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NDT - A Continueing Responsibility in the Future of AerospaceGary Georgeson
Manufacturing Research & Development
Maintenance, Modifications & Repair
Boeing Phantom Works
Seattle, Washington, USA.
As the aerospace industry soars into the 21st century, the NDT community that supports it faces a variety of exciting challenges. These challenges include the manufacturing evaluation of the newest aerospace structures and materials and the in-service inspection and monitoring of aging aircraft. In addition, the aerospace industry is heading toward a cradle-to-grave approach to aircraft maintainability that will require ongoing advances in NDT. The traditional "find and fix" approach, where damage is found on an aircraft using NDT and immediately fixed, will be replaced with a "predict and manage" approach, where damage is predicted using models, and managed with the support of NDT over a period of time. The damaged structure is repaired or replaced only when the damage reaches a threshold. This paper suggests how NDT technology and application must grow and change in order to effectively support the ever-evolving aerospace industry.
Keywords: Non-Destructive Testing, Non-Destructive Evaluation, In-service Inspection, Ultrasonic, Composites
Nondestructive testing/evaluation (NDT/E) currently plays a significant role throughout the lifetime of an aircraft. NDT involvement begins at the research and development stage, with the evaluation of new materials and structural designses. NDT methods are also often utilized at various in-process manufacturing stages. For example, a multi-layer core blanketa septumized core assembly might be inspected for flaws before being bonded to a skin or other structure. Post-fabrication inspection of production hardware requires some form of NDT. This may be a simple visual inspection, but often requires specialized equipment with trained inspectors. After an aircraft enters service, aging effects brought on by the flight environment, fatigue, or operational extremes may cause faults anomalies to develop. Periodic NDT is required to help quantify these effects and to determine if the aircraft is fit for service. Over time, NDT continues to be an assessment tool until the replacement of the part, or retirement of the entire aircraft.
Currently, the aerospace industry utilizes a variety of NDT methods for in-process and post-fabrication inspection. For metals inspection, visual, fluorescent dye penetrant and film x-ray are the most common methods of choice. Eddy current is used for crack inspection and temper validation and ultrasonic testing is conducted on structures not conducive to x-ray techniques. Magnetic particle inspection is utilized on magnetic materials. Composite structures are inspected primarily with ultrasonic and film x-ray techniques, but infrared thermography and shearography/holography are becoming more common.
NDT costs are generally quite high, so it makes sense, as we look toward the future, that we will see the development of more affordable NDT methods for production inspection. Some of the costs can be saved by putting NDT in the earlier stages of a manufacturing process until that process is controlled enough to reduce or eliminate that inspection stage. One NDT technology that can be expected to make some headway is airborne ultrasonics. Recent advances in transducer technology and signal processing have boosted the signal-to-noise ratio for airborne UT, and made it a more viable inspection technique for composites. This is particularly true for in-process inspection, where water or other couplants used in traditional UT can contaminate the structure.
NDT costs will also be saved by modifying or upgrading NDT systems. Advances in computer processors and data storage are allowing us to do more with less. For example, Eddy current arrays, which reduce the need for raster scanning, are replacing single eddy current transducers. Much more area can be covered in the same amount of time. Another arena in which change is expected is in the transition from film X-ray to non-film digital methods. Digital Radiography will significantly reduce inspection costs associated with film use, setup time and hazardous waste (due to chemicals used in film processing). Additionally, the increase in non-film methods will reduce the current need for film storage. Boeing is leading the American Society for Testing and Materials (ASTM) action committee towards developing electronic digital reference images for use with non-film X-ray inspection systems. These images will allow manufacturers to inspect production hardware such as castings using non-film digital technology.
The aerospace industry is increasing its utilization of castings in general, as a way to reduce production and assembly costs. The castings are becoming larger and more complex, and more difficult to inspect. Traditional film x-ray is not always sufficient. For example, structural titanium castings are difficult to inspect because of their large grain structure and often complex shapesgeometries. Phased array ultrasound - developed for use initially in the medical industry - is gaining popularity in the aerospace industry. Almost all UT inspection today is based on probes which produce fixed ultrasonic beams. Phased array probes have beams which that sweep through the inspection volume, allowing highly detailed real-time images to be produced. Potential advantages of phased array UT include improved defect sensitivity, more accurate defect sizing, and easier-to-interpret ultrasonic signals (displayed in an image format). Recent developments with phased arrays mean that they are now available for wider use.
Many new manufacturing methods are being considered for aerospace applications that produce structures withgenerate significant cost and performance advantages. These new structures often have special NDT requirements which that cannot be met with current NDT techniques. For example, aerospace companies are taking a real look at the utilization of bonded primary structure as a way to save on the costs and weight of composite aircraft. Historically, the use of bonded structure has been limited to non-critical parts of an airplane. While flaws such as voids or disbonds within a bondline could generally be found, a weakness in the bondline could not. A bond could appear "good" using traditional NDT methods, and yet be perilously close to falling apart. Although there has been some limited success in specific cases, no general NDT methods have proved successful for bondline assessment. The aerospace NDT community is currently studying bondline characterization and flaw detection using alternative ultrasonic methods (such as guided wave or plate wave UT). Other techniques, such as laser shearography, and ESPI, and holography, are being studied which involve stressing the bond and observing the response. The extent to which bonded structure will be applicable in the future will depend upon the effectiveness of NDT methods for verifying the integrity of the particular type of bond being used.
In general, the need for alternative methods will tend to push NDT technology ahead. The research and development being done in the private and public sectors should yield some surprising new NDI techniques in the years to come.
In-service NDT faces a different set of challenges than those associated with manufacturing inspection. First of all, access to the part under inspectionbeing inspected is generally limited, because it is attached to other hardware. Disassembly of the component to be inspected is always time-consuming and costly, and has to be avoided whenever possible. The aircraft must be inspected thoroughly, and structural damage and material aging must be quantified and monitored for repair or replacement.
Another challenge for in-service inspection is that, in addition to planned maintenance inspections, new in-service inspection procedures will be added to an aircraft over its lifetime. Inspection procedures must be developed quickly after the initial detection of an aircraft fault. Also, personnel working on in-service inspection problems must develop and apply NDT techniques, compatible with limited airline or military maintenance capabilities. Issues of cost-effectiveness, reliability, and credibility of the inspection data also come to bear in transferring inspection technology to maintenance personnel.
Two years ago the FAA initiated a program known as the Safer Skies initiative, to reduce the occurrence of fatal aviation events by 80%. One of the programs in the initiative requires in-service inspections of the safety critical turbine engine parts each time they are fully disassembled. Turbofan engines contain heavy, rapidly rotating parts. If one of these parts cracks and fails, it can lead to an uncontained failure. A review of the last 15 years of aviation data revealed that a significant number of the parts that failed were either not inspected at prior overhaul shop opportunity or were inspected incorrectly. To remedy this situation the FAA, in partnership with engine manufacturers, airlines and regulatory agencies, has developed a prioritized approach for mandating in-service inspections of the most important features on the most safety critical parts. These requirements affect the entire US fleet of turbofan powered large commercial aircraft. Most of the newly developed inspections use eddy current or focused fluorescent penetrant (FPI) techniques. FPI continues to be the wide field method of choice for these parts eddy current inspection is often used for the inspection of critical features and deep holes. These requirements affect the entire US fleet of turbofan powered large commercial aircraft. Most of the newly developed inspections use eddy current or focused fluorescent penetrant (FPI) techniques. FPI continues to be the wide field method of choice for these parts eddy current inspection is often used for the inspection of critical features and deep holes.
The most common in-service inspection issues in the aerospace industry revolve around cracks or corrosion in metals, and impact damage or material degradation in composites.
Cracks in metals have historically been inspected using visual, dye penetrant, x-ray, ultrasonic or eddy current methods. As the commercial fleet ages, however, we are finding the need for development of new NDT techniques. One of the challenges facing us is the detection of cracks beneath external repair doublers. Also, we need to be able to readily detect cracks in multi-layer or thick structure. In general, increasing sensitivity to smaller cracks will lead to larger inspection intervals, and therefore, lower costs.
The infamous Aloha Airlines incident of 1988, and similar structural failures of aircraft have precipitated industry efforts to address widespread fatigue damage (WFD). WFD is caused by multiple small cracks that link up, producing long cracks. The small cracks are generally sub-detectable by traditional means. Since there is not yet any In general, current NDT methods are not effective NDT method for detection of WFD, so mandatory modification of susceptible areas that are susceptible to it must be modified, just in case it occursis an approach being contemplated. The FAA has chartered a working group to study the problem.
There are a variety of emerging electro-magnetic NDT methods that are being assessed for sub-surface crack and WFD detection. Among them are pulsed and multi-frequency eddy current, magneto-resistive sensors, EMATS, SQUIDS and meandering wire magnetometer arrays.
As the fleet ages, corrosion detection is becoming increasingly important in the commercial airline industry. Eddy current, film radiography and IR thermography have shown to be effective, but there is a need for accurate quantification of the extent of corrosion, and to find corrosion hidden under attenuative material or multiple layers. The NDT methods that prove to be successful for corrosion quantification will eventually dominate commercial inspection.
Impact damage is a common problem for aircraft, due to hail, tool dropsdropped tools, and runway debris that is kicked up by the wheels. Impacted composite structure is prone to interply delaminations and skin-to-core disbonds. Finding impact damage in metals is often a simple visual task-just look for the dent! Impact damage in composites, however, can hide under undamaged material. Boeing and Airbus require only visual inspections to detect impact damage in honeycomb composites. The simple coin tap or tap hammer test is often used to find hidden impact damage or determine the limits of visible the damage. However, this method is not only time-consuming, but it is subject to inspector error, limited by noisy environments, and provides no quantitative data. An electronic tap hammer, developed and liscensed by Boeing several years ago, give provides a digital read-out of the tap duration, providing a quantitive measure of the local stiffness. Hand-held pulse/echo ultrasonic testing is used to find and size delaminations in solid laminate skins. Low frequency bond testers, such as the Sondicator and Bondmaster are very effective with finding skin/core disbonds in composites. Laser Shearography and Thermography are also very effective in identifying disbonds or delaminations in composites with thin skins.
Automation is also slowly making headway into in-service inspection. The Mobile Automated Scanner (MAUS IV) developed by Boeing in St. Louis, was designed to be a platform for a variety of NDT sensors. The MAUS allows on-aircraft rastered scanning of an otherwise hand-held method, such as resonance UT, pulse echo UT, or eddy current. Instead of having just point data to review, the inspector is able to look at a 2-D data image of the region containing the flaw.
Material degradation is an issue with the life-extension of composite structures, which may not indicate degradation visually. Heat Damage can reduce the strength of a composite structure. Microcracking can occur over time due to mechanical or thermal stresses.
Composite repair inspection is also becoming an important issue. Composite patches made of boron/epoxy are currently being used on aluminum aircraft. Although IR thermography is effective with thin patch repairs of this type, in general, no effective means of repair inspection exists aircraft, as well as composite structure. Traditional composite-to-composite scarf repairs and metallic-to-metallic bonded patches also suffer from the lack of an effective means of evaluation. This is indeed a significant NDT issue. InIn order for patches to be more utilized, however, development is needed for NDT techniques which that quantify patch goodness and adhesion.
The face of aerospace in-service NDT is changing. There are some exciting trends that are worth mentioning.
Hand-held inspection is time-consuming and costly. It can also be physically dangerous for the inspector. Automated inspection, using a robot of some type to traverse the inspection area, will become more and more commonplace. The implementation of robotic inspection platforms allows remote access and rapid, large area inspection.
Scanners, crawlers, and robots are being developed to enable automated inspection and imaging. Much developmental work is needed. Position sensing/tracking techniques are required, as is automated defect recognition, documentation of the scan area and defect location.
Much of in-service inspection is heading towards non-destructive monitoring (NDM) using sensors of various types on board the aircraft. The airframe, engines, and various components will be fully monitored. Key structural parameters can be assessed without costly downtime. Damage-prone or limited access area will contain "smart sensors." Corrosion conditions, such as moisture, temperature, PH, corrosion products, and cross section loss will be measured with interactive models and sensors. Various NDI NDT modalities could be collected and integrated to provide the best assessment possible. This would be followed by periodic downloading of NDT data that is then analyzed and compared with earlier results. The number of costly inspections can be reduced.
An example of a step in the on-board monitoring direction is Lockheed Martin Aeronautical Systems' use of multi-channel acoustic emission instrumentation to monitor crack growth on a full-scale P-3 Orion aircraft undergoing spectrum fatigue cycling. (The system, called AESMART 2000, is built by Dunegan Engineering Consultants.) The primary objective is to detect crack initiation and analyze crack growth rates in complex components and in areas of the aircraft that are difficult or impossible to inspect by traditional methods.
Original and in-service inspection data will eventually be combined with on-board monitoring data to provide a complete picture of all flight-critical hardware. NDT techniques that are capable of collecting a great deal of data over a large area, can be used. The expansion of computer media storage and increases in processing speed will allow handling of very large databases, which will follow the aircraft from cradle-to-grave. Access to this "digital airplane" would allow efficient and timely logistical support.
Another area that appears to have tremendous potential is the computer modeling of inspections. Substantial cost savings can be experienced if inspection feasibility can be determined beforehand. In order to improve reliability, we must develop probability-of-detection data and analyses for NDT methods and emergent applications. Modeling will allow defect detectability assessment (What size and shape defect can we see at various depths?) and method optimization (What method, techniques, and specific test set-up will work the best?). It can also be used at the beginning of the manufacturing cycle, to help designers engineers design inspectable aircraft. Modeling and analysis can reduce or eliminate situations where certain locations cannot be cost-effectively or adequately inspected.
New NDE Technologies
We will certainly continue to develop better NDT technology. NDT must play an ever-increasing role in ensuring aircraft are safe to operate throughout their lifetime. We will continue to develop appropriate inspection technology for in-service inspection requirements at the lowest possible cost. This requires the ongoing investment in research and development R&D to ensure the development of new inspection technologies for in-service monitoring and inspection.
Wide area inspection techniques should emerge which provide more data at a faster rate. This will reduce both the length and frequency of downtimes. In-service inspection will also benefit from more portable but automated inspection systems that enable inspections to be made more efficiently. The use of scanning platforms, such as the MAUS, will allow digital images to be collected and compared from a variety of emerging technologies.
There are many potential non-traditional in-service applications for NDT as well. Some of the potential applications include aircraft subsystems, brakes, wire bundles, and air ducts. These and other inspection needs will continue to drive the development of new NDT technologies. However, in order for these new technologies to be useful for in-service inspection, they must ultimately be cost effective and relatively easy to use. Not all maintenance or depot facilities will be able to afford emerging NDT technology, if it requires significant training, higher cost personnel, or expensive equipment.
There are many challenges and opportunities ahead for those of us in the NDT community who support the aerospace industry. Production NDT will be needed to ensure part quality of an ever-widening array of complex structures. In-service NDT must take some leaps forward in order to keep aircraft in the air longer and to improve passenger safety. It is an exciting time to be working in our field!
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