Residual stresses or locked-in stresses can be defined as those stresses existing within a body in the absence of external loading or thermal gradients. In other words residual stresses in a structural material or component are those stresses which exist in the object without the application of any service or other external loads.

If the residual stresses are strong enough to overcome the structural integrity of the part, the part will warp upon eject, or later crack when external service load is applied

Residual stresses have the same role in a structure’s strength as common mechanical stresses. However, while stress due to external loads can be calculated with a degree of accuracy, residual stresses are difficult to foresee. It is, therefore, very important to have a reliable method able to measure them directly with minimum damage to the surface. Residual stresses can play a significant role in explaining or preventing failure of a component at times.

The hole-drilling method (ASTM Standard E837) relies on stress relaxation when a hole is drilled into the center 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.

A special high speed air turbine drill is used to first locate the drill to within 0.001”of the rosette center 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 in those situations where the residual stress is relatively uniform over the drilling depth. Thus, it is not intended for situations where the residual stress is superficial.

The accuracy of the hole drilling method is directly related to the ability of locating the hole accurately in the center 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.

DMI’s RSM solution provides a more accurately feasible application vs. conventional rosette strain gage applications. DMI’s RSM peel and stick gage is wire free, and takes seconds to apply. The patent pending gage can take 36 simultaneous measurements and two unknown measurements which translates into more accurate data than conventional rosette strain gages.

DMI’s Residual Stress Measurement (RSM) solution consists of a self powered hand held reader and a portable tablet PC. The reader includes DMI’s patented and patent pending software and unique optic reading process for accurate and quick data retrieval.

• Scientifically proven measurement system
• No wires, or soldering required for DMI RSM gage application
• Can be used in laboratory AND field environments