Introduction to surface Engineering
Bulk and surface Bulk Surface Must have sufficient creep resistance and fatigue strength at the service temperature Must possess sufficient resistance to oxidation and hot corrosion under service conditions. Crack initiation resistance on stage I Fatigue 3/3/20 2
Mechanical Surface Modification 3/3/20 4
Shot peening – A promising surface modification to optimize mechanical properties
Introduction • For the first time shot peening was commercially used by Jhon O Almen to enhance the fatigue property of automotive valve springs in 1940s 3/3/20 6
Mechanism • Shot peening works under the cold working principle • It will increase the dislocation density by bombardment of shots on the surface of the material and also imparts residual stress in the surface • The shots (small spherical media) bombarded on the material surface will result in causing the indentation or a dimple in the surface • Fiber below the indentation will tend the material to return to its original position as shown the Fig 1 3/3/20 7
Continuation… • This will lead to develop the compressive stress in surface and near surface area as in the fig 1 • This compressive stress will remain even after the shot peening process thus it is called as residual stress. • This residual stress will counter act the tensile stress caused during the service condition which results in hardening of the surface. Fig 1 3/3/20 8
Residual stress Effect of residual stress can be easily proven by investigating the S-N curve of the Ni Cr alloy under notched ( X ) , notched and shot peened( ⟥ ) and polished and smooth ( • ) . It clearly witnessed from the Fig 2 that notched and shot peened ( ⟥ ) samples have equal property possessed by smooth and polished ( • ) samples. This is effect is due to the effect of residual stress imparted by the shot peening Fig 2 3/3/20 9
Distribution of residual compressive stress Where SS - Surface stress (stress measure at the surface) Cs max - Maximum compressive stress induced (Magnitude) d - Depth of the point at which the compressive stress crosses the neutral axis and become tensile stress Ts max - Maximum tensile stress at the core of the material. This tensile Stress is developed to balance the compressive stress developed at the layer (Mostly residual stress distribution will exist at 0.025 to 0.5 mm from the surface of the material ) Fig 3 3/3/20 10 Through thickness profile for stress distribution profile
Residual stress influencing factor Residual stress will be strongly influenced by the factors such as shot peening intensity and hardness of the material which has to be peened The following graph (Fig 4) shows the variation of the depth of compressive layer along with the shot peening intensity for Steel with hardness R c 31, Steel with hardness R c 52 and Ti-6Al- 4V.
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- Fall '15
- Materials Science, Tensile strength, Shot peening