A series of advancements in computers, electronics, material science and other interdisciplinary
fields made major impact on all or many of the NDE methods. Accepting the reality that no
single method can provide all the necessary NDE information, efforts are being made to integrate
several methods. The complimenting capabilities offer greater detectability and the overlapping
ones enhance the reliability. Data fusion techniques [Gros, 1996] are being developed to allow
effective data-acquisition and processing as well as provide sound interpretation of test
parameters in relation to the material integrity. Instruments are now commercially available that
used a common platform for ultrasonic and eddy current testing with interchangeable transducers
and modules. Also, software packages are available to process data obtained from various NDE
methods where, once data is acquired and an image is produced, the results can be analyzed and
manipulated using the same software features. The increased processing speed and improvement
in hardware is allowing real-time imaging of all the wave-base NDE methods including
radiography, ultrasonics, shearography, etc. Using analytical tools, finite element analysis as
well as computer hardware and software, test procedures can be developed graphically by
interactive process simulation. Moreover, progress in microelectronics led to the development of
pocket size instruments. The following discussion covers technologies that affected the field of
NDE in general.
Information Highway
In the relatively short time since Internet became the world-wide-web as we know it, this
information highway significantly contributed to the advancement of many fields including NDE
[Bar-Cohen, et al, 1996]. Internet is now the information communication tool of choice for
multimedia (data, files, text, programs, drawings, pictures, video and sound) at speeds, efficiency
and content that cannot be matched by any other known method. NDE experts around the world
rapidly recognized its power as a form of information exchange and archival. The technology is
simplifying and helping to expedite the development of international standards and process
specifications, as well as enabling the centralization and easing access to information achieves
and databases. Formed in 1995, the global NDE Newsgroup [nde@coqui.ccf.swri.edu], which is
maintained on a server at South West Research Institute, is widely used by NDE experts around
the world as an electronic bulletin board. Subscribers are added electronically and they receive
via e-mail inquiries, data, information and announcements of general interest. Global efforts and
initiatives of individuals and companies are contributing greatly to the field. As an example, the
electronic publishing forum, NDT.net, is a highly active and effective web-system that combines
an electronic journal, information archive and monthly technical forums of information exchange
[http://www.ndt.net/newsweb/newsweb.htm]. To take advantage of the various web capabilities
the author formed the JPL's NDEAA webhub [http://ndeaa.jpl.nasa.gov/] with clickable
animation to aid understanding various mechanisms and with links to downloadable recent
publications. Moreover, to quickly find and access the growing number of homepages of
international technical societies, the author formed the Global NDT Internet Superhub (GNIS)
with clickable countries on a globe map [http://eis.jpl.nasa.gov/ndeaa/nasa-nde/gnis/gnis.htm].
The technology reached a point where "companies do not exist unless they have a homepage".
To find some of the major homepage addresses, one can use the NTIAC webpage that has links
to over 300 NDE site [http://www.ntiac.com/ ]. Through its homepages, ASNT is offering
society information such as various services, conferences, and other relevant activity and
announcements [http://www.asnt.org/ ].
Inspection Simulation
Ray tracing is an essential tool for investigating the travel path of waves and developing test
procedures. With the progress and increased speed of computer graphics it became feasible to
investigate ray tracing using rapid interactive simulation. Simulation software can perform 3D
ray tracing, and examine the wave interaction with the test structure geometry while accounting
for the materials that are involved. Effective tools were developed by such research institutes as
the Center for NDE at Iowa State University and the Canadian National Research Council as
well as commercially by UTEX (Ontario, Canada). Computer models were used to develop
user-friendly, accurate and rapid simulation of such methods as radiography, ultrasonics, and
eddy current. Numerous test parameters were included in the models, e.g., for X-ray simulation
(see example in Figure 1) some of the parameters are X-ray source, film type, part geometry,
setup distances, exposure value, material absorption, etc. The part structure can be described
using 3-D CAD models, and many types of defects can be inserted anywhere into the model to
form a realistic simulation of the test process. In the case of modeling ultrasonics, the reflected,
transmitted and refracted waves can be used to produce simulated A-, B-, and C-scans. Further,
for eddy current the real and imaginary components of the impedance-plane output as a function
of the probe position can be simulated in response to crack-like defects.
 a. A honeycomb sandwich |
 b. A penetrameter gauge |
| Fig1: Simulated radiographs using computer code and interactive graphics (Iowa State University). |
Fig 2: Simulation of the ultrasonic wave interaction with a complex
configuration using ray tracing and computer graphics [Iowa State
University]
Fig 3:Figure 3: Inspection of an IIW block using simulation software (UTEX's Imagine3D)
| |
Ultrasonic test procedures can be developed using complex real-world parts (see example in
Figure 2); a complete ultrasonic inspection can be simulated including the test results; difficult
problems can be anticipated and diagnosed by confirming the source of the returning echoes; the
sound paths in the part can be visualized; the sound field of the transducer can be evaluated to
make an effective choice of the transducer; and inspectors can be easily and effectively be
trained. Specifically, A-scan displays for various angles of incidence can be simulated to assist
inspectors in developing ultrasonic test procedures. Longitudinal and shear mode conversions
can be identified by the operator and assist in explaining the origin of echoes that appear on an
A-scan display. Moreover, simulated B-scans can display how stray sound modes and part
geometry might accidentally shadow flaws. If fixturing is required, the simulation software
provides the necessary information for the probe angulation, spacing, delay line or water-path
distances. Transducer modeling features can simulate both contact and immersion coupling
using various frequencies and bandwidths as well as various focal geometries (circular, elliptical
or rectangular). Tests can be viewed in both pulse-echo and pitch-catch and the transducer can
be manipulated to follow the part surface at a fixed distance and/or inspection angle. In Figure 3,
an example of the application of simulation software is shown for the inspection of an IIW
calibration standard as a means of determining the resolution of the test technique. The operator
can use the simulation and the results of the actual calibration to verify the efficiency of the
inspection procedure. The technology significantly simplified the development of test
procedures and its application to process development is highly cost effective.
Miniaturization
Progress in microelectronics enabled the miniaturization of NDE hardware and the production of
portable instruments that can be carried to the field and reach difficult to access areas. Pocket
size ultrasonic thickness gauges are commercially available from most of the leading
manufacturers of ultrasonic instruments. The technology is leading to reduction in cost as well
as in instrumentation weight and size with greatly enhanced capability. Data acquisition cards
that can be plugged into a laptop computer have been available for several years (e.g., Wesdyne
International, California) and credit card size plug-ins that conform to the PCMCIA type 2
standard is one of the forms in which this progress is expressed. The use of such cards allows
converting laptops to small ultrasonic pulser/receiver and with appropriate software they can be
operated as a complete ultrasonic data acquisition and imaging system. Plug-in cards are
available for motion control (encoder) interfaces, high-resolution A/D converters and signal
processors for potable scanners. Increasingly, systems that are battery operable are being used
with wireless communication capabilities.
Fig 4: An instrumentation spider at the University of Tokyo illustrates the potential of NDE in terms of mobile sensors
|
The technology of miniaturization has impacted also the size of sensors and their support
electronics. For over ten years, the trucking industry has been using tires with imbedded sensors
that wirelessly communicate every several minutes the individual tires' identity and pressure.
Using effective power management, these sensors can operate for periods of more than 8 years
without needing to change battery. This technology is intended to help truck drivers avoid tragic
and costly accidents that can result from flat tires. Another area of automotive that benefited
from miniaturized technology is impact sensing and the activation mechanism of airbags.
Further, insects such as bees are commonly tracked with the aid of miniature transmitters that are
installed as backpacks. Such capabilities can be transitioned to the field of acoustic emission and
other NDE if miniature wireless stick-on sensors are used. The size of electronics has become so
small that insects can be instrumented to perform tasks that used to be viewed as science fiction.
Example is shown in Figure 4, where a spider at the University of Tokyo, Japan, was
instrumented as a locomotive to carry a backpack of wireless electronics, and CCD imager.
Development in actuation technology is expected to lead to insect-like robots that can be
launched into hidden areas, such as aircraft engines, and perform inspection and maintenance
tasks. Beside miniaturization of conventional actuators, such as motors, electroactive polymers
(EAP) are being developed to offer the closest emulation of biological muscles. One of the
applications that is currently being explored include a dust-wiper using the bending
characteristics of ion-exchange type EAP materials operating similar to an automotive
windshield wiper [Bar-Cohen, 2000a].
Figure 4: An instrumented spider at the University of Tokyo
illustrates the potential to NDE in terms of mobile sensors
[http://www.leopard.t.u-tokyo.ac.jp/].
Rapid field inspection
Performing NDE using such methods as ultrasonics and eddy current, which require a probe to
obtain data and scanning to cover large areas, is time consuming and involves difficulties when
applied in field conditions. Rapid inspection of large structures is an ongoing challenge to the
NDE community. The need for such a capability grew significantly in recent years as a result of the increase in the numbers of aircraft with composite primary structures and the large number of aging aircraft still in service. Generally, metallic structures are susceptible to corrosion and fatigue cracking whereas composites are sensitive to impact damage that can appear anywhere on the structure. Using manual scanning, field inspection is labor intensive, time consuming and
susceptible to human error, whereas removal of parts from an aircraft for a lab test is costly and may not be practical. Effective field inspection requires a portable, user friendly system that can rapidly scan large areas of complex structures. In recent years, various portable inspection systems have emerged including scanners that are placed at selected locations and sequentially repositioned to fully cover the desired areas [Bar-Cohen, 2000b]. An example of such scanner is shown in Figure 5, where vacuum suction cups are used to secure the attachment of the scanner-bridge to the test structure. The development of such scanners requires multidisciplinary expertise including NDE,telerobotics, neural networks, automated control, imbedded computing and materials science. The capability of the developed systems followed the technology evolution and the overall trend is towards full automation with a desire to have a completely autonomous inspection.
Increasingly, crawling devices are reported as a solution to the need for rapid inspection and the use of suction cups has become a leading form of controlled adherence. In contrast to aerospace use of suction cups, magnetic attachment techniques are used in pipelines, marine and other ferromagnetic structures. Several successful mobile portable scanners have emerged in the last several years, including the Automated Non Destructive Inspector (ANDI) and the Autocrawler
[Bahr, 1992, and Siegel, 1998]. Recently, JPL developed the Multifunction Automated Crawling
System (MACS) offering an open architecture robotic platform for NDE boards and sensors [Bar-
Cohen & Backes, 1999]. MACS is a small, highly dexterous crawler designed to perform
complex scanning tasks. It uses suction cups for controlled attachment and ultrasonic motors for
mobility. MACS (shown in Figure 6) was designed for inspection of large structures particularly
in field and depot conditions and it established the foundations of "walking" computer platforms
using standard plug-in NDE boards. This concept offers larger pool of companies and individuals
the opportunity to become producers of NDE modules without the need to fabricate instruments.
Thus, a significant cost reduction can be materialized from such a focus, equivalent to
manufacturers of modems and other computer components. It will be possible to novel concepts
with rapid transition of to practical use. Such a trend can lead to rapidly improving, affordable and
tailorable systems with potential success similar to the personal computers.
Fig 5: A scanner is secured to a test structure using suction cups (QMI, Costa Mesa, CA). |
Fig 6: The JPL developed MACS crawling on the C-5 aircraft. |
Remote monitoring is one of the avenues of future development where centrally located experts
can be responsible for the inspection and they will be equipped with know-how, database,
analytical tools, CAD drawing, and accept/reject criteria. These experts will need to deal only
with questionable information where the redundant non-defective areas will be screened by the
crawler system. Controlling the crawler remotely via Internet using password will allow
authorized users to simultaneously view and control its operation. Inspection needs can be
addressed rapidly, particularly in cases of crisis where it is necessary to examine a full flight of a particular aircraft model all over the world. A combination of visual, tap testing, eddy current, and ultrasonics are expected to be the leading NDE capabilities for integration into MACS. The evaluation of flaws using multiple NDE methods requires data fusion [Gros, 1996] and neural network data interpretation. Programming the travel on complex structures can employ
telerobotic capabilities similar to the one developed for the exploration of Mars, including the
rover of the Mars Pathfinder mission. MACS is currently using an umbilical cord for power,
control, and communication as well as pressure tubing for ejection and activation of the vacuum
suction cups. Further enhancement will be the development of an autonomous crawler for
operation during aircraft idle time to reduce the need to ground aircraft for inspection. This will require miniature on-board vacuum pump, power and computing capabilities. To protect aircraft elements that rise above the surface from accidental damage, a vision system and collision avoidance software need to be used. Employing local Global Positioning Systems (GPS) can provide absolute coordinates without the need for complex, costly and heavy encoders. The
information regarding the location of the crawler on the aircraft in relation to the detailed
drawings can help assessing flaws criticality.
Testbed for Validation of New NDE Techniques
Fig 7: An aging aircraft with well-documented flaws at AANC(Sandia National Labs). |
The need for a testbed to demonstrate new NDE techniques and instruments hampered the
transition to practical use. In August 1991, an FAA center for NDE was formed at Albuquerque,
NM, to offer a validation testbed consisting of aging aircraft with known flaws. This Sandia
National Laboratories' Airworthiness Assurance Nondestructive Inspection Validation Center
(AANC) was facilitated with a series of aging civilian aircraft and a library of structural parts
with documented flaws [http://www.sandia.gov/aanc/pubs.htm, Smith & Shurtleff, 1997 and
Shurtleff, et al, 1997]. The center arose out of the Aviation Safety Act of 1988, passed by
Congress after the midair structural failure of the Aloha Airlines Boeing 737. Sandia's role has
since been expanded to other areas covering aircraft overall safety system design such as fire
protection, information system management, and accident investigation support. A view of one
of the aircraft that is available at the AANC facility is shown in Figure 7.
The continuing need for improved NDE methods resulted from the fact that any of the known
methods has certain capabilities and limitations in detecting flaws and/or determining material
properties. The selected method(s) and inspection requirements depend on the inspected material
structure and the life cycle stage. While in-service inspection of metallic structures requires
mostly the detection of fatigue cracks and corrosion, composite structures require the detection
of delaminations and impact damage. Generally, unless a crack is located in a stack of plates
beyond the second layer, many methods are available for its detection. Cracks need to be
detected above a critical size and their size and depth need to be determined. In contrast to this
relatively simple requirement, corrosion detection and characterization and NDE of composite
structures are more complex.
Metallic Structures in Service
In service, two major types of flaws are commonly sought: fatigue cracks and corrosion.
Generally, fatigue cracks are initiated at high cyclic stress areas and therefore it is relatively easy
to determine when and where to expect them and their geometry is relatively simple. On the
other hand, the issue of corrosion damage is much more complicate and it can consist of complex
geometry and variety of types (see Table 1). The damage is a relatively slow material
degradation process [Hagemaier, Wendelbo & Bar-Cohen, 1985]. Corrosion is a general term
that describes the oxidative degradation of metals caused by a local galvanic cell between the
base metal (acting as anodic sites), at sites of defective protective coating, having the passive
sites sustaining cathodic reaction. The corrosion process converts the metal into its oxide or
hydroxide forms resulting in deterioration of its mechanical properties. Corrosion in aluminum
alloys and plated steel surfaces can often be recognized by dulling or pitting of the area, and
sometimes by white or red powdery deposits. It may also be the origin of, or revealed by,
delamination, cracking, metal thinning, fretting, etc. Corrosion can appear in many forms,
depending on the type of metal, how it is processed, its surrounding structure and service
conditions. Corrosion results from exposure to humid or corrosive environments and involves
primarily electrochemical action at chemical/metallurgical/physical heterogeneity with dissimilar
potentials. Table 1 lists the various corrosion types that may appear in aircraft metallic
components, their sources of formation and by-products.
Aircraft are designed and manufactured with built-in corrosion-prevention features.
However, most metals used in aircraft structures are subjected to degradation due to exposure
to adverse environments including humidity-induced stresses and wide temperature
excursions. These conditions may cause localized corrosion attacks in various forms.
Corrosion-protection systems are widely in use and they consist of a combination of
materials, sealants, paints, design details, drainage, assembly practice, and preventive
maintenance. The corrosion-prevention system cannot be guaranteed to work. The severity
of corrosion attacks varies with aircraft type, design techniques, operating environments, and
operators' maintenance programs.
In spite of careful maintenance programs to assure a lower rate of progress of corrosion damage
(proper sealing or cleaning in galley area, proper draining, etc.), corrosion does occur and
effective NDE techniques are needed to detect them as early as possible. Detection of each of
the corrosion types described in Table 1 may require a different NDE approach due to the unique
characteristics that are involved. Recent efforts are directed to avoiding removal of the paint or
coating prior to inspection in order to minimize the associated environmental impact as well as
cost. Several NDE methods are widely used for corrosion detection and evaluation. When the
inspected area is physically accessible, visual tests are commonly used for periodic checks in
search for cracks, change of color, change of texture, or bulges. Sometimes, tools such as
magnifying glasses or boroscopes are employed for further evaluation or for less accessible
areas, respectively. Surface corrosion at its embryonic stage can be visually detected from
localized indication such as discoloration, faint powder lines, pimples on the paint and paint
damage. Concealed corrosion is very difficult to detect since in most cases the characteristics of
the damage are not sufficient to trigger an indication in conventional NDE tests. Generally,
several NDE methods are available to detect and characterize hidden corrosion including X-ray
and neutron radiography, ultrasonics, eddy current, and acoustic emission. All existing NDE
methods for detection of corrosion are limited in capability and sensitivity. Frequently,
corrosion is detected only after several subsequent inspection schedules, in which case the
damage is fairly extensive and may require the replacement of the structural component
involved.
| Corrosion type
| Source
| Appearance
| By-Product
| Notes
|
| Crevice
| Afflicts mechanical joints, e.g., coupled pipes or threaded
connections. Triggered by local environment composition differences (O2 concentration).
| Localized damage in the form of scale and pitting.
| Same as scale and pitting.
| - Caused by differential aeration.
- Difference in oxygen concentration
produces potential difference and leads to
flow of electrical currents across aerated
(cathode) and derated (anode) portions of
the metal.
- Causes localized corrosion failure.
|
| Filiform
| High humidity around
fasteners, skin joints or
breaks in coating cause
an electrolytic process.
| Meandering, fine, thread-like trenches spreading from the source.
| Similar to scale.
|
|
| Galvanic Corrosion
| Corrosive condition that results from contact of different metals.
| Uniform damage, scale, surface fogging or tarnishing.
| Emission of mostly molecular hydrogen gas in a diffused form.
| - Slow growth rate. Expressed as
penetration/year or weight loss per unit-area/day, e.g. the rate for aluminum
in open atmospheric conditions of Los Angeles, CA is 0.02-mil/yr.
- For Ti and Al alloys the rate is slow and therefore it does not pose serious
structural problems.
- The metal with the most negative potential
suffers the most damage.
|
| Inter-granular
| Presence of strong potential differences in grain or phase boundaries.
| Appears at the grain or phases boundaries as uniform damage.
| Produces scale type indications at smaller magnitude than stress corrosion.
| - The severity and rate of growth depends on
the material microstructure crystalinity and segregation.
- This type of corrosion can merely result increased susceptibility to attack in the
form of pitting or stress cracking.
- Exfoliation occurs in layered grains (e.g. rolled sheets) in the form of laterally
extended damage.
- Aluminum is susceptible to such attack mainly in an environment of chlorine ions
and dissolved oxygen.
|
| Microbial
| Bacterial, fungus or yeast in contaminated kerosene-type jet engine fuel.
| Appears in integral fuel tanks.
| Combination of by-product of pitting and scale.
| Can be eliminated by a proper maintenance of the fuel in the tank.
|
| Pitting
| Impurity or chemical discontinuity in the paint or protective coating.
| Localized pits or holes with cylindrical shape and hemispherical bottom.
| Rapid dissolution of the base metal.
| - Expressed in terms of pitting depth (i.e. pitting factor).
- Pitting can be critical to the structural integrity.
- Can be detected by AC impedance or electrochemical impedance spectra analysis.
- Usually pitting is accompanied by an order of magnitude change in the local resistance and capacitance.
|
| Stress Corrosion Cracking
| Mechanical tensile stresses combined with chemical susceptibility.
| Localized micro- macro- cracks at shielded or concealed areas.
| Produces initially scale type indications at a large magnitude that
progresses to cracking.
| - Causes critical failure of structures.
- Failure rate is determined by the stress levels.
- Corrosion fatigue occurs under cyclic stresses.
- Stress and rubbing action remove protection and lead to fretting corrosion as a result of contact of the metal surface with particles introduced by the abrasion process.
|
| Thermo-galvanic Corrosion
| Caused by thermal gradients parallel to the metal surface.
| Localized attack correlated with temperature distribution.
| Produces scale indications.
| Hot portion of the metal serves as cathode whereas the cold portion as anode.
|
| Table 1: Corrosion types, which can inflict damage to aircraft strictures, and their characteristics.
|
NDE of Composite Materials
The high stiffness to weight ratio, low electromagnetic reflectance and the ability to embed
sensors and actuators have made fiber-reinforced composites an attractive construction material
for primary aircraft structures. These materials consist of fibers and a polymer matrix that are
stacked in layers and then cured. A limiting factor in widespread use of composites is their high
cost - composite parts are about at least an order of magnitude more expensive than metallic parts.
The cost of inspection is about 30% of the total cost of acquiring and operating composite
structures. This large portion of the total cost makes the need for effective inspection critical not
only to operational safety but also to the cost benefit of these materials [Bar-Cohen, et al, 1991].
Currently, there are several critical issues that are still challenging the NDE community with regards
to inspection of composites. These issues include:
- Defect Detection and Characterization: Throughout their life cycle, composites are
susceptible to the formation of many possible defects mostly due to the multiple step
production process and their non-homogeneity with brittle matrix. These defects include
delaminations, cracking, fiber fracture, fiber pullout, matrix cracking, inclusions, voids, and
impact-damage.Table 2 lists some of the defects that may appear in composite laminates and
their effect on structural performance. While the emphasis of most practical NDE is on
detection of delaminations, porosity and impact damage, Table 2 shows that other defects can
also have critical effect on host structures. Therefore, it is essential to be able to characterize the
flaws in order to determine their degradation effect on the structural integrity.
- Material Properties Characterization: Production and service conditions can cause
degradation of properties and sub-standard performance of primary structures. Sources for such
degradation can be the use of wrong constituent (fiber or matrix), excessive content of one of
the constituent (resin rich or starved), wrong stacking order, high porosity content, micro-
cracking, poor fiber/resin interface aging, fire damage, and excessive environmental/
chemical/radiation exposure. Destructive test methods of determining the elastic properties are
using representative coupons. These methods are costly and they are not providing direct
information about properties of the represented structures.
| Defect
| Effect on the material performance
|
| Delamination
| Catastrophic failure due to loss of interlaminar shear carrying capability. Typical acceptance criteria require the detection of delaminations that are³ 0.25-inch.
|
| Impact damage
| The effect on the compression static strength- Easily visible damage can cause 80% loss
- Barely visible damage can cause 65% loss
|
| Ply gap
| Degradation depends on stacking order and location.
For [0,45,90,-45]2S laminate:
- 9% strength reduction due to gap(s) in 0o ply
- 17% reduction due to gap(s) in 90oply
|
| Ply waviness
| - Strength loss can be predicted by assuming loss of load-carrying capability.
- For 0o ply waviness in [0,45,90,-45]2S laminate, static strength reduction is:
- 10% for slight waviness
- 25% for extreme waviness
- Fatigue life is reduced at least by a factor of 10
|
| Porosity.
| Degrades matrix dominated properties
- 1% porosity reduces strength by 5% and fatigue life by 50%
- Increases equilibrium moisture level
- Aggravates thermal-spike phenomena
|
| Surface notches
| - Static strength reduction of up to 50%
- Local delamination at notch
- Strength reduction is small for notch sizes that are expected in service
|
| Thermal Over- exposure
| Matrix cracking, delamination, fiber debonding and permanent reduction in glass transition temperature
|
| Table 2: Effect of defects in composite materials |
- Rapid Large Area Inspection: Impact damage can have critical effect on the capability of composite structures to operate in service. This critical flaw type can be induced during service life anywhere on the structure and it requires detection as soon as possible rather than waiting for the next scheduled maintenance phase. Using conventional NDE for the assurance of the
structural integrity can be very expensive and takes aircraft out of their main mission. Since
impact damage can appear anytime and anywhere on the structure, there is a need for a low-cost
system that can be used to rapidly inspect large areas in field condition. The use of a robotic
crawlers can potentially offer effective platform for rapid inspection of composite structures.
- Real-Time Health Monitoring : Structurally integrated health monitoring systems are needed to reduce the need for periodic inspection and temporary removal of aircraft from service.
Fundamentally, such health monitoring systems emulate biological systems where onboard
sensors track the structural integrity throughout the life cycle. Such systems can monitor
changes in the characteristics of critical parameters and activate an alarm when certain values
are exceeded.
- Smart Structures: The availability of compact actuators, sensors and neural networks has made it possible to develop structures that self-monitor their own integrity and use actuators to avoid or timely respond to threats. The changing environment or conditions can be counteracted by adequate combination of actuators and sensors that change the conditions and/or dampen the threat. Sensor fusion, neural network and other artificial intelligence capabilities can be used to assure making the most effective and quick response. An example of the application of smart structures is the reduction of vibrations that lead to fatigue.
- Residual Stresses: Current state of the art does not provide effective means of nondestructive determination of residual stresses. Technology is needed to detect and relieve residual stresses in structures made of composite materials.
- Weathering and Corrosion Damage: Composites that are bonded to metals are sensitive to
exposure to service fluids, hygrothermal condition at elevated temperatures and to corrosion.
Particular concern rises when aluminum or steel alloys are in a direct contact with composites
that consist of graphite fibers or carbon matrix. Graphite is cathodic to aluminum and steel
and therefore the metal, which is either fastened or bonded to it, is eroded. In the case of
graphite/epoxy the metal deteriorates, whereas in the case of graphite/polyimide defects are
induced in the composite with the form of microcracking, resin removal, fiber/matrix interface
decoupling and blister (e.g. delaminations).
For many years, the multi-layer and the anisotropic nature of composites posed a challenge to the NDE research community. Pulse-echo and through-transmission are still the leading standard NDE methods of determining the quality of composites. However, these methods provide limited and
mostly qualitative information about defects and material properties. The discovery of the Polar
Backscattering and the leaky Lamb wave (LLW) phenomena in composites enabled effective
quantitative NDE of composites [Bar-Cohen, et al, 1991].
The beginning of the third Millennium provides a milestone point to look back at the progress
that was made in the field of NDE and take a snapshot of its state-of-the-art as well as determine
the challenges that are still constraining the technology. Like many other fields, improvements
were made in every aspect of the NDE science and engineering where computers and internet
contributed greatly to the rapid advancement. Efforts are increasingly being made to integrate
several methods to form multi-mode systems that take advantage of the complementary
capabilities to increase the functionality and the overlapping capabilities to improve the
reliability.
Some technologies affected most or all the NDE methods and those include internet, interactive
inspection simulation, miniaturization, portable robotics, and others. Inversion techniques were
developed to extract flaw characteristics and material properties using nondestructive
measurements. In addition to developing improved methods, various sensors are now available
to perform a wide range of inspection tasks. The available sensors can be grouped as follows:
- Remote sensors - Eddy current, magnetic, visual, dry-couple ultrasonics, etc.
- Attached sensors - Cracking fuse, resistance gauging, strain gage, acoustic emission,
ultrasonic, eddy current, fiber optics
- Sensitive coating - Bruising paint indicator, brittle coating, liquid crystals
- Imbedded sensors - Fiber optics, dielectric, eddy current, magnetic, ultrasonics
The most practical sensors currently used are the ones that either can be operated remotely or
attached to the test structures. The manufacturers and users are still not receptive to using
sensors that can be imbedded, permanently attached or coated. This is due to the weight increase
and the potential effect on the structural integrity. The use of stick-on wireless type sensors is expected to emerge in the coming years to allow monitor structural integrity throughout the life cycle of structures without disassembly, redesign or complex wiring.
The use of the crawler technology is offering great potential to rapid field inspection, where
plug-and-ply boards would define the crawler functionality. Employing off-the-shelf
components and standard personal computers bus structure (e.g., ISA, PCI etc.) can lead to
significant reduction in system cost. Currently, NDE hardware manufacturers have to develop a
complete instrument each time new product is introduced. It is envisioned that concentration on
the development of components with focused NDE functionality (e.g., ultrasonics) will have
great payoff. It would lead to substantially greater affordability of future instruments and to a
faster transition of NDE technology to commercial use. Since the 96 ASNT Fall Conference, the
author started holding Sessions on the topic of Robotics and Miniature NDE Instruments. The
intent of these Sessions is to attract industry and academia attention to the topic of developing
generic crawler similar to a PC motherboard as well as related plug-in modules. Recent
government interest in addressing the issue of NDE of corrosion turned the spotlight onto the
JPL's MACS crawler as a potential baseline for robotic multi-sensor platform for rapid scanning
of aircraft structures. In future generations of this technology, micro-electronic mechanical
systems (MEMS) is expected to lead to extremely small NDE instruments and scanners. Insect-
size micro-scanners may potentially crawl into aircraft engines and other hidden areas and
perform inspection or other maintenance tasks.
Inspection of hidden structures, composite materials and corrosion are still posing challenges to the NDE community. In the coming years, advancement in miniature electronics, actuators,
robotics, wireless communication as well as sensors are expected to make great impact on the
field of NDE. The search for smarter methods that can rapidly and inexpensively detect very
small flaws in complex materials and structures at very high probability and repeatability will
continue to be a challenge for NDE. Efforts will be made to further reduce the complexity
associated with inspection procedures, where redundant tasks will be performed by computers
leaving the role of the human operator to critical decision making tasks.
