Catastrophic fracture

The performance of a tool material in a given appHcation is dictated by its response to conditions at the tool tip. High temperatures and stresses can cause blunting from the plastic deformation of the tool tip, whereas high stresses alone may lead to catastrophic fracture. In addition to plastic deformation and fracture, the service life of cutting tools is deterrnined by a number of wear processes, some of which are shown in Figure 2. 

Of the various tool failure mechanisms, fracture is least desirable because it is unpredictable. Most tool material development work is focused on minimizing flank wear and retarding unwanted tool failure modes such as catastrophic fracture, gross plastic deformation, BUE, crater wear, and DOCN. 

Pc> Pb> Pf cumulative number fraction of grid squares that exhibit craze formation, craze fibril breakdown, and catastrophic fracture, respectively probability that a given entangled strand survives craze fibril formation disentanglement time of i strands in a fibril that survive fibril formation craze interface velocity volume fraction of polymer within craze... 

Fig. 30a-c. Optical micrographs of PS film squares during a "fragility test illustrating a craze initiation, b local fibril breakdown and c catastrophic fracture. Tensile stress lies along the horizontal direction (From Ref. courtesy Macromolecules (ACS))... 

Fig. 34a, b. Median strains for crazing 6, (A), fibril breakdown s, ( ), and catastrophic fracture % ( ) versus molecular weight for the unfiltered specimens a strain rate = 5 x 10 s b 3x 10 s". Half-filled squares correspond to catastrophic fracture at p, = 0.25 (From Ref. courtesy Macromolecules (ACS))... 

Fracture Stress and Strain. Yielding and plastic deformation in the schematic representation of tensile deformation were associated with microfibrillation at the interface and stretching of the microfibrils. Because this representation was assumed to apply to both the core-shell and interconnected-interface models of compatibilization, the constrained-yielding approach was used without specific reference to the microstructure of the interface. In extending the discussion to fracture, however, it is useful to consider the interfacial-deformation mechanisms. Tensile deformation culminated in catastrophic fracture when the microfibrillated interface failed. This was inferred from the quasi-brittle fracture behavior of the uncompatibilized blend with VPS of 0.5, which indicated that the reduced load-bearing cross section after interfacial debonding could not support plastic deformation. Accordingly, the ultimate properties of the compatibilized blend depended on interfacial char-... 

Fig.3 shows the bending stress/displacement curves of Cj/SiC-BN composites. Typical non-catastrophic fracture behaviors were observed both before and after oxidizing at 1000°C for 20h but there are some small decreases in both bending stress and elastic modulus after oxidation. 

NDT T -h 68°C—catastrophic fracture does not occur before the ultimate tensile strength is reached. 

The threshold for slow crack growth in a HDPE occurs at Ki = 0.2 MPa Above this threshold, log(du/dr) under static load increases linearfy with logJiTi until catastrophic fracture occurs at Kic 1.6 MPa m . Specific values of do/dt are 1 nm s" at 0.4 MPa m - and 40 nm s at 1.0 MPa m -. Assuming that HDPE contains defects equivalent to 100 (im cracks, and that Y = 1.15... 

The early clinical experience with COC in Europe was associated with high catastrophic fracture rates. The historical fracture rates ranged between 0% and 13% in some studies (Willmann 2003). However, the in vivo fracture rates within the overall patient population for contemporary alumina hip components have been reported by CeramTec to range between 1 and 3% in 10,000 (Willmann 2003). The risk of cement fracture is thus anticipated to be much lower relative to other clinical risks, which have been associated with all joint replacements, including loosening and infection. Other clinical complications of THA typically occur at a rate of 1% or higher. 

Probe of crack growth on an extremely wide range of time scales. This need not be catastrophic fracture and might involve crazing, microcracking, linking of microcracks, and other pre-failure events. 

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