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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5

kinetic energy, the magnitude of plastic deformation and its penetration. It would result in a higher compressed depth as well as higher subsurface compressive stresses.

Fig. 3 (a) Variation of ‘AI’; (b) Contour of ‘AI’ as a function of peening pressure, exposure time.

Fig. 4 Mean effects of factors on ‘AI’

Fig. 4 shows the mean of the two factors at each level indicating the influence of peening pressure on ‘AI’ is more predominant than that of the exposure duration. The ‘AI’ value of the shot peened Ti-6Al-4V sample rises from 0.425 mm to 0.5 mm during the first 10 min. However, insignificant increase in ‘AI’ value is observed from 0.55 mm to 0.57 mmwith further increase in exposure time to 20 min. It is observed that for a different peening pressure conditions the ‘AI’ values significantly increases. The ‘AI’ values increases as the pressure is increased from 1.5 to 3.5 bar. The peening pressure revealed almost a linear trend with the ‘AI’. The collective influence of shot peening parameters exhibited in the Almen curvature arc height may be correlated to the absorbed energy in the Almen strips employed. It is understandable that, a higher peening pressure value generates higher shot velocities, which would induce higher amount energy to the Almen strips used, resulting in increased ‘AI’values. The mathematical relationship between the ‘AI’, exposure time, peening pressure were established by regression equation AI (mm) = 0.095 + 0.0092 T + 0.119 P; (R 2 = 94 %) (2)

Table 5. ANOVA for AI

Factors

Sum of squares

Degree of freedom

Variance 0.015142 0.047908 0.000914

F value

Exposure time (min) Peening Pressure (bar)

0.045425 0.143725 0.008225 0.197375

3 3 9

16.57 52.42

Error Total

- -

15

-

Further, ANOVA method was used to calculate the F that affects the ‘AI’ shown in table 5. The F value of both exposure time and peening pressure is greater than 3.07 and so both have a significant effect on the intensity. 3.3. Surface Roughness(Ra) Fig. 5a shows the influence of shot peening process parameters on Ra by the experimental design. Ra increases significantly after shot peening process, compared to untreated samples (Ra = 0.36 µm). Fig. 5b shows the effect of shot peening process parameters on the evolution of roughness Ra. The trend exhibited by Ra value indicates some qualitative similarity for the case of surface nanocrystallization and hardening (SNH) process reported by Dai, et al. (2004). Ra increases sharply at the beginning of treatment, and then decreases to achieve a saturation magnitude. It is well established that Ra plays a major role on the fatigue performance. Rough surface profiles can act as potential stress raisers, which increase the local stress and there by crack initiation takes place prematurely in diminishing fatigue performance. The mathematical relationship between Ra, exposure time, peening pressure were established by regression equation Ra (µm) = -0.463 + 0.0201 T + 0.681 P; (R2 = 90 %) (3)