Meaning
Mechanical degradation process where the clamping force in a bolted assembly decreases over time. joint relaxation failure occurs when the material at the interface undergoes microscopic deformation or permanent set under high pressure. This loss of tension can lead to the loosening of fasteners and the subsequent failure of the assembly.
Creep Mechanism
Time dependent deformation of materials occurs even at loads well below the yield strength. In joint relaxation failure, the constant pressure from the bolt head causes the gasket or the base material to flow. This flow reduces the thickness of the material between the bolt head and the nut.
Thermal Impact
Variations in temperature accelerate the loss of preload by changing the material properties and causing differential expansion. Rapid heating cycles can induce joint relaxation failure if the bolt and the clamped parts expand at different rates. Softening of polymers or low melting point alloys often increases the rate of relaxation in high temperature environments.
Lubricants applied to the threads can also migrate or degrade, changing the frictional resistance that holds the fastener in place. Vibration further contributes to the degradation by encouraging the microscopic shifts that allow the tension to bleed away.
Assembly Verification
Torque measurement immediately after tightening often fails to detect long term stability issues. Testing for joint relaxation failure requires monitoring the residual stress after a period of environmental exposure.
