21-06-2014, 01:53 PM
LASAR PEEING
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1) FATIGUE
• Fatigue is a type of failure of a material, occurring under alternating loads. Most of the failures of machine details are caused by fatigue.
• Fatigue cracking is one of the primary damage mechanisms of structural components.
• Fatigue cracking results from cyclic stresses that are below the ultimate tensile stress, or even the yield stress of the material.
• The name “fatigue” is based on the concept that a material becomes “tired” and fails at a stress level below the nominal strength of the material.
2) PEEN :
• The word peen means to flatten with a small hammer. (Fig 1) This peening hammer was used by blacksmith William Shenk at the turn of the century.
• Peening is the process of working a metal's surface to improve its material properties,
) WHY PEEN :
• The atoms in the surface of a piece of manufactured metal will be under (mostly) tensile stresses left over from grinding, welds, heat treatments and other stressful production processes.
• Cracks promulgate easily in areas of tensile stress because the tensile stresses are already working to pull the atoms of the metal apart.
• By peening the material you introduce a layer of compressive stress by compacting the material.
• It is well known that cracks will not initiate nor propagate in a compressively stressed zone.
) WORKING OF LASER PEENING :
An output beam, roughly 25 Joules at 18 nanoseconds from a lase beamis projected onto a work piece to induce a residual compressive stress. The area to be peened can be covered with material to act as an ablative layer and simultaneously as a thermal insulating layer, or peened directly onto the base metal which subsequently may require some form of surface removal of a few microns.
A thin stream of water is made to flow over the surface and the laser light transparently passes through the water, the leading temporal edge of the laser pulse is absorbed on the metal surface or ablativelayer. This absorption rapidly ionizes and vaporizes more of the surface material to rapidly form a plasma that is highly
absorbing for the rest of the laser pulse.
A high plasma builds to approximately 100kBar (1 million pounds per square inch) with the water serving to inertially confine the pressure. This rapid rise inpressure effectively creates a shock wave that penetrates into the metal, plastically straining the near surface layer.
The plastic strain results in a residual compressive stress that penetrates to a depth
of between 1mm and 8mm depending on the material and the processing conditions.This deep level of compressive stress creates a damage tolerant layer and a barrier to crack initiation and growth, which consequently enhances the fatigue lifetime and provides resistance to stress corrosioncracking and fretting fatigue.
) SHOT PEENING :
• Shot peening is a cold working process that imparts a small indentation on the surface of a part by impacting small spheres called shot onto the material surface.
• This process creates the same effect that a peening hammer does by causing outer surface to yield in tension. The material directly beneath it is subjected to high compressive forces from the deformation and tries to restore the outer surface to its original shape.
• By overlapping the surface indentations, a uniform compressive layer is achieved at the surface of the material.
• The compressive layer squeezes the grain boundaries of the surface material together and significantly delays the initiation of fatigue cracking. As a result, the fatigue life of the part can be greatly increased.