Stress-corrosion cracking mechanisms crack velocity

The chapter builds on our critical reviews on Mg corrosion [1-4] and Mg SCC [5,6]. SCC [5-8] involves (1) a stress, (2) a susceptible alloy and (3) an environment. SCC is related to hydrogen embrittlement (HE). HE is SCC that is caused by hydrogen (H), which can be gaseous, can come from corrosion, or can be internal from prior processing. HE is often postulated as the SCC mechanism. SCC can be extremely dangerous. Under safe loading conditions, SCC causes slow crack growth. Fast fracture occurs when the crack reaches a critical size. SCC, for any alloy + environment combination, can be characterised by [7,8] the threshold stress, ctscc> threshold stress intensity factor, iscc> the stress corrosion crack velocity. 

To this point, it has been assumed that failure occurs when K =T (or G=R) but, in studies of fracture, it is sometimes found that crack growth can occur at lower values of or G. Thus, kinetic effects must be included in any general formalism. There are several mechanisms that can give rise to sub-critical crack growth, but most attention has been directed to stress corrosion. This behavior has been extensively studied in silicate glasses but it can also occur in many polycrystalline ceramics. Figure 8.72 shows a typical response of ceramics to stress corrosion, with crack velocity v plotted as a function of K (or G). At low values of K, there often appears to be a threshold value of the stress intensity factor below which... 

Stress corrosion by water at the crack tip is the likely mechanism responsible for crack propagation in ceramics. In this respect, it is interesting to develop briefly the model proposed by Lawn [16] for single crystals or glasses. This model is based on a thermally activated reaction of water and ceramic to form complex. For low crack velocity (stage I), V is given by ... 

The experimental results clearly show that the stress corrosion is the key mechanism for crack propagation in alumina ceramics and that a unique crack velocity-crack tip stress intensity factor law can be obtained by taking into account the crack resistance curve in coarse-grain ceramics. 

The fracture behaviour of glasses is also influenced by humidity. An increase of the relative humidity results in an increase of fracture velocity. Wiederhorn [562] provided an explanation of the mechanism of the impact of moisture on the fracture behaviour. Michalske and Freiman [356] discussed the molecular mechanism of stress corrosion in vitreous silica, which depends on the environmental conditions. The increase in the fracture velocity in glasses with increase in environmental moisture content is due to the chemical reaction between glass with water at the crack tip. The transport rate of water to the crack tip influences the fracture behaviour. These effects are of particular importance for the mechanical shaping of glasses in the presence of aqueous lubricants. 

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