Residual stress laboratory and stress analysis center
The hole-drilling method allows to perform experimental stress analysis without too expensive devices in several kinds of materials of engineering interest (metals, polymers and composites).
The hole-drilling method consists in drilling a hole in the center of a special three-grid strain gage rosette installed on the specimens under test: the machining of the hole modifies the mechanical equilibrium of the workpiece allowing a redistribution of the strains (measured by the strain gage).
The hole-drilling strain-gage method has been standardized by the ASTM E 837-13 “Standard Test Method for Determining Residual Stresses by the Hole-Drilling Strain-Gage Method”.
The hole-drilling method works with an high-speed drilling technology that allows to perform the hole without changing the state of stresses in the material. Moreover it also consents to drill holes without inducing new stresses, that’s a very important point during a test.
The high-speed drilling system comprises:
An high speed drilling device (high speed air turbine, 400000 RPM or electric motor, 50,000 RPM);
A tungsten carbide inverted cone.
The main operations involved in performing a residual stress measurement by the HOLE-DRILLING METHOD are:
installing a three-element strain gage rosette;
centering the drilling device with the center of the rosette;
establishing the initial contact between the workpiece and drilling device (zero setting);
drilling a through hole or blind hole in hole depth steps;
recording strains released through residual stress relaxation by each grid of the rosette;
measuring the actual hole diameter;
calculating residual stresses from recorded strains by the method considered most appropriate (ASTM E837-13, Integral method, Kockelmann method, HDM).
In terms of costs, accuracy and versatility, this is one of the most efficient non-destructive method of relief of residual stresses. Due to this process it is also called the “method of the hole“.
Hole drilling method – some details of different operations