Unstable crack growth fracture surface

However, when stable-unstable fracture occurs (between Transitions 0 and I), evidence of secondary crack nucleation has been obtained by SEM on postmortem fracture surfaces. These cracks lead to parabolic markings ahead of the main blunt crack propagating in the plastic zone. Figure 8a shows an example of such secondary cracks developed at the boundary between the stable growth region and the unstable fracture surface in a 2 L45 specimen tested at 0.1 m/s. 

Figure 9.21a shows rapid force oscillations during crack growth in a 5 mm thick Charpy specimen impacted at lms , which contained a sharp pre-crack. The HDPE had a density of 955 kg m . A number of crack arrest locations are visible on the fracture surface (Fig. 9.21b), so the crack appears to advance in an unstable manner. However, as the crack advances fastest at the specimen mid-thickness, it is not possible to monitor its position by photography, which would only record the surface crack growth through the shear lips (S). 

When the work (W) required to create new surfaces is higher than U—which represents the elastic energy lost in the growth of cracks—dUlda is higher than 0 and there is no possibility of the crack to propagate on the contrary, when dUIda < 0, the crack is unstable and the sample can experience a catastrophic fracture (see Figure 12.18). 

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