Fatigue Crack and Porosity Measurement inComposite Materials by Thermographic and

Ultrasonic Methods

James L. Walker, Samuel S. Russell and Michael W. Suits

National Aeronautics and Space Administration

Nondestructive Evaluation Team, ED32

Marshall Space Flight Center, AL 35812

256-961-1784

Gary L. Workman

University of Alabama in Huntsville

Center for Automation and Robotics

Huntsville, AL 35899

256-824-6578

https://ntrs.nasa.gov/search.jsp?R=20040084006 2019-08-23T17:42:38+00:00Z

OUTLINE• PURPOSE

Detect thermo-mechanically induced intra-ply fatigue microcracking andmanufactured porosity in unlined composite pressure vessels

• DEFECT DESCRIPTIONSPorosityMicrocracking

• THERMOGRAPHYOverview of techniqueStrengths and WeaknessesExamples of its use for porosity detection

• RESONANT ULTRASOUND SPECTROSCOPYOverview of techniqueStrengths and WeaknessesExamples of its use for microcracking detection

• CONCLUSIONS

DEFECT DESCRIPTIONS(Porosity)

Cause/Definition: Voids trapped within a laminate during thecuring process due to off-gassing of the resin, air trappedbetween plies, improper cure schedule, etc.

DEFECT DESCRIPTIONS(Microcracking)

Cause/Definition: Cracking of the resin used to support thefibers in the laminate due to combined thermal “cryogenic”and mechanical loading.

THERMOGRAPHIC INSPECTION SYSTEM• Imager: Indigo Merlin Mid

• Detector => Indium Antimonide

• Detector resolution => 256 x 312

• Spectral Response => 3 - 5 mm

• Sensitivity => 0.025 ∞C NEDT

• Software: ECHOTHERM“ 32 (Thermal Wave Imaging, Inc.)

• Lens => 13 mm

Imager

Flash Hood

Controller

CONTROLCOMPUTER

FLASHLAMPFLASHLAMP

INFRA-REDCAMERAINFRA-REDCAMERA

VIDEO MONITOR

SPECIMEN

FLASH POWERCONTROLLER

FLASH POWERSUPPLIES

SYSTEMCONTROLLER

THERMOGRAPHIC ANALYSIS METHOD

Thermography Image Sequence

Image subtraction (Image N – Pretrigger)

Enhanced Image

Averaged Images

THERMOGRAPHIC ANALYSIS METHOD

Clustered Porosity Detected Thermographically

Lead foilmarkers

Porosity

8 inches

Porosity

THERMOGRAPHIC MATERIAL DEGRADATION TRACKING

0

2

4

6

8

10

12

Time (sec) 2.0 4.0 6.0 8.0 10.0Time (sec.)

THERMOGRAPHY TREND

POROSITY IN GRAPHITE EPOXY

RESONANCE ULTRASOUND(ACOUSTO-ULTRASONICS)

0

2

4

6

8

10

12

0.1 0.6 1.0 1.5Frequency (MHz)

Pulser

Receiver Typical Power Spectra

SAMPLE PREPARATION

CRYOGENIC TENSILE TESTING INLIQUID NITROGEN

MICROCRACK COUNTING

SAMPLE PREPARATION AND TESTING

ANALYSIS (Self Organizing Map Neural Network)

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

200 400 600 800 1000 1200 1400 1600 1800

Frequency (kHz)

CoordinateLayer

UltrasoundResonant

Spectrum

Kohonen Layer

-1

-0.5

0

0.5

1

-1 -0.5 0 0.5 1

d-b-before

d-c-after

e-b-before

e-b-after

d-c-before

d-b-after

e-c-before

e-c-after

UltraSpec System

Energy distributions

Self Organizing Map Neural Network Neural Network Data Map

(0, 90, 0, 90)s SAMPLES

0

10

20

30

40

50

60

70

80

90

100

0.00 10.00 20.00 30.00 40.00 50.00 60.00 70.00 80.00 90.00 100.00

Cracks/inch

-1

-0.5

0

0.5

1

-1 -0.5 0 0.5 1

b-before

c-after

c-before

b-after

Kohonen Map

Microcrack Trend

Increasing count

Surface Breaking Feature

(0, 45, 0, -45)s SAMPLES

0

20

40

60

80

100

120

0 3 6 9 12 15

Cracks/Inch

-1

-0.5

0

0.5

1

-1 -0.5 0 0.5 1

b-before

b-after

c-before

c-after

Kohonen Map

Microcrack Trend

CONCLUSIONS

Thermography has been shown to be capable of detectingclustered porosity and shows promise for quantifying generalporosity level

Resonance ultrasound has been shown capable of detecting thepresence of microcracking

The ability to detect microcracking with resonance ultrasound is

dependent upon the number of cracks present

FUTURE WORK

Validate thermographic porosity level assessment

Quantify microcrack detection