What are the indicators of stress corrosion resistance and their testing methods?

Early studies on stress corrosion cracking involved using smooth specimens to determine the time to delayed failure of metal materials under various stresses in specific media. A large amount of data has been accumulated using this method, which has played a role in understanding the issue of stress corrosion failure. However, there are still many shortcomings, mainly: (1) Due to the scattered data, incorrect conclusions may sometimes be drawn. 　　(2) The variation law of the crack growth rate cannot be correctly determined. 　　(3) It is time-consuming and cannot be used for engineering design. 　　Current research on stress corrosion uses specimens with pre-existing cracks. This type of specimen is placed in a certain medium, and under a constant load, the variation over time of the stress intensity factor K due to crack propagation is measured (the specific testing method will be described below); based on this, the material’s resistance to stress corrosion is determined. 　　For example, Ti-8Al-1Mo-1V shown in Figure 5-1 has K1c=100MPa.m1/2. In 3.5% saline solution, when the initial K value was only 40 MPa·m1/2, the specimen failed within just a few minutes. If the value K is slightly reduced, the failure time can be **delayed**. When the K value drops to a certain critical value, stress corrosion cracking actually does not occur. We refer to this K value as the stress corrosion threshold value, denoted as K1SCC (where SCC is short for Stress Corrosion Cracking). 　　(1)K