PSI - Issue 13

Sze Ki Ng et al. / Procedia Structural Integrity 13 (2018) 304–310 S.K. Ng et al./ Structural Integrity Procedia 00 (2018) 000 – 000

309

6

Critical initiation time, ∗ (s) Critical crack initiation energy ∗ (J/m 2 )

Table 3. Initiation time and crack speed with its corresponding G values.

Critical crack speed, ̇ ∗ (mm/s) 1.13 × 10 −1 1.28 × 10 2

Critical crack propagation energy ∗ (J/m 2 )

Materials

1.34 × 10 2 N/A

1.42 × 10 1 N/A

PMMA 1.03 × 10 1 1.04 × 10 4 As suggested from the amorphous PMMA results, the initial load and thus the initial fracture energy applied to the specimens in methanol are significantly lowered than those performed in air. However, there is not enough evidence to suggest that methanol has the same effect on BOPMMA. Once the crack has initiated, the energy required to propagate the crack in methanol is lower than in air until it becomes flow controlled. Above this critical point, the availability of the environment becomes the limiting factor, so the crack growth in the environment is equivalent to ‘ in-air ’ . This happens at the intersection point of the air and methanol curve in the propagation plots. The factor , defined by (Chan 1982), as the ratio between ℎ and , is found to be 0.25 and 0.40 for PMMA and BOPMMA respectively. The results show that BOPMMA has greater resistance to ESC than amorphous PMMA. This is probably due to the aligned polymer chains of BOPMMA having less free volume and hence a lower flow velocity within the polymer. Both critical crack speed, ̇ ∗ and critical crack propagation energy, ∗ have increased two orders of magnitude from its amorphous to biaxially orientated structure. 3.4. Fractography The SEM images in Fig. 4 were taken in vicinity of the relaxation controlled region (region I) with downwards crack growth to examine the fracture mechanism of ESC. In the case of the amorphous PMMA, the presence of methanol has resulted a rougher fracture surface compared to the ‘in -air ’ specimen. BOPMMA

(a) Th e SE M im ag es in Fi g. 4 we re tak en in vic ini ty of the rel ax ati on co ntr oll ed re gi on (re gi

(b)

(d)

(c)

Fig. 4: SEM image of the relaxation controlled region (region I) of (a) PMMA in air, (b) PMMA in methanol, (c) BOPMMA in air and (d) BOPMMA in methanol. The direction of crack growth is downwards.