What type of fatigue occurs due to repeated application of loads that are insufficient to cause failure individually?

The correct answer is thermal fatigue. This type of fatigue occurs when materials are subjected to repetitive thermal cycles which can cause expansion and contraction. Each individual cycle may not exert enough stress to cause immediate failure; however, over time, these repeated thermal stress cycles can lead to fatigue damage, resulting in cracks or failures.

In this context, it's important to understand the processes involved in thermal fatigue. When a material is repeatedly heated and cooled, it experiences differential expansion and contraction, which creates internal stresses. These stresses can accumulate and lead to the gradual degradation of the material, eventually resulting in failure, even if each thermal cycle seems harmless on its own.

The other options represent different types of fatigue mechanisms that are characterized by unique failure modes:

• Surface or pitting fatigue typically relates to the gradual wear and damage at the surface of a material due to cyclic loading but focuses more on the surface conditions rather than thermal cycles.

• Subsurface cracking occurs deeper within the material, often as a result of stress concentrations but is also not directly related to thermal cycles.

• Corrosion fatigue combines corrosion with mechanical loading, leading to a synergistic failure mode where the material is weakened by both corrosion and repetitive loading rather than just thermal influences.

Understanding these distinctions helps clarify why thermal