Dissipation mechanism, brittle

Samples are mechanically brittle at sufficiently low temperatures, normally below the T . They also are seen to be brittle at very short times. Thus, a short sharp impact can shatter a material at room temperature if it is naturally below its Tg or if the material has been frozen. Materials that are rubbery at room temperature can be sufficiently brittle at low temperatures to fail catastrophically. In the space shuttle Challenger, an explosion was caused by uncombusted fuel escaping when a rubber O-ring was rendered brittle by low-temperature weather. The key in all of these examples is the speed of molecular motion. For energy to be dissipated, a stress can cause a local increase in molecular motion. If that route is denied by time constraints (e.g., a fast shock) or temperature control (molecular immobility), then a crack is the only way energy can leak out. Cold toffee... 

This conclusion was only partly confirmed by scanning electron microscopy micrographs of RuC>4 stained surfaces taken at the crack tip of deformed specimens at 1ms-1, where the non-nucleated and /3-nucleated materials showed, respectively, a semi-brittle and semi-ductile fracture behavior. While some limited rubber cavitation was visible for both resins, crazes—and consequently matrix shearing—could not develop to a large extent whether in the PP or in the /1-PP matrix (although these structures were somewhat more pronounced in the latter case). Therefore, a question remains open was the rubber cavitation sufficient to boost the development of dissipative mechanisms in these resins ... 

The force requirement for separation, in a mode in which rigid facing surfaces remain exactly parallel, would be enormous (25). Brittle separation at an interface, in the absence of a dissipation mechanism resembling the mechanism that controls bulk fracture, would occur at a much lower level of force. So it will be necessary to postulate a dissipation mechanism, such as an interfacial craze, for those cases in which a polymer that has a high Tg does not separate easily from a hard solid. 

If the material is not perfectly brittle, i.e., there are energy dissipating mechanisms in addition to the creation of new surfaces, then we introduce a term G, which is an energy (its units are J/m ) representing crack extension by all the available processes. You will find Gc referred to as total work of fracture, crack extension force, and strain energy release rate. 

The toughening observed in Fig. 11 with increasing loading rate is quite surprising from a standard fracture mechanics point of view on the fracture of viscoplastic materials increasing the loading rate results in less energy dissipated by plasticity and a more brittle response is expected (if the failure process is assumed to be rate independent). One could also invoke a failure... 

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