|NDT.net - February 2000, Vol. 5 No.02|
|TABLE OF CONTENTS|
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 [email@example.com], 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/ ].
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.
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.
|Fig 7: An aging aircraft with well-documented flaws at AANC(Sandia National Labs).|
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.
|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.|
|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.|
|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.|
|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.|
|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.|
|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||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|
|Ply gap||Degradation depends on stacking order and location.|
For [0,45,90,-45]2S laminate:
|Porosity.|| Degrades matrix dominated properties
|Thermal Over- exposure||Matrix cracking, delamination, fiber debonding and permanent reduction in glass transition temperature|
|Table 2: Effect of defects in composite materials|
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].
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:
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.
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