What is Ratcheting?
A progressive accumulation of plastic strain, called strain ratcheting, occurs in ductile materials, subject to asymmetrical inelastic cyclic stressing. Ratcheting is characterised by a shift in the stress-strain hysteresis loop along the strain axis and incremental inelastic deformations. The ratcheting phenomenon is of paramount importance in the design procedure of systems, which undergo inelastic cyclic loading.
Furthermore, it has been considered in several design codes such as ASME, EN13445, RCC-MR, R5. Sea waves, road traffic and earthquakes are typical examples of repetitive loading.
The ratcheting phenomenon is usually divided into material and structural types. material ratcheting is governed by micromechanical changes. It occurs when the stress level and its multiaxiality state remain spatially homogeneous in a structure. Structural ratcheting requires an inhomogeneous distribution of stress and occurs even if there is no material ratcheting [1].
The typical evolution of ratcheting strain with number of cycles has been shown in Fig (a) for SA333 CMn steel. Atypical ratcheting curve can be divided into three regions: primary, secondary and tertiary regions. The three distinct regions of ratcheting also can be distinguished from ratcheting strain rate versus number of cycles plot, which is illustrated in Fig(b) for SA333CMn steel [2].
[1]- Uniaxial strain ratcheting of steel butt-welded joints after multiple-repair welding