Under fluctuating / cyclic stresses, failure can occur at loads considerably lower than tensile or yield strengths of material under a static load: Fatigue
Estimated to causes 90% of all failures of metallic structures (bridges, aircraft, machine components, etc.)
Fatigue failure is brittle-like (relatively little plastic deformation) - even in normally ductile materials. Thus sudden and catastrophic!
Applied stresses causing fatigue may be axial (tension or compression), flextural (bending) or torsional (twisting).