PSI - Issue 14

R.K. Kumar et al. / Procedia Structural Integrity 14 (2019) 134–141 S. Anand Kumar / Structural Integrity Procedia 00 (2018) 000–000

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might diminish the compressive stress values owing to the larger accumulated energy at the near surface layer due to repetitive impact of shots. It may result in development of subsurface cracks as reported by Shivpuri, et al. (2009).

Fig. 7 (a) Variation of surface compressive residual stress; (b) Contour of surface compressive residual stress as a function of peening pressure, exposure time

Fig. 8 Main effects of factors on surface compressive residual stress

Table 7. ANOVA for compressive residual stress

Factors

Sum of squares

Degree of freedom

Variance

F value

Exposure time (min) Peening Pressure (bar)

17185 59600 11378 88162

3 3 9

5728 19867 1264

4.53 15.71

Error Total

- -

15

-

Further, ANOVA method was used to calculate the F that affects the RS shown in Table 7. The exposure time and peening pressure, whose F value is 4.53 and 15.71which is greater than 3.07, has a considerable effect on RS based on the level of confidence. 3.5. Sub surface residual stress

Fig. 9 Variation of Residual Stress Distribution in the sub surface

Fig. 10 Variation of Residual Stress Distribution in materials of varying thickness

The stress gradient and depth of the stress-induced surface due to shot peening was measured. It is observed that the plastic deformation zone is significantly affected by the shot peening pressure. With increasing pressure from 1.5 to 3.5 bar, the plastic deformation zone below the surface increases with peening pressure. For all peening pressure condition, it is observed that maximum compressive residual stress is induced at sub-surface of shot peened sample. It is also observed that the reference point at 0.0 µm is positioned to the shot peened surface layer, which means that the residual stresses at these reference points are equivalent to RS. Also, the “hook” shaped profile for compressive residual stress distribution just beneath the surface is noticed after shot peening. i.e, underneath the shot peened surface the compressive residual stresses value approach the peak values at a specific depth below the surface and then it decreases with increasing depth and eventually reaches to the virgin material stress values. Peening with 3.5 bar resulted in a maximum compressive stress values of -764 MPa at the depth of 25 μm and – 870 MPa at the depth of 40 μm respectively (Fig. 9 and 10). At low pressure conditions viz., 1.5 bar the stress free-state is achieved after a