The process of shot peening increases the fatigue life of steel springs mainly because it results in:
(A) surface hardening
(B) increased stiffness of the material
(C) structural changes in the material
(D) residual compression at the surface
The correct answer and explanation is:
The correct answer is (D) residual compression at the surface.
Shot peening is a process in which small, spherical media (called shots) are blasted onto the surface of a material, usually steel. This creates a compressive stress layer on the surface of the material, which is known as residual compression. This compressive stress helps to resist the initiation and propagation of cracks, which are the primary failure mechanisms in materials under fatigue loading.
When a material, such as a steel spring, is subjected to cyclic loading, cracks can develop and propagate over time, leading to fatigue failure. The introduction of compressive stresses by shot peening reduces the material’s susceptibility to fatigue. Compressive stresses at the surface counteract the tensile stresses that occur when the material is under load. Tensile stresses tend to open up cracks, while compressive stresses help close them, thereby preventing crack growth and increasing the overall fatigue life of the material.
While shot peening can result in some degree of surface hardening, its most significant effect on fatigue life is due to the residual compressive stresses it introduces. Increased stiffness or structural changes are not the primary reasons why shot peening increases fatigue life. In fact, shot peening does not significantly alter the material’s stiffness or cause fundamental structural changes. The compressive stress layer is the key factor that enhances the durability and longevity of steel springs by preventing crack formation and slowing down the growth of any cracks that do form.