Fatigue failure characterisation of resistance-welded thermoplastic composites skin/stringer joints

Abstract

An experimental investigation characterising the fatigue failure mechanisms of resistance-welded thermoplastic composites skin/stringer joints is presented. Unidirectional (UD) and quasi-isotropic adherends were welded using stainless steel meshes as heating elements. The specimen geometry consisted of a flange laminate, representing a stringer, welded onto a skin laminate. In order to avoid current leakage to the electrically conductive adherends, a ceramic-coated heating element (TiO 2 HE) was used for welding the UD specimens and some of the quasi-isotropic specimens. The fatigue performance of the welded joints was investigated under three-point bending. An indefinite fatigue life was obtained at 40% and 35% of the static damage initiation load for the UD and quasi-isotropic specimens, respectively. The failure mechanisms were documented based on observation of the fatigue cracks initiation and growth. UD specimens failed at the weld interface while quasi-isotropic specimens showed delaminations both in the flange or skin laminates and at the weld interface. The TiO 2 HE did not show any fatigue mechanical performance reduction. However, debonding at the weld interface was shown to occur between the metal mesh wires and the TiO 2 coating instead of between the laminates and the weld.