Fracture crack initiation

Such degradation of the surface causes little effect on either flexural strength or flexural modulus of elasticity but the influence on the impact properties is more profound. In such instances the minute cracks form centres for crack initiation and samples struck on the face of samples opposite to the exposed surface show brittle behaviour. For example, a moulded disc which will withstand an impact of 12 ftlbf without fracture before weathering will still withstand this impact if struck on the exposed side but may resist impacts of only 0.75 ftlbf when struck on the unexposed face. 

Moulded plastics will also have crack initiation sites created by moulding defects such as weld lines, gates, etc and by filler particles such as pigments, stabilisers, etc. And, of course, stress concentrations caused by sharp geometrical discontinuities will be a major source of fatigue cracks. Fig. 2.72 shows a typical fatigue fracture in which the crack has propagated from a surface flaw. 

In neutral solutions the application of cathodic polarisation prevents crack initiation and this could be taken to indicate that hydrogen embrittlement is not the operative mechanism, since the discharge and entry of hydrogen might be expected to fracture the specimen more readily. The beneficial effect of cathodic polarisation has been interpreted , however, to result from more rapid film repair in the alkaline catholyte generated by the cathode reaction. The film serves as a barrier to rapid hydrogen entry. Consistent with this is the observation that in an environment of low pH (e.g. 10 N HCl) where film formation would not be expected, cathodic polarisation has no effect upon crack propagation. 

To complete the analysis of the fracture toughness the values of and Ic calculated from the maximum load and the energy for crack initiation are summarized in Fig. 52. It becomes obvious that the brittle-ductile transition, which... 

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. 

Gc can be measured directly from fracture experiments by detecting crack initiation and measuring the work in initiating the crack. 

As a representative example, three of the failure modes of internally pressurized polyethylene pipes are indicated in Fig. 8 in the plot of circumferential stress <7f versus time to failure ff. (Not shown is the regime of brittle fracture by unstable crack propagation, since it only occurs at much lower temperatures and frequently requires a special crack initiation procedure). 

Figure 16. SEM images of the fracture surface of an RH-cycled (80-120% RH) sample, showing crack initiation and growth from a craze site near the anode and membrane interface. A crazing site found on Hi-RH cycled samples. The half-penny shaped craze closely resembles crazes formed on free surfaces.

LEFM, for fractures occurring in the elastic domain, leads us to characterize the materials using critical characteristics for crack initiation (or arrest in the case of stick-slip propagation). 

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