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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Fig. 6 shows the mean of the two factors at each level indicating the influence of peening pressure on Ra is more predominant than that of the exposure time duration. Further, the development Ra during the shot peening process found to be dependent on exposure time. Due to very high kinetic energy of impact in shot peening treatment, a considerable increase in Ra parameters is observed in comparison with unpeened sample. The mean Ra values of the shot peened Ti-6Al-4V sample raise from 0.36 µm to 1.5 µm during the first 10min and with further increase in treatment increases the Ra value upto 1.6 µm. Consequently mean Ra values decreases from 1.6 µm to 1.7 µm at treatment duration of 20 min.

Fig. 5 (a) Variation of Ra; (b) Contour of Ra as a function of peening pressure, exposure time

Fig. 6 Mean effects of factors on Ra

Further, ANOVA method was used to calculate the F that affects the Ra shown in table 6. The F value of the peening pressure and exposure time is 284.19, 11.88 which is much greater than 3.07 and hence having predominant role on Ra. Table 6. ANOVA for Ra Factors Sum of squares Degree of freedom Variance F value Exposure time (min) 0.21547 3 0.07182 11.88 Peening Pressure (bar) 5.15568 3 1.71856 284.19 Error 0.05442 9 0.00605 - Total 5.42558 15 - - 3.4. Surface Residual Stress (RS) Fig. 7a shows the influence of shot peening parameters on ‘RS’ by the experimental design is shown in. For instance, for enhancing fatigue performance or fretting wear resistance of a component, the proper combination of parameters (exposure time and peening pressure) resulting in higher magnitude of compressive residual stress could be selected. The ‘RS’ increases significantly after shot peening process. It is well established that surface compressive residual stress influences significantly the fatigue performance of metallic materials. Hence, a higher value of the compressive residual stress to a greater depth is always desirable. Fig. 7b shows the effect of shot peening process parameters on the evolution of compressive residual stress. The localized plastic deformation in the surface layer results in the formation of a high compressive residual stress. Compressive residual stress induced by shot peening process increases monotonically with an increase in exposure time. As exposure time is increased, the plastic deformation introduced in the material by the impacting balls increases resulting in higher the compressive residual stresses. The mathematical relationship between the RS value, exposure time, peening pressure were established by regression equation. RS (MPa) = - 381.168 - 5.545 T – 76.344 P; (R 2 = 92 %) (4) Fig. 8 shows the mean of the two factors at each level indicating that the influence of peening pressure on compressive residual stress is more predominant than that of the exposure time. The surface compressive stress generated during the shot peening process observed to be dependent on exposure time. The mean compressive residual stress values of the shot peened Ti-6Al-4V samples raise to -610 MPa, during the first 10min and with further increase in exposure time the compressive stress values increases to -670 MPa. Consequently, the compressive residual stress reaches to marginal increased value of about -685 MPa at exposure time of 20 min. Further, increase in exposure time,