PRODUCTS & SERVICES
REDUCING VARIABILITY
IN MATERIALS TESTING
Increasing the quality of test data captured
at high temperatures is key in improving
the efficiency of aero-engines
these early cycles are often the most critical,
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AEROSPACETESTINGINTERNATIONAL.COM // JUNE 79
The undertaking of Low Cycle Fatigue
(LCF) testing at elevated temperatures of
up to 1000°C has been an important part
of aero-engine development for decades. The
role LCF testing plays in most programs is to
simulate the aggressive loading conditions
seen by materials in critical turbine
components, generating data for use in
material or process selection, batch release,
or more recently for modelling.
A drive for greater fidelity in simulation of
aero-engines has led to a renewed focus on
LCF test data. One of the underlying
motivations is the demand for lower carbon
emissions. The use of structural composites
has delivered impressive aircraft weight
reductions but improving the efficiency of
aero-engines also offers a viable way for
aircraft to reduce carbon emissions.
A small increase in useable temperature in
the right place can provide a measureable
improvement in efficiency.
All material fatigue behaviour shows
scatter or distribution, so design and
simulation use statistical confidence limit
values to account for this. However, not all of
the scatter is true material variability –
sometimes design allowables can be
improved simply by collecting better data!
Engineers can often focus obsessively on
simple accuracy, but the realization has hit
home that this alone is not enough –
repeatability is also vital. The routes to
improvement distil down to better mechanical
and thermal controls, but their implications
may not be what you expect.
Instron, working in partnership with their
customers, have identified and addressed
three key areas which have challenged
manufacturers and test practitioners over
past decades.
Firstly, gripping and alignment of
specimens is a major source of variability.
Many theoretically sound grip designs have
to “envelope” towards the target loading, yet
when the material is changing most rapidly
(hardening or softening). Tuning the system
controller was historically a skilled operation,
where “good” was highly subjective.
Instron’s latest stiffness based tuning
algorithms feature removes the risk of
overloading or pre-cycling a specimen during
test setup and achieve near-identical test
control between different operators,
machines and even laboratories.
Thirdly, Instron has completely automated
the control of specimen temperature.
Furnace control hardware in most
laboratories has remained surprisingly
primitive, with engineers having to manually
control a furnace’s three zones
independently, a process that requires
extensive, skilled, time consuming pre-test
work. Furnace control systems can be
delivered fully pre-tuned and use multiple
specimen temperature measurements that
can adaptively control the furnace without
any operator intervention.
Each test is automatically conducted at
the intended temperature, with faster
heating times whilst minimising temperature
overshoot and specimen gradient. The
computerised interface means that features
such as thermocouple calibrations can be
added and a wide variety of metrics can be
logged, greatly improving traceability.
These recent advances have addressed
industry’s need to drive improvement in test
equipment, improving the consistency of
company’s test data by removing the hidden
effects of alignment, test control, and
temperature control. \\
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been used in the past, but they do not
necessarily help repeatable loading.
Essentially, the design must use
appropriate locating features on the
specimen, applying tight tolerances to both
grip and specimen, and a clamping action
without any off-axis motion. Simultaneously,
high lateral stiffness of the whole system is
needed to retain that alignment during a test
– it is important to realise that most of the
lateral compliance at the specimen is not
from the load frame, but rather from flexure of
pull rods and drive train.
Secondly, the fidelity of mechanical test
control is important. This may seem obvious,
but it is not trivial to achieve, unless the test
frames control system is tuned effectively
and consistently.
It has been common practice to allow the
first ten fatigue cycles considerable latitude
1 // A temperature
increase within engines
can improve operating
efficiency and reduce CO2
2 // The control system’s
interface allows a variety
of metrics to be logged
with improved traceability
3 // Low cycle fatigue
testing requires precise
control to accurately
simulate aero-engine
performance
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