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

136

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2.2. Shot Peening Treatment The test sample for shot peening treatment are prepared from a rod of 100mm diameter and cut into 18mm thickness. In order to remove the induced stresses due to grinding and machining, the cut samples are heat treated at 550 ⁰ C for 35 min, followed by air cooling. Metallographic polishing is done on the samples first using four different grades of silicon carbide abrasive papers of different grit sizes of 400, 600, 800 and 1000 and subsequently using alumina. Final cleaning was done with acetone in an ultrasonic cleaner for 15min. Ti-6Al-4V disc samples are shot peened at different pressures 1.5, 2, 3, 3.5 bar and for different exposure time durations 5, 10, 15, 20 min using carbon steel shots of 0.6mm diameter. A total of 16 shot peened test pieces are evaluated at different exposure time and pressure conditions. The intensity of shot peening is determined using a standard Almen process using the test strip. To measure the Almen intensity, standard test strips holder and gage are used as per standard SAE J442. This standard test strip is mounted on a holder and subsequently shot peening is performed. After shot peening, the strip is removed and the peening intensity is measured with an Almen gage. A special calibration block is designed to facilitate the investigation of Almen gage accuracy. The block is made by precision grinding to provide a flat surface on one side and a curved surface on the opposite side. The curvature is set to represent the shape of Almen strip having a deflection of 0.024 in. 2.3. Residual Stress Measurement The residual stresses in untreated and shot peened samples are determined using a X-ray diffraction system with Cr Kα radiation (λ=2.28 A°). The X-ray diffraction technique is followed based on Bragg’s law (nλ=2dSinӨ, where λ is wavelength, d is lattice spacing and Ө is diffraction angle). This technique is suggested by Cullity (1978), and was followed by Noyan and Cohen (1987). The depth profiling of residual stress involved sequential chemical polishing and etching using HNO 3 +HF acid bath from the peened surface and stress measurement at regular intervals of about 10 μm. The shot peened layer is stepwise removed, and residual stress is measured. The procedure is repeated till stress relieved, and the thickness of stress induced layer is obtained.

Table 2. Factor levels for the experiment

Factors

Level I Level II

Level III

Level IV

Exposure time ‘T’ (min) Peening Pressure ‘P’ (bar)

5

10

15

20 3.5

1.5

2

3

Table 3. Full factorial design matrix

Sl.No.

‘T’ (min)

‘P’ (bar)

‘C’ (%)

‘AI’ (mm)

Ra (µm)

‘RS’ (MPa)

1 2 3 4 5 6 7 8 9

5 5 5 5

1.5 2.0 3.0 3.5 1.5 2.0 3.0 3.5 1.5 2.0 3.0 3.5 1.5 2.0 3.0 3.5

30 35 40 60 40 45 65 75 80 75 80 95 98 98

0.29 0.35 0.45 0.61 0.39 0.44 0.55 0.63 0.45 0.48 0.60 0.64 0.46 0.49 0.65 0.66

0.72 1.04 1.44 2.04 0.76 1.22 1.52 2.40 0.84 1.34 1.58 2.50 0.88 1.44 1.64 2.50

-482.97 -542.08 -683.72 -715.15 -512.12 -570.09 -670.17 -695.00 -608.00 -669.10 -684.12 -710.01 -620.00 -669.00 -715.00 -715.00

10 10 10 10 15 15 15 15 20 20 20 20

10 11 12 13 14 15 16

100 100

2.4. Design of Experiment Taguchi’s experimental design is used in the present study. These process parameters are listed in Table 2. In the present investigation, the influence of two factors, each at four levels, is studied using an L 16 (4 2 ) full factorial