Fracture stable/unstable

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. 

Type III simple ring fractures (stable). Type IV unstable fracture with ring disruption... 

If we then introduce a flaw into the system, by poking a pin into the inflated balloon, the balloon will explode, and all this energy will be released. The membrane fails by fast fracture, even though well below its yield strength. But if we introduce a flaw of the same dimensions into a system with less energy in it, as when we poke our pin into a partially inflated balloon, the flaw is stable and fast fracture does not occur. Finally, if we blow up the punctured balloon progressively, we eventually reach a pressure at which it suddenly bursts. In other words, we have arrived at a critical balloon pressure at which our pin-sized flaw is just unstable, and fast fracture just occurs. Why is this ... 

Various types of crack propagation, namely unstable, partially stable, and fully stable can be observed during SENB tests [85,131]. Solvent-modified epoxy networks prepared via CIPS with 13-16 wt % and 22 wt % cyclohexane show unstable crack propagation. For solvent-concentrations of 18 wt % and 20 wt % cyclohexane, a partially stable crack propagation is observed. The amount of energy consumption upon crack propagation is only 10% and 14% respectively for these two compositions. The change in the fracture behavior indicates that the... 

Before analysing these results, it is interesting to point out that optical and electron microscopies of the sample region ahead of the crack reveal that a craze develops at the crack tip, whatever the type of fracture regime, i.e. stable or unstable [53]. 

One of the most curious aspects of crack growth in most epoxies is the apparently unstable manner by which propagation occurs, even over wide ranges of temperature and test rate. This behavior is commonly referred to as stick-slip , and is characterized by the crack growing in a series of discrete, unstable jumps. Even some of the earliest works on epoxy fracture report this mode of crack growth. The suspected origins of stick-slip fracture behavior in epoxies is discussed in a subsequent section. Unhke epoxies, thermoplastic polymers, such as poly(methyl methacrylate) and polystyrene, are characterized by stable, continuous crack growth. This mode of fracture sometimes can be observed in epoxies, in particular, when they are tested at fast rates and/or low temperatures. 

Fig. 9a and b. Stable and unstable fracture behavior as observed on Instron recorder traces when testing constant compliance fracture specimens, a) trace, stable crack growth b) trace, unstable, stick-slip crack growth... 

The effects of moisture on epoxy fracture are not conclusive. Scott et al. reported that an amine cured epoxy, normally displaying stick-slip fracture at room temperature and low rates, exhibited stable behavior when immersed in distilled water. Also, they found that the rate necessary to promote the unstable to stable crack growth transition at room temperature was increased by two orders of magnitude in the presence of the water. Yamini and Young , on the other hand, found that testing in water tended to suppress stable behavior and promote stick-slip fracture in an amine cured epoxy over a wider range... 

The unstable fracture of epoxies has been shown by Mai and Atkins to be accompanied by a negative change of the strain energy release rate, G, tvith crack velocity, a. This is in contrast to the positive dG/da which they find characterizes stable fracture. Whether a negative dG/da is the cause or the consequence of unstable fracture is, however, subject to debate... 

When the polymer is more ductile, several steps of stable and unstable crack propagation can successively occur during the fracture process of the sample. Figure 27.2 illustrates the observed fracture surface of such fracture behavior. 

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