Mechanical behaviour and microstructure of short-fibre-reinforced polybutylene terephthalate

2020-05-28T11:11:58Z (GMT) by Dani Abdo
Short-fiber-reinforced thermoplastics are widely used in industry. They are light-weight, have excellent mechanical properties and can be processed via injection moulding. This allows the mass production of high-quality components with excellent geometric accuracy. Their superior isolation properties make them a good choice for electrical housings in the automotive sector. Due to the importance and precise nature of applications, in which such products are employed, a deep understanding of their deformation and fractures under specific conditions is essential. A typical example of such thermoplastics is polybutylene terephthalate (PBT).
In this thesis, an evaluation of mechanical and morphological properties of short-glass-fibre-reinforced (SGFR) PBT and SGFR PBT TPEE (thermoplastic polyester elastomer: an impact-enhancing additive) is performed, emphasising the effect of TPEE on strain-rate-dependent behaviour and plastic deformation together with assessment of energy-dissipation levels. Experimental studies including uniaxial tensile testing performed at different strain rates revealed degradation in the overall tensile properties and a reduced strain-at-break value as result of TPEE addition to PBT. Furthermore, cyclic tests and incremental loading tests showed that the plastic strain at different loading rates increased with introduction of TPEE in the composite. Dynamic mechanical analysis of the studied materials revealed an increase in viscosity, loss-modulus values and damping capability (tan δ) of the TPEE composite over a specified frequency range, while a damage analysis on basis of degradation of the Young’s modulus demonstrated increased damage initiation and accumulation for this composite.
A study of fracture-surface morphology of tensile-tested samples uncovered the effect of microstructure on variations in mechanical properties, observed in the experimentation part of the study. The analysis demonstrated a significant effect of microstructure on mechanical performance of the PBT composites. The increased plastic strain was proved to be a result of higher plasticity observed in the fractographs. Larger ductile-area fractions were measured on PBT-GF10 TPEE fracture surfaces at all strain rates, in addition to increased void nucleation and relatively insufficient fibre-matrix interfacial adhesion in the TPEE composite. This was evident by the stretched gaps around pull-out glass fibres. The increased energy dissipation was linked to higher deformation and plasticity of the fracture surface. Moreover, an increased viscoplastic material flow was observed in the TPEE composite, demonstrated by formation of conic structures around glass fibres, which, in turn, enhanced the damping properties of this composite.