Non-rotational loosening can occur because of deformation of the fastener itself, or the joint, following assembly. The clamp load acting on a joint exists because of the preload created from the axial extension of the fastener. Normally, the fastener extension and joint compression are elastic. After assembly, changes in the fastener extension and joint compression can lead to a preload loss occurring. The changes may be reversible, because of differential thermal expansion of the bolt to the joint, or permanent, because of plastic deformation, creep, or stress relaxation.
There are several causes of non-rotational loosening all of which involve either the bolt additionally elongating or the joint losing some compression following installation. The terms relaxation and settlement are sometimes used as coverall terms to include certain types of non-rotational loosening that can occur in the bolt and the joint. The main causes of such loosening are:
Embedding: This is localised plastic deformation that occurs under the nut face, in the joint faces and in the threads because of plastic flattening of the surface roughness. Preload loss from embedding in bolted joints is discussed more on this webpage.
Stress relaxation: This is a form of creep and occurs when a high stress present in a bolt is relieved over time, usually at an elevated temperature. The stress is relaxed with a subsequent reduction in the bolt’s preload.
Creep: This is the characteristic of a material to deform permanently under a condition of continual stress. Creep can occur to the joint material itself, any gaskets within the joint and any paint on the plates comprising the joint.
Excessive Bearing Stress: If the surface stress caused by the clamp load is too high, surface collapse occurs resulting in a loss of bolt stretch and so a loss of preload.
Differential Thermal Expansion: Bolts are typically tightened at ambient temperature even though they may operate at elevated or cryogenic temperatures. If the joint and bolt materials differ then the bolt preload may increase or decrease depending upon the expansion/contraction characteristics of the joint and bolt materials.
Yielding after tightening: The bolt sustaining stress beyond its yield point can occur due to differential thermal expansion or the joint sustaining a high external load. Under such circumstances, if the bolt experiences yielding, then when the external load or the temperature changes, the preload will reduce.
Over related articles include:
Preload Loss from Embedding in Bolted Joints
Preload Loss from Stress Relaxation in Bolted Joints