Issue 7

S. Bagheri Fard et alii, Frattura ed Integrità Strutturale, 7 (2009) 3-16; DOI: 10.3221/IGF-ESIS.07.01 6 conventional operation is applied [17] . In ABSP, the shot velocity has a narrow distribution and the impact direction of shot to specimen is almost perpendicular [50]. Air pressure, shot size and shot materials are the process parameters that mainly affect the results of the treatment and the characteristic of the NC surface. Figure 3 : Schematic illustration showing the equipment of ABSP. Ultra sonic shot peening In Fig. 4 experimental set-up of USSP called at times (SMAT) is illustrated. In this method shots are resonated by vibration of an ultrasonic transducer and the impact directions of the balls onto the sample surface are rather random. Repeated multidirectional impacts at high strain rates onto the sample surface result in severe plastic deformation and grain refinement progressively down to the nanometer regime in the entire sample surface [64-70]. Obvious enhancement in the overall properties and performance of the materials is observed after the SMAT treatment [15, 47, 48, 70-76] . Figure 4 : Sketch of SMAT Surface nanocrystallization and hardening In SNH method the specimen is loaded at one end of a cylindrical container. The disc is held in place via mechanical locking. Some balls with diameters normally bigger with those used in other peening methods are loaded into the container. The high velocity of the balls is commonly achieved by shaking the container three dimensionally using a Spex 8000 Mill. Such 3D shaking provides kinetic energy to the balls and generates the complex pattern of motion of the balls inside the containe r [77-80]. This method like previous mentioned ones can provide structural metallic components with several desired features, such as compressive residual stress, work hardened surface layer and also nanocrystalline surface [81-85]. High energy shot peening High-energy shot peening (HESP) is another method reported to be capable of synthesis of nanostructured surface layers. The principle of the HESP treatment is very similar to that of the USSP method, but with lower frequency and bigger shots. Similar to previous methods, the entire surface of the sample to be treated is peened by the flying shots with a high energy and the NC layer is achieved using different durations for peening [86]. Compressed Air Shot Vibration Generator Sample Vacuum