Effect of glass fiber content on flexural properties of polybutylene terephthalate composites: Quasi-static, cyclic, and micromechanical analysis

The effect of short glass fiber (GF) content on the flexural response of injection molded polybutylene terephthalate (PBT) composites is investigated. Quasi-static three-point bending tests were performed on the PBT/GF specimens with 0, 5, 10, 15, 20, 25 and 30 wt% GF to determine the variation of the flexural modulus, bending stress at break, and deformation capacity. Increasing the GF content showed a significant stiffening and strengthening effect, i.e. a pronounced increase in the flexural modulus of the PBT-based composites in the range from 2280 MPa for pure PBT to 8570 MPa for PBT/30 wt% GF with a concomitant decrease of the bending strain at break from 3.2% to 1.7%, respectively, which describes a progressive change towards more brittle behaviour. Cyclic flexural tests showed a significant compromise between an increase in load-bearing capacity and a decrease of the endurance of PBT: pure PBT achieved 7 cycles before rupture with a bending stress at break of 65 MPa, while PBT/30 wt% GF only 2 cycles with a bending stress at break of 150 MPa. The evolution of end of cycle stress exhibited a peak before failure indicating stress relaxation in the composite structure. Fracture-surface SEM observations supported these trends with pervasive matrix cracking, fiber–matrix interfacial debonding and pull-out occurring in the tension-dominated areas and local fracture of fibers occurring in the compression-dominated areas. Overall, the results are able to relate the GF content to the coupled variation in flexural performance, cyclic response and fracture mechanism in PBT/GF composites.