Crack boundaries fracture surface

When a crack propagates in polystyrene at low crack velocities, the craze ruptures close to its median plane by a mechanism having the approximate characteristics of viscous flow. Each fracture surface is then covered by a thin layer of craze. At higher crack velocities, however, failure occurs along the boundaries between the craze and the adjacent bulk polymer by practically brittle fracture (I). The change in fracture... 

Fig, 37a—d. Structure of shear fracture surface a SEM-micrograph of a shear fracture in PP 1120 (T = —80 °C) b secondary crack formation (white arrow) in one of the shear bands of type B in fine spherulitic PB-l (T = -196 °C) c traces of shear bands B containing fibrillated polymer substance on a shear fracture plane in fine spherulitic PP 1120 (T = —196 °C) d preferred shear fracture along spherulite boundaries (SB) of coarse spherulitic PP 1120 (T = —80 °C)... 

Further fracture surface features, especially distinct for the highly isotactic, coarse spherulitic PP, are polyhedron shaped regions at which the shear crack has left the shear fracture plane in order to stay in a spherulitic boundary region oriented at a flat angle to the sliding direction (Fig. 37d). These sites often constitute secondary crack nuclei in boundaries perpendicular to the shear fracture plane. 

One study in our laboratories investigated the direct link between PPy film morphology and mechanical properties.126 Observation of the fracture surfaces of the films showed a roughened surface with cone-shaped features (Figure 3.9), similar to those observed in transmission electron microscope (TEM) micrographs of the film cross sections (described earlier). It was concluded from this study that the cone boundaries are points of weakness within the film that allow easier crack propagation. Thus, films with less prominent boundaries should show improved fracture resistance. 

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. 

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