Design of a CFRP-to-steel joint for a bus engine mount and experimental testing of joint relaxation

Sammanfattning: The objective of this study is to investigate aspects of hybrid joint design for a shear connection. Part 1 is design considerations and a case study based on a engine mount. In Part 2, an experimental study is conducted regarding how relaxation and creep affects properties of a hybrid joint between carbon fiber vinylester and steel. Failure modes, loss of clamped force and ultimate strength of the joint and how these are affected by elevated temperature and different clamped forces are presented. Test specimens were 184×45×5 mm CFRP, in double lap joint between two steel plates of same dimensions, with either a single or double M10 hexagon flange screw. Creep/ relaxation testing was preformed over a 28 day period. Results shows that the amount of initial clamped force does not effect amount of loss of clamped force, but increase in temperature significantly does. Average loss was between 8.3 an 8.8% for single joint specimens, initial torque of 28, 37 and 46 Nm, and 10.8% for double joint specimens, 37 Nm. But losses for specimens subjected to heat, 80 °C, was 35.5%. Ultimate bearing strength of the lamina was not effected by any circumstances. Slip load was lower for specimens clamped with lower clamped force, the loss over the 28 day period was not enough to produced any clear results regarding the amount of loss. But tendencies point towards a lowered slip load of the same magnitude as the loss of clamped force i.e around 8-9%. Slip load for specimens subjected to 80 °C did not decreased, even though the clamped force had been lowered. All specimens fail in bearing, single bolted joints had a far more progressive failure than the double joints, which failed catastrophically.