Residual stresses
Cracking requires tensile stress to propagate. High residual compressive stress counteracts tensile stress and hinders the growth of cracks. Any measure which increases residual compressive stress should increase the life of a mandrel by suppressing cracking. Shot peening treatments introduce residual compressive stresses in the surface layer and prolong the fatigue life of a component. Furthermore, the condition of the surface of the tool is extremely important in addressing fatigue failures. High surface finish minimizes surface imperfections where fatigue cracking can initiate. It also reduces friction, which lowers the intensity of the applied load on the mandrel.