Brittle-like failure

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. 

In certain circumstances, a corrosive environment may induce brittle-like failure in materials submitted to mechanical loads too low to cause any significant damage in the absence of the environment. This includes at least five different phenomena, usually referred as environmentally assisted cracking (EAC) or environment induced cracking ... 

The Safety Board asked several gas system operators about their direct experience with brittle-Uke cracks. Four major gas system operators reported that they had compiled failure statistics sufficient to estimate the extent of brittle-like failures. Three of those four said that brittle-like failures are the second most frequent failure mode in their plastic pipeline systems. One of these operators supplied data showing that it experienced at least 77 brittle-like failures in plastic piping in 1996 alone. 

In 1982 and 1986, DuPont fonnally notified its customers about brittle-like cracking concerns with the company s pre-1973 pipe. Safety Board investigators could find no record of either Century/Amdevco, Union Carbide, or any other piping or resin manufacturer formally notifying the gas industry of the susceptibility to premature brittle-like failures of their products. Nor does any mechanism exist to ensure that the OPS receives safety-related information from manufacturers. 

Regarding Federal actions on this issue, the OPS has not informed the Safety Board of any substantive action it has taken to advise gas system operators of the susceptibility to premature brittle-like failures of any older polyethylene piping [65],... 

Much of the plastic pipe manufactured and used for gas service from the 1960s through the early 1980s may be susceptible to premature brittle-like failures when subjected to stress intensification, and these failures represent a potential public safety hazard. 

Advise your members to notify pipeline system operators if any of their piping products, or materials used in the manufacture of piping products, currently in service for natural gas or other hazardous materials indicate poor resistance to brittle-like failure. (P-98-7)... 

Earthquake nests provide perhaps the best setting for understanding the physical mechanism responsible for intermediate-depth earthquakes (waveform data related to a well-defined seismogenic zone are practically available continuously). Mechanisms involving dehydration embrittlement or thermal shear runaway are mostly invoked to explain brittle or brittle-like failure at intermediate depths. 

At temperatures below the Tg of the amorphous phase, the crystallites and associated tie molecules can severely reduce the mobility of the polymer chains and thus tend to embrittle the material. This generally leads to a brittle-like failure (Fig. 13, curve A) though at slow enough rates yielding and drawing may be observed (Fig. 13, curve B). 

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. 

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 possible fatigue failure mechanisms of SWCNT in the composite were also reported (Ren et al., 2004). Possible failure modes mainly include three stages, that is, splitting of SWCNT bundles, kink formation, and subsequent failure in SWCNTs, and the fracture of SWCNT bundles. As shown in Fig. 9.12, for zigzag SWCNT, failure of defect-free tube and tubes with Stone-Wales defect of either A or B mode all resulted in brittle-like, flat fracture surface. A kinetic model for time-dependent fracture of CNTs is also reported (Satapathy et al., 2005). These simulation results are almost consistent with the observed fracture surfaces, which can be reproduced reasonably well, suggesting the possible mechanism should exist in CNT-polymer composites. 

Typical stress-strain curves, recorded in 3-point flexure, for the uncoated and BN/SiC coated fiber-reinforced composites [33-34] are shown in Fig. 12. Also shown for comparison is the stress-strain curve for a hotpressed BSAS monolith. The monolith shows bend strength of 130 MPa, modulus of 98 GPa and fails in brittle mode, as expected. The uncoated fiber-reinforced CMC shows a monolithic-like failure with strength of -200 MPa and failure strain of 0.1 %. This shows that reinforcement of BSAS matrix with uncoated Hi-Nicalon fibers does not yield a strong or tough material. This may indicate a strong bond between the uncoated fibers and the matrix. However, fiber push-in and push-out tests [33, 35]... 

Figure 1.1a is a typical stress-strain curve for a brittle ceramic having only elastic deformation up to the point of fracture. As indicated in the introduction, ceramics fail in a typically brittle manner, due to the ionic nature of the bonds, which prevent slip via dislocation motion. The fact that brittle catastrophic failure in ceramics is likely is an indication that very little energy is absorbed in the process of fracmre. Pure aluminum oxide behaves as indicated in Fig. 1.1b. The fracture strain of a ceramic is 0.0008-0.001. One can state that ceramics at room temperature are Hookean until fracture. In general, ceramic materials experience very little or no plastic deformation prior to fracture. Slip is difficult due to the structure and the strong local electrostatic potentials (a consequence of the ionic or covalent bonds).

As noted earlier, a frequent failure mechanism with polyethylene piping involves crack initiation and slow crack growth. These brittle-like fractures occur at points of stress intensification generated by external loading acting in concert with internal pressure and residual stresses [59],... 

Fatigue failure is brittle-like in nature even in normally ductile metals in that there is very little, if any, gross plastic deformation associated with failure. The process occurs by the initiation and propagation of cracks, and typically the fracture surface is perpendicular to the direction of an applied tensile stress. 

As ean be seen from the above equation, for brittle materials like glass and eeramies, we ean seale the strength for a proposed design from a test speeimen analysis. In a more useful form for the 2-parameter Weibull distribution, the probability of failure is a funetion of the applied stress, L. 

Whilst the aliphatic nylons are generally classified as being impact resistant, they are affected by stress concentrators like sharp comers which may lead to brittle failures. Incorporation of mbbers which are not soluble in the nylons and hence form dispersions of rubber droplets in the polyamide matrix but which nevertheless can have some interaction between mbber and polyamide can be most effective. Materials described in the literature include the ethylene-propylene rubbers, ionomers (q.v.), polyurethanes, acrylates and methacrylates, ABS polymers and polyamides from dimer acid. 

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