The effect of temperature on the lap shear strength (LSS) and failure mechanisms of ultrasonically welded carbon fibre reinforced polyphenylene sulphide (CF/PPS) joints was investigated, correlating the weld performance to the crystallinity degree of PPS at the weldline. The single-lap shear tests were carried out at temperatures ranging from –50 °C to 120 °C on three series, one with amorphous and two with semi-crystalline weldline. The overall trend was decreasing LSS with increasing temperature and the largest LSS reduction was observed above the glass transition temperature. Fractographic analysis revealed that the main failure mechanism at –50 °C was matrix fracture while fibre/matrix debonding became more pronounced with increasing temperature. It was demonstrated that higher degree of crystallinity of PPS at the weldline was beneficial at high temperatures (90 °C and 120 °C) most likely due to the higher fibre/matrix interfacial strength compared to amorphous PPS. The amorphous weldline was shown to be advantageous at -50 °C, probably due to the higher toughness and ductility of amorphous PPS.

The influence of the ultrasonic welding process parameters, namely the force and the vibration amplitude, on the crystallinity degree at the welding interface of CF/PPS (carbon fibre reinforced poly(phenylene) sulphide) joints is investigated. Two different sets of parameters, one representing high force and high vibration amplitude (1000 N, 86.2 μm) and one representing low force and low vibration amplitude (300 N, 51.8 μm), were used in this study. The temperature at the centre of the overlap was measured using K-type thermocouples in order to obtain the cooling rate for each set of parameters. Differential scanning calorimetry (DSC) and X-ray diffraction (XRD) measurements were performed in order to determine the crystallinity degree of PPS at the welding interface. It was found that a force of 300 N and a vibration amplitude of 51.8 μm could obtain a PPS of a moderate crystallinity degree (14.6%) and an average PPS crystallite size of 41.3 Å, showing that it is possible to obtain a semi-crystalline welding interface by appropriately modifying the process parameters.

The influence of ultrasonic welding on the crystallinity degree at the welding interface of carbon fibre reinforced polyphenylene sulphide (CF/PPS) joints was investigated. Two sets of welding force and vibration amplitude were used, (1000 N, 86.2 μm) and (300 N, 52.8 μm), representing short and long welding times, respectively. The evolution of temperature with time at the centre of the joint overlap was recorded using thermocouples while the crystallinity degree of PPS was measured using differential scanning calorimetry (DSC). The cooling rate dependency of crystallinity was determined through fast scanning calorimetry (FSC) measurements. It was found that high force and high amplitude resulted in faster cooling rates and predominantly amorphous PPS, while low force and low amplitude resulted in slower cooling rates and yielded PPS of moderate crystallinity. It is suggested that the capability of PPS to crystallize despite the very fast cooling rates could be attributed to strain-induced crystallization during the welding process.

In this work, the effect of temperature exposure on the strength of resistance welded joints is analysed. Glass fibre reinforced polyphenylene sulphide (GF/PPS) adherends were joined using the resistance welding technique, using a stainless steel mesh as the heating element. Single lap shear tests were performed at temperatures ranging between −50 °C and 150 °C to evaluate the strength of the welded joints. The results showed that the lap shear strength decreased with increasing temperature, except for the region between 50 °C and 90 °C where it remained constant. Fractography analysis revealed that the main failure mechanism was glass fibre/matrix debonding and the connection between the mesh and the matrix was not the weakest link at the interface of the joint at any temperatures under study. The fibre/matrix interfacial strength and the stress distribution at the joint overlap were identified as the main factors influencing the behaviour of lap shear strength with temperature.