Crack initiation external surfaces

Microstructural examinations revealed intergranular, sparsely branched cracks originating on the external surface. Some cracks initiated as transgranular fissures. 

Typical destructions of standard eye specimens after fatigue tests are shown on fig. 3. During the process of cyclic tests of the eye specimens the fatigue cracks initiated on the surface of holes of 20 mm in diameter (stress raisers) and propagated outward to the external part of the specimens. 

Figure 5. Fracture and crack initiation places on the traverse external surface, x 10.

Figure 33 shows characteristic curves for the conventional drying of a wet body. Moisture is removed initially from the external surface of the body which produces the required moisture gradient for outward moisture flow. However, as a wet body looses moisture it shrinks and since moisture is lost non-uniformly throughout the body, shrinkage is non-uniform. If the drying rate is excessive, surface cracking can occur as the dry surfaxie is placed in tension by the still moist interior. 

Qualitative examples abound. Perfect crystals of sodium carbonate, sulfate, or phosphate may be kept for years without efflorescing, although if scratched, they begin to do so immediately. Too strongly heated or burned lime or plaster of Paris takes up the first traces of water only with difficulty. Reactions of this type tend to be autocat-alytic. The initial rate is slow, due to the absence of the necessary linear interface, but the rate accelerates as more and more product is formed. See Refs. 147-153 for other examples. Ruckenstein [154] has discussed a kinetic model based on nucleation theory. There is certainly evidence that patches of product may be present, as in the oxidation of Mo(lOO) surfaces [155], and that surface defects are important [156]. There may be catalysis thus reaction VII-27 is catalyzed by water vapor [157]. A topotactic reaction is one where the product or products retain the external crystalline shape of the reactant crystal [158]. More often, however, there is a complicated morphology with pitting, cracking, and pore formation, as with calcium carbonate [159]. 

ESC is mostly a surface-initiated failure of multiaxially stressed polymers in contact with surface-active substances. These surface-active substances do not cause chemical degradation of the polymer, but rather accelerate the process of macroscopic brittle-crack failure. Crazing and cracking may occur when a polymer under multiaxial stresses is in contact with a medium. A combination of external and/or internal stresses in a component may be involved. 

Figure 8.26 Superposition for the equivalence of K for general loading and crack surface tractions a) unknown crack configuration b) uncracked configuration and c) initial configuration with external loading removed. (Adapted from Rooke et ai, 1981, reproduced courtesy of Elsevier Science Ltd, Kidlington, UK.)...

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