Brittle fracture and fatigue crack propagation in high impact polystyrene

The brittle fracture behaviour of a High Impact Polystyrene (HIPS) has been studied as a function of thickness and it is shown that the modified energy-based bimodal fracture analysis gives a good description of the experimental results. The brittle fracture toughness (Kc) consists of two contributions; one due to the plane stress toughness (KC2) at the surface layers and the other due to the plane strain toughness (Kcl) due to the inside constrained material. KC decreases with thickness and tends to an asymptotic value of l.45 MNm'a/z, which is the plane strain fracture toughness (Kcl) of the material. Study has been made on the fracture behaviour of epoxy resin coated HIPS using single edge-notched specimens. It has been observed that the composite specimens fail in a brittTe fashion. Without the epoxy resin coatings the HIPS specimens fracture with substantial ductile slow crack growth and a large plastic zone at the crack tip. It is proposed that the additional constraint due to the brittle epoxy resin layers promotes brittle fracture in the HIPS. Using a sandwich cracking mode] the brittle fracture toughness of HIPS is estimated to be l.75 MN m'a/z. This vaIue is higher than the Kclvalue of l.45 MN m'a/2 obtained previously indicating that full thickness constraint has not been achieved in the sandwich experiments. Fatigue tests in air have shown that there is no significant effect of initial notch acuity and mean stress on the fatigue crack propagation rate %%3 The effect of frequency is pronounced in that %%-increases with decrease in frequency level. This has been discussed in terms of the visco-eIastic response of the material. By use of the Dugdale undel, the craze stress vaIues can be obtained and it is shown that these results agree with the values determined by independent tensile tests. The modified Williams model can also be used to predict the craze stresses. Two kinds of fatigue crack propagation nechanisms have been noted: in the low AK region crack propagates by a repeated process of slow stable and continuous growth followed by unstable jump and in the high AK region crack propagates by means of slow stable and continuous growth. The effect of four liquid environments, nanely, methanol, ethanol, propanol and butanol, on the fatigue behaviour of HIPS has also been investigated. The presence of these liquids in HIPS has been found to cause fatigue crack to propagate at a much higher rate than air. The reduction in fatigue resistance has been explained with regards to the plasticization effect on the polystyrene matrix and the weakening of the rubber particle interface which results in the detachment of rubber particle from the matrix. This fact is substantiated by the electron micrographs of fracture surfaces. The modified Williams model has also been used to analyse the fatigue crack propagation data in these liquids with reasonable success