Effects of thermal residual stresses on static strength and fatigue life of welded carbon-fibre/epoxy composite joints

Thermoset Composite Welding (TCW) is a process designed specifically for joining composite materials, developed by the Cooperative Research Centre for Advanced Composite Structures (CRC-ACS). The TCW manufacture process is carried out at higher temperatures than those used in service, causing thermal residual (TR) stresses to develop in the joints. An investigation of the strength of single-lap shear joints (SLJs), and the development of laminate free edge microcracks (LFEMs) is presented in this thesis. The reported investigations are primarily experimental. Finite element analysis has been used to understand observations where appropriate. The effect of TR stresses on static failure of TCW SLJs and Cytec FM1515 thin film epoxy adhesive SLJs over the temperature range of -55°C to 71°C is investigated. At temperatures where the joining material is ductile, plastic flow results in the redistribution of TR stresses within the joints, reducing their effect on the failure strength. No such stress redistributions occur at lower temperatures when the joining material is brittle; hence, the TR stresses cause strength reductions. These results were used to propose a method of shear strength improvement by initiating plastic flow in the joint at the time of manufacture.Microcracks are common at the free edges of thermoset composites. These develop preferentially near the weld material interface in TCW laminates, and are termed laminate free edge microcracks (LFEMs) in this study. MicroCT scanning was used to find and characterise LFEMs in TCW joints. The results indicated that TR stresses combined with the free edge sectioning process cause their development outside the joint overlap regions. Microcracks developed within the joint overlaps during mechanical fatigue cycling. LFEMs were also found in FM1515 joints. A fatigue life study is presented for TCW and FM1515 SLJs at -55°C, in which the effect of LFEMs is considered. TCW is a new process. This investigation is the first dealing with the effect of thermal residual stresses on the strength of TCW joints, and the development and effect of LFEMs. The shear strength improvement method is also a novel concept for joints