Acoustic Techniques for the Inspection
of Ballistic Protective Inserts
in Personnel Armor
Valery F. Godínez-Azcuaga, Richard D. Finlayson
Physical Acoustics Corporation
Princeton Junction, NJ
Janet Ward
U.S. Army Natick Soldier Center
Natick, MA
Abstract
This paper presents the results from a study performed to demonstrate the feasibility of using Guided Wave AcoustoUltrasonics (GWAU) to assess the condition (as related to ballistic performance) of personnel armor Ballistic Protective
Inserts (BPI). The BPI is manufactured with ceramic fiber reinforced plastic (FRP) composite technology. Experiments
were performed on BPI damaged ballistically, by mishandling, and in the “as manufactured” condition. The data
collected from these experiments were used to form manual and automated C-scan images. Both manual and automatic
C-scans of the samples revealed clear differences between the damaged and undamaged samples. Also, RF signals were
recorded from different locations on the samples damaged by mishandling, and a frequency analysis was preformed. The
results from this analysis revealed a good correlation with the actual degree of damage in the samples.
Introduction
Selection of SAPI Samples
Primary ballistic protection offered against small arms rounds
is based on a structure that uses Ceramic tiles with FiberReinforced Polymer (C/FRP) backing. These Ballistic Protective
Inserts (BPI), also known as Small Arms Protective Inserts
(SAPI), although very effective against certain small arms
projectiles (depending on the SAPI protection level), are very
sensitive to low velocity impact damage, which can be caused by
mishandling during storage or transportation.
Currently, the U.S. Army has a considerable number of SAPI,
which may have been subjected to mishandling during storage
and whose condition regarding Ballistic Performance (BP) is not
known. In addition, due to their complex structure and, in
general, anisotropic material properties, SAPI are very difficult
to inspect using conventional Non-Destructive Inspection (NDI)
methods and instruments, such as ultrasonics. Moreover,
methods such as x-ray or CT scan are complicated to use in field
units for economic and logistical reasons. Thus there is a real
need for the development of a new NDI technique which can be
used to assess the condition of the SAPI. This NDI technique
should be capable of assessing the overall condition of the SAPI.
This paper discusses the results obtained in a Phase I SBIR
“In-Service Technique for Assessing Conditions of Ballistic
Protective Inserts in Personnel Armor”. This SBIR was awarded
to Physical Acoustics Corporation (PAC) by the U.S. Army
Natick Soldier Center (NSC) to demonstrate the feasibility of
using Guided Wave Acousto-Ultrasonics (GWAU) to assess the
condition (as related to BP) of SAPI.
A generic SAPI structure is formed by a layer of ceramic tiles
which are adhered to an FRP structural plate. The ceramic face
is then covered with a rubber foam layer to prevent spalling of
the ceramic plate in case of ballistic impact, and then wrapped in
a textile material.
U.S. Army NSC and PAC agreed that the research effort on
this study would be concentrated on the type of SAPI most
commonly used by the U.S. Army. This SAPI type was
identified as “Ranger Armor”. NSC provided PAC with samples
of this SAPI type.
The first SAPI group provided to PAC, labeled “Group A”,
Published in SAMPE Journal, September/October 2003 Issue.
(a)
(b)
FIGURE 1. SAPI samples provided by NSC.
(a) Sample 1A intact. (b) Sample 2A damaged by gunfire.
1
consisted of two (2) samples. The first sample, “Sample 1A,”
was in the “as received from manufacturer” condition, while the
second sample, identified as “Sample 2A”, had been damaged by
gunfire. Figure 1(a) and 1(b) show the undamaged sample 1A
and the damaged sample 2A with the five gunfire impact sites
indicated by the green circles.
The second group, labeled “Group B”, consisted of three (3)
samples with a sample, identified as Sample 1B, in the “as
received” condition and two samples, identified as Samples 2B
and 3B, containing damage caused by mishandling. The damage
in Sample 2B was caused by dropping it from a height of 9
meters onto its front surface. The damage in Sample 3B the
damage was introduced by dropping it from the same height but
this time letting it tumble. Figure 2 shows pictures of the front
and back of the undamaged Sample 1B. Visually, Samples 2B
and 3B do not reveal any indication of the sample condition and
therefore are not shown. The samples in Group B had the same
generic structure as the samples from group A but had a spalling
cover thicker than the samples in Group A, and were wrapped in
a thicker textile material.
(a)
(b)
FIGURE 2. Undamaged sample 1B from the second
group of SAPI samples provided by NSC. (a) Front view
and (b) Back view.
Reflected Acoustic
Wave
Rubber Protective
Layer
Ceramic
Layer
Damage
Incident Acoustic
Wave
θ
Incident
Angle
Fiber Reinforced
Substrate
FIGURE 3. Details of the layer-substrate structure used
in the theoretical simulation of wave propagation in C/FRP
SAPI.
damage. For instance, Figure 4(a) indicates that incidence angles
between 2 and 5 degress combined with frequencies between
100 and 250 kHz will increase the detectability of damage in the
ceramic tile if a reflection configuration is used. These angles of
incidence/frequency combinations form an almost continuous
band light blue in color, clearly observed in Figure 4(a). As seen
in Figure 4(b), if a transmission configuration is used there is no
longer a continuous band of parameters that can be used as in the
case of the reflection configuration. The number of combinations
of angles and frequencies is substantially reduced. The contrast
index color map shows an area of high contrast with frequencies
between 220 and 240 kHz and angle of incidence between 1 and
2 degrees. Higher frequencies in the 240-260 kHz range
combined with incidence angles between 15 and 17 degrees
present also good contrast.
From Figure 4, it can be seen the reflection contrast index
color map indicates that an angle of incidence between 2 and 5
degrees and a frequency between 100 and 250 kHz will provide
good detectability. For the transmission configuration, the
sensitivity to the damage will be maximum with frequencies
between 220 and 240 kHz and angle of incidence between 1 and
2 degrees.
Theoretical Prediction of Inspection
Parameters
Experimental Work
PAC performed a theoretical simulation to analyze the
characteristics of wave propagation in ceramic/FRP layered
structures with and without damage. The theoretical model used
in this study is based on a plane wave propagation model, which
employs the Thomson-Haskell transfer matrix for multi-layered
media to obtain the internal distribution of the energy vector
within a layered composite structure. The results from this
model were used to predict the theoretical acoustic response of
the SAPI in the frequency domain.
Figure 3 shows the experimental setup on which the model
was based. The system is an AlO3 (aluminum oxide) ceramic tile
layer over an FRP composite laminate substrate with damage in
the ceramic tile layer. The ceramic tile face is covered with a
rubber foam layer spalling on the ceramic in case of impact.
The parameters resulting from the theoretical simulation are
known as the Reflection and Transmission Contrast Indexes,
Shown in Figure 4. Their physical significance is that their
values indicate the combination of incident angle and frequency
that should be used to increase the probability of detecting
Inspection of SAPI Samples from Group A The first part of
the experimental work in this project consisted of inspecting two
SAPI samples in group A. According to the results of the
simulation presented in the previous section, angles smaller than
5 degrees and frequencies between 100 and 260 kHz are
preferred for inspection of the SAPI in the reflection and
transmission modes. If small aperture sensors, with dimensions
comparable to the wavelength of the acoustic waves, are used in
the inspection the acoustic beam produced by the sensor has a
dispersion angle larger than 10 degrees. Therefore, a
considerable amount of acoustic energy is generated at shallow
angles, between 0 and 5 degrees, when the sensors are
positioned on the surface of the SAPI oriented at an angle of
zero degrees. That is, even at zero degree incidence angle,
considerable acoustic energy will be transmitted at optimal
angles, between 0 and 5 degrees.
The sensors chosen for the experimental part of the study are
PAC’s micro30 acoustic sensors, with a diameter of 0.25 inches.
These sensors have a broad frequency response, 100 kHz to 500
Published in SAMPE Journal, September/October 2003 Issue.
2
Contrast
Index
Frequency (Hz)
Continuous Frequency/Angle Band
Incidence Angle (Degrees)
(a)
Incidence Angle (Degrees)
(b)
FIGURE 4. Contrast Index for a layered structure simulating a SAPI with a crack in the
ceramic tile layer. (a) Reflection Contrast Index and (b) Transmission Contrast Index.
kHz, but respond very well at a frequency of 225 kHz, which is
in the range of frequencies suggested by the theoretical results
for both the reflection and the transmission configuration. At this
frequency, the wavelength of the acoustic signal travelling in the
SAPI is comparable to the size of the transducer, which will
send acoustic energy at small incidence angles also indicated by
the model.
For the purpose of inspection, a rectangular grid of 0.5 inch
resolution was drawn on the front surface of the two SAPI
samples. Two of PAC’s micro30 acoustic sensors were used in
reflection and transmission configuration. For the reflection
configuration, the sensors were glued to a delrin block, separated
0.5 inches from each other, and pressed against the SAPI front
surface. Figure 5(a) shows the sensors in this configuration. In
the transmission configuration, the sensors were positioned on
both sides of the sample and pressed with a pair of callipers.
Figure 5(b) shows the sensor set-up for the trasmission
inspection.
Reflection Configuration
PAC’s Micro 30’s Acoustic
Sensors
(a)
PAC’s Micro 30’s Acoustic
sensors
(b)
FIGURE 5. Inspection of SAPI. (a) Reflection
Configuration, (b) Transmission Configuration.
Published in SAMPE Journal, September/October 2003 Issue.
A five cycle tone-burst of 225kHz central frequency was used
to excite the pulsing transducer in order to couple acoustic
waves to the SAPI sample. These waves travel through the
protective foam layer and are reflected by the C/FRP structure of
the SAPI.
The amount of reflected acoustic energy is a function of the
C/FRP SAPI condition. The acoustic signals reflected from both
SAPI samples on each square of the grid were recorded and the
maximum amplitudes were extracted. These maxima were then
normalized to the maximum amplitude recorded in the
undamaged SAPI sample and a color code was assigned
according to the normalized signal amplitude. The x-y
coordinates of each signal recorded in the grid were used to form
a color map of the SAPI sample. The color maps corresponding
to the undamaged and damaged samples are presented in Figures
6(a) and 6(b), respectively.
The color maps shown in Figure 6 indicate that the amount of
acoustic energy reflected from the Group A SAPI samples is
higher and more uniform for the undamaged sample, Figure 6(a),
than for the damaged sample, Figure 6(b). In the case of the
undamaged sample, the color map indicates that the reflected
signal falls below the 0.4 level in the color scale represented by
the light blue tones in very few points, when compared to the
number of points that show reflection levels higher than 0.4. It is
important to mention that the right- and left-hand side upper
corners that show dark blue tones are not part of the color map
and are only included in the figures to facilitate the image
formation process.
For the damaged sample, Figure 6(b), the color map shows
that most of the locations have reflected energy below the 0.2
3
Transmission Configuration
Signal
Amplitude
(a)
(b)
FIGURE 6. Color maps obtained using the reflection
configuration. (a) Undamaged sample. (b) Damaged Sample.
level (dark blue tones), yet some locations show reflected
amplitudes above 0.6 (yellow and red tones). However, the
overall distribution of the reflected amplitude has a tendency
towards the blue tones produced by low amplitude signals. This
indicates that the GWAU signals have more difficulty
propagating in this structure than in the undamaged sample.
Visual inspection of Sample 2A showed that most likely the
ceramic tiles where shattered at the point of gunfire impact and
in the immediate area around them. Under these conditions,
trying to couple acoustic waves in the reflection configuration
was difficult. Also, it is important to note that the damage
simulated in the theoretical model is flat and oriented parallel to
the plane of the SAPI, Figure 3, while the real damage is not
necessarily flat and is oriented perpendicular to the plane of the
SAPI. Damage of this type would prevent the propagation of the
acoustic waves in the SAPI.
(a)
(b)
FIGURE 7. RF signal recorded using the
transmission configuration. (a) Undamaged SAPI.
(b) Damaged SAPI.
In summary, it is clear that the undamaged Sample 1A color
map shows mostly red and yellow areas, indicative of higher
amplitude reflected signals, while the color map corresponding
to the damaged Sample 2A shows more green and blue areas
indicative of lower amplitude or absence of reflected signals.
Published in SAMPE Journal, September/October 2003 Issue.
A second set of measurements was taken using the
transmission configuration. The idea behind this configuration is
that cracking, or other types of damage, will prevent the acoustic
signal from traveling through the SAPI sample resulting in a low
amplitude transmission signal. This theory was initially
confirmed by recording RF signals in different areas of Sample
2A. The RF signals of Figure 7, show that the amplitude of the
transmitted acoustic signal was greatly reduced from the
undamaged sample, Figure 7(a), to the damaged sample, Figure
7(b).
The transmission configuration inspection followed the same
signal recording and processing method as in the case of the
reflection configuration inspection. The color maps generated
with the transmission configuration are shown in Figure 8.
Signal
Amplitude
(a)
(b)
FIGURE 8. Color maps obtained using the transmission
configuration. (a) Undamaged SAPI. (b) Damaged SAPI.
The transmission color map for the undamaged sample,
Figure 8(a), is very similar to the corresponding color map for
the reflection configuration inspection shown in Figure 6(a). In
general the transmitted levels are higher than 0.4 level in the
color scale, yellow and red tones.
The color map corresponding to the damaged Sample 2A,
Figure 8(b), shows five distinct areas where the transmitted
signal amplitude drops below the 0.2 level in the color scale
(dark blue tones). These areas correspond to the extended
damage around the five points of impact generated during the
ballistic test. When comparing the reflection to the transmission
configuration inspection results, it is clear that the transmission
configuration is more sensitive to the degree of damage.
These results demonstrate that inspections performed in the
transmission configuration using PAC micro30 sensors can of
detecting damaged SAPI. Although the results obtained in the
reflection configuration show some signal amplitude changes
between the damaged and undamaged SAPI samples, the
difference is not as dramatic as in the transmission
configuration. Therefore, the transmission configuration was
considered to be a better candidate for inspection of SAPI.
However, some difficulties that were identified during the
inspections, had to be overcome before implementing the
inspection method. These difficulties were:
• The foam spalling layer on the front of the samples is very
difficult to penetrate acoustically. To perform the inspection,
4
considerable pressure had to be applied to the sensors in order
to compress the spalling layer and couple the acoustic waves
into the samples.
• Manual formation of color maps of high spatial resolution (as
discussed so far) is not practical for inspection of large
quantities of SAPI.
• Determining inspection sensitivity of the technique to different
degrees of damage. The samples inspected presented only two
cases of SAPI condition: undamaged and extremely damaged.
Most of the SAPI that needs to be inspected in the field are in
conditions between these two extremes, with damage caused
by low velocity impact generated by accidental drop of the
SAPI, defective manufacturing processes, inadequate
protection while in storage, etc.
In order to address these issues, different alternatives were
investigated using SAPI samples from Group B.
Inspection of SAPI Samples from Group B
As discussed in the previous section, the feasibility of using
the GWAU technique in the transmission configuration was
demonstrated by the results shown in Figures 7, and 8. There
were however still important issues that needed to be addressed
in order to develop a GWAU-based method for inspection of
C/FRP SAPI.
The most important issue that needed to de addressed was the
ability of the technique to detect the acoustic signals, either
reflected or transmitted from a SAPI sample, and our ability to
determine from these signals the degree of damage in the
sample. To investigate this issue a series of measurements were
performed in the samples of Group B.
The approach chosen for detecting smaller differences
Back
surface
Pulsing
sensor
Receiving
sensor
Front
surface
FIGURE 9. Location of pulsing and receiving
sensors on the SAPI samples from Group B.
Published in SAMPE Journal, September/October 2003 Issue.
between samples with different degrees of damage consisted of
monitoring the changing characteristics of guided waves as they
propagate along the SAPI. These guided waves are known as
Plate Waves (PW) or Guided Lamb Waves (GLW), and one of
their most important characteristics is the interaction with the
complete thickness of the structure they propagate along.
Therefore GLW are sensitive to changes in the conditions of the
structure. In the case of the SAPI, the GLW interact with the
ceramic tiles and the FRP while propagating along the SAPI.
Thus, if the bonding between the ceramic tiles and the FRP
changes, or if damage is introduced in the sample, the
characteristics of the acoustic wave propagation in the SAPI will
change.
There is an additional aspect of this method that needs to be
considered. The acoustic signals recorded in different positions
on the undamaged Sample 1B, would carry information about
the inherent attenuation of the GLW caused by the SAPI
structure. This “natural” decay of the acoustic signal could then
possibly be used as a base line for comparing similar readings
obtained in the damaged samples 2B and 3B.
As in the case of the experiments performed in the Group A
samples, PAC micro30 sensors were used as pulsing and
receiving sensors. The pulsing sensor was positioned at the
upper edge of the SAPI exactly at the middle point along the
width of the sample on the front surface. This sensor was pulsed
with a frequency-modulated chirp of 100-300 kHz, in order to
produce broad frequency acoustic waves that propagate along
the SAPI. The receiving sensor was placed at different distances
from the pulsing sensor, along the SAPI back surface centerline.
The rationale for putting the pulser and the receiver on opposite
surfaces of the SAPI was to ensure that the GLW propagating
along the SAPI were interacting with the full sample thickness.
Figure 9 shows the sensors mounted on the SAPI. The
orientation of the sensors is at normal incidence with respect to
the surface, which is zero degree incidence angle. Using this
orientation, in combination with the 100-300 kHz frequency
range, the generation of Lamb waves is optimized and no
particular guided wave mode is given preference over others.
Amplitude Analysis of Signals Recorded in SAPI
Samples from Group B
Figures 10(a) to 10(d) show the GLW waveforms recorded at
distances of 25, 50, 75, and 100mm away from the pulsing
sensor in the undamaged Sample 1B. The waveforms show the
natural decay of the acoustic signals as they propagate along the
length of the SAPI. The amplitude of the signal remains
relatively large at distances of 25 and 50mm away from the point
where the pulsing sensor was located, as shown in figures 10(a)
and 10(b). However, a large reduction in the signal amplitude is
observed at 75 and 100mm away from the pulsing sensor as
shown by Figures 10(c) and 10(d).
Figures 11(a) to 11(d) show the RF signals recorded in the
same positions but this time in sample 2B, which was damaged
by dropping it from a height of 9 meters on its front surface. The
difference in the RF signals from Sample 1B is clear for the
25mm position, Figure 11(a), where the amplitude of the signal
decreased by 50% when compared to the same spot in Sample
1B. At the 50mm mark, Figure 11(b), the reduction in signal
amplitude is not as pronounced as at the 25mm mark, but is also
observable. In addition to the changes in amplitude, the shape of
the waveform also changed.
5
(a)
(b)
(a)
(b)
(a)
(b
)
(c)
(d)
(c)
(d)
(c
)
(d
)
FIGURE 10. GLW signals recorded on
the undamaged sample 1B at (a) 2.5mm,
(b) 5mm, (c) 7.5mm, and (d) 10mm from
the pulsing sensor.
FIGURE 11. GLW signals recorded on
the damaged sample 2B at (a) 2.5mm, (b)
5mm, (c) 7.5mm, and (d) 10mm from the
pulsing sensor.
This drop in the amplitude of the signal is an indication that
the damage introduced in Sample 2B has changed, not only the
attenuation characteristics of the SAPI, but also the
characteristics of the GLW that propagate in the SAPI. The
signals recorded at the 75 and 100mm marks, Figures 11(c) and
11(d), show an abrupt drop in amplitude, similar to that observed
in Sample 1B at the same distance from the pulsing sensor.
The waveforms recorded in Sample 3B, dropped and
tumbled sample, at the same positions are shown in Figures
12(a) to 12(d). In this case, the amplitude of the RF signals drop
dramatically even at the 25mm position, Figure 12(a), which
indicates that the damage sustained by this sample has seriously
affected the acoustic propagation properties of the SAPI. The
acoustic signals at the 50, 75, and 100mm show similar decay
when compared to the 25 and 50mm inches signals from
Samples 1B and 2B.
The maximum amplitudes of the RF signals shown in Figures
10, 11, and 12 were extracted and plotted against the distance
from the pulsing sensor in order to explore a possible correlation
between the amplitude of the RF signals and the degree of
damage. This plot is shown in Figure 13. The difference in
amplitude of the acoustic signal between the three samples has a
Power spectrum maximum amplitude [A.U.]
3
Undamaged Sample
Sample dropped on the front surface
Sample dropped and randomly tumbled
2
1
0
0
20
40
60
80
100
120
Distance between pulser and receiver [mm]
140
FIGURE 13. Acoustic signal amplitude as a function of
the distance between pulsing and receiving sensors for the
three SAPI samples of Group B.
maximum at a distance of 25mm from the pulsing sensor and the
Published in SAMPE Journal, September/October 2003 Issue.
FIGURE 12. GLW signals recorded on
the damaged sample 3B at (a) 2.5mm, (b)
5mm, (c) 7.5mm, and (d) 10mm from the
pulsing sensor.
difference diminishes as the distance from the pulsing sensor
increases. The plots corresponding to the three samples seem to
converge to an amplitude value close to zero at a distance of
125mm from the source. It must also be noted that the slope of
the three plots, which indicates the rate of decay of the signal
amplitude, becomes practically the same for distances larger
than 75mm. This indicates that beyond that mark, it would be
very difficult to distinguish between the three samples based
solely on the amplitude of the signal.
This last result suggests that regardless of the type of analysis
used in evaluating the degree of damage of a sample, the
separation between the pulsing and receiving sensors must not
be larger than 75mm. In fact, to take full advantage of this result,
the maximum distance between the pulsing and receiving sensor
should not be more than 50mm. Figure 13 indicates that the
difference between the amplitude of the acoustic signals
corresponding to each of the sample is still very clear at this
distance. This result would determine the sensor configuration to
be used in a system to inspect the SAPI base on this method.
Time-Frequency Analysis Using the Short-Time
Fast Fourier Transform
To further investigate the potential of GLW as a tool to
evaluate the degree of damage in the SAPI, some of the GLW
waveforms were analyzed using the Short-Time Fast Fourier
Transform (STFFT). The STFFT output is called a
“spectrogram”, which is a 2D map of the distribution of energy
in a waveform as a function of time and frequency. In the
spectrogram the amplitude of the energy distribution is presented
as color-coded, in a similar way as in the C-scan images of
Figures 6 and 8. This means that high-amplitude wave mode
arrivals will be shown as bright spots on the color maps. Figures
14, 15, and 16 present the spectrograms of the RF signals
recorded at 25mm from the pulsing sensors in samples 1B, 2B,
and 3B, respectively.
The spectrogram in Figure 14, corresponding to sample 1B
undamaged, indicates that practically all the energy of the
waveform is concentrated in a wave mode arriving in the 80-100
microseconds interval and with frequencies between 225 and
275 kHz, with very little energy outside this time and frequency
interval. Figure 15 shows the spectrogram corresponding to
Sample 2B, damaged by dropping, shows several arrivals
dispersed in the time-frequency plane. Wave mode arrivals are
6
FIGURE 14. STFFT spectrogram of the GLW signal
recorded at the 2.5mm mark in the undamaged
Sample 1B.
FIGURE 15. STFFT spectrogram of the GLW
signal recorded at the 2.5mm mark in the damaged
Sample 2B.
FIGURE 16. STFFT spectrogram of the GLW
signal recorded at the 2.5mm mark in the damaged
Sample 3B.
observed in the 40-60 microseconds with frequencies between
125-175 kHz, and between 60 and 140 microseconds with
frequencies between 250 and 300 kHz. In this spectrogram it is
clear that the most energetic arrival is located at 130
microseconds and a frequency of 270 kHz. The results for
Sample 3B, damaged by dropping and tumbling are shown in
Figure 16. This spectrogram shown arrivals in the 10 to 180
microseconds time interval with frequencies ranging from 125 to
Published in SAMPE Journal, September/October 2003 Issue.
300 kHz. The most energetic arrival is located at 140
microseconds with a frequency of 290 kHz.
One of the differences that can be immediately observed
between the spectrograms shown in Figures 14, 15, and 16 is the
spread in the arrival time of the wave modes and the frequency
distribution of the wave energy. This phenomenon, known as
“dispersion”, could be correlated with the different degrees of
damage in the SAPI and used as a damage assessment tool. By
correlating the “picture” generated by the spectrogram of an
acoustic signal propagating in a SAPI sample with controlled
damage introduced in the sample, the basis for a pattern
recognition method to evaluate the damage in the SAPI could be
established. This approach offers a potential solution for the
issue of how to identify different degrees of damage in the SAPI.
Use of Wide-Band Rolling Sensors
Solutions to two issues discussed in at the end of section 4.1,
the difficulty of penetrating the spalling layer acoustically, and
the difficulty to make continuous measurements on the SAPI
samples, are directly related to the type of sensors to be used in
the prototype system.
In order to make continuous measurements, either to generate
C-scan images or to capture waveforms for STFFT evaluation,
or both, a different type of sensor needs to be used. This sensor
must allow for rapid changes of position and at the same time
overcome the problem of coupling acoustic waves into the SAPI
through the spalling cover.
PAC’s wide band, differential, rolling sensor (RSWD) is a
sensor that with some modifications can overcome both
obstacles. The RSWD is a sensor designed to be used in
applications where dry-couplant and wide bandwidth frequency
analysis is required. This sensor offers a key advantage in
automated process applications; there is no need for couplant.
The rolling sensor rubber tire itself makes contact with the
surface of the SAPI and compresses the rubber foam layer, while
maintaining a constant distance from the sample and therefore a
constant sensitivity. This will solve the first issue discussed in
section 4.1 (difficulty of penetrating the spalling cover).
A solution for the second issue, the difficulty of generating
manual C-scan images of the SAPI, is the inspection of sections
of the SAPI using an array of RSWD sensors, which will roll
over the surface of the sample. Moreover, the results presented
in Figure 13 indicate that signals traveling a distance of 50mm,
along the plate, between pulsing and receiver sensors located on
opposite sides of the SAPI, still have a good signal to noise ratio.
Therefore, the signal still carries information related to the SAPI
structure and possible damage. Taking this into account, an array
of 1 pulsing and 4 receiver sensors will allow the evaluation of a
square section of the SAPI of 50 by 50mm. For a typical
“Ranger” SAPI a total of 30 measurements would be necessary.
This would reduce the time needed to inspect the total area of
the SAPI.
C-Scan Imaging of Ballistic Protective Inserts
In order to prove the feasibility of using RSWD sensors to
generate C-scan images of SAPI, a sample with artificial defects
was inspected. This SAPI specimen was provided to PAC by the
University of Delaware Center for Composite Materials (UDCCM). The defects seeded in this SAPI are known as “dry
spots” and they are formed by high porosity levels in areas of the
composite backing plate.
7
Figure 17 shows the sensor setup used for this process, in
which two RSWD rolling sensors, one acting as a pulser and the
other as the receiver, were mounted on a computer-controlled
scanning bridge. The pulsing sensor was excited with a 100-300
kHz chirp signal with duration of 100 microseconds and
amplitude of 20 volts peak to peak. This type of signal was
chosen because it favors the generation of wide band Lamb
waves. The receiving sensor was connected to a 40 dB gain
preamplifier and then to a modified PAC IPR 1210
Analog/Digital Board. Both sensors were oriented “normal” to
the surface since the current design of the RSWD will only allow
for normal incidence scanning.
A C-scan image is generated by capturing an RF signal,
digitizing it, extracting its maximum amplitude, and associating
it with the x-y position at which it was recorded. Using this
procedure, a C-scan image of the SAPI sample shown in Figure
17 was generated. This image represents a section of the SAPI
200mm wide by 220mm long, and is shown in Figure 18. The
resolution of this C-scan is 0.25mm, which means that data are
recorded every 0.25mm along the scanning direction and the
sensors are displaced 0.25mm perpendicular to the scanning
direction before recording data along another line in the
scanning direction.
Pulsing
sensor
Receiving
sensor
Artificial
Defects
FIGURE 17. C-scan imaging of SAPI sample with seeded
defects using RSWD rolling sensors attached to a computer
controlled scanning bridge.
The C-scan of the SAPI sample with the two seeded defects,
indicated in Figure 17, and known as “dry” spots (porosity on
the FRP backing plate of the SAPI) is shown in Figure 18. The
seeded defects are clearly visible as circled areas in the C-scan
image. This image represents a section of the SAPI 200 mm
wide by 225 mm long.
It is important to mention that this image is a composition of 4
separate C-scans of 50 mm wide by 225 mm long each. This was
done to avoid changes in the coupling of the acoustic signal into
the sample due to its curvature. This last aspect is very important
and it will have to be addressed in the design of a SAPI
inspection system. Most likely, such a system will have to
include a mechanism to follow the contour of the SAPI sample.
Published in SAMPE Journal, September/October 2003 Issue.
FIGURE 18. Composed C-scan image from a
SAPI sample with artificial defects (“dry” spots) on
the FRP backing plate.
Conclusions and Future Work
Based on the overall results this study, It has been concluded
that the amplitude, and the time-frequency analysis using the
STFFT offer good methods to analyze an acoustic signal
propagating in the SAPI. The drop in the amplitude transmitted
through the SAPI, and the output of the STFFT can be evaluated
quantitatively and therefore be correlated to the SAPI condition.
The condition of the SAPI could be evaluated by comparing
the signal amplitude drop and/or the time-frequency dispersion
of the GWAU signal energy from SAPI samples of unknown
condition, with that of standard samples with known damage.
Additionally, the fact that the drop in amplitude and the timefrequency analysis results reflect the average condition of the
SAPI section between the pulsing and receiving sensor makes it
ideal for evaluation of the SAPI by sections. This will speed up
the data gathering since only 30 measurements are necessary for
the complete evaluation of a typical SAPI.
Another important conclusion is that the RSWD rolling
sensors solve the problem of acoustic coupling with the SAPI.
Additionally, the rolling sensors are ideal for contour following
since they “roll” along the SAPI surface.
Based on the results discussed in this paper PAC has prepared
a conceptual design for a prototype system for NDI of C/FRP
SAPI, which has been submitted to NSC for consideration.
Acknowledgements
The authors would like the acknowledge the University of
Delaware Center for Material Composites for providing SAPI
samples used in the generation of the automated C-scan images
presented in section 4.3. In particular we would like to thank Dr.
Jack Gillespie and Dr. Aurimas Dominauskas for their generous
cooperation.
For More Information:
Contact PAC's REACT department at
195 Clarksville Road, Princeton Junction, NJ 08550
Phone: (609) 716-4000 Fax: (609) 716-4057
Email: react@pacndt.com
Internet: www.pacndt.com
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