All about Residual Stress, hole drilling rosettes, hole drilling devices, ASTM E 837-08, hole drilling strain gage method, X-ray diffraction, residual stress computation methods, integral method
Residual Stress

The hole drilling strain gage method

There are many situations where X-ray diffraction is not useful for measuring residual stresses. These include non-crystalline materials, large grained materials, nanomaterials, textured, or heavily
deformed metals. In these cases, other mechanical methods such as the hole-drilling method is used.
The hole-drilling method (ASTM Standard E837) relies on stress relaxation when a hole is drilled into the centre of a rosette strain gage such as that shown below. When the material is removed by drilling, the extent of the strain relief is monitored by the gages and the direction and magnitude of the principal stresses can be calculated.

 residual stress device

A special high speed air turbine drill (shown above) is used to first locate the drill to within 0.001”of the rosette centre and then to remove material to a controlled depth. At each depth increment, the strain relief on each of the gages is measured and converted into stress. As subsequent material removals occur, the stress distribution as a function of depth can be estimated. The hole drilling method is used also in those situations where the residual stress is non uniform over the drilling depth.

The accuracy of the holedrilling method is directly related to the ability of locating the hole accurately in the centre of the rosette. As an example, if the hole is no more than 0.001” off center, the residual strain error is less than 3%. In practice, the location accuracy is better than this, so the overall accuracy in residual stress measurements is quite good. Another important consideration in this method is the ability to drill the relief hole so as not to introduce new stresses. This is best achieved in hard materials by use of a high-speed turbine drill which avoids excessive rubbing of the cutting surface against the hole wall. As a result of careful design of the tool, the holes have flat bottoms and straight walls as required by ASTM E837. The hole drilling method has many advantages, but it also has many disadvantages. Of particolar concern is that the method is valid only up to about 50% of the yield strength of the test material. Thus, great care has to be exercised Rosette Gage (Magnification: 4x) when selecting testing methods

Hole drilling strain gage procedure

  1. Install a special three to six strain gage rosette at the point where stresses are to be measured.
  2. Wire the strain gages to a static strain indicator.
  3. Attach and accurately position a drilling device over the target of the rosette.
  4. Balance the gage circuits.
  5. Drill hole in increments, being careful not to generate heat that would induce residual stresses.
  6. Record strains after the strain indicator has stabilized.
  7. Calculate stresses using strain data by means of formulas reported in ASTM E 837-08