Bolt allowable stress decreases with which condition?

• A. Material composition
• B. Coefficient of friction increases
• C. Temperature increases
• D. Bolt diameter increases

Answer: (C)

The main idea is that bolt allowable stress is tied to the material’s strength, which changes with temperature. As temperature goes up, the metal’s yield strength and ultimate tensile strength decrease, and creep can occur under sustained loads. Because the allowable stress is derived from those strength values plus safety factors, it must be reduced at higher temperatures to keep the joint safe. The other factors don’t cause the same temperature-related derating: material composition sets the baseline strength but isn’t a temperature-dependent operating condition; a higher coefficient of friction affects preload and how the joint behaves under load rather than reducing the bolt’s strength rating; a larger bolt diameter increases capacity in terms of load it can carry, but the allowable stress value itself remains determined by material properties and temperature, not by diameter.