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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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at 700 nm
Untreated
Incre
ase
d H
ea
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Figure 8. (A) An Ocean Optics USB4000 spectrometer with handheld integrating sphere. (B) Sample
reflectance spectrums.
A B
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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inflatable radome
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Crown Seam
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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
21
22
23
24
25
26
27
28
29 1
18
3
Ropes on outside
surface?
Figure 14. Montage of IR still shots of radome crown seam.
6
7
8
9 42.3
39.5
42.0
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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