Assessment of Advanced Ultrasonic and
Infrared Inspection Methods to Detect
Delaminations and Water Ingress in
Composite Honeycomb Materials
David G. Moore and Ciji L. Nelson
Sandia National Laboratories
Nondestructive Evaluation and
Experimental Mechanics Department
Post Office Box 5800 MS-0557
Albuquerque, New Mexico 87185, USA
NDCM Conference May 20- 25, 2013
Le Mans, France
Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly
owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy’s National Nuclear Security
Administration under contract DE-AC04-94AL85000. This presentation is declared a work of the U.S. Government and
is not subject to copyright protection in the United States.
Introduction
Sandia National Laboratories is
studying composites due to
their unique structure and
potential for aerospace
applications.
New composite manufacturing
techniques are requiring us to
implement advanced inspection
methods.
Reference standard creation is
key to the development of
material characterization
establishing inspection criteria
limits and assessing equipment
capabilities.
Overview of Composites
Defects found in Composite Structures
•
Generally four types of flaws in composite
materials:
–
–
–
–
•
Disbond
Core Damage
Delamination
Porosity
These flaws can occur due to the
following:
– Impact damage
– Lighting Strike (heat damage)
– Manufactures Defect
Core Damage
Engineering Sample
The specimen is 20.3 by 20.3 cm composed of a Nomex™ honeycomb
structure sandwiched between two carbon graphite laminate weave skins
(3 plies thick). Five flaws were created in the specimen:
1)
2)
3)
4)
5)
2.54 cm diameter epoxy potted honeycomb core (full thickness);
2.54 cm square Teflon shim (located between plies 2 and 3);
12.7 mm diameter Teflon shim (located between plies 2 and 3);
1.27 square disbond (located at adhesive bondline);
2.54 cm diameter disbond (located at adhesive bondline).
Computed Tomography Baseline Inspection
The Computed Tomography (CT)
technique collects penetrating
radiation measurements from the
composite sample’s x-ray opacity
using an amorphous silicon digital
detector array (flat panel).
The source and detector remain constant while
the part is rotated. These slices are then collected
and mapped together to create a three
dimensional CT-density map.
The fraction of the x-ray beam that is attenuated
will directly relate to the density and thickness of
the material through which the beam has traveled.
CT Inspection Results
Deeper
Interface between the
laminate and Nomex
honeycomb structure.
Shallower
Top of the laminate showing
the weave pattern at the
surface of the part.
A Perkin Elmer (2048 by 2048 pixels) amorphous silicon flat panel detector with a 0.20
by 0.20 mm pixel pitch was used. The panel was scanned at a 1-to-1 ratio giving it a
geometric resolution of 20 mils. The x-ray source operated at 160 Kilovolts and 4.4 mA
CT Inspection Results (continued)
Deeper
Midway through the
honeycomb core crushing
Shallower
Honeycomb material
(disbonds detected)
Side view of the
panel showing an
area of core damage
Ultrasonic Properties
Material
Aluminum
Water (20 °C)
Air (20 °C)
Hysol
Teflon
CompositegraphiteEpoxy
Sound
Velocity v
(m/sec)
Density ρ
(kg/m3)
6320
1483
343
2850
1520
2700
998
1.204
1580
2200
Acoustic
Impedance
Z = ρv (kg/m2
sec) x 106
17
1.48
0.00041
4.52
3.3
3070
1450
4.4
Ultrasonic Inspection
Ultrasonic transducers transmit sonic waves into a sample and measure the
reflected responses.
Near surface resolution can be improved with a delay line tip, to provide a time
delay between sound generation and reception of reflected energy.
Low-frequency Resonant, Ultrasonic Transducers transmit ultrasonic waves that
penetrate through the laminate and enter the honeycomb cell wall at the node
bond adhesive interface.
Front Surface Echo
Back Surface
Echo & Bondline
0.22 mm
Ultrasonic (UT) Setup and Results
The Composite Sample was UT inspected
using the below equipment and setup,
- MAUS V™, (Mobile automated UT
scanner) was used to acquire the images.
- A 5 MHz probe, 6.35 mm in diameter.
- Scanner resolution- 0.5 mm.
- The gate was set to monitor the backwall
signal from the laminate.
Amplitude
Gate Image
Depth
Gate Image
Resonance Basics
Resonance inspection requires a narrowband transducer that can be excited at its
natural resonance frequency. A continuous standing wave is coupled into the material.
Resonant inspection can be used to inspect
honeycomb materials and compliment
conventional ultrasonic inspection.
The cursor tracks the shift in the signal phase (X) and amplitude (Y). Any changes in
structural resonance (disbonds, or delaminations) is represented by changes in the
Resonant Frequency at that point in the sample.
Phase
Amplitude
Shift in Resonant Frequency
Resonance Inspection Results
Resonant inspection was moderately successful on the sample at 110 KHz.
Resonance probe could not reliably detect the Teflon inserts but could detect the
epoxy filled honeycomb core.
The signal is attenuated around the perimeter of the Teflon inserts making the defect
features hard to discern.
The use of Teflon inserts in a resonant inspection reference standard should be
carefully considered. Teflon inserts may not be usable for reliable instrument
calibration or establishing a reject criterion.
Phase
(Y) Image
Amplitude
(X) Image
Thermal Material Properties
x
Conduction: energy transfer from a more energetic
particles to less energetic particles within a material.
Interactions between particles are due to a thermal
gradient.
Fourier's law defines time rate of heat transfer
through a material. The heat flux is proportional to
the negative gradient in the temperature and to the
area. The proportionality constant 𝑘 is the transport
property thermal conductivity W/(m °C).
Heat flux q" is the heat transfer rate in direction x per
unit area perpendicular to the direction of transfer.
Since heat transfer rate is a vector quantity it can be
written in general of the conduction rate equation:
q" = −𝑘𝛻 𝑇 = −𝑘
𝜕𝑇
𝒊
𝜕𝑥
+𝒋
T2
T1 > T2
𝜕𝑇
𝜕𝑦
𝒅𝑻
𝒒𝒙 = −𝒌
𝒅𝒙
𝒒 = q"
𝜕𝑇
+𝒌
𝜕𝑧
Thermophysical Properties
Thermophysical properties have two distinct categories: Transport and
Thermodynamic.
Transport properties include the diffusion rate coefficients thermal
conductivity and kinematic viscosity.
Thermodynamic properties are useful to define the state of equilibrium.
Density – ρ (kg/ m3) Heat capacity – c (J/ kg °C) and volumetric heat
capacity ρc (J/ m3 °C ). Solids can store large amounts of thermal energy
when compared to gases.
The ability of a material to conduct thermal energy relative to it ability to
store thermal energy is termed thermal diffusivity α (how fast the material
temperature adapts to the surrounding temperature).
Thermal effusivity (ε) is a measure of a materials ability to exchange
thermal energy with its surroundings
Thermal Properties
Conductivity,
𝑘
W/(m °C)
Specific Heat,
cp
J/(kg °C)
Density,
ρ
kg/(m3 )
Effusivity
ε
J/(m2 °C) √s
1380
Diffusivity,
α
m2/sec
1 x 10 -7
2.174
Phenolic (resin
pressed)
0.3766
1255
Teflon
0.2510
167.36
1004
707.1
2170
2250
1.152
1052
739.6
16317.6
CFRP Parallel
Carbon Fibers
7
1200
1600
36.45
3666.06
CFRP Perpendicular
Carbon Fibers
0.8
1200
1600
4.167
1239.45
Epoxy (hysol)
0.1945
121
1172
875
1210
2780
1.372
497.43
525.271
17156.1
397.48
14.644
384.9
502.1
8940
7920
1155.
36.83
36982.8
7631.1
GRP Parallel
Glass Fibers
0.38
1200
1900
16.67
930.81
GRP Perpendicular
Glass Fibers
0.30
1200
1900
13.16
827.04
Material
Carbon
Graphite
Aluminum 2024 T3
Copper
Stainless Steel 304
807.667
Infrared Basics
Active Thermography (AT) is a technique where a stimulus is applied to a surface
and causes it to heat or cool in such a way to allow the surface characteristics to be
observed by an infrared camera.
IR camera
PC
Lamp
light
emitted IR
IR Image
heat
conduction
Source: Thermal Wave Imaging
defect
EchoTherm Equipment
Source: http://www.thermalwave.com
The EchoTherm System used the
following components and settings:
- FLIR 6106 camera was ran at 60Hz
at an image size of 640 X 512
pixels, with an InSb detector and 14
bit output.
- The hood contained two xenon
flash lamps partially surrounded by
parabolic reflectors and 5000
Joules per lamp.
- The collected images were
processed with Thermographic
Signal Reconstruction (TSR)™
technology in the Mosaiq software
package, to provide a set of images
representing the heat transfer.
“Damage” Limits for Composites
Dents/crushed core: No greater than 76.2 mm (3.0”) diameter or deeper than 12.7 mm (0.050”)
Holes and punctures: No greater than 25.4 mm (1.0”) diameter.
Disbonds between core and skin: (core crush, split core) No greater than 30 cm (12.0”)
Core bond separations: No greater then 45.2 square cm (7.0 square inches).
Liquid Intrusion in cells: No more than 40 cells
Delaminations
Bond separation
Inspect to make sure water and moisture is within limits. No further water
removal efforts are required provided entrapped water is within limits and leak
path is found, repaired, and water drip test confirms repair.
Infrared Data Analysis
Source: http://www.thermalwave.com
Cool
Low
Effusivity
(ε)
Warm
High
Effusivity
(ε)
9.02
High
Diffusivity
(α )
8.36
7.69
Teflon
7.03
Epoxy
6.36
5.69
0.02
0.06
0.27
1.16
4.79
time (sec)
First derivative of reconstructed intensity per unit time.
20.03
Low
Diffusivity
(α )
First and second derivatives yield
substantial contrast at earlier thermal
decay times in the image sequence.
The images will be sharper. (less
lateral diffusion).
IR Results (Epoxy and Teflon)
0.40
-0.10
9.02
0.20
8.36
-0.30
Log of reconstructed
intensity per time.
7.69
0.00
-0.50
7.03
First derivative of reconstructed
intensity per time.
-0.70
-0.20
Second derivative of reconstructed
intensity per time.
6.36
-0.40
0.05
-0.90
5.69
0.02
0.06
0.27
time (sec)
1.16
4.79
20.03
0.05
0.16
0.54
1.79
5.98
0.16
20.03
time (sec)
Intensity Time Plot 0.55
seconds after pulse
0.54
time (sec)
1.79
5.98
20.03
IR Results (Epoxy and Disbond)
9.24
-0.10
0.40
-0.30
0.20
-0.50
0.00
Log of reconstructed
intensity per time.
8.52
7.81
7.09
First derivative of reconstructed
intensity per time.
-0.70
6.38
5.66
-0.40
-0.90
0.02
0.06
0.27
time (sec)
1.16
4.79
20.03
Second derivative of reconstructed
intensity per time.
-0.20
0.05
0.16
0.54
1.79
5.98
20.03
0.05
time (sec)
Intensity Time Plot 1.1
seconds after pulse
0.16
0.54
time (sec)
1.79
5.98
20.03
IR Inspection Results (continued)
9.24
-0.10
0.40
-0.30
0.20
-0.50
0.00
8.52
7.81
7.09
-0.70
Log of reconstructed
intensity per time.
6.38
5.66
0.02
0.06
0.27
time (sec)
1.16
4.79
-0.90
0.05
20.03
First derivative of reconstructed
intensity per time.
0.16
0.54
1.79
5.98
-0.20
Second derivative of reconstructed
intensity per g time.
-0.40
20.03
0.05
0.16
time (sec)
Intensity Time Plot 4.6
seconds after pulse
0.54
time (sec)
1.79
5.98
20.03
Polymer Properties
Hydrophilic polymers characterized by different swelling factors were evaluated
by: Bogomil YOCHEV, Svetoslav KUTZAROV, Damyan GANCHEV, Krasimir
STAYKOV, Technical University – Sofia, Bulgaria for their ultrasonic properties.
- Preliminary experiments show that polymers having water content above 85%
can not be used as solid dry-couplants for practical purposes because of their
low mechanical strength. Might be used as a reference standard for water
ingress into composite materials.
- When the water content increases the longitudinal ultrasonic velocity decreases
and nears that of water – 1480 m/s.
- Hydrophilic polymers have longitudinal ultrasonic velocity significantly lower
than the velocity of PMMA (type-H) – 2750 m/s, polyizoprene rubber (type-F) –
1840m/s and is comparable to that of Aqualene (type-G) – 1580m/s
Reference: Bogomil YOCHEV et.al
Cut from Aircraft Sample
15.2 cm
Aqualene placement
22.9 cm
0.10 mm skin thickness
The aircraft sample was cut
from a secondary load bearing
surface of a general use
commercial aircraft. It has
naturally occurring skin to core
separation and ply damage.
To simulate water ingress an
elastomer (Aqualene) was
placed at the ply to
honeycomb surface. This
elastic polymer is designed
specifically for ultrasonic
probes. The acoustic
impedance is nearly the same
as water. Its attenuation
coefficient is lower than most
elastomers and plastics.
Elastomer Experiment
Log of reconstructed intensity
per time (2.61 seconds)
First derivative of reconstructed
intensity per time (2.61 seconds).
Water Ingress Experiment
Cells were saturated with
water and inspected with
active IR. This technique
works well for development of
the technique but cannot be
used for a long term reference
standard.
Water Ingress Experiment (continued)
Log of reconstructed intensity
per time (2.56 seconds).
First derivative of reconstructed
intensity per time (2.56 seconds).
Water Ingress Experiment (continued)
First derivative of reconstructed intensity per time. These six
images were taken at different soak times from left to right (0,
5, 18 minutes, top row and 53, 74, and 178 minutes, bottom
row). This allows the water to migrate through the composite.
Summary
Core potting is an excellent way to produce a reference standard that
will represent splices or honeycomb repairs. All inspection techniques
were able to detect this manufacturing process. Resonant inspection
and computed tomography could not reliability detect the embedded
inserts between the plies. Ultrasonic resonant inspection can detect
near surface disbonds in the laminate bonded to honeycomb but may
not be able to establish a reference standard threshold. High
frequency ultrasonics can detect the embedded inserts in the plies
with a high signal-to-noise ratio.
Computed tomography was a valuable assessment tool for
evaluating through the thickness of the honeycomb material as well
as the ply to ply variations, but is limited on the size of part that can
be inspected.
Conclusions
Honeycomb removal at the bondline to create a core disbond was identifiable with
computed tomography and infrared inspection. The thermal properties of the epoxy
and air are significantly different from the honeycomb and carbon graphite plies,
however thermal properties of Teflon are not. The use of signal processing
algorithms greatly enhances the detectability of flaws that are hidden in the raw
infrared image. The TSR helps with the evaluation and characterization of indications
in the sample. Flashed thermography can identify flaws within the first three plies and
the bondline interface. The detectability of a Teflon insert embedded into the plies will
depend on its size, depth from the surface and the degree to which its thermal
properties differ from the surroundings.
The use of representative test samples allows the signal process software to
characterize fluid ingress into honeycomb cells and track the migration path. The
elastomer material has material properties that almost simulate the thermophysical
behavior of water ingress into honeycomb cell walls. The key difference in the
thermal behavior is found in the first and second derivatives.
Questions?