Assessment of Advanced Ultrasonic and

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Oct 29, 2013 (4 years and 10 days ago)

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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.

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.

Introduction

Overview of Composites


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





Defects found in Composite Structures

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)
2.54 cm
diameter
epoxy potted
honeycomb
core (full thickness);

2)
2.54 cm
square
Teflon shim (located between plies 2 and 3);

3)
12.7
mm
diameter Teflon shim (located
between plies 2 and 3);

4)
1.27 square disbond (located at adhesive
bondline
);

5)
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 i
s 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
.

Shallower

Deeper

CT Inspection Results

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 m
A

Top of the laminate showing
the weave pattern at the
surface of the part.

Interface between the
laminate and
Nomex

honeycomb structure.

Shallower

Deeper

CT Inspection Results (continued)

Midway through the
honeycomb core crushing

Honeycomb material
(disbonds detected)

Side view of the
panel showing an
area of core damage

Ultrasonic Properties

Material

Sound
Velocity v

(m/sec)

Density ρ

(kg/m
3
)

Acoustic
Impedance

Z =
ρv

(kg/m
2

sec
) x
10
6

Aluminum

6320

2700

17

Water (20
°
C)

1483

998

1.48

Air (20
°
C)

343

1.204

0.00041

Hysol

Teflon

2850

1520

1580

2200

4.52

3.3

Composite
-

graphite
-

Epoxy


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
T
ransducers 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 C
omposite 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 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.

Resonance Basics

Shift in Resonant Frequency

A
mplitude

P
hase

Resonant inspection was
moderately successful on the sample at
110
KHz.
R
esonance
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.

Amplitude

(X) Image

Phase

(Y) Image

Resonance Inspection Results

Thermal Material Properties

C
onduction: 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:

T
1

> T
2

T
2

𝒒
𝒙
=




𝒅𝑻
𝒅𝒙

x

𝒒

= q"

q"

=


𝑘
𝛻

𝑇
=


𝑘

𝜕𝑇

𝜕
+


𝜕𝑇

𝜕
+

𝜕𝑇

𝜕

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/

m
3
)
Heat capacity


c

(J
/
kg
°
C) and volumetric heat
capacity
ρ
c

(J/

m
3

°
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



Material

Conductivity,

𝑘

W/(m
°




Specific Heat,

c
p

J/(kg
°
C)



Density,

ρ

kg/(m
3

)

Diffusivity,

α

m
2
/sec

1 x 10
-
7

Effusivity

ε

J/(m
2

°
C)


s



Phenolic (resin
pressed)

0.3766

1255

1380

2.174

807.667

Teflon

0.2510

1004

2170

1.152

739.6

Carbon

Graphite

167.36

707.1

2250

1052

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

1172

1210

1.372

525.271

Aluminum 2024 T3

121

875

2780

497.43

17156.1



Copper

397.48

384.9

8940

1155.

36982.8

Stainless Steel 304

14.644

502.1

7920

36.83

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

Thermal Properties

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.

Lamp

IR camera

defect

PC

heat
conduction

emitted IR

light

IR Image

Source: Thermal
W
ave

Imaging


EchoTherm

Equipment

The
EchoTherm

S
ystem 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
T
hermographic

Signal
R
econstruction (TSR)™
technology in the Mosaiq software
package, to provide a set of images
representing the heat transfer.

Source: http://www.thermalwave.com


Bond separation

Delaminations

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











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
.

“Damage” Limits for Composites

Infrared Data Analysis

Cool

Warm

First derivative of reconstructed intensity
per
unit time.

First
and second derivatives
yield
substantial contrast at earlier thermal
decay times in the image sequence.
The images will be sharper. (less
lateral diffusion).

Epoxy

Teflon

Source: http://www.thermalwave.com


High

Diffusivity

(
α
)

Low

Diffusivity

(
α
)

High

Effusivity

(
ε)

Low

Effusivity

(
ε)

IR Results (Epoxy and Teflon)

Intensity Time

Plot 0.55
seconds after pulse

Log
of reconstructed

intensity
per

time.

First derivative of reconstructed

intensity
per

time.

Second derivative of reconstructed

intensity
per
time.

IR Results (Epoxy and Disbond)

Intensity Time

Plot 1.1
seconds after pulse

Log
of reconstructed

intensity
per
time.

First derivative of reconstructed

intensity
per

time.

Second derivative of reconstructed

intensity
per

time.

IR Inspection Results (continued)

Intensity Time

Plot 4.6
seconds after pulse

Log
of reconstructed

intensity
per
time.

First derivative of reconstructed

intensity
per

time.

Second derivative of reconstructed

intensity
per
g
time.

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

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.

0.10 mm skin thickness

Aqualene

placement

15.2 cm

22.9 cm

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?