Shot peening residual stresses in Ti-6Al-4V

RESULTS: Stress profile Very good resolution of peening profile Could do even better with better testing conditions We can see balancing tensile stresses (part is 6.45 mm thick)
Titanium specimen: Ti-6Al-4V 6.45 mm (0.254”) thick at slit location 38 mm (1.5”) wide at slit Shot peened on all 4 faces 3.18mm (0.125”) long gauge on back side (opposite the cut)
Cut slot using wire EDM: Cut slot using 100 µm diameter brass wire Slot cut in 51 µm increments (0.002") initially, then coarser Strain at each depth was measured with strain gage on back surface opposite the cut Cutting stopped because slot was pinching shut on edges See inset: already 9 με at 51 µm depth. Good sensitivity!
Used "pulse method" to fit data: Quality of inverse solution for stress can be judged by how well it fits (reproduces) strain data. This stress variation cannot be well fit using series expansion Because sharp variation is followed by nearly flat profile - not easily fit with polynomials Was able to fit well using "pulse" method. See figure: stresses great strain fit

Options and advice for measuring shot peening and other near-surface stress profiles

Option 1: use gage on top surface very near the cut.

• See: W. Cheng et al., "The Compliance Method for Measurement of Near Surface Residual Stresses - Application and Validation for Surface Treatment by Laser and Shot-Peening, " J. Eng. Mat. & Tech., 116, 556-560, 1994. Shot peening in Ti-6Al-4V just like these stresses
• Advantage: top gauge can be very sensitive to near surface stresses
• Disadvantages: top gauge is experimentally harder to achieve (cutting very close and using small gauge), and very sensitive to cutting induced stresses and other errors

Option 2: use back gauge like the experiment on this page

• Advantage: easier experimentally and much more robust to errors
• Disadvantage: less sensitive, however ...
• To improve sensitivity, use a thinner part. Use inherent strain or eigenstrain approach to transfer slitting results from test coupon to structure.

The back gauge measurements on this page could be improved

• Since strains and strain increments are large enough, could use smaller increments to get better resolution
• Smaller wire diameter would also help