FIGURE 9.24 Fatigue crack growth rates for (a) anodized and sealed specimens and (b) anodized, sealed and then corroded 2024-T3 alloy specimens.

The onset of the peaks labeled as T2, T3, and T4 occur at 200, 410, and 500°C, respectively. These represent critical temperatures below which no hydrogen evolution from the respective states is observed, even if the specimen is exposed to a constant, lower temperature for an extensive time period. According to Pres-souyre [49], the existence of a critical temperature classifies states T2, T3, and T4 as irreversible. Trapping state T1 is found to release hydrogen continuously at lower temperatures. Thus, T1 is considered a reversible trap. This trap corresponds to the low binding-energy state reported by Haidemenopoulos et al. [60], who observed hydrogen evolution with mild heating of corroded Al 2024-T3.

TABLE 9.42 Effect of R Values and Existing Corrosion on Paris Equation Constants for Alloy 2024

Stress Ratio R

"Ref 2" 2024

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