Sensor Applications for Structural Diagnostics and Prognostics Anindya Ghoshal Abstract This chapter examines emerging sensor technologies in aerospace structural prognostics health management. A review of existing and emerging in situ sensor technologies for structural health monitoring for aerospace applications has been discussed in details. Details of the sensor selection criteria for the sensor technologies have been stated. For successful implementation of condition-based maintenance of aerospace vehicles, such emerging sensors are key technologies that would be required. Keywords Condition-based structural health monitoring • In situ sensor technologies • Structural sensing • Diagnostics and prognostics 1 Introduction Considerable advancement has been made in the sensor technology development for in situ sensor technologies for structural health diagnostics and prognostics [1]. This chapter is done with the objective of defining and selecting appropriate damage detection sensor(s) for direct monitoring of subcritical fatigue cracks in airframe primary structural elements. The sensor hardware should also provide reliable detection in representative airframe joints and/or attachments. A. Ghoshal (*) ARL, Towson, MD, USA e-mail: [email protected]S. Chakraborty and G. Bhattacharya (eds.), Proceedings of the International Symposium on Engineering under Uncertainty: Safety Assessment and Management (ISEUSAM - 2012), DOI 10.1007/978-81-322-0757-3_30, # Springer India 2013 503
Transcript
Sensor Applications for Structural Diagnosticsand Prognostics
Anindya Ghoshal
Abstract This chapter examines emerging sensor technologies in aerospace
structural prognostics health management. A review of existing and emerging in
situ sensor technologies for structural health monitoring for aerospace applications
has been discussed in details. Details of the sensor selection criteria for the sensor
technologies have been stated. For successful implementation of condition-based
maintenance of aerospace vehicles, such emerging sensors are key technologies that
would be required.
Keywords Condition-based structural health monitoring • In situ sensor
technologies • Structural sensing • Diagnostics and prognostics
1 Introduction
Considerable advancement has been made in the sensor technology development
for in situ sensor technologies for structural health diagnostics and prognostics [1].
This chapter is done with the objective of defining and selecting appropriate
damage detection sensor(s) for direct monitoring of subcritical fatigue cracks in
airframe primary structural elements. The sensor hardware should also provide
reliable detection in representative airframe joints and/or attachments.
S. Chakraborty and G. Bhattacharya (eds.), Proceedings of the International Symposiumon Engineering under Uncertainty: Safety Assessment and Management (ISEUSAM - 2012),DOI 10.1007/978-81-322-0757-3_30, # Springer India 2013
• Time-domain reflectometry, Material Sensing & Instrumentation, Inc., PA
504 A. Ghoshal
• Magnetostrictive sensors: Southwest Research Institute, TX
• Carbon nanotube and graphene-based sensors
The six technologies that have high potential for the structural health monitoring
application are evaluated in details as follows.
4 Eddy Current Sensors
4.1 Principle of Operation
An eddy current sensing system makes its measurement by measuring the electrical
impedance change of the eddy current probe. The probe consists of coils that carry
high-frequency current and generates an electromagnetic field. When the probe is
placed near a metallic structure, the EM field penetrates the conductive surface and
creates an eddy current within the structure. The intensity of the current or the
electric impedance of the coil is a function of the material properties such as electric
conductivity and permeability which is sensitive to the local structure damage or
defects. This material property variation around the measurement point can then be
translated into the structure defects or damage information via either a mathematic
model or calibration against empirical database. The eddy current sensors can be
made on thin polymer film with electric coil printed on it so they can be customiz-
able in shapes, conformable to the surface of the structure, and cannot be easily
mounted onto any complex surfaces permanently.
JENTEK Inc is today a major player in the technology of crack detection by
eddy current sensing [2]. Its Meandering Winding Magnetometer Array (MWM-
Array) system features high-resolution multiple-channel impedance measurement
instrumentation and high-resolution imaging for crack detection. The sensing
element configurations provide improved detection performance along with
reduced calibration requirements and setup time.
4.2 MWM-Sensor Array
AMWM-Array system has a single period spatial mode drive with a linear array of
sensing elements. The MWM-Array provides images of electrical conductivity and
is suitable for crack detection with high special resolution. The MWM-Array
sensors have a primary winding that is driven with a high-frequency current to
produce a time-varying magnetic field with a spatial wavelength that is determined
by the physical spacing between drive winding segments. The MWM-Arrays
typically operate at frequencies from 10 kHz to 15 MHz. At these frequencies,
the wavelength of traveling waves is long compared to the dimensions of the sensor,
so the distance between the drive winding segments defines the shape of the applied
Sensor Applications for Structural Diagnostics and Prognostics 505
magnetic field. The magnetic field produced by the winding induces eddy currents
in the material being tested. These eddy currents create their own magnetic fields
that oppose the applied field. At low frequencies, these eddy currents are distributed
well into the material under test; at high frequencies, these induced eddy currents
are concentrated on a thin layer near the surface of the test specimen. A surface-
breaking crack interferes with the flow of these eddy currents or the impedance
of the winding. When MWM-Array is scanned across, an image of the impedance of
the sensor array is generated which maps the structural or material abnormality of the
test specimen.
Comparing with conventional eddy current sensor, MWM-Array features:
• Absolute sensing configurations (as opposed to differential sensing element
designs) capable of inspecting regions likely to have cracks forming from
micro-cracks into larger cracks.
• Calibration is performed on-site using either “air” or uniform reference parts
without cracks, reducing calibration and training requirements.
• A crack signature is extracted off-site, only once, using either real cracks, EDM
notch standards, or a simulated crack signature. This is an advantage because it
eliminates dependence on crack standards and avoids potential errors encoun-
tered during calibration on such standards.
4.3 Technology and Product Maturity
JENTEK’s MWM-Array sensor system has been under development and
improvement since its inception in 1996 and has reached a certain level of product
maturity. It currently offers a line of products typically off the shelf. The system is
built around the following components: (1) a parallel architecture impedance instru-
ment, (2) magnetic field (MWM) sensor arrays, and (3) Grid Station software
environment, application modules, and tools. JENTEK is currently delivering two
versions of imaging sensor array systems. The 39-channel system is a high-
resolution imaging systemwith comprehensive imaging, decision support, and proce-
dure development tools. The 7-channel system is designed for less image-intensive
applications. These systems are supported by a wide selection of MWM-Array sensor
configurations.
5 Active Current Potential Drop Sensors
5.1 Principle of Operation
Alternating current potential drop (ACPD) or direct current potential drop (DCPD)
is an electrical resistance measurement technique for sizing surface-breaking
506 A. Ghoshal
defects in metals [3]. ACPD works by inputting an alternating current into the
electric conductive object. At the points (I, I0), a constant direct current is supplied.An increase in crack length produces an increase of the potential drop measured
between the potential leads (V, V0). Presence of defect or crack in the material
between these two points will result in a local resistance larger than that of its
vicinity. By comparing potential differences with a reference value, calibrating
against empirical database or FEM modeling results, crack depth and size can be
estimated.
The reference measurement of potential drop is usually required to provide
comparison and needs to be as close to the crack as possible. Because of the skin
effect, ACPD system is more capable of measuring surface crack while DCPD can
measure in-depth crack but with lower sensitivity. The crack size can be estimated
as Crack Depth¼D/2 (Vc/Vr�1), where D is the probe separation and Vc is the crack
voltage. Vr is the reference voltage. The techniques are available for both thick and
thin structures. Custom-made ACPD sensing probe can be either hand-holding scan
probe or wire spot-weld in structure. Similar to MWM-Array sensors, these sensing
wires need to be permanently mounted near the cracks or where cracks would
potentially develop for detection. This means it is only a local crack detection
system.
5.2 Technology and Product Maturity
Among a few vendors, Matelect Ltd of UK has been selected for investigation of
product availability and specification. Matelect has a line of commercial off-the-shelf
ACPD or DCPD products. Their most popular crack detection is CGM-7
microprocessor-based crack growthmonitor systemwith operating frequency ranging
from 0.3 to 100 kHz and current up to 2 A. The system is able to detect cracks of
0.02–10 mil on lab specimen and 40mil on aircraft components. For permanent crack
monitoring, the potential measuring probes are usually spot-weld into structure and
probe canwithstand 600�C temperature. The products have been used for Rolls-Royce
engine turbine disk dovetail crack inspection, and it is recently being tested on CRJ
aircraft structure for crack monitoring. The vendor claimed POD is 85% on 0.004
crack, sensitive to crack orientation.
The summary of pros and cons for use of ACPD/DCPD is as follows:
• High sensitivity to incipient crack and has long history of industrial application.
• Detestability is sensitive to the orientation of crack.
• Electrode sensor needs to be spot-weld into structure for permanent monitoring
and is less suitable for retrofit application.
• Need complex in field calibration.
• Vulnerable to electromagnetic interference.
Sensor Applications for Structural Diagnostics and Prognostics 507
6 Comparative Vacuum Sensors
6.1 Principle of Operation
Comparative vacuum monitoring (CVM) offers a novel method for in situ, real-time
monitoring of structural crack initiation and propagation [4]. CVM makes use of the
principle that a steady-state vacuum, maintained within a small volume, is extremely
sensitive to any leakage of air. It measures the differential pressure between fine
galleries containing a low vacuum alternating with galleries at atmosphere in a simple
manifold. The manifold is directly mounted on structure surface being monitored for
crack. If no flaw is present, the vacuum in galleries will remain at a stable level. If a
crack develops and creates a passage between vacuum and atmosphere galleries, air
will flow through the passage created from the atmosphere to the vacuum galleries.
Sensors may either take the form of self-adhesive polymer “pads” or may form part of
the component. A transducer measures the fluid flow or pressure difference between
the galleries.
CVM has been developed primarily as a tool to detect crack initiation. Once a
sensor has been installed, a base line reading is made. Generally, the differential
pressure between the reference vacuum and the sensor will be approximately 0 Pa.
However, if there is a known existing flaw or crack beneath the sensor, or the
permeability of the test material is high, the fluid flow meter will measure a nonzero
base value. If this nonzero value is constant, it will not affect the ability of the CVM
system to detect an increase in total crack length.
When a vacuum gallery is breached by a crack, molecules of air will begin to
flow through the path created by the crack. Once the system has reached an
equilibrium flow rate, the volume of air passing through the crack is equal to the
volume of air passing through the flowmeter, and the measured differential pressure
will become constant at a higher value. Therefore, the system is very sensitive to
any changes in the total crack size. The CVM method is unable to differentiate
between a single large crack and several smaller flaws, but is sensitive to any
increase in the total crack length. The sensitivity of the sensor is determined by the
gallery wall thickness.
6.2 Technology and Product Maturity
Structural Monitoring System Inc of Australia has developed this sensor technology.
A variety of sensor types have been developed. These include self-adhesive elastomer
sensors for the measurement of surface crack initiation or propagation and sensors
integral within structure, for example, permeable fiber within a composite. The
sensors are produced from a variety of materials. The accuracy of the crack propaga-
tion sensor is governed by the accuracy of the galleries, measured optically at better
than 10 mm. Once the sensor has been installed, the leading edge of the first gallery is
508 A. Ghoshal
determined optically, and from this initial measure, all subsequent gallery positions
are determined.
CVM sensors have been applied to a variety of aerospace structures for crack
detection. Sandia National Lab, in conjunction with Boeing, Northwest Airlines,
Delta Airlines, Structural Monitoring Systems, the University of Arizona, and the
FAA, has conducted validation testing on the CVM system in an effort to adopt
comparative vacuum monitoring as a standard NDI practice. The system has been
tested by on Boeing 737, DC 9, C130 aircrafts, and Blackhawk helicopter by
Australian air force. According to the SMS Inc., the sensor pad adhesive can hold
the vacuum in the galleries for as long as 18 month. Ninety percent probability of
detection with 0.020–0.02500 crack on 2024 aluminum structure has been reported.
The summary of pros and cons for use of CVM is as follows:
• High sensitivity to 0.020 mils.
• The system is lightweight, inert (safe), and less vulnerable to EMI than any
electric-based system.
• Elastomeric sensor is low cost and conformable to any curved surfaces and can
be easily integrated to complex structural surface.
• Easy to operate and calibrate.
• The system is capable of detecting surface break cracks only.
Overall, CVM is a good candidate technology for local crack detection
7 Networked Piezoelectric Sensor
Piezoelectric (PZT) transducer can be used to monitor structures for internal flaws
when it is embedded in or surface mounted to structures. PZT transducer can act
as both transmitters and sensors due to its direct and reverse piezoelectric effect. As
transmitters, piezoelectric sensors generate elastic waves in the surrounding mate-
rial driven by alternating electric field. As acoustic sensor, they receive elastic
waves and transform them into electric signals. It is conceivable to imagine arrays
of active sensors, in which each element would take, in turn, the role of transmitter
and acoustic sensor and thus scan large structural areas with high-frequency
acoustic waves. As global damage sensing, two PZT sensor network-based
approaches are commonly used for structural health monitoring:
• Self-electromechanical (E/M) impedance method for flaw detection in local area
using effect of structural damage on EM impedance spectrum
• Lamb wave propagation method for large area of detection using acoustic wave
propagation and interception by presence of structural damage on the acoustic
path
In the self-E/M impedance approach, pattern recognition methods are used to
compare frequency domain impedance signatures and to identify damage presence
and progression from the change in these signatures. In the cross impedance
Sensor Applications for Structural Diagnostics and Prognostics 509
approach, the acousto-ultrasonic methods identifying changes in transmission
velocity, phase, and additional reflections generated from the damage site are
used. Both approaches can benefit from the use of artificial intelligence neural
network algorithms that can extract damage features based on a learning process.
The Acellent Technologies of Mountain View, California, offers a PZT acoustic-
based structural health monitoring system commercially off-the-shelf [5]. The
system comprises of SMART Layer sensors, SMART Suitcase, and ACESS Soft-
ware®. This structural health monitoring solution is capable of monitoring both
metallic and composite structures. The SMART Layer consists of multiple piezo-
electric sensing elements with wires lithographically imprinted on the Kapton film.
These sensors operate based upon the principles of piezoelectricity and its converse
effects. The sensor suite uses both pulse echo and transmission mode for operabil-
ity, making it capable for deployment as distributed sensor network for the global
damage monitoring sensing system (GDMS). The Smart Suitcase includes the
portable diagnostic hardware and customized form factor. The hardware is capable
of monitoring up to 64 sensor channels simultaneously.
Under active interrogation mode, the system has the capability of generating a
50–500-kHz input excitation with a maximum of 50 V peak-to-peak amplitude in
the form a single-cycle or multiple-cycle pulse through one of the transducer. The
adjacent sensors are used to detect the transmitted signals generated by the traveling
stress waves. This is then repeated sequentially to cover map the whole structure
which is under the sensor coverage. The signals are then compared with historical
data. Both the transmitted signals and the pulse echo signals are used for analysis.
Through transmission, the system bandwidth is 10 kHz to 1 MHz and the pulse echo
system bandwidth mode is 10 kHz to 5 MHz.
Acellent is currently flight-testing its system by deploying the sensor on an F-16
test aircraft landing gear door to monitor the edge crack growth. The system had
been demonstrated to detect 0.53100 crack in 500 cycles in metallic components.
For a flawed composite doubler, the Sandia National Labs tested the system’s
capability to monitor crack length greater than 1 in. Acellent has done considerable
testing on coupons and components under laboratory conditions. This system needs
calibration of crack size. Herein, it should be noted that crack size determination
has not yet been proven along with its robustness. Currently under a separate
program, Acellent is preparing data for flight certification (salt fog, moisture, etc.).
As this chapter is primarily on its global sensing capability, this evaluation of the
Acellent SHM system focuses on its capability of crack locating and sizing. The
conclusions were:
• The hardware system is basically a multiple-channel high-speed data acquisition
with a set of function generating capability typically used in acoustic wave-
based flaw detection.
• Passive and active mode hardware systems are separate systems as they require
different DAQ boards with different sampling frequencies.
• Its standard software, “Access,” comes with a diagnostic imaging plug-in that
enables raw acoustic data imaging and interpreting, but not damage locating.
510 A. Ghoshal
With a network of sensors and current version of Access software, the system is
able to indicate the structural changes in between the sensors due to damage.
• To achieve the damage localization, new algorithm is to be developed and the
Access software must be customized and field calibrated with sufficient number
of tests on a particular specimen.
• The system is currently unable to perform detection on a 3D geometry without
further customization.
8 Fiber-Optic Sensors
There are two types of FO sensors available in the market: fiber Bragg sensors and
Fabry-Perot interferometry sensors (extrinsic and intrinsic) [6, 7]. The following
section discusses them in some details.
8.1 Fiber Bragg Grating
Fiber-optic Bragg gratings utilize a photo- or heat-induced periodicity in the fiber
core refractive index to create a sensor whose reflected or transmitted wavelength is
a function of this periodicity. The biggest advantage of fiber Bragg grating sensors
(FBG) is that they can be easily multiplexed to enable multiple measurements along
a single fiber. One approach for multiplexing Bragg gratings is to place gratings of
different wavelength in a single fiber and utilize wavelength division multiplexing
(WDM). However, the limited bandwidth of the source, as well as that supported by
the fiber, and the range over which the physical parameter of interest is being
measured provide practical limitations on the number of gratings that can be
multiplexed in a single fiber with WDM approaches. The system, based on
the principle of optical frequency domain reflectometry (OFDR), enables the
interrogation of hundreds or thousands of Bragg gratings in a single fiber. OFDR
essentially eliminates the bandwidth limitations imposed by the WDM technique as
all of the gratings are of nominally the same wavelength. Very low reflectivity
gratings are utilized, which allow reflections from large numbers of gratings to be
recorded and analyzed. By trackingwavelength changes in individual gratings, one is
able to measure mechanical- or thermal-induced strain in the grating. United
Technologies Research Center is the original developer of the fiber Bragg sensors
for strain measurements. The recent developments in multi-axis FBG strain sensor
technology offer a distinct advantage in creating a series of fiber sensor types that are
extremely compatible with one another, allowing the usage of similar readout
equipment for a variety of applications. It is possible to configure a fiber grating
system so that each fiber grating is sensitive to different frequency bands. This could
be done in an array that measures multi-axis strain and other key parameters. This
Sensor Applications for Structural Diagnostics and Prognostics 511
sensor array/neural network is nominally intended to predict and/or last the
lifetime of the structure or component regarding mechanical damage tolerance.
“Pre-assembly”of the fiber array could be affected in applique coatings. In all these
potential arrangements, the simplest attachment methods will be developed capable
for re-hooking the fiber array to the readout equipment. FBGs have minimal risk of
electromagnetic interference and high bandwidth/sensitivity and can noninvasively
inquire (passively or actively) into the health of a structure. The disadvantages of the
FBG sensors are the uncertainties in the long-term durability of the sensors, sensor
bonding to the airframe structure, and the fragility of the quartz elements, especially
when the fiber is turned around. Several organizations, for example, NASA Langley
and companies (e.g., Blue Road Research, Luna Innovations Inc., Micron Optics
Inc., New Focus Inc.), are in the development of FBG demodulators for potential
structural health monitoring applications, and the sensors are mainly developed. The
main fiber-optic sensor manufacturers are Canadian-based companies like LXSix
and FISO technologies. The normal FBG sensors are 150–250 mm in diameter and
approximately 5 mm long.
8.2 Fabry-Perot Interferometry
Fiber-optic sensors can be separated into two classes for discrete strain and temper-
ature measurement: cavity-based designs and grating-based designs. Cavity-based
designs utilize an interferometric cavity in the fiber to create the sensor. Examples
include the extrinsic Fabry-Perot interferometer (EFPI), the intrinsic or fiber Fabry-
Perot interferometer (IFPI or FFPI), and all other etalon-type devices. Although
such sensor designs have been utilized in a wide variety of applications such as in
high temperature and EMI environments, they do not allow for multiplexing
capability in a single fiber and thus may be limited for applications requiring
large number of sensors. Originally developed by a team of scientists at Virginia
Tech, this has seen widespread applications into different areas. To measure both
strain and temperature, a broadband light source is transmitted to the cleaved end of
a single-mode fiber. To perform the strain measurements, upon reaching the end of
the fiber, the light is partially reflected while the remaining light travels past the end
of the fiber and is reflected off a secondary reflector. The reflectors (also fibers) are
aligned with the main fiber in a capillary tube and attached to a substrate. The two
reflected light signals interfere with each other forming a fringe pattern. As the
structural substrate strains, the distance between the two fiber end-faces vary,
causing the fringe pattern to change. Using a spectrometer, the changing gap is
measured to obtain the strain. The sensor is less prone to failure because the fiber
itself is not being strained by the substrate. The temperature sensor has a small,
single-crystal chip on the end of the fiber. The two faces of the chip are reflectors.
Precise temperature can be obtained by measuring the temperature-dependent
optical path length through the chip. Different types of fibers are used for making
512 A. Ghoshal
the FO sensors. For temperature less than 700�C, silica fibers are used for FO
sensor, and for temperature greater than 900�C, applications currently single-
crystal sapphire fibers are being investigated. New fiber-optic sensor materials
being developed in this area include photonic crystal fibers and wholly fibers.
8.3 Technology and Products Maturity
Several vendors of fiber-optic sensing have been evaluated for global damage and
load sensing. Luna Innovations of Blacksburg, VA, has two types of commercially
available solution for fiber-optic sensing—(1) Distributed Sensing System and
(2) FiberPro2 (an older version is also marketed known as FiberScan). The
Distributed Sensing System is able to monitor several (10,000 s) FBG sensor
nodes on a single fiber, which gives the ability to measure strain at several locations
on a single fiber. However, the laser scan rate is limited to 10 Hz limiting its utility
for application at 8P load ranges (40 Hz). For the current rotorcraft airframe
application, this system is not useful for either damage monitoring or load moni-
toring. FiberPro2 is the newer version of FiberScan which can monitor both FBG
and Fabry-Perot Interferometer sensor. It can be connected to “MU8” which is a
multiplexer, allowing the ability to individually monitor eight single sensors on
eight different fibers (channels) at the same time. The demodulator can monitor
both fiber Bragg sensor and Fabry-Perot Interferometry sensor. The single FPI
sensors can be used to monitor strain at a higher loading frequency levels. This is
suitable if we want to monitor single optic fiber sensor mounted on separate fibers at
the same time at different locations. However, this is limited to eight channels (i.e.,
eight sensors mounted on eight fibers), which makes application of FiberPro2 also
limited with regards to multisensor array on a single fiber. One of the issues that is
significant is that vibration in connecting fiber can cause drifting in the sensor
readings. This is significant in terms of accuracy in the measurement during flight
and the airframe undergoing variable load history. The system should be able to
zero out the effects of sensor drift.
Alternatives to Luna’s systems are Insensys and Micron Optics. Both of themhave systems which can monitor dynamic loads over 40 Hz. Invensys did demonstratethe capability of monitoring continuous dynamic loading on a cantilever I-beam at
the AIAA SDM Conference held at Newport, April 2006. A significant innovation
was the designed brackets used for mounting the fiber-optic sensors onto the airframe.
Attaching the fiber-optic sensors to airframes and long-term durability of such
mounts are significant challenges. Also the results from a successful observation of
a bird impact strike using the FO system on a winglet spar were presented. Unlike an
electric-based sensing system, FO can actually detect a lightning strike because of its
electromagnetic immunity. The team is talking to Insensys, which is based in United
Kingdom for a follow-up demo in Connecticut.
Micron Optics developed several optical sensing interrogators which are wave-
length division multiplexing (WDM) based. The WDM interrogators work with
Sensor Applications for Structural Diagnostics and Prognostics 513
both FPI and fiber Bragg grating sensors. The si425-500 combines a PC with a high
power, low-noise laser source. It is a stand-alone system, which can provide optical
power and rapid measurement of 512 FBG sensors mounted on 4 separate optical
fibers (128 sensors on a single fiber). The sensors can be placed as close as 1 cm
apart. The system is expandable to 8–12 channels and customizable. The scan rate
frequency is 250 Hz, and the wavelength is 1,520–1,570 nm. The sm130 can
provide power and rapid measurements of several hundred sensors mounted on
four separate optical fibers up to a scan rate of 1-kHz range. This unit is more
applicable for rugged and harsh environment deployment.
The fiber-optic Bragg sensors that would be tested during sensor characterization
part are going to be obtained from LXSix or FISO companies, which are main FO
suppliers of Micron Optics. Clearly for the applications as load monitoring and
damage monitoring sensors, the fiber Bragg sensors are more applicable than FPI
sensors; as for FBG, we have the capability of monitoring several points for strain
using a single fiber, whereas the FPI is a single discrete sensor at a single point
location. Because of the limitation of the scan rate to 1 kHz, it is envisioned that the
FBG sensors are more applicable for load (strain) monitoring at several locations on
the component rather than using them as damage monitoring sensors.
8.4 Time-Domain Reflectometry
Time-domain reflectometry (TDR) is a method of sending a fast pulse down a
controlled-impedance transmission line and detecting reflections returning from
impedance and geometric discontinuities along the line. Time scales are fast, so
reflections occurring at different positions in the line are separated by time-of-flight,
forming a “closed-circuit radar.”
TDR can potentially be used as structural global damage sensing due to its
distributed nature. TDR has gained popularity in recent years in infrastructure
applications. The transmission line is embedded in a bridge or highway structure,
such that a flaw in the surrounding structure causes a mechanical distortion in the line,
which produces an impedance discontinuity, which is located by time-of-flight.
It has been investigated for composite parts defect detection after instrumenta-
tion high spatial location resolution becomes available. TDR structural health
monitoring probes the structural health of a composite part by propagating a fast
electrical pulse along a distributed linear sensor which has been fabricated directly
in the laminate. The sensor is formed from the native graphite fibers already used in
composite manufacture and constitutes zero defects. Fibers are patterned into a
microwave waveguide geometry, or transmission line, and interrogated by a rapid
pulse as shown below. Structural faults along the line cause distortions in wave-
guide geometry, producing reflected pulses similar to radar. Cracking, delamina-
tion, disbonds, moisture penetration, marcelling, and strain can be detected by
propagation delay for sensor lengths up to several meters. These features make
514 A. Ghoshal
TDR appropriate for the permanently embedded and distributed monitoring of the
structural characteristics variation.
Material Sensing & Instrumentation, Inc (MSI) of Lancaster, PA [8, 9], is one of
the major players specializing in TDR concrete and composite cure monitoring. It
also performs R&D in the area of composite debond or delamination detection.
TDR provides a new approach to cure monitoring of advanced polymer composites
fabrication process. Using a high-speed pulse and inexpensive microwave sensor,
TDR cure monitoring provides an alternative between high-frequency fiber-optic
methods and low-frequency dielectric methods, combining optical-style precision
and miniaturization with electrode-based simplicity and robustness.
MSI is a small company which has successfully executed several government
SBIRs. Its primary technical expertise lies on the composite curing monitoring. The
company typically does not/is not able to provide COTS equipment to customers.
The structural health monitoring using TDR, especially metallic structures, is still
under preliminary development. Due to the non-dielectric nature of metallic struc-
ture, further development of this technology is currently necessary.
9 Magnetostrictive Sensors
This sensing approach is based on a novel thin-film magnetostrictive sensor
material that has recently been developed by Southwest Research Institute
(SwRI) for turbine engine applications [10, 11]. This thin-film is 4 mm thick and
achieves high activation efficiency, as well as temperature stability, using alter-
nate crystalline and amorphous nano-layers. Defect detection is accomplished by
activating the magnetostrictive thin-film causing emission of ultrasonic guided
waves into the component that are subsequently backscattered and detected by the
same sensor in “pitch-catch” fashion mostly done in a pulse echo mode. Energy
harvesting and radio frequency (RF) communication enable multiple, individually
addressable sensors to detect andmonitor damage in structural airframe components.
This sensing system provides a low mass sensing system, which does not affect the
dynamic response of the component and high-efficiency sensor in power density
requirements for electromechanical conversion. The robustness, durability, and the
accuracy level of the magnetostrictive sensors for the rotorcraft airframe component
structure need to be reviewed under rotorcraft loading environment. The thin-film
magnetostrictive sensor is still at R&D stage and is not commercially available yet
for the time being. The currently available sensor system is handheld scanning type.
The sensor-related hardware is bulky and not suited for in situ crack monitoring.
However, under the DARPA SIPS program, SwRI have made considerable techno-
logical improvements in terms of the sensor hardware readiness levels for deployment.
Currently, a small company located in Colorado is trying to commercialize the
magnetostrictive sensor technology.
Sensor Applications for Structural Diagnostics and Prognostics 515
10 Conclusions
The critical conclusion drawn out of this study is that for the time being, although
there are some vendors/technologies of the structural damage sensing system
commercially available on the market, there are some technology gaps required to
be covered to reach a sufficient maturity to be able to claim commercial off-the-shelf
(COTS). Substantial customization to both the “standard hardware and software”
has to be made for each particular application. The damage detection algorithms/
software is still semiempirical and lacking of generality, and the sensor system will
need in situ calibration if the structure being monitored is slightly different in
detection ability.
References
1. Fu-Kuo Chang (ed) (2001) Structural health monitoring—the demands and challenges. In:
Proceedings of the 3rd international workshop on structural health monitoring, held at Stanford
