Failure behaviour of short-fibre-reinforced PBT composites: Effect of strain rate

Thermoplastic polyester elastomer (TPEE), one of thermoplastic elastomers, suitable for blending with polybutylene terephthalate (PBT), recently attracted significant attention thanks to its beneficial effects on the impact strength of fibre-reinforced PBT composites. Short-fibre-reinforced PBT/TPEE composites are used for critical components in the automotive industry, including electrical connectors. However, there is a lack of knowledge of their dynamic behaviour and microstructural processes underpinning their failure. The aim of this paper is to study the rate-dependent behaviour of such composites at different strain rates, assessing the strain-rate dependency of mechanical properties of short-glass-fibre-reinforced (SGFR) PBT and analyse the microstructural features of its failure at various loading rates. For the first time, standard SGFR PBT and TPEE-blended SGRF PBT were investigated under uniaxial tensile testing at different loading rates ranging between 2 mm/min and 400 mm/min (corresponding to 0.003 s(-1) and 0.6 s(-1) strain rates). Fracture surfaces obtained at different loading rates were examined with scanning electron microscopy (SEM) to investigate brittle/ductile properties. The results indicated that ultimate tensile stress of PBT composites is strain-rate sensitive. Both damage initiation and a rate of crack growth were significantly affected by blending with TPEE and demonstrated dependence on the loading rate. SEM analysis confirmed the effects of blending and strain rate on failure behaviour manifested in varying character fracture-surface morphology and microstructural features of the matrix. Micrographs demonstrated that interfacial matrix-fibre bonding was weakened by the addition of TPEE. Additionally, a strong relationship between the size of ductile areas of fracture surfaces and the loading rate was observed, indicating higher plastic deformation of the matrix for PBT TPEE composite.