Brittle cracking phenomena

It is possible to confuse SCC with other brittle cracking phenomena. Confirmation of SCC typically requires a metallographic examination. On thin-walled components, the surface from which the cracking originates may not be apparent. In these cases, a formal metallographic examination may be required to assure positive identification of the surface from which the cracks originate. 

This rather useful empirical expression is applicable to many electrodeposited materials [e.g., molybdenum, zinc, steel (10)]. The expression has been able, for instance, to provide an acceptable explanation for the phenomenon of the brittle cracking in chromium electrodeposits. It has been quite helpful in the general study and understanding of the functional connection between hardness and grain size values in many electrodeposits. 

The environment also plays a role in some environments brittle crack failure is strongly promoted. For example, detergents such as synthetic soaps can decrease the time to brittle failure of PE by a factor between 10 and 50 (see Figure 7.21). This phenomenon is known as stress corrosion or environmental stress cracking (ESC) (see further 8.5). 

The term cracking at simultaneous action of stress and environment was introduced for the description of polymers (mainly polyethylenes) brittle fracture, which are present in a stressed state in the presence of mobile polar liquids. It was shown [1], that, what all is said and done, for material strength at this fracture mode is responsible the weakest amorphous part of semi-ciystalline polymer. This allows to connect occurring at cracking phenomenon with polar liquid diffusion into amorphous regions. 

In individual cases even a sudden brittle failure was observed in HDPE geomembranes manufactured from true high density PE-resins, when they were exposed to a stress over a large area (EPA 1992). This phenomenon, however, seems to occur solely in extremely eold weather. The brittle cracks extend and branch out and propagate rapidly over the whole surface. This kind of cracking is dealt with in the technical literature under the term rapid crack propagation (RCP) as an independent meehanism of stress cracking. 

The most vulnerable point in the classical theory for this phenomenon is the quantitative discrepancy between actual values of brittle fracture stress (500-2000 MPa for steel) and the theoretical cleavage stresses of crystals. This contradiction can only be resolved by assuming that the metal always contains stress concentrators, which is not usually borne out in reality. Hence, at present, the prevailing opinion is that brittle crack initiation is induced by plastic deformation. 

Some manufacturers have experienced die above mentioned Ni3S2 scale formation phenomenon under certain gas conditions, which led to die failure of a rotating blade. One such experience involved a fracture dial was distinctly intergranular with evidence of secondary intergranular cracks or grain separation across die fracture. Intergranular facets of die fracture were sharp and distinct with little evidence of any ductile mode. The fracture appeared to have occurred in a brittle intergranular mode. 

Since the surface strains are largely compressive in nature, the tendency to the development of cracking in the surface should be small. However, some surface cracking or shattering does occur in materials which are brittle in bulk, particularly if they cleave easily. The final surface may then be composed, in part or in whole, of systems of cleavage facets (15, 33). This phenomenon has been observed in a number of ionic crystals, such as rock salt and fluorspar, but not so far in metals. 

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