Recent developments to improve the quality and economics of testing for fatigue and fracture
We have developed a number oftechnologies aimed at rendering test equipment affordable and at the same time reliable and well-supported. These include technologies related to actuator servodrives, energy efficient pumps, modular control and instrumentation electronics and open-source software that i...
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Інститут проблем міцності ім. Г.С. Писаренко НАН України
2009
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| Цитувати: | Recent developments to improve the quality and economics of testing for fatigue and fracture / R. Sunder // Проблемы прочности. — 2009. — № 1. — С. 13-28. — Бібліогр.: 5 назв. — англ. |
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Sunder, R. 2013-08-20T04:40:48Z 2013-08-20T04:40:48Z 2009 Recent developments to improve the quality and economics of testing for fatigue and fracture / R. Sunder // Проблемы прочности. — 2009. — № 1. — С. 13-28. — Бібліогр.: 5 назв. — англ. 0556-171X https://nasplib.isofts.kiev.ua/handle/123456789/48480 539.4 We have developed a number oftechnologies aimed at rendering test equipment affordable and at the same time reliable and well-supported. These include technologies related to actuator servodrives, energy efficient pumps, modular control and instrumentation electronics and open-source software that is efficient in real-time operation. Представлены новые технологии, направленные на разработку недорогого, но надежного испытательного оборудования с обеспечением технической поддержки со стороны разработчика. Указанные технологии относятся к сервоприводам, экономичным насосам, модульному управлению, электронным приборам и программному обеспечению с открытыми кодами, работающему в реальном масштабе времени. The author deeply appreciates valuable inputs from partners and colleagues in the industry, R&D and academia in the U.S., Russia, Ukraine, Malaysia and India many of whom shared the concerns that motivated the effort described in this paper. Ramesh Korradi provided some of the data for this review. en Інститут проблем міцності ім. Г.С. Писаренко НАН України Проблемы прочности Научно-технический раздел Recent developments to improve the quality and economics of testing for fatigue and fracture Новейшие разработки, направленные на повышение качества и экономичности испытаний на усталость и разрушение материалов Article published earlier |
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Digital Library of Periodicals of National Academy of Sciences of Ukraine |
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Recent developments to improve the quality and economics of testing for fatigue and fracture |
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Recent developments to improve the quality and economics of testing for fatigue and fracture Sunder, R. Научно-технический раздел |
| title_short |
Recent developments to improve the quality and economics of testing for fatigue and fracture |
| title_full |
Recent developments to improve the quality and economics of testing for fatigue and fracture |
| title_fullStr |
Recent developments to improve the quality and economics of testing for fatigue and fracture |
| title_full_unstemmed |
Recent developments to improve the quality and economics of testing for fatigue and fracture |
| title_sort |
recent developments to improve the quality and economics of testing for fatigue and fracture |
| author |
Sunder, R. |
| author_facet |
Sunder, R. |
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Научно-технический раздел |
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Научно-технический раздел |
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2009 |
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English |
| container_title |
Проблемы прочности |
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Інститут проблем міцності ім. Г.С. Писаренко НАН України |
| format |
Article |
| title_alt |
Новейшие разработки, направленные на повышение качества и экономичности испытаний на усталость и разрушение материалов |
| description |
We have developed a number oftechnologies aimed at rendering test equipment affordable and at the same time reliable and well-supported. These include technologies related to actuator servodrives, energy efficient pumps, modular control and instrumentation electronics and open-source software that is efficient in real-time operation.
Представлены новые технологии, направленные на разработку недорогого, но надежного испытательного оборудования с обеспечением технической поддержки со стороны разработчика. Указанные технологии относятся к сервоприводам, экономичным насосам, модульному управлению, электронным приборам и программному обеспечению с открытыми кодами, работающему в реальном масштабе времени.
|
| issn |
0556-171X |
| url |
https://nasplib.isofts.kiev.ua/handle/123456789/48480 |
| citation_txt |
Recent developments to improve the quality and economics of testing for fatigue and fracture / R. Sunder // Проблемы прочности. — 2009. — № 1. — С. 13-28. — Бібліогр.: 5 назв. — англ. |
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2025-11-24T21:03:16Z |
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2025-11-24T21:03:16Z |
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| fulltext |
UDC 539.4
Recent Developments to Improve the Quality and Economics of Testing
for Fatigue and Fracture
R. S under
BiSS Research, Bangalore, India
We have developed a number o f technologies aimed at rendering test equipment affordable and at
the same time reliable and well-supported. These include technologies related to actuator servo-
drives, energy efficient pumps, modular control and instrumentation electronics and open-source
software that is efficient in real-time operation.
K e y w o r d s : servo-hydraulic test equipment, power packs, control hardware and
software.
In tro d u c tio n . Several factors govern progress o f global industry:
1. Advances in computational techniques and simulation continuously increase
operational stress levels in order to reduce weight. This is aimed at improving
performance, while at the same time reduce the cost o f raw material. Reduced raw
material means lower production cost, reduced recycling overheads and increased
operational efficiency o f the product.
2. Advances in fatigue design improve durability and safety, even at reduced
structural weight.
3. Due to globalization, consumer products as w ell as industrial goods are
increasingly sourced from vendors on the promise o f acceptable quality at lower
prices.
4. As globalization enlarges the num ber o f competitors, time-to-market
assumes greater significance as individual firms attempt to be the first to market a
new product.
A ll the above factors demand more widespread use o f test equipment to
confirm viability o f new designs, their quality, performance, safety as well as
durability.
Until some years ago, m echanical testing machines were a niche market
controlled by a handful o f vendors, who could price at will, citing unverifiable
costs o f developing new and advanced technology. However, the exploding global
demand for test equipment has expanded the stage o f equipment vendors, with
access to advances in digital technology as well as to global partnerships between
small enterprises empowered by the Internet as w ell low cost o f movement of
people, goods and services.
BiSS is one such developer o f test technologies that enjoys a vibrant
relationship w ith partners across the globe. Our objective is to come up w ith test
solutions that are modular, affordable and easily supported, in order that m ore and
m ore enterprises across the globe can gain access to test technology that until
recently appeared unaffordable. The purpose o f this paper is to describe a variety
o f technologies either developed, or, integrated at BiSS Research that contribute
to the stated objective. These relate to hydraulics used in universal testing
© R. SUNDER, 2009
ISSN 0556-171X. Проблемы прочности, 2009, № 1 13
R. Sunder
machines, grips and fixtures used in testing, control and instrumentation electronics
as well as system and application software. The goal is to demonstrate the
different means by w hich test technology and testing itself are being m ade more
affordable to reduce the life-cycle cost o f test equipment and thereby empower
educational institutions, R&D and industry to become more productive and
competitive in an increasingly globalized framework.
1. C o n tam in a tio n Insensitive S ervo -H ydrau lics. Fluid pow er is well
entrenched as the m ost popular means o f achieving the w idest range o f both force
variation as well as velocity in m echanical testing. Fluid pum ped into a hydraulic
actuator causes it to m ove in the desired direction. The desired force is achieved
by building required pressure against specimen resistance. Testing machines
based on this principle have been in use from the late 19th century in order to test
materials used in machines and structures during the boom o f the industrial
revolution [1]. The first m ajor transform ation in these machines occurred during
W W II as electronic force and displacement indicators using strain bridge and
LVDT transducers enabled the direct plotting o f test results onto X — Y recorders.
In the fifties, W illiam Moog invented what w e know today as the two-stage
M oog servo-valve [2]. W ithin a very short span o f a few years, this valve became
a critical component o f aircraft and m issile flight control systems. The innovation
behind this invention (Fig. 1) was in making it possible for m W o f electrical
energy to control m any kW o f hydraulic power required to deflect aerodynamic
control surfaces such as ailerons, rudders and elevators. Their low mass, combined
with the low mass o f wiring required to drive them, significantly increased the
performance o f airborne systems. On the flip side, such valves demanded a
contamination free hydraulics system. Even particles as small as 10-15 microns
can choke the tiny arteries o f the valve’s sophisticated pilot stage. The high cost
o f the proprietary valve, combined with the increased cost o f support components
such as high-pressure filters were easily justified by the returns in terms of
aircraft performance and flight safety.
C O N TR O L SUPPLY ORIFICE
PORT PRESSURE
PORT
Fig. 1. The dual-stage servo-valve [2]. The remarkable features of this valve are the miniature
flapper and microscopic arteries and orifices in the pilot stage that allow high pressure flow control
using miniscule currents. This also makes the valve extremely sensitive to oil contamination.
Over the years that followed, the 2-stage Moog servo-valve became an
indispensable part o f servo-controlled hydraulic systems not only in aerospace
applications, but also in surface transportation and industry. These include steel
14 ISSN 0556-171X. Проблемы прочности, 2009, N 1
Recent Developments to Improve the Quality and
rolling mills, w ind tunnels, injection m olding machines and o f course, servo-
hydraulic universal testing machines.
The first servo-hydraulic testing m achines appeared in the early sixties. Even
though almost five decades have passed since then, the best known manufacturers
o f testing machines continue to use the same expensive dual-stage servo-valve
technology. In the m eantim e some industrial applications switched to the less
expensive and less demanding “direct-drive valves” (DDVs) single stage
proportional valves, where electromagnetic force is used to position the flow
control spool. The eighties saw the appearance o f a new generation o f servo
proportional valves based on high pow er rare earth magnets [3]. The performance
o f these valves is lower than the two-stage valves (Fig. 2), but these are less
expensive, m ore robust and contamination insensitive, m aking their adaptation to
testing machines an economically attractive prospect.
b
Fig. 2. Performance curves for dual-stage valve (a) [4] and single stage direct-drive valve (DDV)
[5]. While the dual stage valve can operate at frequencies up to a few hundred Hz, the DDV (b) is
effective only up to about 100 Hz. Most cyclic test requirements seldom demand more than 50 Hz.
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R. Sunder
In the early nineties, BiSS Research developed its first servo-actuators with
DDVs. W ith suitable changes to electronics and hydraulics, these valves were
found suitable for fatigue cycling at up to 100 Hz and in terms o f precision
control, even superior to the two stage valves because of their reduced hysteresis
and im proved linearity in flow control. Moreover, as they are virtually insensitive
to contamination, high-pressure fine filtration is not required, rendering the
solution less expensive to install and maintain and at the same time, more reliable.
M ulti-stage servovalves using DDVs at the pilot stage deliver several hundred
LPM o f flow and enable test velocities as high as 7 m/s. BiSS experience with
these valves extends from low force actuators dow n to 5 kN and as high as
1000 kN. One m ay note that none o f these applications w ould have exhibited
improved performance with dual-stage valves given their end-use. The application
o f DDVs has reduced the cost o f test equipment by comparison to that o f brand
names that continue to use the older dual-stage technology. W hat is more, even in
the absence o f pressure line filtration, for over fifteen years now and about a
m illion hours o f cumulative usage experience, there have been no reported cases
o f valve failure. This indicates why DDVs are preferred in terms o f life-cycle cost
as well.
In summary, DDVs make test equipment more affordable by the use o f fewer
and less expensive components on a test system, w ithout compromising the
performance and quality requirements o f the test process. They also reduce
operational costs and increase system reliability.
2. V ariable Speed Pum ps. Hydraulic power is usually delivered by gear
pumps, vane pumps, radial or axial piston pumps. The first two are fixed flow,
while the last two m ay be made variable flow through the inclusion o f a swash
plate that controls piston stroke. Piston pumps are typically m uch more expensive
than gear or vane pumps and variable flow technology m akes them even more
expensive. Typically, at flow rates less than 80 LPM variable delivery pumps are
seldom used as they turn economically unviable. M ost universal test systems
require pump capacity between 4 and 65 LPM. Therefore m ost such machines use
fixed delivery pumps, unless m ultiple test systems are connected to the same
pump, justifying a larger capacity variable delivery option.
The pump is usually selected to m eet the highest sustained performance
requirem ent o f the servoactuator. Assuming that over m ost o f its usage, the
machine will be operated at speeds that are m uch less than its required peak
performance, m uch o f the pow er from the pump will be dissipated as heat as
excess high pressure flow is released through the re lief valve. Apart from the
energy wasted in this process, more energy needs to be spent on cooling the
heated oil. These m ake m ost conventional servo-hydraulic test systems rather
inefficient and therefore more expensive to operate. On an average as high as
80% o f the connected power m ay be wasted as heat. Thus, on a 40 LPM pump, as
m uch as 15 kVA o f consumed pow er m ay be dissipated as heat. This leads to the
need to have large, closed loop cooling systems that are expensive to install and
also consume another 2 -3 kVA o f power.
Some eight years ago, BiSS Research developed the concept o f servo-
controlled pumps for its range o f digitally controlled universal testing machines.
Their development is made possible by the appearance in the m arket o f so called
16 ISSN 0556-171X. Проблемы прочности, 2009, N 1
Recent Developments to Improve the Quality and
variable frequency drives (VFDs). W hen combined w ith a suitably designed
servo-circuit that uses system line pressure as feedback, along w ith a set o f safety
limit interlocks to protect both the drive as well as the pump and m otor assembly,
one can achieve constant pressure on the system through automatic regulation of
pump r.p.m. Thus, the pump now consumes only as m uch pow er as is required to
m eet the instantaneous hydraulic flow requirem ent on the servoactuator.
Over the years, BiSS has designed VFD driven pumps with flow rates
ranging from 4 to 400 LPM. Industry feedback suggests that they save up to 70%
in consumed energy, reduce cooling requirements and are also a lot quieter than
conventional pumps. Figure 3 shows an example o f these pumps and their drives.
As will be shown later, sm aller 4 LPM pumps require no cooling at all - the
lim ited amount of heat generated on the system is dissipated by the exposed sides
of its enclosure.
Fig. 3. 4 X 65 LPM fixed delivery pumps (left) driven by four 30 kVA variable frequency drives
(VFDs). The programmable logic controller (PLC) controlling the four VFDs ensures that only that
fraction of installed 120 kVA that is really required is used at any given instant. This results in
considerable reduction in energy consumed and also in energy dissipated as heat. The cooling
system for this power pack is comparable to that used in conventional pumps of considerably lower
capacity. Use of commercially available off-the shelf (COTS) components such as PLCs and VFDs
ensures easy maintenance with readily available spares.
In summary, the technology of servo-control on pump m otor speed makes
servo-hydraulic test systems considerably more efficient than ever before. This
“green” solution also provides m any other benefits such as reduced operational
cost, lower noise in operation and possible increase in pump life. Also, measured
pump parameters can serve condition-monitoring purposes.
3. Self-Aligning Low-Cycle Fatigue G rips. Low-cycle fatigue (LCF) testing
demands the capability to enforce tension-com pression loading far beyond the
theoretical buckling load of the specimen. W ithout this capability, one can neither
obtain the cyclic stress-strain curve for a material, nor determine fatigue properties
under conditions o f cyclic plastic strain (see Fig. 4a).
LCF testing demands near-perfect alignment of the load train to ensure the
absence o f bending component from off-center loading. ASTM E 606 prescribes a
5% limit on bending stresses. To m eet this requirement, conventional test systems
are supplied w ith an alignment kit. The kit includes instruments to measure
coaxiality and colinearity of the top and bottom gripping on the test system and
an alignment correction unit placed between the top crosshead and load cell that
ISSN 0556-171X. npodxeMbi npounocmu, 2009, N 1 17
R. Sunder
permits controlled shifting o f the top axis as well as tilt correction i f required. The
kit also includes an instrumented specimen that can sense dual-plane bending
moments arising from potential m isalignm ent, along with the electronics to
condition the transducer outputs and the data acquisition to measure and display
quality o f alignment.
Be-ucl X (N-min) Strain
d
Fig. 4. (a) LCF data require tension-compression testing under cyclic plastic strain. Excellent
specimen alignment is required to make this possible. (b) Schematic of hydraulic self-aligning grips
developed at BiSS Research. The hydraulically locked spherical seat eliminates specimen backlash
and bending moment at start of test. Hydraulic wedge locks grip to actuator at bottom and load cell
at top. (c) Typical X and Y bending moment during cycling at ±10 kN load using self-aligning
grips. Bending stress from these moments are les than 5% of applied stress, which meets the
requirements of ASTM E 606. (d) Typical cyclic stress-strain response with self-aligning grips.
Note the absence of zero-crossover glitch and symmetry of loading/unloading response.
Once a system is aligned using the above procedure, coaxiality is unlikely to
change because the rigid fixtures are preloaded and locked. However, any
m ovem ent o f the crosshead followed by reclamping m ay introduce angular
misalignment. Further, system alignment by itself m ay not serve as a guarantee o f
specimen alignment. Specim en axiality and threading tolerances m ay introduce
variations in alignment between specimens, that the above alignment procedure is
not in a position to address.
Figure 4b describes an engineering solution developed at B iSS Research that
attempts to address the demands o f specimen-to-specimen realignment. Both the
18 ISSN 0556-171X. npodxeMbi npounocmu, 2009, N 1
Recent Developments to Improve the Quality and
top and bottom grip consist o f three important elements. A hydraulic lock is
provided between grip and actuator/load cell to ensure backlash free grip
mounting. A spherical seat on the specimen pull rod ensures both angular and
lateral axis re-alignm ent of each individual specimen through tensile cycling at
small loads. A hydraulic press with pressure control acts as a hydraulic lock on
the specimen and spherical seat in order to guarantee zero-backlash rigid response
in tension-com pression loading.
Figure 4c shows the results o f random alignment checks using an instrumented
specimen. Without exception, bending stresses appear to be well within prescribed
norms. In addition to the advantage o f a lower cost solution that avoids the need
for additional fixtures and instrumentation, the self aligning grip offers a few
other advantages that are noteworthy. It can be readily converted into a high
temperature version by using temperature resistant pull rods. M ounting and
dismounting the grip or the specimen is m uch faster than on conventional grips.
And finally, the solution guarantees alignment of each new specimen, irrespective
o f its own dimensional variations. This is because in tension-tension cycling at
small load, those very moments that are caused by misalignm ent work towards
re-alignm ent of the spherical seat.
Figure 4d shows typical cyclic stress-strain response under incremental strain
cycling.
4. “ G auge F ree” D isp lacem ent M easurem ent. All analog transducers share
a common shortcoming. Their measurem ent precision is determined by gauge
length. Thus a 0.1% precision on a gauge length o f 10 mm w ould m ean error of
10 m icrons, while on a gauge length o f 100 mm, it would be 100 microns.
It follows from the above that quality test results require that transducer
ranges need to be not m uch greater than what is required for a given test.
Obviously, i f fatigue cycling needs to be perform ed between 2 and 4 kN, a 10 kN
test system will ensure one-tenth the error one would observe on a 100 kN test
system.*
All servo-hydraulic test systems are equipped with LVDTs on the actuator
that provide stroke readout. These are typically between ± 5 0 and ± 150 mm range.
Errors in stroke readout would thus be in the region o f about 100 microns, which
is quite unacceptable when testing specimens for modulus or fracture mechanics.
These tests require additional clip-on or non-contact transducers that measure
displacement over specimen gauge length. Thus, extensometers are used in
tension and LCF testing, while COD gauges are used in fracture mechanics
testing. The error in such transducers is o f the order o f only a few microns
because the displacement range is only a few millimeters.
Room tem perature transducers are affordable. However, high temperature
extensometers and CO D gauges are not only expensive, but also delicate and
therefore prone to damage in repeated usage. I f it were possible to use the stroke
transducer to obtain quality readouts of specimen gauge length displacement, the
cost and effort of testing would be considerably reduced. Also, this would make
* One needs to note in this regard that any attempt to sub range the transducer either through
amplification in electronics or data acquisition will only improve resolution. Noise and drift will
increase proportionately with signal amplification.
ISSN 0556-171X. npo6neMbi npouHocmu, 2009, № 1 19
R. Sunder
testing in aggressive environm ents possible as would be the case with corrosion
bath and also with irradiated specimens that pose a safety hazard in gauge
mounting.
As m entioned earlier, the stroke range in test systems is one or two orders of
magnitude greater than that o f the displacement range required in most mechanical
tests. The other problem associated with a “rem ote” m easurem ent such as stroke
is its sensitivity to the displacement response o f the test system as a whole. As
shown in Fig. 5, application o f load will cause deformation o f not only the test
specimen, but also the load frame as well as the load train including actuator rod
and load cell, pull rods, grips and the several fasteners holding them together. The
latter also introduces a non-linear element into the stroke versus load relationship.
Fig. 5. (a) COD gauge mounted on SE(B) specimen as required by relevant test standards. The
gauge provides accurate measurement of crack opening displacement, but is unsuitable for
aggressive environments that may damage the gauge. (b) COD gauge mounted using special fixture
to accurately measure actuator displacement (stroke). A digital linear encoder may also be used for
the purpose. This location is away from the specimen area and is immune to test environment. (c)
Specimen COD response compared with values deduced (corrected) from remote measurement
using linear encoder on actuator rod. Similar data would be obtained from gauge mounted as shown
in (b). (d) Crack size computed from specimen COD and from actuator position inputs. Note the
similarity in estimates, suggesting viability of alternate measurements.
20 ISSN 0556-171X. npo6neMbi npouHocmu, 2009, № 1
Recent Developments to Improve the Quality and
Assuming a fixed crosshead position and assuming all the load train
components are tightly fastened, one may expect the stroke versus load relationship
to be reproducible even i f non-linear. This still leaves the problem o f stroke
precision which m ay be resolved either by using a shorter range displacement
transducer, or, by resorting to a state-of-the-art digital encoder based measurement
o f stroke that guarantees resolution and precision that is independent o f stroke
range. Presented below are results from attempts to use both approaches. The
experiments were perform ed w ith 3-point single-edge bend (SE-B) specimens
tested in fatigue pre-cracking followed by fracture test.
Figure 5 shows test results using a 0.1 m icron digital encoder measuring
stroke. Similar results were obtained using a COD gauge m ounted on the
servo-actuator to m easure its m ovem ent (Fig. 5b). In both cases, an extremely
stiff un-cracked dummy specimen was used to obtain the stroke versus load
relationship. Then, during testing with the real specimen in place, stroke computed
as a function o f load from this relationship was subtracted from the stroke readout
in order to obtain specimen related displacement.
From the results in Fig. 5c, d, one m ay summarize that reasonable estim ates
o f specimen response can be obtained from actuator stroke measurement. The
quality o f these m easurem ents is determ ined by precision o f the remote
m easurem ent as well as the reproducibility o f the stroke versus load response for
the dummy (stiff) specimen. It appears desirable that a standard practice be
evolved to perm it valid industrial quality tests w ithout having to use gauges on
the specimen. This would reduce the cost o f testing and perhaps also improve
productivity. And it would significantly improve productivity in test labs associated
with irradiated specimens such as in nuclear reactor safety assurance.
5. P rocessor-Independent M od u lar H ardw are . Servo-hydraulic test systems
find applications in the m ost diverse areas. Typical examples include materials
test systems to determine strength and durability o f materials, high performance
testers for elastomers, low rate systems for stress-corrosion, cyclic creep and LCF,
moderate force high velocity test systems for suspension components, biaxial test
systems including axial-torsion systems, single and m ulti-channel structural test
systems using several actuators and tightly coupled high performance test systems
such as Stewart Table (6 actuators) and earthquake simulators (8 actuators).
Control hardware and software are the heart and soul o f all these systems. This
raises the question about how to m odularize both the hardware and software such
that a collection o f common modules can m eet all o f the above requirements.
M odularization is important for several reasons. Use o f common hardware
and software components across the wide variety o f applications described above
implies reduced production as well as spares support cost due to volume and
common inventory considerations. It m eans that hardw are technicians are
invariably familiar w ith hardware set up, support and troubleshooting because it
does not differ between applications. Finally, i f it is indeed possible at all to have
common software modules between applications, it can have a favourable impact
not only on the cost o f application software, but also on the quality o f customer
support, not to mention ease o f migration for the user from one application to
another. In the pursuit o f such goals, the developer needs to ensure that individual
modules seldom contain features that are redundant to m any users. Failure to do
ISSN 0556-171X. Проблемы прочности, 2009, N 1 21
R. Sunder
so w ould escalate costs for users who end up paying for something they do not
have use for.
In an attempt to m odularize both hardware and software to m eet the above
objectives, BiSS Research came up w ith a few hardware and software solutions,
w hich over the years have shown a noticeable degree o f success. A m otherboard
w ith components required on m ost test equipment along w ith six plug-in slots for
add-on cards was developed as a common hardw are platform for all the
applications listed above. A combination o f six different add-on cards were
developed such that a suitable m ix o f these cards can address all o f the
applications listed above. These are essentially digital signal conditioning and
data acquisition cards to handle different transducers used on testing machines,
including LVDTs, load cells and other strain bridges, encoders and accelerometers.
W ith suitable packaging o f the cards and connectors, up to sixteen independent
data acquisition channels along with signal conditioning can be connected to a
single card, expanding instrumentation channel count capacity to over 100 on the
controller. Controller firmware and application software were suitably designed to
handle all o f the combinations that m ay arise from the hardware modularity.
Finally a num ber o f software modules were developed whose functionality is not
affected by channel count or hardware configuration. These include log mode
synchronous data acquisition across up to 150 channels at speeds o f up to 100 kHz
(cumulative), m ulti-channel control and command waveform generation from one
to 32 channels, and multi-panel, m ulti-channel real-time graphics to display
collected real-time data.
6 . Single F o o tp rin t Test System s. Conventional perception o f a servo-
hydraulic universal test system is that o f separate load frame, hydraulic power
pack and control console. N ot readily apparent in the scheme are the need for
three phase pow er supply and water or air cooling utilities to support system
operation.
As shown in the previous section, energy efficient servo-controlled pumps
dissipate less heat and are less noisy. Low perform ance test systems such as LCF
test systems and lower force rating test systems, typically 25 kN and less, require
sm aller pumps. Obviously, such pumps will dissipate even less heat, raising the
possibility o f completely doing away w ith active cooling and its associated cost
and energy consumption. On the other hand, new er pump assemblies that have
appeared in the m arket are extrem ely com pact and com petitively priced, thanks
to the pressure die casting technology that permits integration o f several hydraulic
components into a single compact manifold.
Figure 6 shows how the above technologies perm it the integration o f the load
frame, pump and console into a single footprint scheme. The load frame itself
uses hollow pipe columns that are light weight and also act as conduits for sensor
cables. High strength Al-alloy crosshead and platen coupled w ith these columns
m ake a stiff load frame rated to 25 kN dynamic capacity.
The bottom platen o f the load frame is at an elevated height to suit the
convenience o f the operator. The enclosure below the platen houses the servo-
actuator, hydraulic pow er pack as well as the controller. The sides o f the enclosure
act as radiators to cool the oil. The entire system weight is below 175 kgs and is
packaged for air shipment. It operates at ambient temperatures up to 35° C and
22 ISSN 0556-171X. npodxeMbi npounocmu, 2009, N 1
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Fig. 6. Fully integrated standalone digitally servo-
controlled test system, suitable for tension/compression,
fatigue and fracture testing. Once plugged in to a 220 V
mains socket, the system is ready to use. The load
frame is rated to 25 kN. The hydraulic power pack and
digital controls are located in the lower half of the
enclosure. The system is connected to any industry-
standard notebook or desktop PC. With force rating
between 10 and 25 kN and frequency response up to
100 Hz, the system may be used to perform most
materials testing that is normally carried out on much
larger conventional test systems. Heat dissipation from
the side walls of the enclosure is adequate to cool the
power pack. Weighing in at under 200 kgs, the system
may be readily relocated for demonstrations in a
lecture hall or seminar room.
draws single-phase 220 V mains supply. It is suitable for tension/compression
testing as well as for fatigue and fracture testing at up to 100 Hz.
Systems such as the one in Fig. 6 appear to change the w ay one sets up a
testing laboratory, or even how one budgets experimental research. Unlike
conventional test systems, they do not require special flooring or utilities such as
three-phase mains supply, special grounding and w ater cooling, not to mention
access and m aterial handling equipment that is demanded by heavier machines.
Finally, the absence o f external cabling and disappearance from view o f oil and
water hoses m akes the system an unobtrusive and less conspicuous part o f the
laboratory premises, m uch like other office equipment. The aesthetics o f this
solution combined with its reduced life-cycle cost were made possible by several
recent advances in technology.
7. In teg ra tin g L arge C hannel C o u n t In s tru m en ta tio n w ith Test C ontrol.
M ost m echanical tests involve the use o f standard transducers to measure
gaugelength displacement, applied force, etc. However, researchers sometimes
w ish to strain gauge a test specimen at different locations, or, to measure local
displacement or deflection using a num ber o f gaging LVDTs. As conventional
controllers o f testing machines do not allow too m any data acquisition channels, a
dedicated strain gauge or displacement data acquisition is acquired to m eet
instrumentation requirements. Being a niche market, these data acquisition systems
can cost as m uch as the test controller itself. And the user ends up with the need
to set up and operate a second system, probably connected to a separate computer
system and working w ith a different application program. A t the commencement
o f the test, the user carries the added responsibility o f wiring m achine readouts
such as force and stroke to the data acquisition unit, so that reference readouts are
available against the logged data. Finally, there remains the problem o f controlling
both the systems through the test in order to ensure that not only the test proceeds
successfully, but also, that all useful data are logged on the second system in a
m anner that can be used to reconstruct specimen response. The m odular control
ISSN 0556-171X. Проблемы прочности, 2009, N 1 23
R. Sunder
hardware and software described earlier attempts to address the need to incorporate
large channel count instrumentation into the test system control hardware and
software for the convenience o f the end user and for the affordability o f the
solution. W ith this system, user can log tens o f strain and LVDT outputs along
with the command waveform, stroke and load readouts, without the need for an
expensive dedicated data acquisition unit. For example, a large panel subject to
cyclic loading under complex load sequences can be strain gaged at a num ber of
locations along the path o f a growing fatigue crack in order to study strain
distribution under cyclic conditions as well as their redistribution under subsequent
static loading to fracture (see Fig. 7).
Fig. 7. Aircraft wing skin panel being tested
on a 200 kN test system (left). The panel is
instrumented with ten strain gauges (top) to
track strain distribution under static and
dynamic conditions. Provision of additional
instrumentation channels on the test control
system improves the quality and reduces the
cost of testing.
8 . G lobal D a ta S haring . Consider the prospect o f having to “oversee” the
quality o f test control perform ed by a particular application. A n example would
be the m onitoring o f how accurately, the application program maintains the mean
and amplitude o f cycling in a long duration fatigue test, so that a qualification
certificate based on real m easured data m ay be issued. W ith m ost conventional
test software, one would have to be reconciled w ith the possibility o f accepting
the vendor’s process o f performing the test and collecting test results. Independent
verification o f actual loads applied w ould be effectively ruled out, unless an
attempt is made to connect an additional load cell or at least a data acquisition
system for independent m easurem ent o f applied force versus time.
The problem with m ost “canned” software is that the user has no means of
observing in “real-tim e”, the data that flow through the controller and computer
system. M ost testing machines using host computers rely on the MS-Windows
platform. W ith the advent o f 32-bit m em ory protection starting w ith Windows 98
and N T operating systems (OS), multiple tasks cannot share common data,
because m em ory space belonging to a particular task cannot be accessed by
another task. Data sharing under MS-W indows is implemented by copying data to
an intermediary, usually a resource on the OS, which in turns passes it on as
“legal tender” to the target application (see Fig. 8a). This is how applications,
24 ISSN G556-Î7ÎX. Проблемы прочности, 2GG9, № І
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even those that form part o f M S-Office share data. This m ay serve the purpose of
mem ory protection against corruption by other tasks and the associated overheads
m ay go unnoticed in conventional operations associated w ith PCs. However, the
scheme presents serious shortcom ings w ith regard to real-tim e application.
Considerable slowdown and m em ory “hogging” can occur as multiple copies of
large amounts o f data are passed around to different participating tasks (Fig. 8a).
As these are real-time data, there is no guarantee that all participating tasks “see”
the same data at the same time. Thus a load readout m ay not “appear the sam e” at
a given time across multiple tasks and give rise to conflicting decision making
among tasks.
A
Task 4
1. M achhe-related readouts and statis
2. M U I - M t t real-lroe acquire«! « a
3. Com puted / translated infcnuabon
4. Applicatori specific data (cycling,
material behavior, etc}
Data sharing in
conventional MS-
Windows is thrcwgh
conies of data
- 1 11ttfj r“—— \
1. MacNrtt-ftMM readouts and s'alii
2. dbn(7i
3. Computed f transited iifcrmatkin
4. Appfcafon sp«ci6c delta (&ycfetg.
roaterial behavior, «tc)
G lo b a l D a ta S h a r in g
A ll ta s k s s e e s a m e d a ta
at all t im e s
b
Fig. 8. (a) Conventional data sharing between concurrent tasks in the MS-Windows considerably
slows down real-time multi-tasking. This is the operational mode of the well known Windows data
sharing mechanisms such as dynamic data exchange (DDE) and ActiveX controls. (b) The GDS
environment provides for concurrent tasks to directly access the same physical address space. This
considerably speeds up real-time computing.
a
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R. Sunder
In an attempt to regain the advantages o f multi-tasking without compromising
synchronization and throughput o f real-time data flow among tasks, a data
structure and access scheme was im plemented within the framework o f MS-
Windows that protects the integrity o f the application and at the same time
eliminates the need for data messaging (see Fig. 8b). We call it global data
sharing, or, GDS. All participating tasks directly and concurrently operate on both
static as well as dynamic areas o f a so-called virtual machine (VM). The contents
o f the VM appear as arguments in individual tasks, so m uch so, that when a value
is changed by one o f the tasks, all other tasks will instantaneously “see” the new
value - even i f one or more o f them is in debug or trace mode.
Dynamic data are in the form o f circular buffers carrying incoming or
outgoing data from/to the controller. The buffers are large enough to carry data
covering over a 100 s o f transfer. This means that the different tasks m ay access
them asynchronously without affecting their integrity. It also means that i f the OS
briefly hangs due to tim e critical operations on the disk or on the network, the
real-time data flow even i f interrupted, will not result in lost data.
Up to 64 independent tasks can “register” for access to GDS. The access
process ensures that tasks unrelated to the concerned application cannot corrupt
the GDS. It also enables the operation o f m ore than one GDS. Thus, it is possible
for m ultiple test systems to be connected to a single host com puter and for
multiple sets o f the same or different application to hook up to individual GDSs.
Work is in progress to provide GDS access across the network and also on the
Internet. This opens the possibility o f developing laboratory management software
that can hook up to multiple test systems. It w ill also open the potential o f remote
test control. A major advantage o f GDS is that it is software platform independent.
Thus, code developed on a variety o f development platforms can work together,
allowing for end-users and developers alike to choose the platform o f their choice
to control the test system. This has been confirmed through sample real-time
applications written in Visual C++, VB, M S-Office M acros, Lab-W indows, Lab
View and Delphi. As exclusivity threatens the prospect o f cost escalation without
necessarily improving quality, user-access to the real-time application introduces
an element o f security and control.
9. O pen S ource M S-O ffice In te rface . A n overwhelm ing m ajority of
professionals use computers w ith MS-W indows. A t the workplace, they rely on
M S-Office to perform their day-to-day documentation work. This includes MS-
Excel (XL), which permits reasonably complex processing, arrangement and
storage o f vast amounts o f data. XL provides for data representation in user
-definable report format. It also incorporates the VB-M acro (VBM) facility,
whereby, user-written code in VBM can process and store information in the XL
Worksheet. The GDS environment described above permits XL-based VBM
applications to set up and control a test and also collect, process and store test
data in real-time. Such an application can be created to perform any particular test
how ever complex it m ay appear. Figure 9 shows a typical user interface for an
XL-based fatigue crack growth test application.
The XL-file thus stores the test assignment including operator and customer
details, collected data, processed results as well as the test report. In addition, the
same file also contains the VBM code that was used to execute the test and
26 ISSN 0556-171X. Проблемы прочности, 2009, N 1
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process results. This guarantees a degree o f traceability that m ay not have been
possible without such a file structure and not to m ention without GDS. Finally, if
the code is left unprotected, it becomes open source and therefore amenable to
change, im provem ent as well as exchange within the testing and research
community.
Fig. 9. Typical user interface of an XL-based real-time application written as VB macros. Real-time
commands to perform test control as well as data acquisition are embedded in the XL-file as
open-source code. Data are written directly into the desired XL worksheets. The end user is free to
process the data in a different way if desired either manually, or, by writing new macros. The
source code can also be changed if required to control the test in a different manner and to perform
additional operations as required. Most end users are comfortable working in a software
environment that appears familiar and intuitive.
The concept o f open source software is gaining popularity as users demand
fair trade practices that do not confuse knowledge with technology. The GDS
framework provides a unique opportunity to develop a vast collection o f open
source software that is bound to reduce the cost o f at least routine testing and also
benefit from the collective expertise o f the testing community.
Conclusions. Changing circumstances in the global m arket force demand
m ore affordable solutions that satisfy materials research, product development
and quality assurance. Progress in this area is made possible w ith the appearance
o f lower-cost high performance components, modularization and increased levels
o f open source application software that perm it the engineering o f test equipment
w ith reduced life cycle cost. Advances in this area will im pact the further
expansion o f product development and m anufacturing across an increasingly
globalized world.
ISSN 0556-171X. npoôëeMbi npounocmu, 2009, N 1 27
R. Sunder
Acknowledgments. The author deeply appreciates valuable inputs from partners and colleagues
in the industry, R&D and academia in the U.S., Russia, Ukraine, Malaysia and India many of whom
shared the concerns that motivated the effort described in this paper. Ramesh Korradi provided
some of the data for this review.
1. The B a ld w in T e s te r s : www.instron.com/wa/library/StreamFile.aspx?doc=466.
2. H is to r y o f th e D u a l-S ta g e V a lve: http://w w w .m oog.com /m edia/1/D esign
ofEHValveTechpaper.pdf.
3. J. C. Jones, D e v e lo p m e n ts in D e s ig n o f E le c tr o h y d r a u l ic C o n tr o l V a lves
f r o m T h e ir I n i t ia l D e s ig n C o n c e p t to P r e s e n t D a y D e s ig n a n d A p p lic a tio n s ,
W o rk sh o p on P r o p o r t io n a l a n d S e r v o v a lv e s , M onash University, Melbourne,
Australia (1997).
4. http://www.m oog.com/M edia/1/761seriesvalves.pdf.
5. B o sc h -R e x ro th A G H y d r a u lic C o m p o n e n ts C a ta lo g P a r t T y p e 4 W R P E H 6
(2007), p. 785.
Received 11. 06. 2008
28 ISSN 0556-171X. npo6neMbi nponuocmu, 2009, N 1
http://www.instron.com/wa/library/StreamFile.aspx?doc=466
http://www.moog.com/media/1/Design
http://www.moog.com/Media/1/761seriesvalves.pdf
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