Crack Initiation and Propagation

The process of fatigue failure is characterized by three distinct steps (1) crack initiation, in which a small crack forms at some point of high stress concentration (2) crack propagation, during which this crack advances incrementally with each stress cycle and (3) final failure, which occurs very rapidly once the advancing crack has reached a critical size. Cracks associated with fatigue failure almost always initiate (or nucleate) on the 

It should be emphasized that although both beachmarks and striations are fatigue fractme surface featmes having similar appearances, they are nevertheless different in both origin and size. There may be thousands of striations within a single beachmark. 

Concept Check 8.4 Surfaces for some steel speeimens that have failed by fatigue have a bright crystalline or grainy appearance. Laymen may explain the failure by saying that the metal crystallized while in service. Offer a criticism for this explanation. 

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The chemical species that can lead to EIC in each alloy system are fairly well known although the exact mechanisms of crack initiation and propagation are not thoroughly understood. The species that promote EIC in one alloy system do not necessarily promote EIC in others. A discussion of EIC and the chemical species that promote it can be found in the Hterature (34). 

Separated Anode/Cathode Realizing, as noted in the preceding, that locahzed corrosion is usually active to the surrounding metal surface, a stress specimen with a limited area exposed to the test solution (the anode) is elec trically connec ted to an unstressed specimen (the cathode). A potentiostat, used as a zero-resistance ammeter, is placed between the specimens for monitoring the galvanic current. It is possible to approximately correlate the galvanic current 7g and potential to crack initiation and propagation, and, eventually, catastrophic fail-... 

Highly ductile materials tend to be more resistant to thermal fatigue and also seem more resistant to crack initiation and propagation. 

Duquette, D. J., Mechanisms of crack initiation and propagation in corrosion fatigue , Proc. Int. Conf. on Mechanisms of Environmental Cracking in Materials, University of Surrey, 4-7 April 1977, The Metals Society, pp. 305-21 (1977)... 

Saintier, N., Cailletaud, G., and Piques, R., Crack initiation and propagation under multiaxial fatigue in a natural rubber, Int. J. Fatigue, 28, 61, 2006. 

While a new tire may have excellent resistance to crack initiation and propagation between the steel belts, an aged tire of the exact same construction can exhibit dramatically reduced crack growth resistance, which in some cases may contribute to tire failure. To further underscore the point, the United States National Highway Traffic Safety Administration (NHTSA) determined that tire aging was a main contributor to the Firestone Wilderness AT tire recall [1]. Figure 34.1 shows the failure rate of Wilderness AT tires as a function of tire age (determined from date of tire manufacture). 

As mentioned earlier, fatigue is the result of crack initiation and propagation. Crack growth under fatigue is estimated by the Paris equation ... 

Network defects in the form of unreacted groups serve as sites for crack initiation and propagation. When such defects are non-randomly distributed within the network a nodular morphology will be observed upon fracture or chemical etching of the bulk network. 

The relationship between craze initiation and propagation, and crack initiation and propagation, is not well understood (31). Crazing clearly precedes formation of unstable cracking. For example, in tensile tests on Lustran A-21 S/AN, crazing occurred at 1.8-2.0% elongation, whereas... 

An improvement in the toughness of thermosets can be favored by rubber or thermoplastic particles, which operate both in crack initiation and propagation mechanisms. The different toughening mechanisms can act simultaneously and can be modeled quantitatively. 

In terms of tonnage, polyolefins are by far the most important polymeric materials for structural applications, and there is consequently enormous interest in optimising their fracture properties. A rational approach to this requires detailed understanding of the relationships between macroscopic fracture and molecular parameters such as the molar mass, M, and external variables such as temperature, T, and test speed, v. Considerable effort is therefore also devoted to characterising the irreversible processes (crazing and shear deformation) that accompany crack initiation and propagation in these polymers, some examples of which will given. 

The fracture behaviour of polymers, usually under conditions of mode I opening, considered the severest test of a material s resistance to crack initiation and propagation, is widely characterised using linear elastic fracture mechanics (LEFM) parameters, such as the plane strain critical stress intensity factor, Kic, or the critical strain energy release rate, Gic, for crack initiation (determined using standard geometries such as those in Fig. 1). LEFM... 

X.S. Wang et al Low-cycle fatigue small crack initiation and propagation behaviour of cast magnesium alloys based on in-situ SEM observations. Phil. Mag. 86, 1581-1596 (2006)... 

Experiment shows that when the hydrogen concentration is enough to cause embrittlement, the permeation current builds up with time and then instead of stabilizing to a steady state as it normally does in the absence of a crack-initiation and propagation process, it drops down and only then becomes steady (Fig. 12.83). Thus, the fall in permeation current occurs at the onset of crack propagation and embrittlement. 

A bolt fails when it can no longer provide the support it is designed for. Fracture is the separation of a solid body into two or more parts under the action of stress. It consists of crack initiation and propagation. 

Fig. 7 Development of fatigue cracks in an epoxy/glass contact under gross slip condition (1Hz, displacement amplitude 60 xm) (from [97]). White arrows indicate the occurrence of crack initiation and propagation at the edge of the contact under the action of tensile stresses. The lateral contact stiffness, K, is essentially a measurement of the elastic response of the epoxy substrate within the contact zone. Brittle crack propagation is associated to a drop in stiffness due to the additional accommodation of the imposed displacement provided by crack opening mechanisms...

Figure 27.19 shows the variation of U/A as a function of A, according to the proposed model of crack initiation and propagation energies. From the intercept of this plot, a fracture energy value of 4.7 kJ/m2 can be obtained for ABS. This value is much lower than that determined above, using the method developed for brittle fracture or the constant fracture energy consideration. 

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