3. Small repetitive forces which result in fracture: 1. Corrosion 2. Crystals Fatigue 4. Liquation

The correct answer is 3. Fatigue.<br /><br />Fatigue refers to the weakening of a material caused by repeatedly applied loads. It localized structural damage that occurs when a material is subjected to cyclic loading. Over time, these small repetitive forces can accumulate and lead to the fracture of the material, even if the individual loads are not large enough to cause immediate failure. This phenomenon is particularly relevant in the context of materials and structures that experience cyclic loading, such as machinery parts or aircraft components.