Veqter - exploiting residual stress measurement technique to provide unique structural integrity assessment service (for redaction)
Submitting InstitutionUniversity of Bristol
Unit of AssessmentGeneral Engineering
Summary Impact TypeTechnological
Research Subject Area(s)
Engineering: Manufacturing Engineering, Materials Engineering
Summary of the impact
Residual stresses are the stresses locked into a component during
manufacture. It is essential that the magnitudes of these residual stress
fields are known as they may combine deleteriously with applied loads.
This can lead to premature failure, of a component or structure, at loads
the designer would otherwise view as safe. Researchers at Bristol have
developed a residual stress measurement technique called deep-hole
drilling, which allows measurements of residual stresses both near
the surface and throughout the thickness of the specimen, even for very
large components which other methods are unable to measure. Veqter
Ltd was created in 2004 as a University spin-out company to provide
deep-hole drilling residual stress measurements for industry. The company
has grown [text removed for publication]. Primarily, Veqter is a
service company, undertaking laboratory and on-site residual
stress measurements on safety-critical components using hardware and
analysis algorithms developed at the University. It is the only company
worldwide that offers this facility and its customer base includes EDF
Energy, the Japan Nuclear Energy Safety Organisation, the US Nuclear
Regulatory Commission and Airbus. Veqter's measurements allow these
companies to better understand the structural integrity of safety-critical
The presence of residual stresses can cause components or structures to
fail at lower load levels than anticipated. Residual stress measurements
are therefore needed for safety critical plant, such as those in the
nuclear and aerospace sectors. In thick-sectioned components (perhaps
>100mm thick), however, there was a paucity of reliable measurement
techniques. To address this problem, Bristol's Solid Mechanics
Group has developed the deep-hole drilling technique. This is a semi-
destructive technique requiring only a minimal amount of material removal
from the component being examined. Note, in contrast to destructive
techniques, the overall structural integrity of the component is left
intact allowing further testing or repair and reuse.
Smith (UoB since 1988) started research on the measurement technique and
its basic implementation in the late 1990s. This original technique has
been progressively advanced and refined through the 2000s by Smith, Truman
(UoB since 2000) and Pavier (UoB since 1989). Key underpinning research,
conducted by the University's Group and adopted by its spin-out company
Veqter Ltd for commercial exploitation include the following:
- George (UoB RA 1995-2001) and Smith demonstrated the basic
proof-of-concept measurement technique on welds joining industrial
Welding, the principal joining process between metallic components,
introduces highly tensile residual stresses due to the molten metal
cooling and contracting. This research assesses the accuracy of residual
stress measurements on a selection of components originating from the
nuclear industry, which leads the way in assessing safety critical
components. Whilst a handful of residual stress measurement techniques
already exist, such as neutron diffraction, slitting and ring-core they
are all limited to thin sections. This paper demonstrated that deep-hole
drilling is capable of providing reliable through-thickness measurements
in large metallic components.
- Kingston (UoB RA 1998-2004 and Veqter since 2004), Smith and Truman
developed the technique to measure geometrically complex industrial
The original deep-hole drilling analysis utilised an underlying
elasticity solution for the stresses around a hole in an infinite plate
subjected to far field loading and so was only applicable to simply
shaped components. This research removed these restrictions by advancing
the theoretical and experimental procedures. These advances were
demonstrated by taking accurate experimental residual stress
measurements for a series of complex shaped industrial components, such
as nozzle-pipe interactions and inclined penetration welds.
- Nakhodchi (UoB RA 2005-11), Smith, Truman and Pavier extended the
technique to measure residual stresses in non-metallic materials, e.g.
composites, graphite and glasses .
The underpinning solution used for data analysis in deep-hole drilling
had made the assumption that the component being measured was fabricated
from isotropic materials. In order to extend the method to provide
measurement results in anisotropic materials, such as composites, major
extensions to the data analysis were required. This work provided those
extensions and successfully applied deep-hole drilling to a composite
component originating from the aerospace industry.
- Mahmoudi (UoB RA 2000-2008), Pavier, Truman and Smith developed a
method to measure samples containing near-yield magnitude residual
stresses accurately [5,6].
With improvements in the modelling of residual stresses and the demand
to measure ever more complex components (such as dissimilar metal
welds), it was desirable to move away from employing the underpinning
elasticity assumption. If possible, this would facilitate the
measurement of specimens where residual stress magnitudes were high
relative to the yield stress of the material and triaxial in nature
(with significant residual stress components in all three coordinate
axis directions). This research addressed this problem and provided a
formulation and methodology to apply the deep-hole drilling technique
irrespective of residual stress magnitude and triaxiality.
References to the research
 *D.J. Smith, P.J. Bouchard and D. George, 2000. Measurement and
prediction of residual stresses in thick section steel welds.
Journal of Strain Analysis for Engineering Design, 35(4), 483-497,
 D. Stefanescu, C.E. Truman and D.J. Smith, 2004. An integrated
approach for measuring near and sub-surface residual stress in
engineering components. Journal of Strain Analysis for Engineering
Design, 39(5), 483-497, dx.doi.org/10.1243/0309324041896524.
 S. Hossain, C.E. Truman, D.J. Smith and P.J. Bouchard, 2006. Measurement
of residual stresses in a type 316H stainless steel offset
repair in a pipe girth weld. ASME Journal of Pressure Vessel
Technology, 128(3), 420-426, dx.doi.org/10.1115/1.2218346.
 M.G. Bateman, O.H. Miller, T.J. Palmer, C.E.P. Breen, E.J. Kingston,
D.J. Smith and M.J. Pavier, 2005. Measurement of residual stress in
thick section composite laminates using the deep-hole method.
International Journal of Mechanical Sciences, 47(11), 1718-1739,
 *A. Mahmoudi, S. Hossain, C.E. Truman, D.J. Smith and M.J. Pavier,
2009. A new procedure to measure near yield residual stresses using
the deep hole drilling technique. Experimental Mechanics, 49(4),
595-604, dx.doi.org/10.1007/s11340-008-9164-y (listed in REF2).
 *A.H. Mahmoudi, C.E. Truman, D.J. Smith and M.J. Pavier, 2011. The
effect of plasticity on the ability of the deep hole drilling technique
to measure axisymmetric residual stress. International Journal of
Mechanical Sciences, 53(11), p78-988,
dx.doi.org/10.1016/j.ijmecsci.2011.08.002 (listed in REF2).
* References that best indicate the quality of the underpinning research.
Details of the impact
Undertaking fully through-thickness residual stress measurements in
thick-sectioned industrial components is expensive, whichever technique is
employed. Neutron diffraction measurements are often viewed as the nearest
measurement alternative to deep-hole drilling, as they are able to measure
sub-surface residual stresses in crystalline materials. However, neutron
diffraction is only able to make measurements to a depth of approximately
30mm in steel and commercial costs of neutron beam time are in the region
of £15-20k per day. Bristol's deep-hole drilling technique, in contrast,
can be used to take residual stress measurements on much thicker
components. To date, the deepest measurement - 750mm - was taken on a
steel turbine for Siemens [a]. Costs are primarily determined by the time
to undertake the measurement. As an example, an on-site measurement on a
large nuclear component may take a week to perform.
Using the University's deep-hole drilling technique, and its associated
data analysis methods, Veqter Ltd provide commercial residual stress
measurements for use in structural integrity assessments. The company
undertakes the measurement service on structural-level samples sent to the
laboratory or, for larger components, conducts the service in-situ at the
plant. The company was formed in 2004 by Kingston (UoB 1998-2004) when it
became apparent that the quick turnaround of measurements demanded by
industry, coupled with confidentiality requirements, necessitated the need
for an independent, commercial spin-out company. Smith and Truman remain
non-executive directors on the Board of Veqter. Since the company was
formed, the key impact indicators are [b]:
- The growth from [text removed for publication] employees
- The growth in annual turnover [text removed for publication].
- The steady increase in the client base from across the world,
including Japan, America and France.
- Veqter appointing a dedicated Japanese agent, IIC, in 2008 to extend
operations in this country.
In addition to the direct economic impact for Veqter, its unique
measurement service, often applied to safety-critical hardware, has
significant impact for its clients. The through-thickness residual stress
profile measurements allow the client to undertake detailed fitness for
service assessments or to validate numerical simulations. Examples
relating to the design and repair of components in the nuclear industry
- In 2010-11, deep-hole drilling measurement results were used in a
series of plant life extensions for EDF Energy (formerly British
Energy Generation Ltd) to help extend the life of the current fleet of
advanced gas-cooled nuclear reactors.
As an example, full structural weld overlays have been used extensively
to repair, or to mitigate against, primary water stress corrosion
cracking at dissimilar metal welds in pressurized nozzles. To support an
approved weld overlay design and safety submission, British Energy
commissioned Veqter to perform tests on pressurized water reactor
nozzles. Specifically, a detailed assessment of the effects of a weld
overlay on the through-wall residual stress distribution in these
nozzles was conducted using deep-hole drilling. This study allowed EDF
to determine the most appropriate form of weld overlay to apply to their
plant to maximize confidence in the structural integrity case [b,c].
- The US Nuclear Regulatory Commission are employing deep-hole
drilling to measure and validate through-wall weld residual stresses
that develop at the J-groove welds in nuclear control rod mechanisms
The Commission and the Electric Power Research Institute worked
cooperatively to validate residual stress predictions at welds between
primary cooling loop components. The residual stress distribution for
the prototype component was predicted numerically using different
techniques. Veqter provided experimental measurements for comparison.
The Commission noted, in 2011, that due to the thickness of the
components "The only non-destructive or semi- destructive technique
capable of obtaining full through the wall residual stress ... is the
VEQTER Ltd proprietary Deep Hole Drilling technique" [d]. The
results of this study showed that, on average, analysts can develop
welding residual stress predictions that are a reasonable estimate for
actual configurations, as quantified by Veqter's measurements. However,
the scatter in predicted results from analyst to analyst could be quite
large. These results allowed the Commission to understand the
limitations of numerical weld simulation models and hence place an
appropriate error on numerical results [b,e].
- The Japan Nuclear Energy Safety Organisation has employed
deep-hole drilling in a series of key national nuclear safety projects
As an example, in recent years, the occurrence of stress corrosion
cracking in Alloy 600 weld regions of pressurized water reactor plants
has increased. In order to evaluate crack propagation mechanisms, it was
necessary to estimate the stress distribution, including residual stress
and operational stress, across the wall thickness in the region of the
weld. In a national project, for the purpose of establishing the optimum
residual stress evaluation method, a test model was produced. The stress
distribution, resulting from fabrication processes, was measured by
Veqter using the deep-hole drilling technique. The results showed that
the stress distribution, in the thickness direction at the centre of the
weld line, hardly varied during a hydrostatic test. This insight allowed
the Safety Organisation to validate the numerical safety simulations
performed by plant fabricators Toshiba, Hitachi and MHI [b].
AREVA plc have placed high value on the use of Veqter's
deep-hole drilling measurements in the development of narrow gap TIG
For new nuclear builds, AREVA has developed an advanced narrow gap
welding technique for junctions between heavy section steel components
and stainless steel piping. The company used finite element simulations
to predict the residual stress field of a 29" multipass narrow gap weld.
These numerical results were compared to Veqter's measurements obtained
by the deep-hole drilling technique. This comparison provided data that
allowed AREVA to gain a better insight into the error levels associated
with numerical weld simulation modeling [b,f].
Sources to corroborate the impact
[a] Diesel & Gas Worldwide article Measuring the Stress in
Turbines, April 2012.
[b] Managing Director, Veqter.
[c] Former Principal engineer, EDF Energy.
[d] U.S. Nuclear Regulatory Commission: Weld Residual Stress
Measurements for Reactor Vessel Bottom Mounted instrument and Control
Rod Drive Mechanism Nozzles purchase order announcement, August
[e] Chief - Component Integrity Branch, U.S. Nuclear Regulatory
[f] AREVA Expert in Mechanics & Materials, AREVA.