Yeah four things going on. Design errors Probabilistic nature of the loads applied. Material defects Fatigue Deterioration All structures have a ser

Yeah four things going on. Design errors Probabilistic nature of the loads applied. Material defects Fatigue Deterioration All structures have a service life and it's the service lifetime an experienced engineer is trying to hit. For the impeller in a rocket turbo pump the service life is like 5 minutes. For the impeller in a hydro electric dam it's 50 years. The other thing that one of my professors pointed out was that 80% of engineers end up designing once off designs. Where the NRE cost is a lot more than the material costs. Shaving the safety factor is false economy.