Evaluate what is "new and exciting" in the composites testing world.

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

2. 2
“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