Impact des conditions environnementales sur la performance mécanique de composites thermoplastiques soudés par résistance avec un élément chauffant modifié

Abstract

This work presents the study of the hygrothermal aging and freeze/thaw cycles effects on the lap shear strength of carbon fibres/polyphenylene sulfide resistance welded joints and especially the impact of a silane based coating before and after these conditioning. Resistance welding is a fast and easy to apply joining method but few studies considered the joints performance under harsh environments. The joints failure mode usually involves debonding between the stainless steel heating element and the thermoplastic matrix, an indication of a poor adhesion between the two materials. This weak adhesion can enable moisture infiltration and thus lower the joint performance. The moisture and temperature impact on the single lap shear strength and the failure mode of welded joints is first studied. It is shown that the lap shear strength decreases with the temperature between 21°C and 150°C and that hygrothermal conditions do not have a significant impact on the resistance. However, the fracture surfaces observation shows a degradation of the fibre/matrix interface after conditioning. It also seems that the heating element is lifted off the composite adherends during testing at high temperature or after hygrothermal conditioning. As the failure mode involves a heating element and matrix debonding, a silane-based coating is developed for the heating element. This coating is realized in four steps: the corrosion of the steel with sulfuric acid, its oxidation at high temperature, the development of the silane coating using a sol/gel method and the silane condensation on the steel. A Taguchi design of experiment is used to study the impact of several parameters on the joints lap shear strength. The oxidation temperature, the pH and the solvent of the silane solution are identified as parameters with a significant impact on the joints mechanical performance. The coating improves the lap shear strength of welded joints by more than 30% with an oxidation at 500°C and a silane solution with a basic pH in ethanol. Fracture surfaces microscopic observations show that the matrix remains attached to the heating element. Moreover, double cantilever beam tests between stainless steel plates and the composite show a better adhesion between both materials with matrix and fibres transferring to the steel plate after fracture. Finally, dry and moisture saturated joints, with or without a silane coating are subjected to 1000 freeze/thaw cycles from -40°C to 82°C. In the same manner as hygrothermal aging, freeze/thaw cycles do not have any significant impact on the dry and saturated joints lap shear strength. In general, the coating still provides superior lap shear strength values compared to untreated joints after the same environmental conditioning. Fracture surfaces and cross-section analysis of the joints show that both fibre/matrix and coated steel/matrix interfaces are degraded by the freeze/thaw cycles.

Date	28 Jul 2020
Original language	French
Awarding Institution	École de technologie supérieure
Supervisor	Martine Dubé (Supervisor) & Louis Laberge Lebel (Co-supervisor)