Fracture crack propagation

In linear-elastic fracture, crack propagation (after Lcr has been reached) proceeds roughly at the speed of sound in the material (of order 1 km s-1). This gives rise to a snapping sound, typical for fracture of brittle materials. 

Metals that fracture with a relatively small or negligible amount of plastic strain exhibit brittle fracture. Cracks propagate rapidly. Brittle failure results from cleavage (splitting along definite planes). Ductile fracture is better than brittle fracture, because ductile fracture occurs over a period of time, where as brittle fracture is fast, and can occur (with flaws) at lower stress levels than a ductile fracture. Figure 1 shows the basic types of fracture. 

A special pneumatic multiposition low-temperature impact testing machine was developed to allow investigation of impact strength with samples inside the coolant bath, containing liquid helium. The impact test machine is shown in Fig. 6. It enables investigation of the temperature dependence of fracture, crack propagation, and maximum and proportionality limit loading. 

Analysis of the data of the fractured specimens reveals that a fracture crack propagated from the points where additional stress concentrations were present. This confirms the assumption that the fracture of a loaded ceramic specimen starts from a small crack ahead of a machined notch root. It is beheved that is influenced more by the sharpness of the notch root, rather than by its shape. The data in Table 2.1 are from three to four-point flexure tests performed on several monolithic ceramics. Table 2.2 shows the Kic values at RT and high temperatures attained by the SEVNB method for these notched specimens. 

Practure toughness is another way to characterize the strength of a material. It measures how well a material resists crack propagation and is expressed as the stress needed to enlarge a crack of a specific size. The room temperature fracture toughness of clear, vitreous sihca is approximately 0.75 - 0.80 MPa-mT2 (87,163). 

The parameter is a crack propagation velocity and n(e) is a crack activation law driven by the bulk tensile strain e and specified by the Weibull fracture theory... 

Fig. 23.9. Brittle fracture the largest crack propagates when the fast-fracture criterion is satisfied.

It has been demonstrated that with SBR polystyrene blends the rubber should exist in discrete droplets, less than 50 p.m in diameter where a good finish is required, within the polystyrene matrix. It is believed that in such a form the rubber can reduce crack propagation and hence fracture in various ways. The most favoured current explanations of this were discussed in Chapter 3. Suffice it to say here that the following features appear necessary for a suitable blend ... 

In metals, inelastic deformation occurs at the crack tip, yielding a plastic zone. Smith [34] has argued that the elastic stress intensity factor is adequate to describe the crack tip field condition if the inelastic zone is limited in size compared with the near crack tip field, which is then assumed to dominate the crack tip inelastic response. He suggested that the inelastic zone be 1/5 of the size of the near crack tip elastic field (a/10). This restriction is in accordance with the generally accepted limitation on the maximum size of the plastic zone allowed in a valid fracture toughness test [35,36]. For the case of crack propagation, the minimum crack size for which continuum considerations hold should be at least 50 x (r ,J. 

Not all fracture is by crack propagation. Highly ductile materials stressed at high temperature will eventually break by the growth, through absorption of lattice vacancies, of plastic voids. This shades into the phenomenon of superplasticity, which was examined in Section 4.2.5. 

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