Effect of plasma nitriding on fatigue crack growth on AISI 4140 steel under variable amplitude loading

► The effects of plasma nitriding on fatigue crack growth (FCG) were investigated. ► Digital image correlation (DIC) method was used to observe FCG rate. ► The constant amplitude loading FCG limits were improved by the plasma-nitriding. ► Plasma nitriding under variable amplitude loading decreased FCG retardation period.