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

N Koutras,R Benedictus,I Fernandez Villegas

doi:10.1016/j.jcomc.2020.100093

N Koutras, R Benedictus + Show 1 more

Open Access

https://doi.org/10.1016/j.jcomc.2020.100093

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Abstract

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.

Highlights

To date, the effect of temperature on the mechanical performance of thermoplastic composites (TPC) welded joints has not been studied extensively
Cohesive failure was the dominant failure mechanism at -50 °C but its occurrence diminished with increasing temperature, whereas fibre/matrix debonding became more pronounced
It seems fair to suggest that the lap shear strength (LSS) of ultrasonically welded (UW) carbon fibre reinforced polyphenylene sulphide (CF/PPS) joints can be related to the fibre/matrix interfacial strength and that the latter deteriorated with increasing temperature, contributing to the reduction in LSS

Summary

Introduction

The effect of temperature on the mechanical performance of thermoplastic composites (TPC) welded joints has not been studied extensively. Rohart et al [2] found that resistance welded CF/PPS joints exposed to a wide range of temperatures above room temperature (RT) primarily failed at the interface between the heating element and the matrix. In both studies, the largest reduction in lap shear strength (LSS) occurred at temperatures higher than the glass transition temperature (Tg) of PPS. Considering the effect of temperature on the elastic modulus of the matrix and the fibre/matrix interfacial strength, as well as the effect of temperature on resistance welded TPC joints, it seems plausible to expect that temperature could have a detrimental effect on UW TPC joints as well

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