Define the term “fatigue cracking.”

The term "fatigue cracking" specifically refers to cracks that develop in materials as a result of repetitive loading and unloading cycles. When a material is subjected to repeated stress, even if the stress levels are below the material's yield strength, microscopic flaws or cracks can initiate and propagate over time. This process is often exacerbated by factors such as the number of cycles experienced and the magnitude of the loads, ultimately leading to failure.

It's essential to understand that fatigue cracking is primarily related to the mechanical fatigue of materials, rather than issues stemming from environmental factors, maintenance practices, or thermal effects. Other options describe different types of cracking. For instance, poor maintenance could relate to issues like corrosion or erosion, while thermal expansion pertains to changes in material dimensions due to temperature variations. In contrast, fatigue cracking is distinct in that it is fundamentally a result of mechanical stress cycles over time.