Live Webinar, September 2013

Composites Fatigue Testing using

Specimen Self-Heating Control for WaveMatrix

Click icon to add picture

2

Overview

• Composites Fatigue

• Problems with current methods

• A solution

• Rewards vs Concerns

• Case Study

• SSHC live demonstration

• Questions

• Temperature measurement / monitoring

• Options and set-up demonstration

• More questions ?

3

Composites Fatigue

• Cyclic loading tests on coupons

• Roughly equivalent to HCF for metals

- No consistent methodology yet for crack growth

• Highly sensitive to structure in the same “type” of material

• Rapid commercial uptake by wind energy sector

• Cost vs lifecycle is of critical importance

• System qualification is less complex than aerospace

• Other sectors gradually catching up

4

Industry Interest

• Turbine manufacturers

• Vestas, Siemens, GE

• Composite materials producers

• Resins - Hexcel, DSM

• Fibres - LM Glasfiber, Teijin, Sabic

• Aerospace & automotive are deciding what to do…

• Boeing, Airbus, Fokker, BMW, Jaguar, and others

• Test houses

• Researchers

5

Current Standards

• Standards include:

• ASTM D 3479

• ISO 13003

• Similar AITM, Boeing and SACMA

• These standards specify:

• Fixed frequency

• Temperature monitoring

• They also recommend:

• Frequency < 5 Hz

• Temperature rise < 10°C during test

6

An easy opportunity?Growing industry with international standards….

7

A Major Problem

• Composite specimens heat significantly under cyclic loading

• Composites fatigue tests are slow

• 8 weeks for a reasonable S-N curve

• Run at higher frequency ?

• Test temperature fluctuates severely

• Room temperature at low load

• >20° C rise at high load

• Run at even lower frequency ?

8

Cyclic Heating Effect

• Woven CFRE – thermal imaging• 80% UTS, R = 0.1, 5 Hz

t = 0 1 min 2 min 4 min 8 min

16 min30°C 35°C 40°C

9

Customers’ Current Methods

• General practise:• Pick a frequency based on 1 or 2 materials

• Use it for everything

• Not worry too much about temperature rise, because they can’t control it.

• Result:• Very slow test; > 4 weeks for reasonable S-N curve

• Often exceeds 10 °C temperature rise

• Unpredictable specimen temperature

• Temperature varies for different loading

• Temperature varies during test

10

Not quite so easy after all !

11

Specimen Self-Heating Control

• New WaveMatrix module to improve specimen throughput

• Automatically adjusts frequency to balance the heating and cooling rates at a user selected specimen temperature

• Faster tests where cycles-to-failure is high

• Maintains stable temperature for all tests

• Patent pending – our competitors cannot do this

• Good comparability with fixed frequency tests

12

Simple Control Tab

13

Optimising Frequency

Stable temperature by reducing frequency

tan δ → Increasing heating rate

* GFRE loaded to 40% UTS at R = 0.1, target temperature 21.0 ± 0.5 °C

~3x higher frequency

Acceptable temperature

14

Preventing Over-Heating

Stable temperature by reducing frequency

tan δ → Increasing heating rate

* GFRE loaded to 60% UTS at R = 0.1, target temperature 28.0 ± 0.5 °C

15

How do we sell it ?Neat technology

16

System Requirements

• Specimen Self-Heating Control Module for WaveMatrix

• Available from WaveMatrix version 1.8• Requires Console 8.2.149 or later

• Requires temperature sensor

• either a Thermocouple interface • National Instruments USB DAQ supported in WaveMatrix

• Does not need SCM

• Supports multiple channels

• or an IR camera/probe input via strain channel• Tested with Optris camera and spot sensors

• Need to allow a spare SCM (+ CP to interface?)

17

Does the customer even know they have

a problem?

• Customers already working on composites fatigue

• Have an idea of the problem

• Often do not continuously monitor temperature

• Don’t know how badly it affects their results

• Customers new to the field

• Have probably read the standards

• Don’t know how badly it affects results

18

Rewards and Concerns

• Not mentioned in ASTM or ISO standards

• But many do not work strictly to standards anyway!

• Data may differ from an existing method

• But may indicate longer fatigue life

- versus -

• Even a small increase in frequency on lower loading gives a big time saving.

• Don’t waste time and specimens trying out different frequencies.

• Controlled temperature can reduce spread of data.

19

Convince me!Sounds good, but…

20

A Case Study

• UK National Composites Certification and Evaluation Facility

• Used “standard” thermocouple temperature measurement

• ISO 13003 tension-tension test

• Complete repetition of S/N curve

• > 25% reduction in machine time

• Comparable or reduced scatter

• Comparable fit – better intercept

• < 4% difference for 10 million cycles prediction

21

Comparative Case StudyWoven CFRE prepreg, Tension-Tension (R=0.1)

1,000 10,000 100,000 1,000,000 10,000,000 55

60

65

70

75

80

85

4 Hz fixed frequency

Specimen Self-Heating Control

Cycles to Failure

σm

ax

[%

UT

S]

22

Case Study: Fitting and Prediction

Gradient Intercept (% UTS)

Fit quality

(“R²”)

Predicted Stress at 107 cycles (%UTS)

Predicted Stress at 108 cycles (%UTS)

4 Hz – all data 3.156 106.7 0.844 55.9 48.6

4 Hz – exclude outliers 2.886 104.4 0.931 57.9 51.2

Adaptive frequency 2.651 100.7 0.966 57.9 51.8

Logarithmic fit: σc = ‒ a ln(N) + c

23

Case Study: Time Saving

Frequency variation

Total time for S/N curve

Equivalent time at 4 Hz

Time saving vs 4 Hz

< 62 % 40 days 55 days 27.5 %  

Load (%UTS)

Specimen Self-Heating Control Fixed Frequency 4 Hz

  Frequency * (Hz)

Time per specimen* (hours)

Equivalent time per specimen** (hours)

80% 3.6 0.93 0.84

75% 4.0 3.9 3.9

70% 4.6 22 24

65% 5.0 290 349

60% 6.5 642 946

  * averaged across duration of test ** from cycles to fail

24

Conclusion

• SSHC offers a unique enhancement for coupon based fatigue testing of composites

• Quick simple set-up

• Comparable results to existing methods

• Better use of machine time by automatically optimising frequency

• Reliable temperature control

• Reduced test time

• Excellent lock-out opportunity – our competitors cannot

do this due to patent pending!

25

Catalogue Entry

• In catalogue for all territories

• Released and shipping since June

• Dynamic Systems: Software• 2495-915 F1 – new systems

• 2495-965 F1 or F2 – upgrades

• Dynamic Systems: Composite Testing Accessories• CP112383 – Thermocouple USB integration (4 channel)

• Basic solution from NI to allow you to specify a whole system with SSHC

• PoD, white paper, and press release available

26

Questions?Technical Background and Case Study

27

Specimen Self-Heating Control

Live demonstration

28

Simple Control Tab

29

Live test screen

* target 25°C

30

Temperature MonitoringOptions and setting up for temperature measurement and SSHC

31

Temperature Measurement

• Temperature measurement is important

• WaveMatrix has good capability

• Standard product is limited for now

• Integrated temperature recording is a great feature anyway especially for composites & polymers

• Even if you are not going to use SSHC

• Even if you are operating in a chamber

32

Thermocouples

• Universally understood

• USB daq our preferred option

• Single point measurements

• Standardised one-shot CP112383

• Attachment to specimen can cause problems

• Measurement is taken by WaveMatrix not 8800

• Values are not available as Live Displays

33

Infrared Transducer

• Single point measurement(s) • spot size 1mm to >>10mm

• Popular & established in industrial process control

• Desirable for non-contact measurement

• Connect as strain channel to 8800 or using USB daq

• No standard product or one-shot

• Good for relative monitoring but…

• Absolute measurement is difficult because it depends on the specimen

34

Infrared Camera

• Full thermal imaging

• Similar benefits to IR spot sensor

• Useful features such as averaging and hot-spot detection

• Observe/capture localisation

• Connect as strain channel to 8800

• Strongly recommend customer source directly

• x10 more expensive than thermocouples and IR spot

• Response still depends on specimen

35

Questions?Temperature Measurement and Monitoring