Corrosion fatigue stress intensity

In common with many of the alloy-environment systems described so far, if the alloy is not susceptible to stress-corrosion cracking under constant stress or stress intensity, then little or no effect of environment on fatigue crack growth is observed. In these cases, frequency, R ratio and potential within the passive or cathodically protected ranges for titanium have no effect on growth rates. 

Structural failure may occur when the overall structural cross-section cannot support the applied load or, when the critical flaw size ac is exceeded by preexisting discontinuity or by reaching the critical crack size through fatigue, stress corrosion cracking or creep mechanisms. Using fracture mechanics the stress at a crack tip can be calculated by a stress-intensity parameter K as,... 

Cyclic load frequency is the most important factor that influences corrosion fatigue for most material environment and stress intensity conditions. The dominance of frequency is related directly to the time dependence of the mass transport and chemical reaction steps involved for brittle cracking. 

Figure 6.51 Corrosion fatigue crack propagation rate (da/dN) as a function of the cyclic crack tip stress intensity range (AK) (Phull)5...

In addition to their successful applications to fracture stability problems, the stress intensity factors have been widely used as correlation parameters in analyzing the subcritical crack growth rates da/dn (in fatigue) and dafdt (in corrosion), n and t referring to the number of... 

Fig. 7.123 Corrosion-fatigue-crack-growth rate as a function of stress-intensity range for X-65 line pipe steel in air and at the free corrosion potential in 3.5% NaCl at indicated frequencies and R = 0.2. Corrosion potential = -440 30 mV (SHE). (Note Original reference includes data on which these lines are based.) Source Ref 1 69...

The effects of corrosion and fatigue on crack growth rate (CGR) are presented in Fig. 9.12. The increment of growth during each fatigue cycle is defined as the CGR and is given in Eq. (9.13) as a function of stress-intensity amplitude, AK, by the power law relationship ... 

The best tools for weighing the various aspects are quantitative expressions of properties and performance data valid under various conditions, such as corrosion rate and distribution, lifetime in eorrosion fatigue, mechanical or electrochemical threshold values (Kiscc, Kthi Ep etc.), compared with corresponding quantified requirements or service conditions, i.e. specified lifetime, actual stress intensity factors and functions, and corrosion potential. 

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