A derailment of a passenger train was caused by the failure of one of the forged steel wheels. A chemical analysis indicated that the wheel met the specifications for a Class A passenger wheel. Its carbon content was $0\mathrm{.}56$wt $\%.$ In contrast, the carbon content of freight car wheels is more like $0\mathrm{.}75$wt $\%.1)$ If the wheel had been shot$-$peened by the manufacturer, what type of residual stress is developed? $(2)2)$ What method should we use to measure the stress distribution? $(3)3)$ How to find out the reasons to generate the failure? $(3)$