Bonding of composite and titanium using thermoplastics: toughening by bridging

Assemblies made of dissimilar materials are becoming more and more common in the aeronautic industry. Thermoplastic (TP) joints present a good alternative to mechanical fastening, the historical solution, as it reduces the weight of the joint and permits a better stress distribution. This work focuses on the joining of titanium and carbon fibers reinforced polymer (CFRP) made with RTM6, which are two common materials used in this industry. It is divided into two parts, the study of PEI and CFRP bonding using co-curing and titanium TP welding. The problem of PEI CFRP co-bonding is its low fracture toughness. Different toughening techniques such as bridging and crack trapping are studied and analyzed. Fishing lines are inserted in the joint to cause bridging and to have RTM6-PEI interphase that forms outside the carbon fibers. This interphase is known to trap the crack and toughen the joint. The fracture toughness of the different samples is determined via DCB testing and DIC analysis. A curve fitting method has also been used to obtain the fracture toughness. The samples with different conditions displayed fracture toughness between 345 and 508 [J/m2]. Titanium TP welding has been studied with two different thermoplastics polyetherimide (PEI) and polyamide 6 (PA6). Two series of tensile tests have been conducted, the titanium underwent a Sol/Gel surface treatment in the first one and underwent mechanical surface treatment in the second one to study the effect of the surface rugosity on the adhesion behavior. The Sol/Gel treatment was damaged by the curing temperature and the resulting tensile strengths did not exceed 16 MPa for PEI and 32 MPa for PA. The mechanically treated titanium series displayed higher tensile strengths with a maximum of 45 MPa for PA and 71 MPa for PEI.