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ADVANCED MATERIALS & PROCESSES •
JUNE 2014
17
Dynamic Testing Market
for Composites
Sees Growth and Innovation
W
hile today’s composites encompass
a broad and well-established family
of materials, the commercial market
for high-performance, structural composites
has actually existed for well over 30 years. Euro-
pean automotive manufacturers have made
considerable use of lower performance glass
fiber reinforced polyester (GFRP) bodywork
since the 1950s. In addition, adoption in the
aerospace industry has resulted in a wide range
of static test methods offering reliable results.
However, there is still only limited consensus,
so test houses and machine manufacturers
must maintain an extensive catalogue of fix-
tures in order to meet diverse international and
industry standards. It might seem that not
much is happening with regard to composites
testing innovation, but some exciting trends are
now developing.
Fatigue testing
The wind energy industry is leading the way
when it comes to composites testing develop-
ment. In the past six to eight years, major man-
ufacturers quickly adopted fatigue testing as a
key part of composite materials evaluation. The
nature of this industry has greater freedom
than others in terms of qualification and adop-
tion of new materials. In addition, significant
levels of elastic strain are acceptable in turbine
blades, and correctly balancing design life and
cost can achieve a significant competitive edge.
The most common implementation for fatigue
testing of composites is in tension-tension
mode. This is generally due to its simplicity in
terms of specimen preparation, gripping, and
setup. Research establishments are emphasiz-
ing the importance of compression-compres-
sion, or fully reversed fatigue tests on
composites, because these are more represen-
tative and aggressive tests. However, they pres-
ent a greater challenge in terms of equipment
and specimen preparation, to avoid buckling or
mixed mode loading.
For all fatigue load cases, there is major
concern because composites exhibit an un-
pleasant combination of temperature sensitive
behavior and heat generation during loading
and failure (Fig.1). As a result, the testing
process can severely affect measured proper-
ties. In static tests, these problems can be mit-
igated by a carefully controlled environment
combined with very low load rates. In cyclic
tests, this is impractical because they often test
to failure or run-out at above one million cy-
cles. At the 1 mm/min. rate of displacement
preferred in static tests, this would take roughly
two years for a single specimen.
General practice is, therefore, based around
practical methodologies derived from metals
fatigue, using a sinusoidally varying applied
stress at 3-5 Hz frequency, typically described
as peak stress and stress ratio. Compared with
metals fatigue these are very low frequencies,
yet tend to increase the surface temperature by
20°-40°C during testing, while less loaded spec-
imens run <2°C above ambient temperature.
The outcome is a lengthy test schedule of four
to eight weeks of machine time to produce an
SN curve for a single composite material, in one
particular layup, with a poorly controlled spec-
imen temperature and high degree of scatter.
Frequency optimization (automatic or man-
ual selection of frequency for different stress lev-
els) is sometimes used to reduce total test time.
Slightly decreasing the test frequency for highly
stressed specimens avoids overheating, while a
small increase for lower stress levels saves con-
siderable time. Researchers at the National Com-
posites Certification and Evaluation Facility,
University of Manchester, UK, found that it took
55 days of continuous running at 4 Hz to prepare
an SN curve for woven carbon fiber composites
and the average specimen temperatures ranged
Peter Bailey
Instron
Buckinghamshire, UK
Some exciting
trends are
developing in
the world of
composites
testing.
Fig.1 —
Thermal image of heating at
failure of static tensile test (1 mm/min) of
an impacted composite specimen.
Fig. 2 —
Thermal image of self-heating of
fatigue specimen 10 min into test (woven
carbon fiber epoxy, 5 Hz test frequency).
40°C
35°C
30°C
40°C
35°C
30°C