NDE of Lap Seams in Composite Laminate Fabric Structures

Eric C. JOHNSON, Dhruv N. PATEL, Christopher J. PANETTA, Oscar ESQUIVEL, and

Yong M. KIM

Space Materials Laboratory, The Aerospace Corporation, Los Angeles, CA 90009, USA

Abstract

Woven structural fibers can be infused with PTFE (polytetrafluoroethylene) to form composite laminate panels.

These Teflon®

-matrix, composite panels serve as choice material for a number of applications including roofing

(airport terminals), chemical protection shelters and suits, radomes and antenna cover systems. They are also

being considered for use in futuristic hydrogen airships designed for hydrogen transport and as a means of luxury

travel. To construct some of these structures, air- and water-tight lap seams to join panels are formed through

use of a heat seaming process. The process needs to be tightly controlled to reduce the possibility of seam

defects such as unbonds, overheated material and/or insufficient overlap. Inevitably, the overall integrity of the

structure depends on the quality of the seams. Here we assessed UV-Visible Reflectometry and Thermography

as potential NDE methods for assessing the quality of such seams in structures comprised of composite laminate

fabric panels.

Keywords: Composite laminate fabrics, PTFE–matrix composites, thermography, heat seamed lap joints

1. Background

Fabrics comprised of woven structural fibers such as aramid (Kevlar®

) or fiberglass can be

infused with Teflon®

to form composite laminates panels. The fiber orientations within

different layers of these panels can be chosen to make the panels stronger and/or stiffer in

chosen directions. The result is a relatively light material that exhibits strength, flexibility and

weather resistance making it a choice candidate for a number of applications[1,2,3]

including

roofing, chemical protection shelters and suits, radomes and other antenna protection systems

and possibly even futuristic airships.

For some of the aforementioned applications, to join panels, a heat seaming process is used to

create airtight, weather-resistant lap seams. Given that the overall integrity of a seamed

structure depends highly on the quality of the seams, the seaming process should be tightly

controlled to reduce the possibility of seam defects. In addition, NDE techniques should be

made available to check for process escapes. This study was focussed on developing NDE

techniques for inspecting the seams of a structure built with composite laminate panels

comprised of Kevlar®

fibers in a Teflon®

matrix. The seams were formed by melting the

Teflon®

matrix while pressing the two lap seam constituents into each other. Teflon®

melts at

approximately 621°F.[4]

When heated to this temperature, Kevlar®

fibers exhibit a small

reduction in tensile strength and modulus that grows with heat exposure time. As one would

Figure 1. Examples of structures constructed from composite laminate fabric panels. (A) Denver

International Airport Terminal Building, photo by R. B. Pan. (B) CSU-CHILL Radome and Airlock,

used with permission.

A B

13th International Symposium on Nondestructive Characterization of Materials (NDCM-XIII) , 20-24 May 2013, Le Mans, France

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expect, at higher temperatures, which still work for seaming, this reduction is more

pronounced and it is possible to overheat the seams. Other possible defects include regions of

the seam that do not fuse properly (unbonds) and regions where the overlap is insufficient.

The goal of this study was to develop and apply (1) a reflectometry method for determining if

the seam material strength has been significantly degraded due to overheating and (2) a

thermographic method for verifying seam overlap widths and detecting unbonds.

2. Test Material

Specimens for this study were cut from RAYDEL

® Q65 composite laminate panels.

RAYDEL®

Q65 is a high performance product of Saint-Gobain Performance Plastics

Corporation.[5]

As indicated in their published data sheets, the RAYDEL®

product is a

“microwave transmissive composite designed specifically for use in microwave applications”

that is comprised of Kevlar®

fabric in a PTFE matrix and is a quadriaxial, multi-ply laminate

with fibers in directions designated Warp, Fill, D1 and D2.

3. Reflectometry Study

3.1 Initial Observations

The supplied RAYDEL®

Q65 fabric was opaque with a white PTFE coated exterior surface

and a tan uncoated interior surface. Upon heating of the material, it was observed that the

interior surface darkened somewhat monotonically with increased heat exposure at

temperatures in the range used for creating seams.

Preliminary tests suggested that Ultraviolet-Visible (UV-Vis) reflectometry measurements

could be used to quantify the degree of material darkening of the seams. A test plan was,

therefore, devised to determine if fabric strength degradation due to the heat exposure could

be correlated with the UV-Vis results.

increased heat exposure

Figure 3. Heat exposure in the temperature range used for seaming resulted in darkening of the

interior surface of the supplied RAYDEL®

Q65 fabric.

PTFE Layer Warp Yarn Fill Yarn

Warp Direction

D1 & D2 Yarns

Fill Direction

PTFE Layer

Warp Yarn

Fill Yarn

D1 & D2 Yarns

Carrier Yarn

Figure 2. Optical micrographs of RAYDEL®

Q65 composite laminate cross-sections.

3.2 Test Method

Samples were prepared and tested according to the following plan:

1. RAYDEL®

Q65 fabric tensile test specimens were prepared and subjected to various

prescribed degrees of heat exposure.

2. Reflectometry measurements were then performed on the heat-treated specimens to

quantify the darkening of the fabric.

3. Untreated specimens were tensile tested to failure to obtain baseline properties and

validate the tensile testing methodology through comparison with the manufacturer’s

published data.

4. Each heat-treated specimen was tensile tested to failure.

5. The UV-Vis reflectometry results were plotted against the observed failure loads.

3.2.1 Specimen Heat Exposure

The tensile test specimens consisted of 1.5 X 7 inch strips of RAYDEL®

Q65 fabric oriented

for pull in the warp direction. The 2 inch grip regions of each specimen were sandwiched

between 2 x 3 x 0.5 inch blocks of ceramic leaving the center 2.5 inches of the strip fully

exposed as depicted in Figure 4. Thermocouples were used to monitor the fabric temperature

beneath the ceramic. Two steel platens designed to be in surface contact with the specimen in

between the blocks of ceramic were preheated in a convection furnace. These platens were

instrumented with thermocouples placed near the contact surface. The oven was set to the

desired temperature. Once the platens equilibrated at this temperature, the specimen, with

ends insulated, was placed in the furnace with its exposed center sandwiched between the

steel platens. Weights were used to increase the platen pressure to 15 psi. After the

prescribed exposure time, the sample was removed from the furnace and allowed to cool.

After analyzing the thermocouple data for a few exposures of various time durations, it was

decided to use six-minute “hits” to expose the samples. Between these hits, the sample would

be removed from the furnace and permitted to cool. This approach prevented the fabric

beneath the insulators from getting too hot. A specimen that was exposed to one six minute

hit at 660 °F is depicted in Figure 5 along with plots of the thermocouple readings during the

exposure. The discoloration of the heated center of the fabric specimen is quite apparent. The

thermocouple data indicate that the sample center was relatively evenly heated at 660 °F while

the sample ends did not rise much above 300 °F in temperature.

fabric sample insulator

steel platen

thermocouple mounting hole

Figure 4. Tensile test specimen readied for heat exposure (see text for details).

3.2.2 Fabric Strength Tests

Fabric strength tests were performed in accordance with ASTM D4851-07 (Standard Test

Methods for Coated and Laminated Fabrics for Architectural Use) and ASTM D5035-11

(Standard Test Method for Breaking Force and Elongation of Textile Fabrics – Strip Method).

An Instron Model No. 55R1115 Universal Testing Machine and Model No. 2511-303 11240

load cell were used to pull the specimens. The crosshead displacement rate was set to 2.0

inch/min and data was accumulated at a rate of 10 points per second. The specimen was held

between a set of wedge grips and an Instron Series 2620 Extensometer with 2.0 inch axial

gage length was attached to the back surface of the specimen to allow for measurement of

sample strain as depicted in Figure 6.

Photographs of one of the heat-treated and failure-tested specimens are presented in Figure 7.

This specimen was subjected to two 6 minute hits at 760 °F. The failure occurred at a load

corresponding to a material strength of 1200 lbs-ft/in. The failure mode, typical of nearly all

the samples tested, consisted of several horizontal breaks across the specimen within the heat-

treated gage region. The fabric strength for untreated specimens was found to be 2235 lbs-ft/in

suggesting that the material strength for this specimen was degraded by 46.3%.

6 min @ 660 °F

Thermocouple Readings for 6 min Exposure

0

100

200

300

400

500

600

700

800

900

1,000

0 1 2 3 4 5 6 7 8 9 Te

mp

, °F

Time, mins

Top Platen Bottom Platen

Insulated Sample End-Bottom Insulated Sample End - Top

Put Platens

on Sample Remove from

Furnace

Figure 5. Specimen following 6 minute, 660 °F heat exposure and corresponding thermocouple data.

Instron Universal Test Machine Mounted Sample

wedge grip

extensometer

wedge grip

Figure 6. The specimens were tensile tested to failure using an Instron Universal Test Machine.

3.2.3 UV-Vis Reflectance Measurements

An Ocean Optics USB4000 spectrometer and handheld integrating sphere with an internal

light source was used to measure the surface reflectance of the specimens. This spectrometer,

depicted in Figure 8, houses a grating and linear CCD array. Light entering the unit is

diffracted over the CCD for intensity measurements at wavelengths ranging from ~ 350 nm to

~ 1050 nm. The measurements were made relative to a Spectralon®

plug that was corrected

against a NBS 2019D diffuse reflection calibration standard. The reflectance spectrums for

samples of various heat exposures, also shown in Figure 8, revealed that 700 nm is a

reasonable wavelength for reflectance comparisons.

3.2.4 Results – Fabric Strength vs. Reflectance

The reflectance and failure-strength data for 23 specimens are plotted in Figure 9. The fabric

strength degradation is simply the percentage decrease in failure strength relative to that of the

untreated fabric. The reflection data is normalized against the measured reflection for the

untreated fabric (44.75%). As indicated earlier, the “hits” were of 6 minutes duration. While

After 1st 6 min 760 °F Hit After 2nd 6 min 760 °F Hit After Failure

Figure 7. Typical heat-treated and failure-tested specimen. The failure mode consisted of

several horizontal breaks across the gage region which show up as a thin crack line

(evidenced by a thin crack line on the right-most photograph.

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there is an obvious trend of reduced reflectance for increased fabric strength degradation,

there is also considerable scatter in the data. The scatter was likely due to differences in how

the heat was conducted from the platens to the specimen. Were the tests repeated, this

parameter could possibly be more tightly controlled. The data suggests field reflectance

measurements could be used to (1) crudely grade the degree of discoloration at specific points

on a seam and (2) estimate the fabric strength degradation of the material in the vicinity of the

seam to within ± 15% or better.

3.3 Application of Reflectometry Method to a Radome Seam Specimen

RAYDEL®

Q65 side seam specimens were obtained from a dismantled inflatable radome.

The specimens had seams with a nominal 5 inch overlap between the two joined panels and

included a thin, 8 inch wide, outside overtape layer comprised of the same constituent

materials a shown in the schematic on the right of Figure 10.

Degradation vs. Reflection Ratio

Figure 9. The data are scattered, but still suggest a monotonic relationship between the Reflection

Ratio and Fabric Strength Degradation for the range of heat exposure studied.

0.0 % 1.3 % 1.9 % 2.2% 1.2 %

4.8 % 8.3 % 11.0 % 11.7 % 9.8 %

27.9 % 24.5 % 21.1 % 20.0 % 19.3 %

21.7 % 16.6 % 11.0 % 8.3 % 5.4 %

outside overtape

interior visible panel edge

hidden panel edge

Seam Cross-Section

Figure 10. Predicted % fabric strength degradation at designated points along the seam of a specimen

taken from a dismantled inflatable radome.

Reflectance measurements were made on the seam specimen depicted in Figure 10 at the

points indicated with green circles. The predicted % fabric strength degradation indicated for

each point was derived using a crude linear fit to the Degradation vs. Reflection Ratio data

presented earlier in Figure 9. The highest observed degradation level for this specimen was

27.9 ± 15 % at the seam edge where the material transitions from two layers + overtape to one

layer + overtape.

4. Thermography

4.1 IR testing of a Radome Seam Specimen

To thermographically examine a seam specimen from the dismantled radome, it was gently

warmed uniformly by waving a quartz halogen lamp over it. An Amber Radiance 1 imager

was then used to capture an image ~ 30 s after application of the heat. The sensor in this

imager is active in the 3-5 !m range. Images were acquired from both sides of the sample,

first with the heat applied on the same side of the sample as the imager and then with the heat

applied on side opposite the imager. In all four instances, the images showed clear indications

of the seam edges and an unbond that was present in the seam. Images of the interior side of

the sample are presented in Figure 11.

4.2 Thermographic Inspection of an Inflatable Radome

Inflatable radomes can be as large as 100 ft in diameter making it a challenge to gain access to

many of the seams of an in-service radome. The through-heat transmission results, presented

in the left image of Figure 11, were of sufficient quality to prompt an attempt at the remote

viewing approach suggested in Figure 12 for a field inspection. In this approach, the sun

striking outside of the radome provides a constant thermal stimulus that works against the air

conditioning within the radome. The hope was that there would be enough of a steady state

thermal differential across the fabric to permit thermal imaging of the seams though use of a

telephoto lens on the imager.

seam width

seam unbond

indication due to air gap at hidden edge

!  IR imager: interior surface !  Heat Applied: outside surface

!  IR imager: interior surface !  Heat Applied: inside surface

seam width

seam unbond

indication due to air gap at hidden edge

Figure 11. Thermographic images of a side seam specimen taken from a dismantled inflatable radome. For

reference, the geometry of the seam is the same as that provided in the cross-sectional drawing in Figure 10.

The radome chosen for the field test housed a dish antenna. The plan was to image the

radome crown seam. The IR imager was positioned on the highest stair platform about 50 ft

from the seam as depicted in Figure 13. Using a 100 mm lens, a resolution of ~ 8.6 pixels per

inch was achieved.

Still shots were then captured to create the montage of the entire seam presented in Figure 14.

Careful inspection of the image reveals a number of salient features of the visible seams. It is

noteworthy that indication due to the air gap at the hidden panel edge changes from light to

dark depending on which side of the dome the sun was on. There are also notable shadowing

effects. For instance, the visible edge appears as a darker and thicker line for sectors on one

side of the dome. Features in addition to the seam edges were also evident such as the

overtape edges, seam overlaps, various splices and climbing ropes lying on the exterior of the

dome.

Figure 12. Remote IR imaging of radome seams via solar heating.

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Figure 13. Radome Interior. The IR imager was positioned on the platform and focussed to capture images

of the crown seam (left). An enlargement of the crown seam is with sectors numbered for reference (right).

The data was of sufficient quality to detect seam width variation of less than 1/4 inch. For

example, in the span depicted in Figure 15, a seam width variation of 2.5 pixels = 0.29 inch is

observed.

2

4

5

6

7

8

9

10

11

12

13

14

15 16 17

19

20

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Figure 14. Montage of IR still shots of radome crown seam.

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Figure 15. Image analysis tools were employed to provide average pixel profile maps at points along the seam.

The number of pixels between the extrema associated with the overlap edges could then measured to determine

the seam width at each point.

5. Conclusions

Two potential methods for evaluating welded lap seams in structures comprised of Kevlar®

-

fiber/Teflon®

-matrix composite laminate panels were investigated. The first was a

reflectometry method for determining if the seam material strength had been significantly

degraded due to overheating and the second was a thermographic method for verifying seam

overlap widths and detecting unbonds.

Preliminary results for the reflectometry method were presented. As noted, the data scatter

was large, possibly due to inconsistencies in the heat treatment of the test specimens. It

remains to be seen if improvements in the method of heat exposure would lead to a tightening

of the data. The untreated fabric for these tests exhibited a reflection value of 44.75%.

Apparently, this parameter varies noticeably among lots of the same material. All the

reflection data were normalized to this value, which was characteristic of the material used in

this study. Tests need to be performed to see how much variation in the reflection of the

untreated material affects the end results. It should also be noted, that this measurement

method requires contact with the inner surface of the seam of interest.

The thermographic method proved to be quite successful for the measurement of seam widths

and detection of unbonds. An actual field inspection of a radome was attempted and it was

found that the inspection could be quite effectively accomplished remotely from within the

radome with solar heating as the stimulus. Several improvements over what was done here

could be easily incorporated. A higher magnification lens could be used to decrease the field

of view and improve the resolution. The inspection could be repeated at several distinct times

during the sunny part of the day (e.g., mid-morning, high noon, and mid-afternoon). This

would increase the data set and make it easy to correct for shadowing effects. Finally, a

standard camera could be mounted in conjunction with the IR imager to capture a

synchronous optical photograph for data overlay.

Acknowledgements

The assistances provided by Mr. Thomas V. Albright in the preparation and strength testing of

specimens and of Dr. James P. Nokes in making the radome field inspection measurements

are most gratefully acknowledged.

References

1. Craig G. Huntington, The Tensioned Fabric Roof, Reston, VA: American Society of Civil

Engineers, 2004.

2. Tensile Fabric Roof Structures – PTFE Coated Glass Cloth, Architen Landrell Associates

Limited, www.architen.com

3. Miriam Euni Son, The Design and Analysis of Tension Fabric Structures, Thesis (M. Eng)

-- Massachusetts Institute of Technology, Dept. of Civil and Environmental Engineering,

2007.

4. DuPont™

Teflon®

PTFE 6C, Product Information, 249415B, 2005.

5. RAYDEL® Q65 Material Specification, 2004, fabricatedsystems.saint-gobain.com.

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