Lamb Waves for Composite Health MonitoringNon-Destructive Testing – Laurens Stevaert2Ma Chemical & Materials Engineering – VUB/ULB 2012-’13

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COMPOSITESProperties, (Dis)Advantages & Inspection

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Composites: Properties• Widely used• Aerospace• Automotive• Naval

• Advantages• High specific strength• Light weight• Fatigue and corrosion resistance• Design freedom – tailored properties

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Composites: Properties• Disadvantage: impact damage• Low through-thickness strength• Even low velocity!• Bird strike• Tool dropped during servicing• Runway stones

• Damage• Indentation• Delamination• Fibre/matrix cracking• “Barely Visible Impact Damage”

Detect, locate & characterize damage!4

Composites: N-D Inspection• Loads of methods• Visual inspection• Optical methods• Eddy current (E-M waves)• Thermography (input heat energy)• Ultrasonic (high E acoustic waves)• Etc.

• But…• Cost & time• Bulky transducers• Part has to be removed, sometimes placed under water• Point scan 5

LAMB WAVESProperties & Application

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Guided Wave Testing

High detection range 100m

wikipedia.org

• Mechanical stress waves• Guided by geometry• Super low freq. (10-100 kHz)

• Other advantages:• Elastic waves: reversible deform. mech. properties• Through thickness scanning• Imaging internal hidden defects

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Lamb Waves: Properties• Discovered in 1916 but only recently applied• Complex mathematics• Need for computational power

• Elastic wave in solid plates• plate plane• propagation direction• (Guided by geometry, travel long distances)

• Infinite number of modes, only two used• Symmetrical S0

• Asymmetrical A0

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Lamb Waves: Testing• Normally: transducers on the outside• Good coupling required!• Contact mode• Air is not a good medium• Immersion in water part has to be removed…• Water jets very sensitive…

• Non-contact mode• Easier option for testing• Often expensive

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Lamb Waves: Smart Systems• Small transducers permanently attached• To the surface• Embedded in composite laminate• Constantly monitor the structure, on demand info

• Piezoelectric Wafer Transducers• Transmitter: electrical E mechanical E (elastic waves)• Receiver: mechanical E (propagated wave) electrical E• E.g.: PZT – Lead Zirconate Titanate

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eetimes.com

• Different ways to analyze signal – depends on application• Examples• TOF measurement: defect location

• Defect material with different prop.• Wave: different velocity (slower)• Comparison of wave peak locations

• Laser vibrometer: defect location• Non-contact vibration measurement: Doppler shift of laser frequency

due to surface vibration• 3D lamb wave, follow peak-to-peak amplitudes

• Finite element-based technique: defect size• Measure reflection and transmission coefficients• Predict these coefficients for set of damage parameters• Parameter optimization defect geometry

Lamb Waves: Analysis

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FUTURE WORKWeak points & Improvements

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Future Work• Some disadvantages• Single mode: dispersion properties needed difficult for

composites!• Low frequency = large wavelengths small defects not correctly

measured• Analysis over long time influenced by T, loading, bad coupling…• Anisotropy

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Commercial applications limited… for now

Conclusion• Lamb waves• Special properties• Propagate through plate geometries• Detection over large distances

• Smart systems• Active structural health monitoring• Monitor damage (evolution)• While in-service!

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Promising technique!

QUESTIONS?“Lamb” Wave – Vague de “Agneau”?

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Sources• Diamanti, K. et al (2010) Structural Health Monitoring

Techniques for Aircraft Composite Structures. Progress in Aerospace Sciences, Vol 46, pp. 342 – 352

• Staszewski, W.J. et al (2008) Health Monitoring of Aerospace Composite Structures – Active and Passive Approach. Composites Science and Technology, Vol 69, pp. 1678 – 1685

• Castaings, M. et al (2011) Sizing of Impact Damages in Composite Materials Using Ultrasonic Guided Waves. NDT&E International, Vol 46, pp. 22 – 31

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