Relaxation

During the relaxation phase of a bolted joint, creeping and relaxation phenomena occur simultaneously after the joint has been embedded.

Relaxation in bolted joints refers to the time-dependent reduction of stresses at constant longitudinal strain, caused by microplasticity or viscoelastic flow, particularly at contact and sealing surfaces. It differs from creeping, in which a change in length occurs under constant stress. Relaxation leads to a gradual loss of preload force, which can impair sliding resistance and clamping force transmission and result in a loss of function. This effect is exacerbated by high temperatures, soft intermediate elements, poor surface finishes, and high assembly pressures.

VDI 2230 Sheet 1 accounts for relaxation mathematically as a settling phenomenon, whereby appropriate material selection and design measures are intended to minimize the loss of preload. Relaxation effects generally occur within weeks to months.