What two primary factors may cause the development of stress corrosion cracks?

The development of stress corrosion cracks is primarily influenced by a combination of a corrosive environment and sustained tensile stress. In this context, a corrosive environment provides the chemical conditions necessary for corrosion to occur, often exacerbating the process of material deterioration. Sustained tensile stress, which is the continuous pull or stretch applied to a material, can lead to the initiation of cracks in susceptible materials. When these two factors are present simultaneously, the likelihood of stress corrosion cracking increases significantly as the protective oxide layer on metals can be compromised, allowing the corrosive agents to penetrate and react with the material more readily.

The other options touch on factors related to material failure but do not accurately capture the specific interaction that leads to stress corrosion cracking. For example, low humidity and vibration may contribute to mechanical fatigue but do not directly correlate with the conditions required for stress corrosion cracking. Similarly, high temperatures and metallic fatigue, while critical in other aspects of material behavior, do not specifically address the dual influence of environmental corrosion combined with tensile stress. Excessive load and impact fatigue are more associated with fracture mechanics rather than the slow, progressive failure process inherent in stress corrosion cracking.