Shot-peeningA Bulwark Against Fatigue Failures.SOLUTIONS with Effective, Practical Advice
(Sponsored Links) Impacting at high velocity, the individual shot particles deform plastically the metal surface introducing a small indentation or dimple. The plastically deformed slight depressions result stretched radially, while the metal underneath is not deformed. The local stress equilibrium resulting is such that the outermost layers are in compression, and the deeper ones in tension. The surface compressive stresses distributed in a thin affected layer, are many time higher than the tensile stresses below surface, spread on a thicker layer. The compressive stresses effectively offset service stresses that put the surface under tension (like those on the convex side of a bent beam). The Shot-peening effect remarkably improves the resistance of metallic parts to the initiation and propagation of fatigue cracks, thereby increasing parts life. That is because cracks can appear only on surfaces stressed in tension, not in compression. Therefore, whenever there is concern for the fatigue life of parts, Shot-peening is routinely employed, as well as in cases where previous processing, like grinding, leaves the surface stressed in tension. Also forming, machining or heat treating may leave certain surfaces under tension and suitable for life improvement through the considered application of Shot-peening. Besides extending fatigue life, Shot-peening has useful applications in relieving the tensile stresses that undermine materials subject to stress corrosion cracking, in certain forming and straightening applications and for testing the adherence of silver plating on steel test pieces. The amount of compressive stresses induced by Shot-peening can be about half of the yield stress. For parts like springs, subjected to unidirectional service loads, a modification of the process can provide higher residual stresses approaching the full yield stress. This consists in Shot-peening performed while the part is strained in tension and is called Strain peening. To provide repeatable and tested results, shot is standardized Two propulsion methods are used for generating the shot stream, one mechanical, where the high velocity is provided by a spinning bladed wheel, the other using compressed air. Both methods have advantages and limitations, essentially influencing the economy of operation depending on size of treated parts and production quantities. Shot-peening control is achieved by establishing the required parameters and by testing the efficacy of treatment using Almen strips per SAE J444 SAE J442 Following Shot-peening, the strips, originally flat, tend, after being peened from one side only, to bend freely, by an amount related to the intensity of the impingement. The measure of the arc height gives an easy way to determine the amount of residual compressive stress introduced in the parts. For treating delicate materials glass beads are used, with dry or wet methods. Advanced processes known to provide effects similar to those obtained by Shot-peening, but to be applied to more delicate structures (like airfoils of gas turbine compressors) are called Laser Shock Wave Peening and Cavitation Peening. Laser Shock Wave Peening is applied by using a high energy pulsed laser that creates a high amplitude shock wave on the surface to be treated. As the stress wave propagates into the material, it causes the surface layer to yield and to deform plastically, developing on the surface a residual compressive stress layer. Cavitation Peening is performed by sweeping ultra high pressure water jets over the surface to be treated so that cavitation bubbles form and collapse on the workpiece. It is the shock of the collapsing bubbles that causes the formation of the sought residual compressive stresses. Different methods of applying the cavitation generating water jets were proposed, either in air, in a water filled chamber or in air but concentrically within a slower speed water jet. This last method is claimed to reach more favorable residual stress patterns. The processes described require that the parts be presented in the peening region of equipment built for the purpose. There is therefore a limit to the size of parts that can be introduced into the machines. Large welded constructions like bridges that could benefit from gains in fatigue life, cannot be treated by the standard Shot-peening methods. In those case a special hand held tool can be used, that produces the peening action by using floating pins put in motion by ultrasonic energy from a small generator. The method, called Ultrasonic Impact Treatment or Ultrasonic Peening, is based on conversion of harmonic oscillations of the ultrasonic transducer into peening impulses of ultrasonic frequency. In practice Shot-peening is the first remedial action to which one resorts to improve the behavior and the life of parts known to be subject to fatigue failures. Any questions or comments or feedback? Write them down and send them to us by e-mail. Click on the Contact Us button in the NavBar at top left of every page.
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