Automatic system for measuring residual stresses
by the hole-drilling strain gage method
MTS3000-Restan: the automatic system for hole-drilling measurements
How does the method work?
The MTS3000-Restan is an automatic system able to measure residual stresses by the hole-drilling strain-gage method, developed and patented by SINT Technology.
The hole-drilling strain-gauge method for residual stress measurement is a semi-destructive mechanical relaxation method that works by drilling a small hole in the centre of a 3-grids strain gauge rosette.
The drilling process is performed using drilling increments: for each drilling step, the strains relaxed by the tested material, are acquired and used to calculate the residual stresses.
The hole-drilling method is standardized at world level by ASTM E837, the last release of which is dated 2013.
The standard method defines:
the distribution and number of the drilling steps,
the sizes (diameter, grid length and width) of the strain gauge rosette
the coefficients used for calculating residual stresses starting from the acquired strains.
The Hole Drilling Strain Gauge Method & the ASTM E837-13 standard
The MTS3000-Restan system works in accordance with the ASTM E837 standard.
The most recent version of the standard is dated 2013 and it introduced some modifications and innovations to the calculation of the residual stress using the hole drilling method.
The standard defines 2 main different algorithms:
a uniform stress field calculation mainly suggested to determine a representative size of the residual stresses that are present in the materials;
a non-uniform stress field calculation for the evaluation of the stresses in the total depth of analysis.
Some limitations connected with the type of strain gauge used are still present, as well as the maximum allowable eccentricity error (±0.004 D, ±0.02mm for standard gage) , the maximum stress level that can be measured (80% of the yield strength for thick specimens) and the calculation of intermediate thickness specimens.
The MTS3000-Restan system offers a wide range of solutions for hole drilling measurements and provides several accessories to extend the range of applicability of the method including polymeric and composite materials.
To calculate the residual stresses in compliance with the ASTM E837-13, SINT Technology has recently developed a new version of the EVAL software. It is also able to minimize the aforementioned limitations, thanks to the introduction of new coefficients and correction algorithms.