What is the primary cause of material failure in structural engineering over time when subjected to consistent loads?

Material failure in structural engineering over time under consistent loads is primarily caused by fatigue. Fatigue refers to the weakening of a material caused by repeatedly applied loads, which may be below the material's ultimate tensile strength. Over time, these cyclic stresses can lead to the initiation and growth of cracks, ultimately resulting in failure.

This phenomenon is particularly critical in structures that experience fluctuating loads, such as bridges or beams subjected to traffic or wind. The microscopic damage accumulates with each load cycle, and even materials that are generally considered strong can fail due to this gradual accumulation of damage over time.

Other factors such as creep, cracking, and stress concentration play roles in material performance but are not the primary cause in the context of consistent loading. Creep is the slow deformation of materials under constant load, significant in high-temperature applications but not necessarily the main cause of failure for consistent loads. Cracking can result from various types of stresses but is often a symptom rather than a direct cause of failure from consistent loading. Stress concentration refers to localized increases in stress and can lead to failure, but it typically doesn't encompass the long-term effects of repeated loading that fatigue does.