Apollo-Clad uses the Hole-Drilling Strain Gauge Method of determining near surface residual stresses up to a 1mm (0.040") depth. The test uses a high speed drill to remove material in 50um (0.002") steps and measure the relaxation from a chemically bonded strain gauge. The drill is orbited to ensure a circular hole is achieved. Microstrain values are then converted to stresses using standard transformations outlined in ASTM E837.

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