Composite Thermal Damage Measurement with Handheld FTIR

November 14, 2013 B.D. Flinn, T. Howie, A.C. Tracey, D. Pate,

J.T. Morasch, E. Roberts University of Washington

Composite Thermal Damage Measurement with Handheld FTIR

§  Motivation and Key Issues §  Damage detection in composites requires different

techniques than metals §  Incipient thermal damage occurs below traditional NDE

detection limits

§  Objective §  Determine if handheld FTIR can detect thermal damage and

guide repair §  Approach

§  Characterize panels with controlled thermal damage and perform repair based on FTIR inspection

1

FAA Sponsored Project Information

§  Principal Investigators & Researchers §  Brian Flinn (PI) §  Ashley Tracey (PhD student, UW-MSE) §  Tucker Howie (PhD student, UW-MSE)

§  FAA Technical Monitor §  David Galella (year 3) §  Paul Swindell (year 1 & 2)

§  Industry Participation §  The Boeing Company (Paul Shelley, Paul Vahey) §  Sandia National Lab (Dennis Roach) §  Agilent (formerly A2 Technologies)

2

Background §  Continuation of existing project (year 3 of 3) §  Years 1 and 2 (A2 Technologies, Boeing and U of DE)

§  Characterization of homogeneous thermal damage §  Ultrasound §  Short beam shear (SBS) §  Microscopy §  Handheld FTIR (ExoScan)

§  Calibration curve for FTIR detection of thermal damage (SBS data)

§  Mapped surface of localized thermal damage §  Year 3 (UW and Boeing)

§  3-D characterization of localized thermal damage §  Contact angle and fluorescence spectroscopy §  FTIR guided scarf repair §  Test repair

3

Thermal Damage vs. Detection Method

§  SBS, ultrasound, and microscopic analysis of composites with thermal damage §  Properties degrade before detection possible à need

method to detect incipient thermal damage (ITD)

Short Beam Shear Strength Retention vs. Temp./Time – Epoxy 1

Onset of crack

development visible in

micrographs,

No cracks visible in micrographs

Damage becomes visible in C-Scans

4

Thermal Damage vs. Detection Method

§  SBS, ultrasound, and microscopic analysis of composites with thermal damage §  Properties degrade before detection possible à need

method to detect incipient thermal damage (ITD)

Short Beam Shear Strength Retention vs. Temp./Time – Epoxy 1

Onset of crack

development visible in

micrographs,

No cracks visible in micrographs

Damage becomes visible in C-Scans

4

Summary of Work Completed

FTIR Contact Angle

Fluor-escence

Thermal Damage ✔ ✗ ✗ Resin rich (tooled) surface oxidation peaks characterized

Fiber rich (sanded) surface

oxidation peaks absent à multivariate analysis (MVA)

Fiber orientation signal varies with orientation

Surface finish

can only compare surfaces with MVA when surface finish

is same

5

Summary of Work Completed

FTIR Contact Angle

Fluor-escence

Thermal Damage ✔ ✗ ✗ Resin rich (tooled) surface oxidation peaks characterized

Fiber rich (sanded) surface

oxidation peaks absent à multivariate analysis (MVA)

Fiber orientation signal varies with orientation

Surface finish

can only compare surfaces with MVA when surface

finish is same

5

Experimental Overview

§  Thermally Damaged SBS samples §  FTIR measurements on SBS samples §  Develop calibration curve for FTIR from

SBS values §  Predict evaluation set to validate model §  Composite panel locally damaged §  Panel Mapped using FTIR §  Panel cut up for mechanical testing

§  SBS §  Tg (DMA)

6

Materials and Process

§  Toray T800/3900 composites with various levels of thermal damage §  SBS samples provided from Year 1 & 2 research §  SBS samples thermally exposed in air §  Locally damaged panels heated in air – UW/Boeing

§  Sand SBS surfaces with 180 grit Al2O3 sanding pads §  Measure sanded surface with diffuse reflectance FTIR

§  3 measurements per sample §  3 samples per time/temp

§  Use MVA to develop calibration curve for thermal damage §  GRAMS IQ software

9 measurements

7

Materials and Process – FTIR

§  Mid-IR data region: 4000 cm-1 to 650 cm-1 §  Diffuse reflectance sampling interface §  Data collection: 90 coadded scans with 16 cm-1

resolution for background and specimen

An infrared beam path for diffuse reflectance

Mirror

Substrate Reflected

Beam

IR beam from interferometer

θi = 0

θr

To detector

ExoScan FTIR

8

Resin Rich vs. Fiber Rich Surfaces

§  Resin rich surfaces: oxidation peaks increase with damage

§  Fiber rich surfaces: oxidation removed by sanding Ø  Need MVA to determine differences in spectra and

correlate to SBS data

0

0.1

0.2

0.3

0.4

0.5

0.6

1200 1300 1400 1500 1600 1700 1800 1900 2000

Abs

orba

nce

Wavenumber (cm-1)

High Damage

No Damage

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

1200 1300 1400 1500 1600 1700 1800 1900 2000

Abs

orba

nce

Wavenumber (cm-1)

Fiber Rich Surface Resin Rich Surface

9

Spectral Analysis

§  FTIR spectra of CFRP surfaces complex §  Multiple constituents à many spectral peaks

§  How to analyze spectra with confidence? §  Multivariate analysis!

§  Principal Component Analysis (PCA) §  Exploratory to

identify trends §  Peak locations and

intensities §  Used to develop

models

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8

650 1150 1650 2150 2650 3150 3650

Abs

orba

nce

Wavenumber (cm-1)

10

Effect of Sanding Variables on FTIR

§  Variables: temperature, method (hand vs. orbital), direction of sanding, grit size

-0.01

-0.005

0

0.005

0.01

0.015

-­‐0.015   -­‐0.01   -­‐0.005   0   0.005   0.01  

PC

2

PC1

1 hr @ 400 °F

1 hr @ 500 °F

Temperature Grit size

11

-0.006

-0.004

-0.002

0

0.002

0.004

0.006

0.008

-­‐0.01   -­‐0.005   0   0.005   0.01  

PC

2

PC1

180 grit

240 grit

320 grit

Effect of Sanding Variables on FTIR

Ø  FTIR Results influenced by sanding technique Ø  Measure consistent surfaces and develop model

-0.005

-0.0025

0

0.0025

0.005

-­‐0.015   -­‐0.01   -­‐0.005   0   0.005   0.01   0.015   0.02  

PC

2

PC1

Parallel to Fibers

Perpendicular to Fibers

Direction of Sanding Sanding Method

12

-0.004

-0.002

0

0.002

0.004

-­‐0.015   -­‐0.01   -­‐0.005   0   0.005   0.01   0.015   0.02  P

C2

PC1

Hand sanding

Orbital sanding

Developing FTIR Model

§  SBS samples sanded and measured with FTIR §  FTIR spectra processed to remove baseline

effects §  1st derivative with Savitzky-Golay 7pt smoothing

§  Partial Least Squares model developed using MVA on processed spectra

Raw Spectra Processed Spectra

13

FTIR Model

ü  Data fits model

R² = 0.9244

50

60

70

80

90

100

110

50 60 70 80 90 100

Pre

dict

ed S

BS

Ret

entio

n Va

lue

(%)

Actual SBS Rentention Value (%)

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Model Validation

§  Model used to predict SBS retention in independent evaluation set

§  Error in model determined

%100*%actual

actualpredictedError −=

146

14 11

< 5 % 5 % ≤ x ≤ 10 % > 10 %

Nu

mb

er o

f S

amp

les

% Error

15

Localized Heat Damage – Process

16

Localized Heat-Damage Mapping

§  Different levels of thermal damage detected by FTIR Ø  Cut panels into SBS and dynamic mechanical analysis

(DMA) samples for mechanical testing

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

650 850 1050 1250 1450 1650 1850

Abs

orba

nce

Wavenumber (cm-1)

Outside Heated Region Edge of Heated Region Center of Heated Region

0 50

100 150 200 250 300 350 400 450 500

0 25 50 75 100 125

Tem

pera

ture

(°F)

Time (min)

17

SBS and DMA Testing

§  Damaged panel cut into SBS and DMA coupons

§  Testing in progress §  Compare SBS and Tg to

determine best method to correlate to FTIR

18

Summary

§  FTIR measurements sensitive to surface finish §  Need to test samples with consistently

sanded surfaces §  Calibration model developed from SBS

samples §  Model predicted evaluation set well

§  Panels created with localized thermal damage and surface mapped with FTIR

§  SBS and DMA testing in progress to correlate mechanical damage to FTIR spectra

19

Future Work

§  Map thermally damage panels provided from Years 1 & 2 with FTIR

§  Determine mechanical test to correlate damage to spectra

§  Characterize thermally damage of panels provided from Years 1 & 2

§  Perform scarfed repair guided by FTIR §  Test scarfed repair

20

Looking Forward

§  Benefit to Aviation §  Improved damage detection §  Greater confidence in repairs

§  Future needs §  Application to other composite

systems §  Other applications of handheld FTIR

§  Chemical damage §  Surface prep for bonding

21

Fluorescent Thermal Damage Probe

1 inch

Thermal exposure on composite Fluorescence emission

Fluorescence inspection

Funded by: The Boeing Co.

Probe-doped coating 1 hr @ 450 °F

22

Acknowledgements

FAA, JAMS, AMTAS Boeing Company

Ø  Paul Vahey, Paul Shelly, Greg Werner, Megan Watson, Jim Chanes

Sandia National Labs Agilent Technologies UW MSE

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