Detection of Incipient Thermal Damage of CFRP Using Fluorescent

Thermal Damage Probes

Tucker Howiea, Zhengwei Shia, Sei-Hum Janga, Alex Jena, Gary Georgesonb, and Brian Flinna

a University of Washington, Materials Science and Engineering, Seattle, WA b The Boeing Company, Seattle, WA

Thermal exposure on composite Fluorescence emission

Fluorescence inspection

Thermal Degradation of Composites v  CFRP composites are susceptible to thermal degradation because of the

polymer matrix

v  Thermal degradation of the matrix may cause delaminations, fiber-matrix debonding, embrittlement of the matrix, and reduction of Tg which can significantly reduce mechanical properties such as: v  Flexural strength v  Compression after impact (CAI) v  Interlaminar shear strength (ILSS)

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Hypertac Hypertronics www.hypertronics.com

Need for Incipient Thermal Damage Detection v  Ultrasound techniques are the most common method for detecting non-

visible damage to composites v  Below a certain thermal exposure threshold ultrasound techniques are not

capable of detecting parts with significant property degradation v  Damage is below resolution of ultrasound (molecular scale) v  This level of damage is termed to as incipient thermal damage (ITD)

3 Chart courtesy of Dennis Roach Sandia National Laboratory

Motivation v  Develop a detection method that is capable of detecting ITD of CFRP

v  Ideally the inspection method would possess the following characteristics: v  Fast and large area inspection capable of easily locating localized

damage v  Provide information on depth of damage to guide repairs v  Be independent of matrix resin system

Scarf Repair

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Methods to Detect ITD

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FTIR

Thermo-elastic Characterization

Laser-induced Fluorescence (LIF)

Fisher, et al., Mater. Eval, 1997, 55 Image courtesy of Dennis Roach Sandia National Laboratory

Sathish, S., et al., Rev. Sci. Instrum. 2012, 83

Our Approach: Fluorescent Thermal Damage Probes

v  Dope thermally activated fluorescent probes into matrix v  Fluorescence of probe is turned on only in areas of thermal exposure

v  Large contrast between on and off states, easy to see affected areas v  Extrinsically doped fluorescent probes offer several advantages:

v  Thermal response of probe can be characterized and can be utilized in many resins

v  Fluorescent probes are tunable for application (time-temperature, wavelength)

v  High fluorescence quantum yield → strong fluorescence signal

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Thermal exposure on composite Fluorescence emission

Fluorescence inspection

Mechanism for Fluorescent Thermal Damage Probe

v  Thermal damage probe AJNDE16 starts in “Off” state, which has a green fluorescence v  Conjugation between donor molecules is disrupted by adduct molecule

(sphere) v  Applying sufficient thermal energy causes the AJNDE16 molecule to split into

two molecules: v  Fluorescent molecule AJNDE16a “On” state (orange fluorescence)

v  Fluorescence changes due to restoration of conjugation bridge v  The molecule disrupting the conjugation bridge (sphere)

v  Reaction is irreversible due to decomposition of sphere → stable fluorescence emission

7 Images provided by Dr. Zhengwei Shi

v  AJNDE16 (off state) v  Absorbs in UV range (λ ≤ 350 nm) v  Emits green fluorescence with λmax = 510 nm

v  AJNDE16a (on state) v  Absorbs in UV range and blue wavelengths v  Emits orange fluorescence λmax = 595 nm

Fluorescence of AJNDE16 and AJNDE16a

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Fluorescence UV-Vis Absorbance

AJNDE16 AJNDE16a

Results and images provided by Dr. Zhengwei Shi

Fluorescence Measurement Setup

Excitation (390 or 470 nm)

Fluorescent Emission

LED

sample

Detector

Fluorescence Measurement Procedure 1.  As-cured sample spectrum obtained 2.  Sample exposed at desired temperatures (204ºC, 232ºC, and 260ºC) for discrete

time intervals 3.  Spectrum obtained after exposure

Sample

Probe

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Filter

Fluorescence Images of Exposed Probe-Doped Epoxy and Composite Samples

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Exposure Time

Bright field

Fluorescence 470nm excitation

v  0.05 -0.1 wt% probe in epoxy. v  Large contrast between off state (as-

cured) and on state (after exposure)

v  Discoloration of epoxy is much more difficult to observe in presence of carbon fibers

v  Fluorescence emission behavior of exposed composite is still observable and provides better contrast than bright field

Exposure Time

Bright field

Fluorescence 470nm excitation

Samples exposed at 232 ºC

Fluorescent probes are feasible for detecting thermal exposure of composites!!

Probe-doped epoxy (2.5 cm x 2.5 cm) Composite (0.5 cm x 0.5 cm)

v  Samples exposed at 232ºC for discrete time intervals v  Emission in probe-doped composite sample is similar to that of probe-doped

epoxy v  Weaker due to lower epoxy content

v  As exposure times increases, fluorescence emission is quenched and red-shifts v  Decomposition of probe molecule or due to change in matrix absorbance?

Fluorescence Emission of Probe-Doped Samples

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Probe-Doped Epoxy Probe-Doped Composite

Increasing exposure time

Increasing exposure time

Localized Heating Detection v  Localized heating is easily detected v  Size of fluorescent area grows with increasing exposure time v  Superposition of AJNDE16 and autofluorescence causes shift of fluorescence

emission v  Fluorescence is still visible around quenched areas

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5 min @ 232 ºC 15 min @ 232 ºC 30 min @ 232 ºC

Removal of Oxidation Layer

v  Sanding removes the oxidation layer restores the fluorescence

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1 hr at 232 ºC After Sanding

450 500 550 600 650 700 750

PL In

tens

ity (A

.U.)

Wavelength (nm)

before sanding After sanding

Summary of Preliminary Results v  Fluorescent probe AJNDE16 responds to temperature in the range

of ITD

v  Fluorescence changes in response to thermal exposure were characterized in probe-doped epoxy and epoxy/CF composite specimens

v  Fluorescence emission was quenched with increased exposure time at elevated temperatures. v  Related to oxidative degradation of the matrix v  Removing oxidation layer restores fluorescence

v  Probe was capable of detecting localized thermal exposure even in cases where areas of the fluorescence were quenched

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Acknowledgements I would like to acknowledge the following:

v  Flinn Research Group

v Curtis Hickmott, Dana Rosenbladt, Ryan Toivola, Ashley Tracey, and Gary Weber

v  Jen Research Group

v Zhengwei Shi, Sei-Hum Jang

v  The Boeing Company

v  Gary Georgeson

v  Funding: Boeing BL8DL Witness Surface Coatings

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v  Hysol EA 9390 is a epoxy resin commonly used in composite repairs

v  Under 470 nm excitation epoxy does not exhibit autofluorescence after thermal exposure

Epoxy Matrix

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Part A: tetraglycidal-4-4’-diaminophenylmethane (TGDDM)

Part B: 2,2'-dimethyl-4,4'- methylenebis(cyclohexylamine)

v  Samples exposed in argon show little change in absorbance v  Samples exposed in air exhibited significant darkening

v  Darkening caused by oxidation products

Effect of Exposure Atmosphere As-cured

Exposed in air

9390 9390 w/AJNDE16

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9390 9390 w/AJNDE16

Exposed in Argon

9390 9390 w/AJNDE16

0

0.05

0.1

0.15

0.2

400 450 500 550

Effect of Exposure Atmosphere on Fluorescence Emission of Probe-Doped Epoxy

v  Increase in matrix absorbance due to oxidation appears to quench fluorescence emission

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500 550 600 650 700 750

PL In

tens

ity (A

.U.)

Wavelength (nm)

9390 + AJNDE16 1 hr @ 204 ºC in air

9390 + AJNDE16 1 hr @ 204 ºC in argon

Probe-Doped Coatings v  Probe-doped coatings have some potential applications

v  Existing parts v  Localized areas

v  DGEBA-DETA was chosen as the coating material because it is a model epoxy system and cures at room temperature

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diglycidylether of bisphenol A (DGEBA) diethylenetriamine (DETA)

505

450 500 550 600 650 700 750

PL In

tens

ity (A

.U.)

Wavelength (nm)

As-cured

120 min @ 70 ºC

60 min @ 204 ºC

v  DGEBA-DETA displays autofluorescence upon thermal exposure

v  λmax = 505 nm (shorter λ than probe)

v  Behavior is similar to thermal damage probe, but activates at lower temperature

390 nm excitation

Thermal Exposure Probe-Doped DGEBA-DETA v  After short term exposure there are two peaks at 537 nm and at 560 nm

(yellow-orange fluorescence) v  As exposure continues λmax shifts to shorter wavelengths (blue-green

fluorescence) before quenching v  Shifts due to superposition of DD autofluorescence and probe

fluorescence

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505

537 560

522

450 500 550 600 650 700 750

PL In

tens

ity (A

.U.)

Wavelength (nm)

As-cured 5 min @ 232 ºC 15 min @ 232 ºC 30 min @ 232 ºC

390 nm excitation

Thermal Exposure Probe-Doped DGEBA-DETA v  After short term exposure there are two peaks at 537 nm and at 560 nm

(yellow-orange fluorescence) v  As exposure continues λmax shifts to shorter wavelengths (blue-green

fluorescence) before quenching v  Shifts due to superposition of DD autofluorescence and probe

fluorescence

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505

537 560

522

450 500 550 600 650 700 750

PL In

tens

ity (A

.U.)

Wavelength (nm)

As-cured 5 min @ 232 ºC 15 min @ 232 ºC 30 min @ 232 ºC

505

566

560

450 500 550 600 650 700 PL

Inte

nsity

(AU

) Wavelength (nm)

DD autofluorescence

Probe AJNDE16a

DD + AJNDE16a

Short Exposure

505

566

522

450 500 550 600 650 700 PL

Inte

nsity

(AU

) Wavelength (nm)

DD autofluorescence

Probe AJNDE16a

DD + AJNDE16a

Thermal Exposure Probe-Doped DGEBA-DETA v  After short term exposure there are two peaks at 537 nm and at 560 nm

(yellow-orange fluorescence) v  As exposure continues λmax shifts to shorter wavelengths (blue-green

fluorescence) before quenching v  Shifts due to superposition of DD autofluorescence and probe

fluorescence

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505

537 560

522

450 500 550 600 650 700 750

PL In

tens

ity (A

.U.)

Wavelength (nm)

As-cured 5 min @ 232 ºC 15 min @ 232 ºC 30 min @ 232 ºC

Long Exposure

Localized Heating Setup v  Localized heating was achieved by placing composite samples with

AJNDE16 doped DGEBA-DETA coating in contact with a heated aluminum rod (D = 18.4 mm).

v  Temperature was measured using 3 thermocouples placed at 7.5 mm, 10 mm, and 12.5 mm from the center

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