Which failure mode in materials is characterized by the gradual weakening of a material over time?

The failure mode characterized by the gradual weakening of a material over time is creep. Creep occurs under conditions of constant stress and elevated temperature, leading to time-dependent deformation. This process is predominantly observed in materials subjected to high temperatures for extended periods, as the atomic structure of the material allows for incremental displacement over time.

In creep, the material does not fail instantaneously; rather, it experiences a slow and progressive deformation that can ultimately lead to failure if the conditions persist. This phenomenon is critical in applications like power plants and aerospace components, where materials are exposed to high thermal environments for long durations. Understanding creep behavior is essential for engineers to ensure material selections and designs can withstand operational stresses over the intended lifetime of the component.

Other failure modes, like ductile fracture and brittle fracture, involve immediate and often catastrophic failure rather than a prolonged weakening process. Fatigue, while also a time-related failure, refers to the process of fracture from repeated cyclic loading rather than the slow weakening under constant load.