Thermal effects on the performance of ultrasonically welded CF/PPS joints and its correlation to the degree of crystallinity at the weldline

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.