11624 CF Crack Growth CFCG

The vast majority of CF studies have involved the use of precracked specimens with corresponding results plotted in the form of crack growth rate (da/dN in, e.g., millimeters/cycle), versus stress intensity factor range, AK (MPa ^/m). This type of plot is also known as the Paris curve [30] (see Fig. 11.15).

Figure 11.15 shows that an enhancement in crack growth occurs for fatigue cycling within the environment, particularly in the mid-AK range. At high AK values, CFCG rates approach the air fatigue crack growth rate as mechanical effects dominate. At low AK values, the threshold value for crack growth in an environment may be lower or higher than that for air depending upon effects such as closure (i.e., corrosion product blocking the crack enclave). Alternatively anodic dissolution at the crack tip may create a notch-like feature, thereby increasing the stress concentration and making slip easier or conversely blunting

Stress intensity factor range, aK MPa.vm

FIGURE 11.15 Schematic showing typical fatigue and corrosion fatigue crack growth curves.

Environment

Stress intensity factor range, aK MPa.vm

FIGURE 11.15 Schematic showing typical fatigue and corrosion fatigue crack growth curves.

of the crack tip may occur decreasing the effective crack tip stress intensity and reducing CFCG rate.

Effect of Variables on CFCG Rates of CFCG are affected by changes in both mechanical and electrochemical variables, for example.

frequency It is clear that if the CFCG rate is dependent upon both stress and chemical reactions occurring inside the crack, then crack growth rates will be influenced by the time allowed for chemical reactions to occur. Changes in frequency have two effects. First, there will be a change in crack tip strain rate with changes in frequency, and, second, the time interval during which the crack is fully open will also change. In general, a decrease in frequency causes an increase in crack growth rate.

environmental composition In general, the corrosion fatigue crack growth rate increases as the aggressive nature of the environment increases. However, a general prediction of the effects of environment is difficult and trends may be restricted to specific environmental changes, for example, pH, oxygen concentration, relative humidity, and the like.

temperature As corrosion rates increase with increasing temperature, that is, an approximate doubling of rate for every 10° C rise in temperature, it is not unexpected that crack growth rates will also increase as the environment temperature rises, particularly where mass transport/diffusion processes control crack growth rate. However, it should also be recognized that as temperature increases the solubility of oxygen decreases and where the cathodic reaction is that of oxygen reduction, a decrease in crack growth rate may occur.

loading waveform The type of loading experienced by the component can be one of a number of forms, that is, sinusoidal, square wave, triangular, and positive or negative sawtooth form. Where initial rise times are sufficiently slow, as in sinusoidal, triangular, and positive sawtooth waveforms, reactions at the crack tip can take place during the loading half-cycle leading to an effect on crack growth rate [31]. Conversely, negative sawtooth and square waveforms, which have fast rise times, result in a negligible effect on crack growth rate.

scc contributions During corrosion fatigue cycling, it is possible that other static modes of fracture, that is, SCC, contribute to crack growth. For this to happen it has been argued that the material must exhibit an SCC tendency, and any perturbations to normal corrosion fatigue crack growth will only occur when the stress intensity at the crack tip, during the cycle, rises above the threshold stress intensity for the onset of SCC, that is, KISCC.

As a result of these factors, it is possible that the Paris curve may take on one of three forms [32]: (i) true corrosion fatigue (TCF), (ii) stress corrosion fatigue (SCF). In this case, perturbations occur when the AK value of the cycle is equal to or greater than KISCC, and (iii) true corrosion fatigue on stress corrosion fatigue. This type of behavior will occur when the material exhibits both CF and SCC.

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