Time and repetition: creep, relaxation and fatigue

Creep is time-dependent strain under sustained stress. It depends on material, stress, temperature and exposure. It is usually small in structural steel at ordinary temperatures, but can matter at elevated temperatures. Concrete, timber and polymers can exhibit appreciable creep in ordinary service.

The primary, secondary and tertiary stages describe a classical creep-to-rupture curve. Not every service-creep model follows those stages: concrete and timber require their own time, moisture and loading descriptions.

Relaxation is the same underlying process seen from the other side. Instead of holding the stress and watching strain grow, hold the strain and watch the stress fall.

The practical case is prestressing. A tendon stretched to a set extension and locked off will lose force over time — the steel relaxes — and the concrete around it creeps and shrinks as well, shortening the member and slackening the tendon further. Prestress losses of this kind are not a defect; they are predicted and allowed for from the start, and a design that ignored them would end up with far less prestress than intended.

The distinction is worth keeping crisp:

• Creep: stress constant, strain increases.
• Relaxation: strain constant, stress decreases.

Fatigue damage develops under repeated loading. Cracks may initiate at local stress concentrations or pre-existing defects, then grow until the remaining section can no longer sustain the loading. Growth per cycle and visible fracture markings are not uniform.

Stress range Δσ = σ_max − σ_min is central to many fatigue methods. Mean stress, detail geometry, residual stress, environment and loading sequence can also matter. Compare cases only on a consistent basis.

S–N curves relate a defined stress measure to cycles to failure, often using piecewise straight segments on logarithmic axes. Any endurance threshold applies only within the stated material, detail and environmental assumptions.