Answer
Stress relaxation is the time-dependent transition of elastic strain into plastic strain under constant deflection. In a bolted joint, if a wave spring is used to maintain tension, relaxation will result in a decrease in the applied force $P$. The rate of relaxation follows an Arrhenius-type relationship: $d\sigma/dt = A \cdot e^{(-Q/RT)} \cdot \sigma^n$, where $Q$ is the activation energy for creep and $T$ is temperature. At $150^{\circ}C$, standard carbon steels may relax up to $10-15\%$ of their initial load within the first $1000$ hours. To mitigate this, engineers should specify a 'Heat Setting' process during manufacture, where the spring is compressed to its working height and exposed to a temperature exceeding the operating environment, effectively 'pre-relaxing' the material.
