RESIDUAL STRESS MEASUREMENT USING A MINIATURISED
DEEP HOLE DRILLING METHOD
X. Ficquet, D. J. Smith and C. E. Truman
Department of Mechanical Engineering, University of Bristol
Queen’s Building, University Walk
Bristol, BS8 1TR, UK
ABSTRACT
This paper presents the details and recent developments of the deep hole drilling technique and then goes on to
present recent research directed towards reducing the overall size of the existing measurement equipment. Using
our existing experimental arrangement, which has been extensively developed over recent years [1-4], it was
possible to undertake residual stress measurements to depths in excess of 500mm, although this deep throughthickness capability meant that the rig had an overall length of about one meter, which limited its use in confined
spaces. It was decided to miniaturise the machine to allow residual stress measurements to be made in
environments that were hitherto not amenable to such measurements, e.g. on the inside of a pipe. The trade-off
to the miniaturisation process was a reduced measurement depth capability of approximately 50mm. In addition to
permitting residual stress measurements to be made in confined spaces, the miniaturisation process also
provides improved transportability, which in turn, means that measurements made be made more readily away
from the laboratory. The paper will begin by describing in detail the new, miniaturised deep hole drilling system
and then go on to use the rig to make a residual stress measurement in a stainless steel component.
INTRODUCTION
There are many methods for determining the stress in material. For example, there are a wide range of
experimental techniques available to measure surface or through-thickness stresses, particularly residual
stresses, in engineering components. These methods can be classified generally into three categories: noninvasive, semi-invasive and totally invasive. The degree of destruction depends on the extent that the component
remains intact.
Non-invasive techniques include mainly conventional X-ray diffraction, neutron diffraction, ultrasonic methods and
electromagnetism. The depth of residual stress measurement is confined mainly to near the surface, from several
micro-metres (conventional X-ray diffraction) up to 5mm using high-energy synchrotron X-ray in transmission
mode. Ultrasonic methods are useful to a depth of about 2 mm in steel. The semi-invasive and fully invasive
methods all rely on measurement of strain relaxation resulting from material removal. Semi-invasive techniques
include centre-hole drilling (about 2-4mm depth), ring coring (up to 15 mm). The deep-hole drilling method is
capable of measuring stress at depths up to 500 mm in steel. These semi-invasive methods allow several
measurements to be made on the specimen but not at the same location.
Finally, invasive techniques such as the slotting, the block removal, splitting and layering and the inherent strain
methods enable through-thickness measurements of residual stress to be made. However, they destroy the
specimen completely so that no further measurements can be obtained.
THE DHD MEASUREMENT TECHNIQUE
The deep-hole drilling method was developed as an extension of the standard hole drilling technique to allow the
full through-thickness stress field of a specimen to be obtained. The deep hole drilling technique determines the
residual stresses in a component by measuring the distortions of a reference hole in the component after a
column of material containing the reference hole as its axis is removed. This procedure can be divided into four
steps, see Figure 1. Step 1, Figure 1(a), consists of drilling a small reference hole through the component of
interest. Step 2, Figure 1(b), consists of accurately measuring the diameter of this reference hole using an air
probe. Diameter measurements are made at many angular positions and at many depth intervals. In step 3,
Figure 1(c), a column of material containing the reference hole as its axis is coaxially trepanned free of the
component using an electro-discharge machining (EDM) technique. Finally, step 4, Figure 1(d), consists of remeasuring the reference hole diameter at the same angular positions and depths using the same air probe. The
distortion of the reference hole diameter in the plane normal to the reference hole axis is used to determine the in-
plane residual stress field. It is assumed that the stresses relieved by the introduction of the reference hole are
negligible and that the trepanned core completely relaxes in a linear elastic manner after trepanning. The analysis
also assumes that the trepanned core may be considered as many independent block-lengths. Each independent
block-length may be viewed as an infinite plate containing a hole subjected to a uniform, uni-axial stress. Further
details of the DHD method may be found in [5-7].
Figure 1. Schematic diagram of the DHD technique
MINIATURISATION OF DHD SYSTEM
One of the significant advantages of the DHD technique over others is that being a semi-destructive technique the
experiment may be conducted “on-site”, allowing a component or structure to be withdrawn for testing and
subsequent repair. The object of this work is to reduce the overall size of the current portable rig. This rig is
capable of measuring depths up to 500mm, although, it is still over 1m in length and 400mm in diameter, which
limits its access in confined spaces. It was decided that the maximum thickness of any sample the new rig would
test would be up to 50mm, and that only a 1.5mm drill bit would be used. The reason for these restrictions is to
allow the rig to be inserted into pipes to test outwards from the inside, as well as providing a small and readily
transportable rig. This allows the test to be taken more easily for structures unable to be transported to the
laboratory.
Figure 2. Layout of the miniaturised DHD rig
The miniaturised test rig can been split up into four main parts, the platform, the tools holder, the rotary motion
and the mounting system. A layout of the machine is shown in Figure 2. The platform was designed to ensure that
linear translation has the main DHD components attached. The drive consisted of a stepper motor driving a lead
screw with a carriage connected. The linear table had 73mm of travel. The solution adopted for the miniaturised
rig was to make the coolant union and the tools holder as one part. Instead of using different modules for each
process, the miniaturised rig uses a common system where only the tools are exchanged. The motor used was a
th
Brushless-Servomotor with an incremental encoder. The resolution approached by this motor is up to 1/1000 of
a revolution. Speed range is from 10 to 10,000rpm for a maximum torque of 47mNm. The bush system has been
designed to achieve three functions: firstly, aligning the rig to the correct position for measurement, also guiding
the different tools to the correct location and finally for collecting and removing fluids used in the drilling and
trepanning processes.
The miniaturised rig performs only a DHD measurement using a 1.5mm reference hole, which gives the ability to
use a less powerful motor. The motor used is about three times smaller and lighter. Added with a hall sensor and
encoder this motor can be used for all three processes (EDM, Air probe and gundrill), as a result set up is much
quicker. This gain in terms of weight due the reduce size motor, the coolant union and the specific design allows
the rig to be self attach by its bush system. The machine setting-up consists of the alignment of a simple cylinder
bush to the specimen, where the rig will be fixed on. The software sets all readings from the measurement into an
excel file. This file transforms automatically the readings into diameter, strain and residual stresses.
APPLICATION OF MINIATURISED DHD SYSTEM
The validation component considered in this paper originated as part of the NET (Neutron Evaluation Techniques)
European network. Four nominally identical base-plates were machined to the dimensions shown in Figure 3
(180mmx120mmx17mm) from a single piece of solution heat treated AISI Type 316L austenitic stainless steel
plate of nominal dimensions 600mmx150mmx50mm. The base plates were then re-solution heat treated in air to
eliminate any machining residual stresses. A single weld bead of nominal length 60mm was deposited along the
centre-line of each plate parallel to the longest edge using an automated Tungsten inert gas (TIG) process.
During welding, the plates were held horizontally across the 120mm dimension by lightly clamping in a vice. This
weld geometry produces a strongly three-dimensional residual stress distribution, with similar characteristics to a
weld repair in a compact portable specimen which is amenable to measurement using diverse techniques. The
single weld pass is relatively straightforward to model numerically. However, because the bead is deposited onto
a relatively thin plate, the predicted stresses are very sensitive to key modeling assumptions such as total heat
input and thermal transient time history, making measurements even more necessary.
This specimen had two principal attractions when viewed as a validation specimen for the new DHD rig. Firstly, a
previous DHD measurement had been performed using the existing DHD rig (at the position labeled DHD1 in
Figure 3). Secondly, an abundance of experimental and numerical data was available from many research
laboratories across Europe for comparison. The paper will concentrate on the application of the DHD procedure to
location DHD2 in Figure 3 (the stop end of the weld) and will present the measurement results and their
comparison to other measurements of residual stress at the same spatial location. The results also permit a
comparison to be made between the through-thickness residual stress distributions at weld mid-length and at the
stop end of the weld. The comparison supports the hypothesis from numerical models that the residual stresses
are greatest at the stop end of the weld, a result which has clear implications when viewed in the context of
structural integrity assessments or repair welds.
Prior to drilling, bushes were adhered to the outer and inner surfaces of the plate for both of the DHD
measurements. Reference holes with diameters of 1.5mm were then gundrilled through the bushes and the plate,
proceeding from the welded surface and penetrating fully through the thickness. The bushes were used to provide
a stress free calibration measurement and to provide reference points to aid in the depth alignment of the
measured
d (! )
and
d ' (! )
values. The diameter of the gundrilled reference hole was then measured at 18
angular positions (in equal intervals of 10°) for each depth increment of 0.2mm using a compressed air probe.
The airprobe provided a direct reading of the reference hole diameter by measuring air pressure, which was in
turn calibrated using pre-fabricated high precision discs containing holes of well-defined radius. A cylindrical core
of material containing the reference hole as its axis was then trepanned using a hollow copper EDM electrode
with a diameter of 5mm. After trepanning, the reference hole diameter was re-measured through the thickness of
the header using the air probe. The measured reference hole distortions were due to the relaxation of residual
stress field contained within the core of material. The analysis technique described in [4] was used to interpret the
distortion measurements and derive the in-plane residual stress components.
A Young's modulus, E, of 171GPa for the weld metal and 195.6GPa for parent material was assumed in the
analysis. For the first DHD measurement, DHD1, weld metal was assumed to penetrate to a depth of 2.9mm. For
the second DHD measurement at the stop end of the weld, DHD2, the core of material extracted as a result of the
measurement procedure was sectioned and polished and revealed a weld penetration depth of 1.49mm. This
depth was accordingly used in the analysis.
Figure 3. Diagram of the NET specimen, showing DHD measurement locations
Figure 4 shows the residual stress measurement of the NET specimen at the centre of the weld and the stop end
of the weld. The residual stresses are shown for the directions transverse and longitudinal to the weld as well as
the associated in-plane shear stress. The residual stresses are shown as functions of depth through the specimen
from the outer weld surface.
350
300
Residual stress, MPa .
250
200
150
100
50
Transverse DHD1
0
Longitudinal DHD1
Longitudinal DHD2
-50
Transverse DHD2
-100
-1
1
3
5
7
9
11
13
15
17
Depth through specimen from the top surface, mm
Figure 4. DHD residual stress measurement results on NET specimen
The longitudinal and transverse residual stresses were similar in profile and both tensile throughout the thickness
of the specimen. The longitudinal residual stresses were the most tensile throughout the thickness of the
specimen. The transverse residual stresses started at a tensile value of about 20MPa at the top surface before
increasing rapidly to reach a peak value of 237MPa at a depth of 1.80mm. The transverse residual stresses then
decreased to reach a compressive value of -40MPa at 10mm after which they gradually increased to reach
50MPa at 15.5mm to drop to zero stress near the unwelded surface.
The longitudinal residual stresses started at a tensile value of about 0MPa at the weld surface before increasing
to reach a peak value of 328MPa at a depth of 1.8mm. The longitudinal residual stresses then decreased to reach
0MPa at 10mm after which they gradually increased to reach 84MPa at 15.5mm and drop into 0MPa near the unwelded surface. It can be seen that the magnitude of the in-plane shear stresses is zero; hence, the principal
stresses are approximately equal to the transverse and longitudinal residual stresses.
Due to the sampling area of the DHD measurement equipment, the residual stresses measured at depths within
0.4mm of the specimen surfaces are prone to large errors. At these depths, surface edge effects occur and the
accuracy of the measured residual stresses was unknown. Therefore, the measured residual stresses over these
depths (i.e. 0.0 – 0.5mm and 16.4 – 17mm) were omitted from the graphical results to avoid confusion.
Figure 5 and Figure 6 show a comparison of the measurement done using the DHD technique (line with
diamond), with FEA (in full line) undertaken by British Energy and a neutron diffraction measurement (in dot line
with square) undertaken by the Open University. It can be seen that there is excellent agreement between the ND
and DHD measurement for the longitudinal and transverse residual stresses.
Comparison of longitudinal residual stress results for C(STOP)-D line
350
BE-FNC nl-kin both
250
OU ND measurements
stress (MPa)
Bristol Univ DHD2 measurements
150
50
-50
0
2
4
6
8
10
12
14
16
y (mm)
Figure 5. Longitudinal residual stress comparison
Comparison of transverse residual stress results for C(STOP)-D line
270
BE-FNC nl-kin both
OU ND measurements
stress (MPa)
170
Bristol Univ DHD2 measurements
70
-30
-130
-230
0
2
4
6
8
10
12
14
16
y (mm)
Figure 6. Transverse residual stress comparison
DISCUSSION
Through-wall profiles of in-plane residual stresses have been measured using the DHD technique in a stainless
steel plate specimen consisting of weld and parent material at a location through the weld centreline and at the
stop end of the weld. The DHD measurements were performed using the existing rig (DHD 1) and the
miniaturised rig (DHD 2), even if the measurements were taken at different location, a very good agreement was
observed between the two machines. The longitudinal residual stresses were found to be tensile throughout the
thickness of the specimen; the transverse is mainly tensile with a small compressive area in the middle and the
end of the weld. A maximum tensile stress of approximately 327MPa was measured in the longitudinal direction at
a depth of 1.8mm from the top weld surface.
Overall, there was acceptable, but not convincing, agreement between numerical predictions and measurements.
The finite element analysis generally overestimated the magnitude of the longitudinal residual stress component.
It should be noted, however, that the numerical predictions of residual stress described in this paper were lower in
magnitude than finite element predictions made by other partners. This was thought due to the use of an
advanced hardening model in the finite element analysis. It also suggested that by further refinement of the
hardening model, better agreement could be attained between predictions and measurements. Further
discrepancy between predictions and measurements may have resulted from the fact that the measurements
were obtained on a different specimen to the one used to provide input numerical data.
CONCLUSION
In order to further develop the deep hole drilling residual stress measurement technique, a miniaturised rig has
been developed which is smaller enough to allow measurements to be made in confined spaces, such as on the
inside of pipes. The layout and principles of this new measurement rig have been presented and discussed. The
new rig was then used to perform residual stress measurements on a European project, NeT, round robin
specimen. The specimen consisted of a 316L stainless steel plate with a single weld bead deposited on the
surface. An initial DHD measurement had been undertaken on this specimen at the weld centre, and the new rig
was used to perform a measurement at the weld stop end. Results were consistent with finite element predictions
and made with other measurement techniques.
References
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