Considering the ratio between cost versus depth of analysis, this is one of the most efficient non-destructive methods for the measurement of residual stresses: performing a core with an internal diameter of 14mm and an external dimeter of 18mm it is possible to evaluate the residual stress up to the total depth of about 5mm.
The complete Ring-Coring measurement is made up by the following procedure:
- Installation of a three-radial grid strain gauge rosette and protection of the sensitive elements.
- Positioning the mechanical device in the testing area, laying the strain gage rosette cables through the hollow shaft and finding the zero-depth position by electric contact.
- Drilling a core around the strain gage rosette.
- Measurement of strains produced by relaxation of the residual stresses.
- Calculation of residual stresses from the strain measurements.
The Ring-Coring strain-gage method has not yet been standardized at a world level but there are several procedures, recognised in industrial application (KWU method) for the evaluation of residual stresses mainly in forged parts.
The bigger advantages of the Ring-Coring method are:
- Higher depth of investigation
- Higher sensitivity to the strains
- Lower sensitivity to the eccentricity error
Moreover, there are several scientific papers and experimental activities regarding the evaluation of residual stress by the ring-coring method using different calculation strategies. In particular it is possible to evaluate non-uniform residual stress fields according to the Integral method: in that case, thanks to the cooperation with well reputed universities worldwide, sets of numerical coefficients for determining the value of residual stresses were evaluated by finite element method (FEM).
With the strength of many years of experience in the field of strain gage measurements, mechanical design engineering and software development, SINT Technology has developed and patented a fully automated system (MTS3000-RingCore) for measuring residual stresses.