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Shot-peeningA Bulwark Against Fatigue Failures.SOLUTIONS with Effective, Practical Advice
Shot-peening is a method of cold working. It introduces residual compressive stresses onto exposed surfaces of metallic objects. This most needed result is obtained by submitting the surfaces to the impingement of a stream of shot, under well defined conditions. Shot consists in small rounded parts made of steel, cast iron, glass or ceramic. Impacting at high velocity, the individual shot particles deform plastically the metal surface producing small indentations or dimples. 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 increases fatigue life. That occurs because surface compressive stresses improve the resistance of metallic parts to the initiation and propagation of fatigue cracks. 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. It is also advantageous in cases where previous processing, like grinding, leaves the surface stressed in tension. Forming, machining or heat treating may leave certain surfaces under tension. These are candidates, suitable for life improvement through the considered application of Shot-peening.
Besides extending fatigue life, Shot-peening has useful applications
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 by SAE J444. Its size is defined by numbers expressing the distribution of the weight percentage allowable, remaining on given screens, after shaking for a standard time. All parameters including,
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. Tests per SAE J442 uses Almen strips, submitted to the treatment with production parts. SAE J 444 SAE J442 Following Shot-peening, the strips, originally flat, tend, after being peened from one side only, to bend freely. The amount of bending is 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, are called Laser Shock Wave Peening and Cavitation Peening. These are applied to more delicate structures (like airfoils of gas turbine compressors). 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:
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. It uses floating hard 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. An Article on Ultrasonic Impact Treatment was published (11) in Issue 56 of Practical Welding Letter for April 2008. An Article on Relaxation of Shot Peen Induced Residual Stresses was published (7) in Issue 84 of Practical Welding Letter for August 2010. The analysis of residual stresses produced by peening methods is performed by the methods described in the Residual Stress page. The following video from a commercial company is presented as a demonstration. No recommendation or endorsement is intended. Other suppliers may provide equally acceptable instrumentation.
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