of these practical considerations were seen to have varying effects on squeeze flow within TCW joints; the results indicated that non-Newtonian material properties and the spew fillet analysis were of most significance. With each of the practical considerations and their effects presented, the chapter culminated in an analysis of the squeeze flow between the foot of a TCW top-hat stringer and a skin. The difference between the simplified cases modelled, and the TCW stringer itself were largely negligible, with the largest difference seen in the shape of the spew fillets formed. An experimental investigation into the effect of joint width, initial joint surface roughness, and joint rigidity on void migration within the welded interface of a TCW joint is presented in Chapter 5. The results of this chapter indicated that for joint widths larger than 25mm, a textured initial surface roughness profile provides assistance in removing trapped air. This investigation is supported by the findings reported in Chapter 4; as the trapped air identified within the 70mm wide welded stringers was observed in the squeeze flow analysis of the elastic upper adherents. Chapter 6 presents an experimental investigation into the effect of welding temperature and welding time on the quality of a welded interface. Weld quality was measured using Single Lap Shear (SLS) specimens to characterise the welded interface. This study was aimed at providing recommendations for minimum recommended welding times and temperatures for TCW joints. A minimum welding temperature of 185°C and a welding time of at least 40 sec is required to form a fully welded interface. It was observed that the welding temperature is the major controlling mechanism for the welding of TCW joints, in comparison to the welding time.

Finally, Chapter 7 provides a detailed summary of the work conducted in this PhD thesis; as well as detailing important design recommendations to be applied to the application of TCW to aircraft and the aerospace industry in the future. These design recommendations provide an important tool for the manufacturing of TCW joints, which until now had not been identified. They also contribute towards decreasing the assembly time of composite joints on aircraft, ultimately saving time, money, and resources for the end user of this technology. Thermoset Composite Welding is not only restricted to the aerospace and aeronautical industries, it is also highly applicable in industries such as automotive and rail; ultimately any field in which composites joining is required.