Recent Advances in Inspection

1
Recent Advances in Inspection Health
Monitoring of Aerospace Materials Structures
for General Aviation

• Jay Amos (Cessna Aircraft)
• NDE Materials Processes Engineering

2
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
  2009

• 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

3
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
  countries
• 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

4
Cessna Facilities

• Wichita - Mid-Continent Headquarters and jet
  assembly, 3.5 million square feet Wichita -
  Pawnee
• 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
  service.
• 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)

5
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
  ies
• NDE SUSTAINING - Design review, DADT support,
  test article support
• Engineering drawing review
• Define NDI requirements for service-related
  inspections
• Inspection support for component test articles
• PRODUCTION OPERATIONS  Support for NDI methods
  at all facilities

6
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
  documented.
• Eliminates expensive acquisition costs for
  customers.
• Pay on a per use basis - easy to understand ROI.
• Mitigates Quality Control issues and lead times.

7
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)

8
Cessna Gantry UT Systems

• Gantries w/up to four TTU nozzle pairs and 28 ips

9
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.

10
Projects

• Slight Hidden Corrosion in thin metal bonded
  structures
• 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
  instrument
• Physical Test Monitoring
• Acoustic Emission
• Metal Bond Process Monitoring
• PAA/Bond Primer Thickness Automated anodize trace
  analysis
• Semi-automated contamination detection system
• Residual stress after cold-working
• NDE Modeling
• Technique optimization

11
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
12
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)

13
X-ray Modeling

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

14
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
  real-time.
• 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.

15
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, et.al.
  1999) suggesting that SHM could save up to 44 of
  current inspection time on modern fighter
  aircraft.
• Ultimate concept imitates the human nervous
  system, though SHM will better since structures
  are monitoring directly, measuring the effect of
  damage.
• Compared to conventional NDT, SHM has many
  advantages
• No access to the inspection area necessary
  fewer access panels component removal
  requirements
• 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
16
CBM Approach SHM Potential

• Time spent inspecting the structure to assure
  continued airworthiness increases as aircraft
  age.
• 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
  so.
• 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
Structures
17
SHM Applications

• Difficult to access inspections
• Hot spot monitoring

18
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
19
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

20
Challenges to SHM

• Develop and demonstrate SHM technologies that can
  be used to monitor structural integrity in
  service conditions with high reliability
  durability.
• 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
  system.
• 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.

21
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
  particularly
• Easy installation and application on surfaces
• Accuracy and reliability when used on painted
  surfaces
• No corrosive damage to surfaces where applied
• No delaminating between sensor and monitored
  structure
• Suitable for metal, composite, sandwich
  structures
• 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
22
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
  processor.

H. Speckmann, Materials Processes - Testing
Technology, Airbus
23
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
  flight.
• 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).

24
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
  development.
• 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.

25
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
  contact).
• 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
26
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
27
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
  frequency.
• 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
  indication.
• Also the false call rate and POD can be
  problematic

28
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
  structures.
• 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.

29
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
  emission.
• 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
  sufficient
• savings to make the system viable and was
  recommended
• to be further investigated.

T. Ewbank, Cranfield University, Application Of
Condition Based Maintenance On Aerospace
Structures
30
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.

31
PZT tests on Boron Epoxy patch

• Similar to conventional UT, PZT data analysis can
  include one or more of the following
  measurements
• 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

D. Roach, Sandia Labs, HEALTH MONITORING OF
AIRCRAFT STRUCTURES USING DISTRIBUTED SENSOR
SYSTEMS
32
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
  insensitive.
• 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, et.al. 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
  classification.

33
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,
  fiber-optic
• sensors are not subject to interference by EM
  fields,
• so do not require elaborate shielding.

Cross-section of embedded fiber
34
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
  sensitivity.
• 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
  processing.

35
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.

36
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
37
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)
  modes.
• 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.

38
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
  detection.
• 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.)

39
Directions

• Aircraft structural design optimization is the
  ultimate benefit of SHM
• Structural maintenance inspections are a
  significant factor in operators Direct Operating
  Costs.
• 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
  variability)
• Immediate benefit in faster and less costly
  inspection
• No difficult or dirty access to critical
  inspection zones

40
Conclusions

• 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.