Fretting fatigue mechanism of 40CrNiMoA steel subjected to the ultrasonic surface rolling process: The role of the gradient structure

•The fretting fatigue (FF) mechanism of gradient-nanostructured steel was revealed.•Decreased FF initiation angle and increased FF propagation path were explained.•The nanolamellar martensite can inhibit FF crack propagation.•The influence mechanism model of 40CrNiMoA steel fretting fatigue was established.