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Recent Advances in Inspection


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Title: Recent Advances in Inspection

Recent Advances in Inspection Health
Monitoring of Aerospace Materials Structures
for General Aviation
  • Jay Amos (Cessna Aircraft)
  • NDE Materials Processes Engineering

Cessna Aircraft
  • Cessna is part of Textron, a multi-industry
    company with strong brands such as Bell
    Helicopter, Kautex, Lycoming, AAI Corporation,
    E-Z-GO Jacobsen and Greenlee.
  • Citation X 
  • Cessna's Citation X is the fastest business jet
    in the world (0.92 Mach), capable of flying from
    New York to London in just under 6 hours.
  • 300th production Citation X rolled off the
    production line in 12/08 fleet has exceeded 1M
    flight hours
  • Citation Sovereign
  • Since the first delivery in late 2004, more than
    250 Sovereigns have been delivered.
  • Citation XLS
  • FAA certification was achieved in 5/08 (EASA in
    3/09) first retail XLS delivered in 12/08
  • With a global fleet of more than 700 and more
    than 1.4M flight hours (as of 8/08), the
    Excel/XLS is the best-selling business jet model
  • Citation CJ4
  • CJ4 prototype completed first flight in 5/08
    First flight of the production CJ4 in 8/08.
  • Plan to begin customer deliveries in the first
    half of 2010.
  • Mustang
  • The first retail Citation Mustang entered service
    in April 2007, and Cessna delivered 45 in 2007
  • About three of every five Mustangs go to
    international customers,
  • Certified in 60 countries through April 2009
    Fleet reached 100 in August 2008 and 200 in May
  • Corvalis and Corvalis TT
  • The Corvalis TT received EASA certification
    February 2009 
  • Corvalis TT  the fastest fixed-gear
    single-engine piston aircraft on the market 
  • Acquired certain assets of the former Columbia
    Aircraft Manufacturing Co. in December 2007
  • Skycatcher
  • The Skycatcher Achieves ASTM Compliance in 7/09
    Clearing the Way for Deliveries
  • Skyhawk
  • Cessna delivered the 40,000th Model 172 in 2008 -
    the most produced aircraft in history

Cessna History
  • Cessna Aircraft Company is the world's largest
    manufacturer of general aviation airplanes in
    terms of units delivered. In 2008, Cessna
    delivered 1,301 aircraft, including 467 Citation
    business jets.
  • Leading designer and manufacturer of light and
    mid-size business jets and utility turboprops,
    and a leading manufacturer of single-engine
    piston aircraft
  • Since the company was originally established in
    1927, some 192,000 Cessna airplanes have been
    delivered around the world, flying in nearly 80
  • including more than 24,000 twin-engine, 2,000
    military jets and 6,000 Citations, making it the
    largest fleet of business jets in the world.
  • Employment of 8019 as of 9/09.
  • Most complete line of business jets in the
    industry - a Citation takes off or lands
    somewhere in the world every 20 seconds.
  • 2009 approximate delivery rates (projected)
  • 400 single-engine pistons
  • 100 Caravans
  • 275 Citation business jets

Cessna Facilities
  • Wichita - Mid-Continent Headquarters and jet
    assembly, 3.5 million square feet Wichita -
  • Component production, 1.5 million square feet
    Wichita Citation Service Center 477,000 square
    feet one of the largest buildings in Kansas
    Room for some 100 Citations for service, w/5,270
    service orders in 2007Independence, Kan. SEP
    and Mustang assembly, Opened in 1996, 528,000
    square feet Columbus, Ga. Component
    production, Opened in 1996, 241,500 square feet
    Chihuahua, Mexico Component Production, Opened
    in 2006, 206,000 square feet
  • Citation Service Centers Nine Cessna-owned
    Citation Service Centers or participating
    authorized facilities around the world.Customer
    Support for piston products Propeller-driven
    Cessnas are supported through a worldwide network
    of more than 350 independent service facilities
    operating with high standards of customer
  • Cessna Pilot Centers CPC network consists of 264
    domestic and 13 international affiliates. CPC has
    been the leader in flight training since 1973.
    Since 2000, CPCs have delivered private and
    instrument pilot training to more than 100,000
    pilots around the world.
  • CitationAirOffers the full range of
    transportation solutions, ranging from Jet Cards,
    Jet Shares (fractional), Jet Management
    (turn-key, cost-effective way to optimize
    aircraft ownership with guaranteed use of the
    aircraft to support or back-up the CitationAir
    fleet and revenue generation for the aircraft
    owners), Corporate Solutions (fleet access
    without whole aircraft ownership)

MP Engineering - NDE
  • ADVANCED TECHNOLOGY - AD, Experimental,
    Manufacturing and process sensing support
  • Support process specifications
  • Evaluate new methods of testing inspection
  • Support implementation of new techniques/technolog
  • NDE SUSTAINING - Design review, DADT support,
    test article support
  • Engineering drawing review
  • Define NDI requirements for service-related
  • Inspection support for component test articles
  • PRODUCTION OPERATIONS  Support for NDI methods
    at all facilities

Citation NDI Certification / Licensing
  • Citation NDI Certification Program
  • Established in 1985 to ensure independent
    facilities/personnel meet Cessna NDI requirements
    (reflects NDT industry standards).
  • Provides Citation owners/operators a means to
    confirm a facilities NDI qualifications.
  • 103 Facilities Worldwide.
  • 61 Independent Domestic Facilities
  • 34 International Facilities
  • 8 Service Centers
  • NDI License Program
  • Established May 2001 wherein Cessna owns all
    Citation NDI calibrations standards.
  • Cessna provides control over the use and
    distribution of the standards.
  • Preserves intent of the Certification Program.
  • Ensures only approved NDI facilities are
    performing inspections.
  • Ensures standards are controlled, certified and
  • Eliminates expensive acquisition costs for
  • Pay on a per use basis - easy to understand ROI.
  • Mitigates Quality Control issues and lead times.

NDE Methods
  • Liquid Penetrant
  • Magnetic Particle
  • Radiography
  • Eddy Current (arrays)
  • Ultrasonic (Squirter, Air-coupled, Manual, PAUT)
  • Six 2.5D UT squirter gantries (up to 50)
  • Air-coupled UT 8 ch high speed flat scanner
    (Prod) 2 ch (Expr
  • Shearography (6x6x20 vacuum chamber w/heat
    excitation robot scanner)
  • Scanners
  • GenScan free-scan system with Mimeo feedback
  • 2-axis portable scanner for UT ET
  • MOI
  • Thermography (PE TTU)
  • Acoustic Emission (16 ch)

Cessna Gantry UT Systems
  • Gantries w/up to four TTU nozzle pairs and 28 ips

Cessna Air-coupled UT System
  • Reduced inspection time
  • Well-suited for composite or metal bond
  • Inspected over 2M sq. ft. of bonded assemblies
    since installed in 00.

  • Slight Hidden Corrosion in thin metal bonded
  • RFEC, ET array, phased array UT
  • Multi-Layer Cracking up to 4 layers 1.25 thick
  • Pulsed Eddy Current, RFEC, MS GMR
  • Angle beam UT scanning w/portable scanner
  • Physical Test Monitoring
  • Acoustic Emission
  • Metal Bond Process Monitoring
  • PAA/Bond Primer Thickness Automated anodize trace
  • Semi-automated contamination detection system
  • Residual stress after cold-working
  • NDE Modeling
  • Technique optimization

EC Modeling
Model Part Probe Flaw Output (b)
MEC03 Plate/Halfspace Air-core Complex (a) B,C
PLATE07 Multi-layers General Single crack, tight/open A,B,C
BEM07 General General Single crack, tight/open A,B,C
(a) Flaw type Complex multi-ligament
cracking, open or tight. (b) Output options A
incident EC fields, B part geometry signal, C
flaw signal Iowa State University CNDE
UT Modeling
  • Model can predict voltage of the defect echo,
    given info about the metal (density, velocity,
    surface), defect (size, shape, location) and
    system (waterpath, probe characteristics,
    reference echo, etc.).
  • Diffraction, attenuation, transmission/
    reflection coefficients, near/far fields, freq.
    dependencies, focused or flat probes, lenses
    mode conversions
  • SNR estimates can be made if noise properties of
    the microstructure are known (UT scatterer model
    for grain noise)

X-ray Modeling
  • Simulation can predict radiographic density of
    various materials, flaw composition, flaw
  • POD estimation for 3D components with multiple
    shot orientations.

What is SHM
  • Structural Health Monitoring is the continuous
    monitoring of structures/components using
    integrated or applied sensors.
  • Aimed at assuring structural integrity of the
    aircraft, replacing on-event and periodic
    inspections to detect damages resulting from
    fatigue, corrosion, excessive loads, impact ...
  • Monitoring of structures does not necessarily
    mean knowing the status of the structure in
  • Structures are designed with acceptable margins
    such that, after normal or exceptional events,
    maintenance tasks can be planned at next
    appropriate inspection.
  • Systems are available for aircraft condition
    monitoring - mostly for loads (accelerations,
    flight parameters, etc.) and enable decisions to
    be made based on actual flight load levels.
    Indirect surveillance of the structure is not
    comprehensive or reliable enough to avoid
    interval inspections.

Conventional vs. Condition Based Management
  • Currently, NDT is applied starting with visual
    inspections followed by for more subtle or hidden
    flaws, procedures are defined based on eddy
    currents, ultrasonics, x-rays, etc
  • Inspection intervals are usually based on
    knowledge of the structure residual strength,
    operating environment, applied loads, damage
    growth rate and failure consequences.
  • Of course, inspections result in downtime and
    inaccessible areas of structure often require
    significant effort to remove equipment or strip
    protective coatings for access, which then must
    be restored after the inspection.
  • Monitoring activity comes at a considerable cost
    and accounts for an average of 44 of all
    on-aircraft maintenance man-hours for commercial
    aircraft (Andresen, 2006). In terms of life
    cycle cost, a US DoD study attributed 27 of the
    total cost of an aircraft being maintenance
    related with structural inspection being a
    significant driver of this cost (Kudva,
    1999) suggesting that SHM could save up to 44 of
    current inspection time on modern fighter
  • Ultimate concept imitates the human nervous
    system, though SHM will better since structures
    are monitoring directly, measuring the effect of
  • Compared to conventional NDT, SHM has many
  • No access to the inspection area necessary
    fewer access panels component removal
  • No physical operation in the area - safe
    inspection of hazardous areas
  • No use of scanners necessary eliminating time
    consuming setup
  • Sensors used in the inspection are integral to
    the structure
  • Automated process - no human factors influence on
    inspection POD
  • Interrogating many locations or wide field at
    once - significant time saving

H. Speckmann, Materials Processes - Testing
Technology, Airbus
CBM Approach SHM Potential
  • Time spent inspecting the structure to assure
    continued airworthiness increases as aircraft
  • To allow for statistical variation of the real
    life of the structure, a safety factor is applied
    to the demonstrated lives of components.
  • To reduce the inspection burden, some industries
    have introduced automated on-line structural
    health monitoring systems with maintenance only
    being carried out when the health of the
    structure indicates a need for it.
  • CBM approach to maintenance if applied to
    aerospace structures has the potential to not
    only reduce the time spent inspecting these
    structures, but also improve airworthiness by
    detecting damage at an earlier stage than
    possible during discrete periodic inspections.
  • For safe life components it would also be
    possible to detect early failures and withdraw
    them from service ahead of their expected life,
    or for healthy components continue to use them
    beyond the design life and only withdraw them
    when their health indicates a requirement to do
  • Potential to both improve airworthiness and gain
    economic benefits was originally conceived within
    the context of a rotorcraft Health and Usage
    Monitoring System (HUMS) but is applicable to any
    health or usage monitoring system.

T. Ewbank, Cranfield University, Application Of
Condition Based Maintenance On Aerospace
SHM Applications
  • Difficult to access inspections
  • Hot spot monitoring

Desirable SHM Attributes
  • Impact damage in composite
  • Wireless/passive sensing
  • Appropriate for in-situ embedded or attached
    robust sensors during component manufacture
  • Cost effective, lightweight sensors (optical,
    acoustic, electromagnetic, etc.)

Acoustic Emission
Acellent Technologies
SHM Outcomes
  • Physics-based material properties measurements to
  • determine material state throughout life cycle
    allow design conservativeness to be minimized
  • Intelligent structures (self-diagnostic,
    self-healing, health monitoring diagnostics,
    manufacturability w/sensors, etc.)
  • Leading to design optimization, weight saving,
    less fuel consumption environmental impact

Challenges to SHM
  • Develop and demonstrate SHM technologies that can
    be used to monitor structural integrity in
    service conditions with high reliability
  • As in conventional NDT, a single technology will
    not be suited for the entire range of
    applications, based on different materials,
    component geometries and damage scenarios.
  • Diagnosis must have high reliability over the
    aircraft lifetime, since un-justified maintenance
    actions are quite costly to the operator and
    spurious warnings degrades confidence in the
  • Accuracy and reliability may even be more
    stringent, since further optimization of
    structural design will rely on SHM with better
    knowledge of actual flight loads condition.

System Qualification
  • Components shall qualified, as part of aircraft
    certification, meaning they shall perform the
    specified function while withstanding the
    specified environmental conditions.
  • Include large variations of temperature,
    vibrations, impacts, Electro-Magnetic Hazards,
    chemical fluids, etc as per RTCA DO 160 and
    aircraft integrator directives.
  • Components qualification shall demonstrate that
    the system performs reliably in the specified
    environmental conditions, in all the aircraft
    operational conditions over its lifetime.
  • Specific issues need to be considered
  • Easy installation and application on surfaces
  • Accuracy and reliability when used on painted
  • No corrosive damage to surfaces where applied
  • No delaminating between sensor and monitored
  • Suitable for metal, composite, sandwich
  • Suitable for various damage cracks, corrosion,
    delamination, de-bonding
  • Clearly different requirements will apply onto
    the system components sensors, processors,
    computers, wiring, power supply, depending on
    the technologies, architecture and installation
    location retained.
  • Micro-Nano-Technologies have the potential for
    supporting qualification requirements and the SHM
    business case.

H. Speckmann, Materials Processes - Testing
Technology, Airbus
Implementing SHM
  • SHM is not a new concept - it is already
    implemented on military aircraft, with a
    different rationale but some converging features.
  • Still the constraints of airworthiness
    certification and the existing cost/benefit have
    limited its introduction in commercial aviation
  • There is currently no specific FAA or EASA policy
    on CBM for civil aircraft. However, some guidance
    is provided in FAR-29 (FAA, 2003) on achieving
    maintenance or airworthiness credits with HUMS
    that could be developed.
  • US DoD stated the requirement to transition to a
    CBM program by the end of 2015.
  • Confidence needs to be built that SHM will bring
    the expected benefits, while maintaining or
    improving the safety and efficiency of modern
    aircraft by progressive introduction and
    proving the reliability benefits.
  • 1st generation of SHM shall target maintenance
    cost reduction and increased aircraft
    availability - technology will allow saving cost
    and time in regulatory inspections.
  • 2nd and 3rd generations of SHM shall integrate a
    new certifiable design philosophy and will permit
    weight reduction.
  • Sensors and their local processors would be more
    integrated with microelectronics allowing more
    decentralized architecture where local processors
    perform record the first level of SHM
    processing until transmission to the upper level

H. Speckmann, Materials Processes - Testing
Technology, Airbus
USAF Experiences
  • LAHMP Health Monitoring System F-15 Flight Tests
  • In 2003, the Army awarded an SBIR Phase II
    contract to TRI/Austin to develop a
    diagnostic/prognostic system that could monitor
    aircraft and rotorcraft structural components in
  • focused on ruggedizing the system, optimizing
    performance, reducing power draw, refining the
    prognostic/diagnostic algorithms and building a
    system for test.
  • successfully conducted third-party independent
    testing included acceleration testing of up to 6G
    on 6 axes as well as RFI/EMI testing. In-house
    thermal testing showed the system to be
    operational in the specified range of -40 to 85C,
    including thermal shock.
  • effort culminated in a successful flight test of
    the LAHMP system acquiring data from three areas
    on the F-15.
  • developed patented algorithms to determine
    structural health from on board sensor readings.
    In addition, we designed the health management
    platform to be fully customizable for a wide
    variety of aircraft."
  • JSF program has CBM features within the
    aircrafts design with rudimentary corrosion
    sensors installed and strain gauge monitoring of
    loads on a limited number of aircraft
  • loads are then coupled with flight data
    monitoring to allow parametric usage monitoring
    as a tool for CBM across the entire fleet using
    maneuver recognition algorithms to determine the
    loads on aircraft that are not monitored (Reed,
    JSF CBM Features, 2007).

Current Technology State
  • Of various fatigue damage detection technologies
    being researched to enable CBM on aerospace
    structures, Comparative Vacuum Monitoring
    acoustic emission are most mature and are
    currently marketed as commercial structural
    health monitoring solutions however both have
    application limitations
  • CVM has the capability to detect the presence,
    location and extent of damage, which when
    combined with a usage monitoring and a prognostic
    system could provide a full CBM capability.
  • However, at present CVM falls into a grey area
    between on-line structural health monitoring and
    NDT as the sensors are permanently installed but
    the vacuum and flow detector are only connected
    on the ground for off-line damage assessment.
  • Given the simplicity of this process it still
    offers significant advantages, especially for
    inaccessible structure. However, it is limited
    to areas of structure where the damage mechanism
    is well understood and predictable (localized
    damage detection)
  • Nevertheless, it is a elegantly simple concept
    that is gaining mainstream acceptance from
    aircraft OEMs and operators.
  • Corrosion sensing technology is generally crude -
    moisture detectors that could be placed in areas
    of corrosion prone structure are under
  • However, apart from using UT to measure the
    reduction in plate thickness, this and all the
    present techniques give an indication of the
    probability of corrosion on the structure that
    must be verified by visual inspection and to
    quantify its extent.
  • Of the currently developing damage detection
    technologies, guided waves and electrical
    impedance measurement appear to have promise but
    need to be tested in realistic structures under
    environmental conditions.
  • Fiber Bragg Gratings are also promising for
    increased structural coverage with minimal
    calibration requirements.

Comparative Vacuum Monitoring (CVM)
  • CVM sensors work on the basis of differential
    pressure - pressure changes in a system of small
    capillaries provide an indication of structural
    defects (cracks, corrosion and loss of bonding
  • Each sensor, which is 125 mm thick, is
    perforated with fine galleries alternately
    containing air and a vacuum. The presence of a
    crack or other defect in the monitored material
    creates a connection between the two types of
    gallery, altering the distribution of pressure
    inside the sensor at this point.
  • Used by Airbus in acceptance testing of GLARE
    (GLAss-fibre REinforced aluminum) composites - a
    laminate consisting of three layers of Al held
    together by intermediate layers of glass-fiber
    reinforced epoxy resin (A380 upper skins)
  • Since CVM sensors were permanently attached,
    inspection was done in a fraction of the time
    required by conventional testing methods
  • each measurement could be reproduced under
    exactly the same conditions
  • onerous task of installing and removing sensors
    only had to be carried out once, which saved a
    great deal of time, especially in the less easily
    accessible parts of the airframe
  • To monitor cracking from the fastener holes, CVM
    sensors were positioned inside the lap joint
    before riveting began
  • able to detect cracks of a magnitude of 1-2 mm
    cracks that most other test methods were
    incapable of detecting

H. Speckmann, Materials Processes - Testing
Technology, Airbus
CVM Sensitivity Durability
  • Sandia led project to mount a series of 26
    sensors on structure in four different DC-9, 757,
    and 767 aircraft in NWA and Delta fleet
  • Periodic testing is being used to study the
    long-term operation of the sensors in actual
    operating environments
  • compliments lab flaw detection as part of an
    overall CVM certification effort.
  • In conjunction with Boeing, Northwest Airlines,
    Delta Airlines, Structural Monitoring Systems,
    the University of Arizona, and the FAA,
    validation testing conducted on the CVM system in
    an effort to adopt Comparative Vacuum Monitoring
    as a standard NDI practice.
  • Fatigue tests conducted on simulated aircraft
    panels to grow cracks in riveted specimens while
    the vacuum pressure within the various sensor
    galleries are simultaneously recorded.
  • Crack is propagated until it engages, and
    fractures, one of the vacuum galleries such that
    crack detection is achieved (sensor indicates the
    presence of a crack by its inability to maintain
    a vacuum).
  • In order to properly consider the effects of
    crack closure in an unloaded condition (i.e.
    during sensor monitoring), a crack was deemed to
    be detected when a permanent alarm was produced
    and the CVM sensor did not maintain a vacuum even
    if the stress was reduced to zero.

Unpainted 0.040" Skin 0.040" Skin w/Primer
0.002-0.030 long cracks 0.002-0.010 long cracks
Acoustic Emission
  • An arbitrary mechanical excitation applied to a
    plate will generate a multiplicity of Lamb waves
    carrying energy across a range of frequencies -
    such is the case for the AE wave.
  • The challenge is to recognize the multiple Lamb
    wave components in the received waveform and to
    interpret them in terms of source motion.
  • This contrasts with the situation in UT, where
    the first challenge is to generate a single,
    well-controlled Lamb wave mode at a single
  • But even in UT, mode conversion takes place when
    the generated Lamb wave interacts with flaws, so
    the interpretation of reflected signals
    compounded from multiple modes becomes a means of
    flaw characterization
  • AEs fundamental limitation is the ability to
    only detect the growth of damage and not reliably
    give a measure of its extent, which makes the
    assessment of current and future load carrying
    capability impossible to reliably determine.
  • Consequently, once damage is detected a second
    method is required to confirm and assess the
    extent of the damage, which with current
    available technologies will require manual NDT.
  • this may still yield benefits but will need to be
    combined with another damage detection technology
    such as guided waves to be a truly on-line CBM
    enabling technology.
  • Many applications have struggled because it is
    difficult to determine exactly the position of an
  • Also the false call rate and POD can be

AE Applications
  • Airbus A320 fatigue test certification of inner
    wing (Staszewski, et al., 2003), and
    the monitoring of the A340 Landing Gear Support
    Structure during the full scale fatigue test of
    the A340-600 (Lloyd, et al., 2003).
  • In both cases the AE system was used to identify
    the presence and source of damage with
    conventional NDT being used to both confirm and
    quantify the extent of the damage. The A340 test
    was conducted with Ultra Electronics BALRUE
    system, which used 24 narrow bandwidth ceramic AE
    sensors at 300 kHz.
  • During the one year trial, all damage detected by
    conventional NDT was also detected with the AE
    system.  Furthermore, several damage sites were
    detected by AE before being found by conventional
    NDT techniques.
  • This system has now been modified and qualified
    for airborne use and is now marketed by Ultra
    Electronics as the AAIMS as an additional tool
    for SHM.
  • Apparently only one operator implemented, on one
    P-3 fire fighting aircraft (Aero Union)
  • 12 similar sensors were installed on the front
    spar to monitor the spar structure between the
    fuselage and inboard engine.
  • Data collected by these sensors was then stored
    together with 28 several other flight parameters
    such as spar cap strain, indicated airspeed, tank
    volume and vertical acceleration to a Data
    Acquisition Unit (DAU).
  • The equipment was installed on the P-3 aircraft
    during depot level maintenance and after just 47
    flying hours the analysis showed emissions.
    Further analysis of these results showed
    consistent crack growth in several areas, which
    were then confirmed by conventional NDT.
  • A 12 mm crack on the lower spar cap, almost
    certainly present during maintenance and not
    detected by the NDT carried out, was found to
    correlate with the damage. It was located 20
    cm away from the true site of the damage and
    identified the need to use an alternative
    technique for damage location in realistic
  • Approach requires the characterization of an
    installation by inserting 300 kHz signals into
    the structure in known locations during the
    systems installation, and then using signature
    recognition and a 3D model of the structure in
    the analysis, which has improved the positional
    accuracy to within 2 cm.

AE Application on MLG Fitting
  • For the Tornado GR4 retraction jack fitting, the
    only technically viable damage detection method
    to provide an on-line condition monitoring
    capability was considered to be acoustic
  • With CVM, flanges on the bushings may preclude
    damage detection and the movement of the
    retraction jack was likely to damage the sensors.
  • Given the limitations of CVM for wide area
    unpredictable damage mechanisms
  • fatigue damage may occur in virtually any
    location of the retraction jack fitting and take
    a number of directions.
  • Over a 10 year period the labor savings of
    introducing an AE on-line condition monitoring
    capability for the Tornado GR4 retraction jack
    fitting are estimated to be approximately 800k
    in NPV terms.
  • estimated to increase aircraft availability by 61
    aircraft days per year across the fleet.
  • When acquisition and design incorporation costs
    of an AE system are taken into account to provide
    an on-line condition monitoring it is unlikely
    that the system would be cost effective in terms
    of labor savings alone.
  • However, the increased availability may yield
  • savings to make the system viable and was
  • to be further investigated.

T. Ewbank, Cranfield University, Application Of
Condition Based Maintenance On Aerospace
PZT Network System using Lamb Waves
  • Acellent Technologies uses built-in network of
    piezoelectric transducers embedded in a thin
    dielectric carrier film.
  • system includes the PZT network connected to
    portable, diagnostic hardware and software.
  • Performs in-situ monitoring, data collection,
    signal processing, and real-time data
    interpretation to produce a two-dimensional image
    of the structure being interrogated.
  • Software controls the actuators to generate
    pre-selected diagnostic signals and transmit them
    to neighboring sensors.
  • wave types including 3, 5, and 10-peak narrow
    band frequency waveforms, chirp, random, and user
    defined excitations
  • Software links each sensor with its neighbors to
    form a web, or network, covering the area of
    interest and collects responses from each of the
    sensor sets as each PZT is activated.
  • Changes in Lamb waves generated within the
    structure are used with triangulation methods to
    detect the presence of anomalies and to determine
    size location.

PZT tests on Boron Epoxy patch
  • Similar to conventional UT, PZT data analysis can
    include one or more of the following
  • Time of wave transit (or delay), path length,
    frequency, phase, amplitude and angle of wave
    deflection (reflection refraction)
  • A series of excitation frequencies were used to
    optimize detection 50 kHz, 200 kHz, 350 kHz, and
    500 kHz.
  • Results revealed that disbond flaws were most
    strongly detected with 50 kHz, while the crack
    growth was monitored best with the highest 500
    kHz excitation
  • Signal attenuation, corresponding to disbonds
    between the patch and metal skin were apparent
  • Both flaws from one complete disbonded due to a
    Teflon insert, and a weak bond produced by a mold
    release agent

Lamb Wave Applications
  • A wingbox was tested (Grondel, Assaad, Delebarre,
    Moulin, 2004) with delamination of the plate
    sections of the composite structure and disbonds,
    with stringers being readily detected using
    amplitude analysis.
  • However, one key conclusion drawn was the need to
    identify the Lamb wave propagation modes possible
    in the structure at the frequency used
  • In this case there were 4 modes at the 400 kHz
    transducer frequency with wavelengths ranging
    from 3.75 mm to 15 mm. Higher frequency modes
    were very sensitive to damage, whereas the modes
    with the longer wavelengths were relatively
  • Use a frequency region where only fundamental
    propagation modes exist.
  • Chose a propagation mode and frequency where
    dispersion (wave velocity is a function of
    frequency and thickness of the plate) is kept to
    a minimum in order to simplify signal analysis.
  • Damage Localization in a Stiffened Composite
    Panel (D. Chetwynd, University of
    Sheffield) work conducted as part of the Aircraft
    Reliability Through Intelligent Materials
    Application (ARTIMA) EU project.
  • Case study of damage detection in a curved
    carbon-fiber reinforced panel with two omega
    stiffeners investigated using UT Lamb waves.
  • Outlier statistical analysis was used as a way of
    pre-processing data prior to damage
    classification. Multilayer perceptron neural
    networks were used for classification and
    regression problems of damage detection.
  • It was then investigated whether using wavelet
    analysis to perform prior wavelet decompositions
    of experimental data could facilitate damage

Fiber Bragg Gratings (FBG)
  • Fiber-optic sensors with elastic properties
    similar to those of the tested material
  • Can be used to monitor temperature, thermal and
    mechanical stress, damage caused by collision or
    impact, and delamination.
  • Fiber-optic sensors operate in similar manner as
    strain gauges.
  • As the material under test expands due to the
    effect of temperature or mechanical forces,
    properties of the sensor fibers vary in an easily
    measured way.
  • In a strain gauge, the electrical resistance
    varies in proportion to its distension
  • In fiber Bragg gratings, the characteristics of
    the reflected light change based on the position
    of tiny mirrors that make up the Bragg grating
    and with which the optical fiber is doped using a
    laser technique.
  • Up to 25 measurement points can be integrated in
    a small single fiber
  • To achieve the same number of measurement points
    using a strain gauge, it would be necessary to
    lay 25 thick multi-wire cables a hardware
    density that is already too high to be of
    practical use in a test configuration.
  • As a result, the network of measurement points
    used in conventional testing is correspondingly
    widely spaced FBGs would allow more closely
    spaced data resolution.
  • Quality of data would also be enhanced, using far
    less elaborate means
  • Unlike more conventional types of sensors,
  • sensors are not subject to interference by EM
  • so do not require elaborate shielding.

Cross-section of embedded fiber
FBG Applications
  • Betz, Staszewski, Thursby, Culshaw Structural
    Damage Identification Using Multifunctional Bragg
    Grating Sensors Damage Detection Results and
    Analysis, 2006).
  • Work attempted to determine sensitivity to
    temperature variation and, most significantly,
    give an indication of damage detection
  • Included the use of two driving frequencies at
    260 kHz and 460 kHz to explore the effect of
    frequency on damage detection.
  • Also involved the use of several analytical tools
    and the use of both piezoelectric and fiber optic
    Bragg Grating (FBG) sensors for the ultrasonics.
  • Results suggest that analysis of amplitude and
    the propagation period of the first two Lamb wave
    packets received gave the best damage detection
    correlation with the ability to discriminate
    between damage sizes of 0.8, 1.4 and 2 mm but for
    damage greater than 15 mm in size, a saturation
    effect was observed.
  • When using the analysis of amplitude and the
    propagation period of the first two Lamb wave
    packets both the piezoelectric and FBG sensors
    worked equally well, and it was possible to
    correlate damage size with both of the driving
    frequencies used.
  • Although analysis did show some sensitivity to
    changes in temperature, these effects were very
    minor and therefore well suited to further

Acousto-Ultrasonics (AU)
  • Technique that sends acoustic waves into the
    structure and intercepts them when they emerge on
    the other side.
  • Deviations from the expected wave pattern
    indicate the presence of cracks or delamination.

Eddy Current
  • Eddy Current Testing Foil Sensors (ETFS) are
    suitable for use on metallic structures.
  • Cracks and corrosion alter the electromagnetic
    field induced by the eddy current generated by
    the sensor (flexible).
  • Micro Eddy Current Sensor - Sandia Labs is
    developing a customized eddy current sensor for
    crack detection in thick steel structure. The
    probe must be able to detect deep, second-layer
    cracks as much as 0.5 below the surface.
  • Impedance bridge and other differential circuits
    were explored to maximize the magnetic flux
    density and corresponding eddy current strength.
  • Successful crack detection was achieved with a
    dual coil configuration that combines a pancake
    excitation inductor with a co-located pickup coil
    to produce a transducer that requires very little
    drive current (75 mA) and operates in the desired
    10 kHz range.
  • Excellent crack detection was achieved even when
    inspecting through composite repair doublers
    approaching 0.5 thickness.
  • More sophisticated rugged electronics package
    including digital signal processing to filter and
    detect phase shifts - to further improve probe
    sensitivity were being developed.

GE Inspection Technologies
In-situ Sensors Durability
  • Advantage/disadvantage compared to established
    NDT techniques is that future sensors remain
    permanently attached / embedded in place,
    required to withstand many decades of aircraft
    service life
  • mechanical structural stresses and ensure sensor
    performance substrate bonding
  • hot/cold and wet conditions
  • Embedded sensors can integrate well with
    composite materials - piezoelectric fibers or
    fiber-optic sensors can be fabricated with CFRP
    or GFRP (mitigating risks of sensor debonding)
    however two difficulties arise
  • if component is replaced due of wear or damage,
    the embedded sensor is also
  • maintaining the sensor is difficult, virtually
    impossible to repair, and not replaceable
  • Once installed, sensors provide a simple means of
    monitoring even areas that are difficult or
    dangerous for inspectors to access such as fuel
    tanks, wing spars, engine beams, etc. areas
    that are difficult to detect microscopic cracks
    or corrosion.
  • Detection might be in online (measured
    continuously in flight) or offline (data
    downloaded at next inspection or maintenance)
  • In-situ SHM sensors would be capable of spotting
    defects much faster, leading to considerably
    shorter inspection times.
  • Still, SHM should not entirely replace
    conventional NDT inspection practices
  • Conventional maintenance checks are rarely
    limited to the precise area specified in the
    maintenance manual - maintenance technicians
    typically take a careful look at surrounding area
    outside the actual inspection range.

Signal Analysis Power Mgt
  • Processor capabilities now are not limiting
    damage detection signal processing, but weight /
    power are always concerns.
  • Data must be managed in a robust system to ensure
    value and translated into useful structural
    health and usage information. DO-178B provide
    some structured development guidelines, which is
    recognized by FAA EASA
  • Many published works recognize patterns from
    known damage rather than identify unknown damage.
    Parametric recognition systems are beginning to
    appear (UK published policy in 07 to guide
    development for future military aircraft).
  • Parallel processing appears to be mature for some
    limited applications such as maneuver recognition
    in usage monitoring, but not prevalent in damage
  • Energy harvesting methods have been studied that
    can power sensors systems by converting
    structural stresses (strain energy harvesting)
    into electrical power via piezoelectric
    transducers (Sandia Labs, Kansas State, etc.)

  • Aircraft structural design optimization is the
    ultimate benefit of SHM
  • Structural maintenance inspections are a
    significant factor in operators Direct Operating
  • Minimize conventional NDI for periodic interval
    inspections for airworthiness and due to unusual
    events (hard landing, impact, lightning strike,
    etc.) since structural health data is available.
  • Inspection intervals are calculated
    conservatively based on fatigue and corrosion
    growth models. SHM will allow optimizing these
    assumptions with actual aircraft flight data.
  • SHM sensors can greatly simplify inspections
    since affixed permanently, can be activated
    quickly and reliably (without operator
  • Immediate benefit in faster and less costly
  • No difficult or dirty access to critical
    inspection zones

  • Some technologies are well ahead of others in
    terms of development maturity reliability,
    robustness, durability and data analysis are
    still key issues.
  • Future innovative approaches are being developed
    in microelectronics, nanomaterials, MEMS, etc.
    which will improve effectiveness and costs.
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