1. Lamb Waves for Composite Health Monitoring
Non-Destructive Testing – Laurens Stevaert 2Ma Chemical & Materials Engineering – VUB/ULB 2012-’13

2. Properties, (Dis)Advantages & Inspection
Composites

3. Composites: Properties
Widely used
Aerospace
Automotive
Naval
Advantages
High specific strength
Light weight
Fatigue and corrosion resistance
Design freedom – tailored properties

4. 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!

5. 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

6. Properties & Application
Lamb Waves

7. Guided Wave Testing Mechanical stress waves Guided by geometry
Super low freq. ( kHz)
Other advantages:
Elastic waves: reversible deform.  mech. properties
Through thickness scanning
Imaging internal hidden defects
High detection range m
wikipedia.org

8. 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

9. 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

10. 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
eetimes.com

11. Lamb Waves: Analysis
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

12. Weak points & Improvements
Future Work

13. Future Work  Commercial applications limited… for now
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
 Commercial applications limited… for now

14. Conclusion  Promising technique! Lamb waves Smart systems
Special properties
Propagate through plate geometries
Detection over large distances
Smart systems
Active structural health monitoring
Monitor damage (evolution)
While in-service!
 Promising technique!

15. “Lamb” Wave – Vague de “Agneau”?
Questions?

16. 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 – 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