Ring Core Method

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Residual stress measurement
Ring Core Method

Ring Core Method for residual stress measurement

Main strain gage methods used for measuring residual stresses include the Ring Core Method.
Ring Core method consists in drilling a core (approx. D = 18mm, d = 14 mm, h = 4-5 mm) around a special 3-element strain rosette. The core changes the initial strain allowing redistribution of the residual stresses originally existing in the material. The alternative best method is the Hole drilling strain gage method, that consists in drilling a small hole (approx. 1.8 mm x 2.0 mm) into the centre of a special 3-element strain rosette.

RING CORE METHOD: TEST PROCEDURE

The test procedure used for measurement of residual stresses by the ring-core method with the MTS3000 – RingCore system follows these steps:

Apply a strain gage rosette with three overlapping grids in the test area
Protect the strain gage leads with heat-shrink tubing
Protect the rosette with protective coating
Place the mechanical unit of the instrument on the area to be measured, lay the cables from the strain gage rosette through the hollow drilling spindle and secure the instrument with the magnetic clamps (or with proper adhesive)
Center the crown milling cutter and strain gage rosette using the Webcam installed on the instrument and the centering software
Zero depth is automatically detected by the electrical contact technique
An annular groove is machined around the strain gage rosette in depth increments of 0.25 mm, and at each step the strain values measured by the three grids are automatically acquired.

To get the perfect measurement, the previous step is repeated to reach a minimum depth of 4 mm. To analyze the test results, the Eval –RC post-processing software is used. It allows to read correctly all the test data and to choose among different calculation options.