**3. Experimental Support for the Above Concepts**

Figure 8 shows experimental threshold data under corrosion fatigue for steels with different heat treatments resulting in different yields stresses (from Usami [31]). Interestingly the change in the yield stress affects only the endurance limit but not long crack growth thresholds. Why this is so is not clear. With increasing yield stress, the material's behavior is increasingly elastic, as one should expect. Correspondingly, the internal stress triangle becomes smaller with increasing yield stress, indicating that the needed contribution from the local internal stress decreases. When the yield stress exceeds the fracture stress, the material becomes brittle. Figure 9 shows another example based on Hiroshi–Mura data [32] on stress corrosion of 4340 steel in H2(SO)4. The data are plotted in the form of the Kitagawa–Takahashi diagram. The endurance stress is similar to the minimum failure stress, σth, of a smooth specimen loaded in the corrosive environment. The mechanical equivalent of chemical internal stress is defined in the figure. The extent of experimental data on smooth and fracture mechanics specimens in corrosive media available in the open literature is limited. Nevertheless, the analysis shows that transition from short cracks to long cracks and the role of internal stresses in accentuating the crack initiation and growth process are general for all subcritical crack growth processes in materials.

**Figure 8.** Corrosion-fatigue crack growth in alloy steels with varying yield stress, from Usami, 1981.

**Figure 9.** Modified Kitagawa–Takahashi diagram for stress corrosion crack growth in 4340 steel in H2SO4 solution, extracted from Hiroshi–Mura data.

## *3.1. Application to Fracture Toughness*

The above concepts are relevant not only for subcritical crack growth, but also to fracture toughness (Figure 10). Experimental data from Bucci (1996; [33]) are shown in Figure 10a, and the data are plotted in terms of the modified Kitagawa–Takahashi diagram for two selected alloys (Figure 10b), one with low and the other with high fracture stresses. The horizontal portions of the plot correspond to the tensile fracture stress, while the inclined lines represent the K1C lines. The extension of the K1C lines defines the internal stresses needed to initiate a crack in a smooth specimen. The internal stress triangle is large for the low yield stress 2024-T3 alloy in comparison to the high yield stress 7075-T6 alloy, as expected.

**Figure 10.** Application to K1C, fracture data. (**a**) Experimental data from Bucci, 1996. (**b**) Representation of the data in terms of the modified Kitagawa–Takahashi diagram showing the internal stress triangle for the two cases, 7075-T6 and 2024-T3 Al-alloys.
