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Los Alamos National Laboratory The Slitting Method for Residual Stress Measurement:  : Shot peening
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Shot peening residual stresses in Ti-6Al-4V

RESULTS: Stress profile

  • Very good resolution of peening profile
  • We can see balancing tensile stresses (part is 6.45 mm thick)
Stress profile

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)


Titanium specimen

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!
Strain data

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
fit to strain data

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

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

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