Deep Hole Drilling

Abstract

The Deep Hole Drilling (DHD) method belongs to the class of mechanical strain relaxation techniques designed to measure stresses and residual stresses in materials. The technique described in this chapter is now used extensively to measure both applied and locked-in stresses in many manufactured engineering materials and components. Its origins as a technique rise from its use in rock mechanics [1,2]. The procedure involves drilling a pilot borehole into the rock to the depth required for stress measurement (Figure 3.1). A device, often called a strain cell, is inserted into the borehole and then pressurized to ensure good contact with the sides of the borehole or glued into the borehole. Then material surrounding the pilot borehole is then over-cored to relax the stresses in the surrounding rock, with the strain cell monitoring the relaxed strains. An elasticity analysis is then used to convert the strains to stresses. Rock mechanics practitioners also refer to using "soft or hard inclusion" analysis for determining the stresses from the strains measured, [2][3][4] with the analysis depending on the type of strain cell used. An example of its application in rock mechanics is provided by Martin and Christiansson [3]. They describe borehole and over-coring methods where the pilot hole was 38 mm diameter and the over-core diameter 96 mm.The process of introducing a pilot hole, inserting a strain cell and then over-coring has also been developed and applied to civil engineering structures, such as bridges and concrete constructions. Ryall [5] explains a "hard inclusion" method based on overcoring. A solid steel bar, representing the "hard inclusion", is bonded into the borehole using high-modulus cement grout. The steel bar is strain gaged to permit measurement of strains during over-coring. Borehole and over-core diameters were about 42 mm and Practical Residual Stress Measurement Methods, First Edition. Edited by Gary S. Schajer.

Determination of residual stress within complex-shaped coarse-grained cobalt–chromium biomedical castings

“…. The deep-hole drilling method 63 it not suitable as stress-relaxation associated with drilling the initial pilot hole means that results obtained from the initial ∼1 mm depth are uncertain. 64 Residual stress determination methods measure strain from which stress is calculated utilising a variation of Hooke's law:…”

“…This method requires a minimum core diameter of 15 mm to accommodate a 12 mm diameter strain gauge rosette. 59,60 The slitting method, 61 for which compliance functions can be determined for complex geometries, 62 is not suitable as the spatial resolution of the technique is too coarse. The deep-hole drilling method 63 it not suitable as stress-relaxation associated with drilling the initial pilot hole means that results obtained from the initial ∼1 mm depth are uncertain. 64 …”

“…The deep-hole drilling method 63 it not suitable as stress-relaxation associated with drilling the initial pilot hole means that results obtained from the initial ∼1 mm depth are uncertain. 64…”

Two-Dimensional Mapping of In-plane Residual Stress with Slitting

“…Such a map might be constructed from a series of deep hole drilling measurements [6] in thick parts or a series of slitting measurements [7] in thin parts. Recently, a technique comparison of inplane residual stress mapping using hole drilling, neutron diffraction, and slitting was performed in a thin slice taken from a dissimilar metal weld [8].…”

Axisymmetric residual stresses in a solid cylinder of finite length

“…The practical measurement of the residual strains or stresses in solids is typically based on either destructive or non-destructive testing. The destructive techniques imply the complete or partial destruction of structural members in order to perform the measurements (e.g., the hole drilling and ring coring techniques [ 5 – 7 ], the slitting and contour methods [ 8 – 10 ], etc.). On the other hand, the non-destructive methods (e.g., the X-ray and neutron diffraction [ 11 – 13 ], the magnetic, ultrasonic, and optical methods [ 14 – 16 ], etc.)…”