The progressive accumulation of incremental plastic strain in a structure or component subjected to cyclic loading with a non‑zero mean (bias) that produces net deformation per cycle, leading over many cycles to geometric distortion, loss of function, and eventual failure.

Principle

When cyclic stress or strain excursions cross the material's yield surface in a manner that produces asymmetric plastic flow (nonrecoverable strain) each cycle adds a residual increment; if the increments accumulate faster than any relaxation or shakedown mechanism, net ratcheting deformation grows until limit states are reached.

Demonstration

Illustrative scenario: A pressurized thin‑walled pipe subjected to repeated internal pressure cycles with a tensile mean stress develops a small incremental increase in hoop strain each cycle. Over thousands of cycles the radius grows and supports shift, producing misalignment and eventual through‑wall thinning or local collapse at a stress concentration.

Misapplication

Confusing ratcheting with high‑cycle fatigue: fatigue describes crack initiation and propagation from cyclic stresses, whereas ratcheting is an accumulation of plastic deformation without necessarily producing cracks until later; they are distinct mechanisms that can interact.

Consequence

Progressive dimensional change, loss of alignment or preload, increased local stresses and stress concentrations, reduced clearance or functional interference, and eventual structural failure either by collapse, excessive deformation or by accelerating fatigue crack formation due to stress redistribution.

Reversal

If mean stress is removed or the loading reverses sufficiently (elastic shakedown) the incremental plasticity can halt and the structure may stabilize; thermal annealing, material yielding path changes or design modifications that reduce mean load can reverse or arrest ratcheting.

Boundary

Clearly within: a gasketed bolted flange under cyclic internal pressure with nonzero mean tensile load that shows monotonic increase in bolt stretch. Boundary case: low‑amplitude cycles near yield where some microplasticity occurs but net accumulation is slow and shakedown may occur. Clearly outside: purely elastic cyclic loading with no residual strain accumulation, or isolated fatigue crack growth without measurable global plastic ratchet.

Semantic Tension

Designers must balance allowable plasticity for energy absorption and ductility against the risk that nonzero mean cycles will accumulate deformation; increasing yield strength reduces ratcheting but may promote brittle failure modes or reduce toughness.

Synthesis

Ratcheting is the long‑term geometric consequence of asymmetric cyclic plasticity: controlling mean load, accommodating controlled plastic zones, or ensuring elastic shakedown are the primary strategies to prevent progressive distortion and the secondary failures it causes.