This fact was proved by the Knoop microhardness test carried out using a 100 g

This fact was proved by the knoop microhardness test

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strength, making difficult the intrusion and extrusion micromechanism of crack nucleation. This fact was proved by the Knoop microhardness test, carried out using a 100 g load at the shot peened region, as shown in Figure (5), whose values are presented in Table (4). As can be seen in this table, the maximum microhardness values were found for the shot peening condition at 200ºC.
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1000 10000 100000 1000000 0 50 100 150 200 250 300 TEMPERATURE ( ºC) CYCLES TO FRACTURE Dureza 491HV (2,85) Dureza 528HV (2,77) Sem Jateamento SDS Figure 1. Mean fatigue life versus shot peening temperature. 2,50 3,00 3,50 4,00 4,50 5,00 5,50 6,00 0 50 100 150 200 250 300 Roughness, Ra ( ± m ) Temperature (ºC) Figure 2: Roughness, measured in terms of Ra, as function of the shot peening temperature.
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-620,00 -595,00 -570,00 -545,00 -520,00 -495,00 -470,00 -445,00 -420,00 0 25 50 75 100 125 150 175 200 225 250 275 300 325 Temperature ( ºC ) Residual Stress (MPa) . Figure 3. Residual stress mean value versus temperature of shot peening. Figure 4. Knoop microhardness indentation close to the shot peened surface.
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Table 4.: Knoop microhardness results, load of 100g. Level 2 Level 2 Level 3 Level 4 Level 5 Temp. [ºC] Mean. SD Mean SD Mean SD Mean SD Mean SD 100 444.2 8.8 467.1 42.4 511.2 16.5 497.1 5.2 429.3 13.1 200 549.3 11.1 541.7 4.5 575.70 36.3 527.7 45.9 494.3 30.3 300 338.5 13.4 483.6 37.8 487.9 24.2 456.5 49.5 444.5 78.6 4 CONCLUSIONS The results from this work, allowed a very important conclusion related to the shot peening effect on fatigue life. As well known for SAE5160 spring steel, the shot peening process at room temperature definitely improve fatigue life of leaf springs. However, as proved in this work, it can be further increased by execution of shot peening at approximately 200 ºC. It was found that the main reason for the increased fatigue life at this shot peening temperature, relay mainly on the strain hardening effect that overcame the effect of the decrease in the compressive residual stress level and the increase in the surface roughness 5 REFERENCES Courtney T. H., 2000, “Mechanical Behavior of Materials”, Second Edition , Chapter 12, McGraw-Hill, p.574-583. Tange, A., Koyama, K., Hiroshi, H., Tsuji, S. and Hiroto, K., 1999, “Shot Peening for Suspension Coil Spring”, S.A.E. Technical Paper Series, International Congress and Exposition Detroit, Michigan, March 1-4. Totten, G., Howes, M. and Inoue, T., Handbook of Residual Stress and Deformation of Steel. March 2002. ASM International.
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