Bolt torque attenuation, often referred to as preload loss or relaxation, is a critical phenomenon where the clamping force (preload) in a bolted joint decreases over time after initial tightening. This is a major cause of joint failure. The attenuation isn't of the "torque" itself, but of the preload that the torque was meant to create. This loss occurs due to several key mechanisms:
1. Embedment and Creep (The Primary Cause):
This is the most significant factor. Microscopic surface irregularities (peaks and valleys) on the bolt head, nut, and between the joined parts flatten under high pressure. This settling, called embedment, allows the bolt to stretch slightly, reducing the tension and preload. Creep is a similar slow, plastic deformation of the materials (including the bolt itself) under constant stress, especially at elevated temperatures, further contributing to relaxation.
2. Vibration and Shock Loads:
Dynamic loads can cause the nut and bolt to undergo tiny relative movements (micromotion). If the friction under the nut or bolt head is overcome, this can lead to rotation, causing the fastener to loosen. This is why lock washers, prevailing torque nuts, and thread-locking adhesives are often used in vibrating environments.
3. Thermal Expansion and Contraction:
If the bolt and the clamped materials have different coefficients of thermal expansion, temperature cycles will cause them to expand and contract at different rates. This can either increase or decrease the preload. Repeated cycling often leads to a net loss of preload over time.
4. Gasket Relaxation:
In joints with gaskets (e.g., in flanges), the gasket material itself can compress and relax over time, leading to a direct loss of clamp load that the bolt must maintain.
How to Mitigate Torque/Preload Attenuation:
Proper Design: Following established guidelines for torque, surface preparation, and bolt strength.
Use of Locking Elements: Applying lock nuts, lock washers, or thread-locking fluid.
Re-torquing: For critical joints, especially after the first operational heat cycle or period of vibration, re-torquing after a short interval can compensate for initial embedment loss.
Controlled Tightening Methods: Using methods that directly measure or control preload (like torque-angle or hydraulic tensioning) are more accurate than simple torque control alone, leading to more consistent initial preload and less susceptibility to relaxation.