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Wire Electric Discharge Machining for Residual Stress Measurement
In the Crack Compliance Method we measure residual
stress by using wire EDM to incrementally cut a slot into a part while
measuring released strains.
In Wire EDM (electric discharge machining, a.k.a. spark erosion) a wire
that is electrically charged with respect to the workpiece is used to
make a cut. As the wire approaches the workpiece, a spark jumps the gap
and electrically erodes away the material.
![Sketch of EDM and Strain Gauges](images/edm.gif)
Advantages of Wire EDM for Measuring Residual Stress
- It introduces very little stress if done with the correct settings
- It makes a very fine slot, a 0.002" diameter wire can generally be
used and will produce a slot about 0.0035" wide. Wires as small as 0.001"
diameter have been used by researchers measuring residual stress (References
#24)
- Because the part is bathed in deionized water, the temperature is
very stable resulting in very stable strain readings
- The wire EDM machine used in our ring test
was an anti-electrolysis machine which produces even less stress than
a regular EDM machine.
Aren't EDM machines expensive?
It doesn't really matter. They are available everywhere. If you're at a
big company or university, you probably have at least one. If not, most
urban areas will have machine shops with EDM machines that will work with
you for an hourly rate.
Other cutting methods for crack compliance measurements
Other methods have been used for making the cut
- Conventional EDM, also known as plunge EDM or ram EDM
- Milling cutter
- Saw blade
For more info on measuring residual stress with wire EDM see:
W. Cheng, I. Finnie, M. Gremaud, and M.B. Prime, "Measurement of Near Surface
Residual Stresses Using Electrical Discharge Wire Machining," J. Eng.
Mat. & Tech., 116, 1-7, 1994. Sorry, I don't have
an electronic version. |
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