system
Given the following post, respond as if you're commenting on Reddit:
human
Why don't steel bridges e.g. the Sydney Harbour Bridge, suffer metal fatigue?

Millions of tons of moving, vibrating trains, trucks, busses and cars every day. Forces of wind and rain. Fierce heat and cold yet no-one ever talks about fatigue in the steel. Especially in cables like Brooklyn bridge or the Golden Gate Bridge?
gpt
If you keep the stress below a certain limit, you can go an infinite number of cycles (N) below a certain stress (S) for steel. A material’s S-N curve describes how many cycles to failure at a given stress. The curve for steel levels off and stays flat at a certain S, heading to infinite N (horizontal  axis). Most materials are not like this, e.g. aluminum always has a finite fatigue life, so you have retire or refurbish your airframe eventually. Note that even if you design to stay in infinite regime for steel, you still have to maintain and inspect, because of corrosion, trauma, etc. It’s important to keep a bridge well painted.