How it works

The Ring-Coring method allows accurate experimental stress analyses at higher depth of analysis.

Ring-Coring method is considered a semi-destructive mechanical method for measuring residual stresses using strain gages.

The Ring-Coring method, in analogy with all the mechanical methods, works on the basis of removing material from the component to release stresses and hence disturb the state of residual stress equilibrium; the remaining component material then deforms to reinstate equilibrium. The resulting deformations or strains of the remaining component material are recorded using a strain gage rosette and compliance functions are used to back-calculate the original residual stress present. The semi-destructive techniques remove only a small amount of material from a component, keeping its overall structural integrity intact for further testing, repair and/or use.

The Ring-Coring method consists in drilling a core groove using a dedicated electric motor to induce an alteration in the stress state involving a change and a redistribution of forces within the structure. This change can be measured by a strain gauge rosette with three grids, acquiring and elaborating the data to obtain the measurement of the residual stresses.

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.