- Dynamic testing of composites is critical for design but still lags behind metals testing. While fatigue testing of composites has begun, it has mostly involved simple tension-tension testing without temperature control. More realistic testing is needed that involves compression, reversed loading, and fixed strain rates. - Strain rate testing is also important as materials behave differently at higher strain rates. Recent work has developed high strain rate compression testing up to 100/s but challenges remain in reducing noise and interpreting results. - New analysis tools like digital image correlation and thermoelastic stress analysis allow more data-rich testing of failure modes like crack propagation but composites testing still has headroom for development.

1. P. B. S. Bailey, PhD
Manchester Conference Centre, November 2015
Composites on the Move:
The Need for Dynamic Testing

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“New and Exciting”?
• Frequent improvements in resins and fibers
• Various “revolutionary” new processing
techniques
• Many effective test methods
• Too many?
• Or too few…
• Established structural composites market
for 30 years

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New and Exciting
• Primary structures
• High-volume products
• Truly composite-specific design
Aluminum CFRP

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Early Fiber Composites in UK Transport
• Cost-Efficient Body Materials
• 1936 Vickers Wellington
• 1940 De Havilland Mosquito
• 1946 Jowett Bradford
• 1957 Ford UK Thames Trader © Crown Copyright: IWM
Source: Redsimon at English WikipediaSource: Les Chatfield, Brighton, England

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UK Innovation Pedigree
• Fiber Processing
- 1779 spinning jenny – Samuel Crompton
- 1784 power loom – Edmund Cartwright
- 1792 mechanised braiding
• The foundations of the industrial revolution

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Leaders in Science and Engineering
• Testing & Materials Physics
- 1858 Iron & Steel Testing, 1874 Testing Works
- David Kirkaldy
- Fatigue & Fracture
- 1849 HM Govt procures research report from E. Hodgkinson
- 1903 J.A. Ewing – origin of fatigue failure
- 1920 A.A. Griffith
- 1950s G.R. Irwin, R.S. Rivlin, A.G. Thomas
Source: Richard Rogerson

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Why the History Lesson?
• There is a huge weight of evidence behind
metals design
• Fibre technology has been around for a
while too!
• Structural composites have caught up a
lot in just a few decades
• But they are still some way behind…

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Catching Up with Metals
Metals
Tests
Metals
Theory
Metals
Design
Composite
Tests
Composite
Theory
Composite
Design
Static      
Impact      
Fatigue     ? ?
Strain rate    ?  
Crack propagation     ? 

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How Does Testing Fit into this Now?
On Measurements of Small Strains in the
Testing of Materials and Structures
Source: J. A. Ewing, Proceedings of the Royal Society of London,
Vol. 58 (1895), pp. 123-142
AVE2 Video Extensometer
Instron, 2014

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What Do You Think About Testing?
• Costly?
• Difficult?
• Interesting?
• Useful for R&D?
• Central to your business success?
• Critical?

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Yes …To All of Those!
• Testing composites is critical to the
continued success of the sector
• Testing and design methods need to fully
address the effects of dynamic behavior

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Fatigue
• “Between 80% to 90% of ALL mechanical
service failures can be attributed to fatigue.”
(ASM – Metals Handbook 2008)
• >186 years of observation!
• Do we have enough data to make a
comparison for composites?

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Fatigue: Early Industry Adoption
• Lead by wind turbine industry
• Recognition of the problem
• Need for minimal cost
• Lower regulation

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Fatigue: Early Compromises
• Tension-tension tests only
• Sinusoidal loading
• Minimal temperature control

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Is this Representative?
• Incomplete, but not incorrect…
• Where all science starts out!
• What is the next step?
• Compression or reversed loading would
be more aggressive and more realistic
• Constant / fixed strain rate would give
more comparable characterization

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The Thermal Problem
• Cyclic heating effect
• Severe test temperature change
• Symptom/cause of break at grip?
t = 0 2 min 8 min
25°C 30°C 35°C
26°C
27°C
28°C

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(Non-)Isothermal Behavior
• Traditionally assume constant temperature
• Reason for very-low, quasi-static test
speeds on composites
• Not an accurate description
• Representative test?

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Strain Rate Testing
• Materials exhibit different mechanical
behavior at different strain rates
• Viscoelastic behavior in polymers
• Geometric & friction effects in textiles
• Crack propagation mechanisms
• Quasi-static data are insufficient to model
crash behavior
• Even for metals

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Very High-Speed Servohydraulic Test

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Significant Effects of High Strain Rate
• Cold rolled steel

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Renewed Interest
• Organic growth of research in:
• Composites performance
• Composites modelling
• 2010 onwards
• Automotive:
• Before 2014, interest…
• Now converted to urgent need

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High Strain Rates on Composites
0.00044 s-1
0.044 s-1
4.4 s-1
0.44 s-1
44 s-1
* Gude et al, Mech. Comp. Mat., Vol.45, No.5
High on-axis stiffness
Off-axis stiffness low in static, but
increases with strain rate… 2.5x
Strength increases
by factor of 2
in “crash” condition strain rate
Woven Glass Fibre Reinforced Epoxy – modulus measurement
** Schloßig et al, Polymer Testing, Vol.27, p893-900

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Measurement Challenges
• Strain rates over 100-s
• At >10 m/s this is a very fast test
• Looking at events on order of sound speed
• Resonance ≠ Signal Noise
• Careful interpretation needed
• Effective fixturing must be highly tailored

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Recent Test Development
• High rate compression & crushing
• Measuring the critical properties for crash
structures in composite
• Servohydraulic
• Drop weight
• In drafting for ASTM method
• Barnes et al at Engenuity Ltd.

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Crushing Data
Good Stable Crush
Performance
Intermediate
Performance
Poor Crush
Performance

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Successful Implementation

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Is That All We Need?
• Good news or bad?
• This is all that design methods can handle
yet… or nearly
• But there is plenty of headroom in
measurement technology

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Where are We Heading?
• What is the next level?
• More data-rich tests
• Crack propagation?
• Low-cycle fatigue?
…somewhere in Dallas, Texas, USA

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New Analyses
• Quantifying “Failure”
• Digital Image Correlation
• Thermoelastic Stress Analysis
Images reproduced by courtesy of the University of Southampton
Crump et al, Engineering Integrity, No.35. 2013.
T.S.A.
Stressmap
D.I.C.
Strainmap

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Experiments with TSA
Few Cycles
Damage Onset? “Failure”?
(images courtesy of J E Thatcher, University of Southampton)

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It’s Not all Bad News
• Developing Effective Modelling
• AMSCI Datacomp project
• Engenuity Ltd.
• Industry Investment in Test Capability
• Automotive (Germany)
• Aerospace (e.g. Boeing, COMAC…)
• Materials (e.g. Hexcel, Borealis…)
• Developing Tests and Theory

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We have the tools…
What do you want to test?
What do you need to
measure?

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Questions?
Contact: Peter Bailey, PhD
Senior Applications Specialist
Instron
peter_bailey@instron.com
+44 7880 187 716