* Importance of surface and its relation to failures

* Surface improvement techniques

* Laser Shock Peening

> history, process, layout, examples, etc.

* Experiment planned

* References

> Nearly all the fatigue and stress corrosion failures originate at the surface of the component. (long term failures)

> Surface of a material (or material near the surface) has unique properties. Surface grains deform plastically at lower stress level than those interior grains that are surrounded by neighboring grains because they are subject to less constraint.

(a) dislocations terminating at a clean surface will move under a lower stress than those internal dislocations which are anchored at both the ends.
(b) minute surface irregularities act as dislocation sources.

Due to these reasons mechanical failures mostly occur/initiate on the surface. Hence, surface plays a vital role in the service of a component.

> In most modes of long-term failure, the denominator is tensile stress.

> Tensile stresses attempt to stretch or pull the surface apart and may eventually lead to crack initiation.

> Because crack growth is slowed significantly in a compressive layer, increasing the depth of this layer increases crack resistance.

Prevention Idea: Generate compressive stresses on the surface.

Mechanical working of materials:

> Hammer

> Shots

> Laser

> Shot peening is a cold working process in which small spherical media called shot bombard the surface of a part.

> During the shot peening process, each piece of shot that strikes the material acts as a tiny peening hammer, imparting to the surface a small indentation or dimple.

> To create the dimple, the surface of the material must yield in tension.

> Below the surface, the material tries to restore its original shape, thereby producing below the dimple, a hemisphere of cold-worked material highly stressed in compression.

> The overlapping dimples from shot peening create a uniform layer of compressive stress at metal surfaces.

> Shot peening provides considerable increases in part life because cracks do not initiate or propagate in a compressively stressed zone.

> Compressive stresses are beneficial in increasing resistance to fatigue failures, corrosion fatigue, stress corrosion cracking, hydrogen assisted cracking, fretting, galling and erosion caused by cavitation.

> The maximum compressive residual stress produced just below the surface of a part by shot peening is at least as great as one-half the yield strength of the material being shot peened.