Ductile-like failure

TEM images of fatigue failure surfaces (a) very tips of pulled-out SWNTs on a fracture surface. The pulled out SWCNT bundles showed a dendrite-like morphology, believed resulted from splitting of SWCNT rope during fatigue, (b) kink-induced failure of a SWNT bundle, (c) brittle-like failure and (d) ductile-like failure of SWNT bundle. 

By analogy with the mechanical fracture (discussed in the next chapter), the case (i) corresponds to a brittle failure and the case (ii) to a ductile failure. Although there is no definite answer, it is believed that in the case of percolation disorder, the electrical failures are mostly brittle-like failures the voltage (or the current) for the first failure is often the voltage (or the current) for the failure of the whole sample, especially for disorder concentrations near the percolation threshold. We shall see later a different type of disorder which can give ductile failure. 

SWNT fractures are observed at the very tips of pulled-out SWNTs (Fig. 13.13(a)), and within SWNT bundles away from the tips. Some of die SWNT failures are characterized with rather flat fracture surfaces, a characteristic of brittle-like failure (Fig. 13.13(c)). It should be mentioned that die fractured bundle shown in Fig. 13.13(c) may contain many SWNTs. In Fig. 13.13(d), tearing fractures are seen, where SWNT ropes are tom apart, leaving a ductile-like fracture surface. For some SWNT ropes, stepwise, abmpt changes in diameter are seen, indicating that SWNTs within a rope may not be broken at the same time or at the same location. 

The axial tensile strength of many woods is around 100 MPa - about the same as that of strong polymers like the epoxies. The ductility is low - typically 1% strain to failure. 

Environments. Among the environmental factors that can shorten life under thermal fatigue conditions are surface decarburization, oxidation, and carburization. The last can be detrimental because it is likely to reduce both hot strength and ductility at the same time. The usual failure mechanism of heat-resistant alloy fixtures in carburizing furnaces is by thermal fatigue damage, evidenced by a prominent network of deep cracks. 

Laminated composite plates under in-plane tensile loading exhibit deformation response that is both like a ductile metal plate under tension and iike a metai plate that buckles. That is, a composite plate exhibits progressive faiiure on a layer-by-layer basis as in Figure 4-34. Of course, a composite plate in compression buckles in a manner similar to that of a metal plate except that the various failures in the compressive loading version of Figure 4-34 could be lamina failures or the various plate buckling events (more than one buckling load occurs). 

This situation indeed occurs frequently, for instance with PVC pipes, where high internal pressures give rise to explosive brittle fractures, while with lower pressures, after a shorter or a longer time, balloon-like fractures occur. At even lower pressures brittle hair cracks are formed, also with PE, causing the pipes to leak. This happens after very long times of use (e.g. some years), but considerably earlier than predicted from extrapolation of the curve for ductile failure. 

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