The hole-drilling test allows accurate experimental stress analyses at moderate costs. This method is considered a semi-destructive mechanical technique for measuring residual stress using strain gauges.
All the mechanical methods work 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 are then recorded using a strain gage or a strain gage rosette and compliance functions are used to back-calculate to the originally existing residual stresses. The semi-destructive techniques remove only a small part of material from a component leaving its overall structural integrity intact for further testing, repair and/or use.
The hole-drilling method consists in drilling a small hole using high speed technology 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 stress.
In terms of costs, accuracy and versatility, this is one of the most efficient non-destructive methods for the measurement of residual stresses. Due to this process, it is also called the “method of the hole” and it can be carried out in different testing configurations depending on the dimensions of the strain gage rosette and the related endmill used to perform the hole: by changing these parameters it is possible to increase the accuracy close to the surface or in alternative the total depth of analysis. In particular, the hole-drilling method can give non-uniform residual stress results up to 2mm in the depth or, in alternative, a very small resolution close to the surface (5-10 µm). The most used testing configuration requires a hole with a diameter of around 2mm and it provides non-uniform residual stress results up to 1mm in the depth.
The complete hole-drilling method test is made up by the following procedures:
Installation of a three-radial grid strain gauge rosette.
Drilling of a small hole through the centre of the rosette.
Measurement of strains produced by relaxation of the residual stresses.
Calculation of residual stresses from the strain measurements.
The hole-drilling strain-gage method has been standardized at a world level by the ASTM E837-20 “Standard Test Method for Determining Residual Stresses by the Hole-Drilling Strain-Gage Method”.
The ASTM E837-20 specifies the number of the required drilling increments, the numerical coefficients to determine the value of residual stresses, the data processing method and the measurement-related uncertainty.
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