Experimental Characterisation of Ultrasonically Welded COFA Joints

Abstract

The Composite Fitting Attachment (COFA) system was developed by Cooperative Research Centre for Advanced Composite Structures (CRC-ACS) to join thermoplastic (TP) brackets to carbon fibre reinforced plastic (CFRP) using ultrasonic welding. This complex process means experimental characterisation of the joint is crucial. Three aspects of the joint formation were investigated: weld development over time, temperature in the weld and surrounding laminate, and failure loads of the joint. Weld development over time was investigated using cross sectional micrographs, differential scanning calorimetry and hot stage. This indicated under a given set of weld parameters, the energy director of the COFA joint started to collapse after 0.025 secs of ultrasonic vibration and collapse distance is directly proportional to the input actuator energy. Various spherulite sizes were observed at the joint interface divided by striations suggesting varying cooling rates in the TP during resolidification. Minimal percentage crystallinity changes using various cooling rates were measured. Interface and through thickness laminate temperatures were measured using thermocouples. Measured interface temperatures were in excess of 500°C and 300°C for joints welded at 20kHz and 40kHz, respectively. Maximum times for which any portion of the laminates measured above 200°C were under 1 second and just over 1 second for joints welded at 20kHz and 40kHz, respectively. These short times are expected to be insufficient to allow thermal degradation of the laminates during welding. Failure test of COFA joint were performed under axial, transverse and shear loads. The shear load test case was newly developed to fully characterise the load-bearing capabilities of the joint. All tests passed the 200N requirement. Transverse loading applied at 90° to the CFRP's first ply is the critical load case. Three different failure mechanisms were observed including CFRP laminate damage through first ply fibre pull out.