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Non-Destructive Testing of Fibre-Reinforced Plastics Composites

Non-Destructive Testing of Fibre-Reinforced Plastics Composites. John Summerscales School of Engineering University of Plymouth. Introduction. Composite Materials Non-Destructive Testing electromagnetic chemical spectroscopy mechanical vibration Conclusions. Composite materials.

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Non-Destructive Testing of Fibre-Reinforced Plastics Composites

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  1. Non-Destructive Testing of Fibre-Reinforced Plastics Composites John Summerscales School of Engineering University of Plymouth

  2. Introduction • Composite Materials • Non-Destructive Testing • electromagnetic • chemical spectroscopy • mechanical vibration • Conclusions

  3. Composite materials • FRP = fibre-reinforced plastic • reinforcement • fibres/fabrics • polymer matrix • thermoset/thermoplastic/elastomer • interface/interphase

  4. Fibres • aramid (Kevlar, Twaron) • boron • carbon • glass • polyethylene (Dyneema, Spectra)

  5. Matrix • Thermoset • polyester • vinyl ester • epoxy • Thermoplastic • polypropylene • nylon • PEEK

  6. Interface/interphase • sizing agent • coupling agent • resin rich regions between layers • orientation close to the fibre surface

  7. Voids • “… regardless of the resin, fibre type or fibre surface treatment, the interlaminar shear strength of a composite decreases by about 7% for each 1% of voids up to a total void content of about 4%” • Judd and Wright • SAMPE Journal , January 1978

  8. NON-DESTRUCTIVE TESTING Functions • initial inspection of test samples • confirmation of structural integrity • monitoring sample tests in progress • monitoring components under service loads • especially where changes occur over time • analysis of reasons for failure

  9. Non-destructive testing for • structural integrity • global inspection/proof tests • fibre • orientation/breaks/waviness • matrix • state-of-cure/porosity/cracking • interface • debonding/delamination/moisture ingress

  10. From Bjorn Backman (Elsevier 2005)Composite Structures, Design, Safety and Innovation

  11. ELECTROMAGNETIC SPECTRUM • radiography • light • heat • microwaves • eddy current • dielectric • electric

  12. Radiography • X-ray • electrically generated • broad band of wavelengths • gamma, beta or neutron beams • generated by radioactivity/synchrotron • discrete frequencies • shadow pictures on fluorescent screen/film • tomography available

  13. Periodic table (strong absorbers)

  14. Periodic table (reinforcements)

  15. Periodic table (resin matrix)

  16. Nick Veasey’sx-ray images http://content.time.com/time/photogallery/0,29307,1719207,00.html US$5 million machine employed by U.S. customs on the Mexican border

  17. White light • observation • triangulation • shadow and projected fringes • photoelasticity • fibre optics

  18. Lasers (coherent light) • interferometry • speckle • holography • shearography • fibre optics • Image from Insight, January 2001, v44, n1, p6 • (Loughborough University)

  19. Optical fibres • Step-index multimode optical fibres • thick fibre - thin skin of different optical properties • carries signals in different propagation modes • uses reflection from the skin-core interface • Graded-index multimode optical fibres • thick fibre with a gentle change in refractive index • carries signals in different propagation modes • Small-core monomode optical fibres • thin fibre core - thick skin • carries signals is a single propagation mode

  20. Optical fibres • Step-index multimode optical fibres • Graded-index multimode optical fibres • Small-core monomode optical fibres

  21. Optical fibres (embedded sensors) • crack detection • strain measurement • interferometry • Bragg gratings • acoustic emission sensor • interfacial corrosion

  22. Optical Fibre Bragg Grating (OFBG) Image from http://en.wikipedia.org/wiki/Image:Fbg.GIF

  23. Thermography • EATF = externally applied thermal field • thermal paints and coatings • infra-red cameras • SGTF = stress generated thermal field • vibro-thermography • thermosonics • SPATE: Stress Pattern Analysis by Thermal Emission/TSE: thermally stimulated emission

  24. Thermography of B737 radome Image from Insight (Br J NDT), November 1996, 38(11), 771.

  25. From the “moulds” lecture < top, heating top, at temp > < back90°C dwell insulated -ditto-, no insulation > Thermograms of an electrically heated ‘skeg’ mould

  26. TeraHertz (~1012 Hz) • relatively new field • facilitated by the development of • femtosecond pulsed lasers, and more recently • semiconductor Quantum Cascade Laser (QCL). • problem of sources with significant power output • attracting significant interest in the context of security applications

  27. Microwaves • precise distance & roughness measurement • analysis of complex vibrations • detection of internal flaws • dielectric properties • density, porosity, state-of-cure, ageing, orientation, homogeneity and moisture content

  28. Microwaves • potential health hazard at high power! • interference from communications networksleading to poor signal/noise ratios • used by radome manufacturers to confirm electromagnetic performance of structures

  29. Eddy current • AC induces eddy currents in conductors • limited use with carbon fibre composites • not applicable to GRP/aramid • thickness measurement of insulators

  30. Dielectric • relative permeability • dissipation/loss factor • sensitive to • state-of-cure • moisture content • micro-fabricated inter-digitated sensors • parallel plate configuration

  31. Dielectric • moisture meters used by marine surveyors • manufacturers use different frequencies and electrode spacings • consequently values of moisture content and depthdiffer between instruments

  32. Electric • Resistance is a function of • dimension • fibre orientation • state-of-cure • ion-graphing/phaseometry • moisture content • thermal decomposition • crack geometry and growth • Imaging • electrified particles • applied potential tomography

  33. CHEMICAL SPECTROSCOPY • ultra-violet • Raman (including optical fibres) • infra-red (including optical fibres) • electron spin resonance • nuclear magnetic resonance • magnetic resonance imaging (tomography)

  34. To be continued …

  35. To be continued ...

  36. MECHANICAL VIBRATION • Scanning acoustic microscopy (GHz) • Ultrasonics (MHz) • Acousto-ultrasonics (high kHz) • Acoustic emission (low kHz) • Vibration (Hz)

  37. Ultrasonics 0.5-50 MHz • widely practised in industry • significant attenuation of signal by composites (cf metals) • anisotropy of properties

  38. Ultrasound wave propagation • Bulk waves • longitudinal/compression • transverse/shear • Surface waves • Rayleigh Stoneley • P-waves: head waves (pressure waves) • S-waves: SH or SV (shear waves: horizontal or vertical) • creeping waves • Plate waves • Lamb Love rod waves

  39. Ultrasound transducers • Normally piezoelectric crystals • PZT = Lead (Pb) zirconium titanate • able to send and receive signals • produces a compression wave • converted to shear wave by wedge mounting • Phased array • multiple transducer elements in one package • usually 8-, 16-, 32- or 64 elements • Array controller configures beam • Can simulate several transducers configurations

  40. US A-scan without/with defect(voltage vs time trace) Transducer Void << no defect Backwall echo Transmission pulse

  41. US A-scan without/with defect(voltage vs time trace) << no defect << full width defect - no backwall echo Defect echo

  42. US A-scan without/with defect(voltage vs time trace) << no defect << full width defect << part width defect

  43. Ultrasonic displays • A-scan: voltage/time at one position • B-scan: size & position vs probe movement • C-scan: attenuation/x-y position • D-scan: attenuation/x-z position • F-scan: feature scan • P-scan: projection/tomography • S-scan: sector scan (phased array technique)  Flash Movie animation from Wavelength-NDT

  44. Convergent flow fronts (video)

  45. Ultrasound C-scan convergent flow

  46. Acousto-ultrasonics ~500 kHz • pulser-receiver in the plane of sample • energy in/out through rubber wheels • waveform received interpreted as • stress wave factors (SWF)

  47. Acoustic emission (broad band noise) • transient elastic waves produced as a response to active defects • rapid release of stressfrom local sources within the material • also known as • stress wave emission (SWE) • micro-seismic activity • materials scientists stethoscope • clinking (martensitic transformations) • tin cry (twinning), dunting in ceramics • creaking ships’ timbers/pit props

  48. Acoustic emission detection • microphones • piezo-electric transducers • strain gauges • optical fibres • lasers

  49. Typical AE burst type event Amplitude Threshhold Time Event duration

  50. AE count (+ve crossings) Amplitude Time Counts vs time

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