Stress, types cleavage

Figure 6.1 Types of stresses, (a) Normal (or direct) stress, (b) shear stress, (c) cleavage stress, (d) peel stress.

Aldehyde Formation. Several investigators observed a marked dominance of hexanal in the volatile products of low-temperature oxidation. At the higher temperatures, however, 2,4-decadienal was the major aldehyde formed (19,20,21). Both aldehydes are typical scission products of linoleate hydroperoxides. Swoboda and Lea (20) explained this difference on the basis of a selective further oxidation of the dienal at the higher temperature, while Kimoto and Gaddis (19) speculated that the carbon-carbon bond between the carbonyl group and the double bond (Type B) is the most vulnerable to cleavage under moderate conditions of autoxidation, while scission at the carbon-carbon bond away from the olefinic linkage (Type A) is favored under stress such as heat or alkali. 

Figure 1 Schematic representation of the receptor-mediated (extrinsic) and the intracellular stress-mediated (intrinsic) pathways of caspase activation. Death-receptor signaling may involve direct caspase-8-mediated caspase-3 activation (type 1 cells) ora Bid-cleavage-dependent mitochondrial amplification step (type 2 cells).

Crack-opening mechanisms have been proposed that simply relate to the effect of environment and local alloy composition on the atom-to-atom bond strength at the crack tip. Reduction in this bond strength has been attributed to stress-induced changes in alloy composition as just described and to adsorption of atoms from the environment. Since dislocation movement is not considered in the mechanism, breaking bonds in the plane of the crack propagation leads to a cleavage-type rupture (Ref 159). 

Examples of this type of application abound in the literature. Segregation of impurities in metals and alloys often occurs at grain boundaries. A study of the embrittlement and stress-failure of a tungsten sample showed a nearly uniform distribution of phosphorus across the sample, except for certain grains that appeared to be completely free of it this complete absence of phosphorus can be interpreted as related to cleavage failure at these points. 

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