Continuing trends toward reduced procurement of new aircraft is forcing the United States Air Force (USAF) to extend the operational life of its current aircraft. In the past, the USAF operator was able to replace fleet aircraft on a fairly regular basis. This process has been drastically altered by the significant reductions in the Defense Department budget as a result of the end of the Cold War. The requirement to extend the fleet's operational life is placing greater importance on the ability to find, characterize, and ameliorate the deleterious effects of operation and maintenance. In addition, many aircraft are being asked to operate with changed mission requirements that were not envisioned when they were originally procured. The life management of the aging fleet is interwoven with the ability to utilize nondestructive evaluation (NDE) to identify and characterize changes in the materials and structures throughout their lifetime.
Nondestructive evaluation has been used in civil aviation for 50 years. Until the arrival of the jet era it was mostly applied to component inspection. Since the damage-tolerant design philosophy was introduced by mandate for large transport aircraft, it has become an integral part of their design and maintenance. In the near future its role in the maintenance of aging small transport aircraft is expected to grow significantly. The most important factor contributing to the growing importance of NDE is the fact that the industry has been operating its aircraft much longer than originally envisioned, making it necessary to carefully monitor their structural condition to assure their airworthiness. NDE is helping making it economically feasible to operate aircraft for extended life times. Another major factor is the increased use of advanced materials, such as composites. Again, monitoring has to assure integrity. More recently, in an industry which has become highly competitive, NDE is becoming an important tool in the quest for reducing maintenance costs. The importance of NDE is expected to grow further.
The Science and Engineering Lab at McClellan Air Force Base, Sacramento, Calif. has been involved in the development and use of computer-based scanning systems for NDE (nondestructive evaluation) since 1985. This paper describes the history leading up to our current applications which employ eddy current and ultrasonic scanning of aircraft structures that contain both metallics and advanced composites. The scanning is performed using industrialized computers interfaced to proprietary acquisition equipment and software. Examples are shown that image several types of damage such as exfoliation and fuselage lap joint corrosion in aluminum, impact damage, embedded foreign material, and porosity in Kevlar and graphite epoxy composites. Image analysis techniques are reported that are performed using consumer oriented computer hardware and software that are not NDE specific and not expensive
Monitoring aging aircraft for hidden corrosion is a significant problem for both military and civilian aircraft. Under a Wright Laboratory sponsored program, Boeing Defense & Space Group is investigating three novel methods for detecting and monitoring hidden corrosion: (1) atmospheric neutron radiography, (2) 14 MeV neutron activation analysis and (3) fiber optic corrosion sensors. Atmospheric neutron radiography utilizes the presence of neutrons in the upper atmosphere as a source for interrogation of the aircraft structure. Passive track-etch neutron detectors, which have been previously placed on the aircraft, are evaluated during maintenance checks to assess the presence of corrosion. Neutrons generated by an accelerator are used via activation analysis to assess the presence of distinctive elements in corrosion products, particularly oxygen. By using fast (14 MeV) neutrons for the activation, portable, high intensity sources can be employed for field testing of aircraft. The third novel method uses fiber optics as part of a smart structure technology for corrosion detection and monitoring. Fiber optic corrosion sensors are placed in the aircraft at locations known to be susceptible to corrosion. Periodic monitoring of the sensors is used to alert maintenance personnel to the presence and degree of corrosion at specific locations on the aircraft. During the atmospheric neutron experimentation, we identified a fourth method referred to as secondary emission radiography (SER). This paper discusses the development of these methods.
Infrared computed tomography (IRCT) is a promising, non-contact, nondestructive evaluation tool used to inspect the mechanical integrity of large structures. We describe on-site, proof-of- principle demonstrations of IRCT to inspect defective metallic and composite structures. The IRCT system captures time sequences of heat-stimulated, dual-band infrared (DBIR) thermal maps for flash-heated and naturally heated targets. Our VIEW algorithms produce co- registered thermal, thermal inertia, and thermal-timegram maps from which we quantify the percent metal-loss corrosion damage for airframes and the defect sites, depths, and host- material physical properties for composite structures. The IRCT method clarifies the type of defect, e.g., corrosion, fabrication, foreign-material insert, delamination, unbond, void, and quantifies the amount of damage from the defect, e.g., the percent metal-loss from corrosion in metal structures, the depth, thickness, and areal extent of heat damage in multi-layered composite materials. Potential long-term benefits of IRCT technology are in-service monitoring of incipient corrosion damage, to avoid catastrophic failure, and production- monitoring of cure states for composite materials.
The successful application of emerging NDT technologies for specific aging aircraft inspections requires an integration of efforts between aircraft operators, airframe manufacturers, NDT equipment designers, and government regulators. This paper describes the development process that was followed to establish an alternate inspection technique for a DC-10 crown skin butt joint inspection. Initial investigation, intermediate development, and final evaluations are discussed.
It is now possible to reduce the costs and time needed to perform radiography of aging aircraft, space vehicles, and aerospace components. Recent advances in high resolution x-ray detectors, high fidelity low light level camera systems, image enhancement techniques, and robotic inspections have made the use of real time techniques more attractive from the standpoint of reducing film costs, reducing waste products from film processing, obtaining more reliable inspections, and gaining computer assisted interpretations. This paper summarizes ongoing efforts in support of the U.S. Air Force Wright Laboratories to develop a semiautomated high resolution digital real-time radiographic (HRRTR) inspection system to detect anomalous conditions on aircraft structures, turbine engine components, and composite materials. Examples of detection of corrosion and cracking in various aircraft structures using this system are reported.
Computed radiography incorporates innovative phosphor technology and scanning laser stimulated luminescence to convert X- or gamma- radiation into a digital file that can be arranged and viewed like a radiograph on a computer display. This relatively new approach to radiography represents a more affordable and practical alternative to film as an imaging process for some nondestructive inspection applications. This presentation describes the computed radiography (CR) `process' with special emphasis focused on the CR phosphor. The similarities and specific advantages that the CR process has over a film based system are presented. CR has been identified to be more practical and affordable than either film radiography or ultrasound in five aerospace NDT applications. Laboratory evaluations of these applications are discussed.
The McClellan Air Force Base's Nuclear Radiation Center (MNRC) consists of the most extensive aircraft neutron radioscopic facilities in the world. The neutron radioscopic facility's primary function is the detection of low levels of moisture and corrosion in aircraft structures. These inspections are accomplished using two independent but complementary systems. The first system is the Maneuverable Neutron Radiography System (MNRS). The MNRS is used to inspect intact aircraft structures for moisture and corrosion with minimal disassembly of the airframe. The system is comprised of two source manipulating robots, one being an overhead six-axis gantry robot and the other four-axis floor mounted robot. Each robot utilizes 50 milligrams of Californium-252 as the neutron source. The Californium-252 source is robotically manipulated around the aircraft to the desired inspection location. The second independent inspection system is based around a one-megawatt TRIGA nuclear reactor with four neutron beam tubes which terminate in four separate concrete bays. Two of these bays are each equipped with a six-axis component positioning system (CPS) robot and radioscopic imaging equipment. This combination allows for high throughput production inspection of aircraft components for moisture and corrosion.
The McClellan Nuclear Radiation Center's (MNRC) staff in conjunction with a Cooperative Research and Development Agreement (CRDA) with the U.C. Santa Barbara facility has developed a system that can be used for aircraft inspection of jet engine blades. The problem was to develop an inspection system that can detect very low concentrations of hydrogen (i.e., greater than 100 ppm) in metal matricies. Specifically in Titanium alloy jet engine blades. Entrapment and precipitation of hydrogen in metals is an undesirable phenomenon which occurs in many alloys of steel and titanium. In general, metals suffer a loss of mechanical properties after long exposures to hydrogen, especially at high temperatures and pressures, thereby becoming embrittled. Neutron radiography has been used as a nondestructive testing technique for many years. Neutrons, because of their unique interactions with materials, are especially useful in the detection of hydrogen. They have an extremely high interaction cross section for low atomic number nuclei (i.e., hydrogen). Thus hydrogen in a metal matrix can be visualized using neutrons. Traditional radiography is sensitive to the total attenuation integrated over the path of radiation through the material. Increased sensitivity and quantitative cross section resolution can be obtained using three-dimensional volumetric imaging techniques such as tomography. The solution used to solve the problem was to develop a neutron tomography system. The neutron source is the McClellan Nuclear Radiation Center's 1 MW TRIGA reactor. This paper describes the hardware used in the system as well as some of the preliminary results.
Neutron radiography has proven to be the most efficient nondestructive method for the detection of residual core material in air-cooled turbine blades. The method relies on the addition of gadolinium compounds to the core material and the subsequent thermal neutron radiography of the casting after the core removal process.
Over 80 percent of the inspections on large transport category aircraft are visual inspections. On small transport aircraft the ratio is even greater and on general aviation aircraft, virtually all inspection is visual. Visual inspection is usually the most economical and fastest way to obtain an early assessment of the condition of an aircraft and its components. Most of the defects found on aircraft are found by visual inspections, and the air frame manufacturers and users depend on regular visual inspections to ensure the continued airworthiness of their aircraft. Consequently, it is important that visual inspection methods be understood and properly applied by those responsible for the continued airworthiness of aircraft. Proficiency in visual inspection is crucial to the safe operation of aircraft. Such proficiency is gotten from experience, but also by learning the methods developed by others. This document outlines some of those methods and the way they are used in the various inspections carried out on aircraft.
The most common tool used by aircraft inspectors is the personal flashlight. While it is compact and very portable, it is generally typified by poor beam quality which can interfere with the ability for an inspector to detect small defects and anomalies, such as cracks and corrosion sites, which may be indicators of major structural problems. A Light Shaping Diffuser TM (LSD) installed in a stock flashlight as a replacement to the lens can improve the uniformity of an average flashlight and improve the quality of the inspection. Field trials at aircraft maintenance facilities have demonstrated general acceptance of the LSD by aircraft inspection and maintenance personnel.
An optical triangulation technique (D Sight) for high speed inspection of aging aircraft for lap joint corrosion is presented. Specific examples of using D Sight equipment on aircraft structures are presented along with comparisons to other NDE methods. The specific advantages of this technique are rapid inspection, high sensitivity, and computer based record keeping for future reference.
The in-flight structural failure of an Aloha Airlines 737-200 in April of 1988 brought international attention to the aging aircraft issue and prompted operators to improve inspection and maintenance procedures for their fleets. The use of nondestructive visual inspection equipment such as borescopes, fiberscopes, and videoimagescopes allow maintenance personnel to inspect internal aircraft structure for corrosion and fatigue without costly and time consuming disassembly. Some special purpose scopes have been designed for ultra violet applications using dye penetrant or even grinding and blending of corrosion from a remote location. These devices, coupled with sophisticated digital image processors, provide a permanent visual record of the inspection while allowing for three dimensional defect measurement, trend analysis, image enhancement, and video telephone link-ups. The use of this equipment can enhance and in some cases replace existing maintenance procedures, providing the most reliable and cost effective approach to aging aircraft program implementation. In cooperation with Dr. Richard Shagam of Sandia National Laboratories, sample inspections were accomplished at the Aging Aircraft NDI Validation Center (AANC) in Albuquerque, New Mexico on August 23 - 27, 1993. This paper provides the results of the testing along with detailed descriptions of each inspection procedure. Additional applications for visual inspection in aging fleet programs are seemingly endless and should be thoroughly investigated by program administrators throughout the industry.
Preliminary results of shearographic inspections of the shuttle external tank (ET) spray-on foam insulation (SOFI) and solid rocket booster (SRB) Marshall sprayable ablative (MSA-2) epoxy-cork thermal protection systems (TPS) are presented. Debonding SOFI or MSA-2 damage the orbiter `belly' tile and exposes the ET/SRB to thermal loading. Previous work with the ET/SRB showed promising results with shearography. The first area investigated was the jack pad close-out, one of many areas on the ET where foam is applied at KSC. Voids 0.375 inch were detected in 1.75 inch thick foam using a pressure reduction of less than 0.4 psi. Of primary interest are areas of the ET that directly face the orbiter tile TPS. It is estimated that 90% of tile TPS damage on the orbiter `belly' results from debonding SOFI during ascent. Test panels modeling these areas were manufactured with programmed debonds to determine the sensitivity of shearography as a function of debond size, SOFI thickness and vacuum. Results show repeatable detection of debonds with a diameter approximately half the SOFI thickness at less than 0.4 psi pressure reduction. Preliminary results are also presented on inspections of MSA-2 and the remote manipulator system (RMS) honeycomb material.
Laser shearography using real-time electronic image processing has been in use for the inspection of aircraft structures since 1988 when it was introduced on the USAF B-2 program for the nondestructive evaluation (NDE) of honeycomb control surfaces. Since that time, shearography techniques for NDE of aerospace structures have matured considerably to provide both more repeatable and quantifiable results but also broaden the applications considerably. This paper explores shearography techniques currently in use and applications on aerospace vehicles.
The advent of the B-2 program brought many unique and challenging demands upon the nondestructive inspection (NDI) organization. One of the most important was the verification of bondline integrity on bonded composite structural assemblies. Skin-to-core bondline integrity inspections of low `Z' composite materials used on the B-2 program presented formidable challenges to conventional NDI techniques. In addition to these difficult anomaly detection requirements, a projected high-rate of inspection (14,000 sq ft/month) necessitated that an innovative, rapid inspection method be implemented to support critical production schedules.
Aluminum honeycomb panels fabricated in accordance with spacecraft fracture control guidelines must be evaluated to a 90/95 POD/CL (probability of detection/confidence) level for detection of the critical initial flaw (CIF) size. Severe weight limitations can drive the CIF to a size of one cell diameter, or smaller. Additionally, producibility (low or no type II errors) must be maintained, and inspection costs minimized. To assure these goals, a reliability demonstration program was undertaken on thin skin panels for the Space Station Electric Power System ORU (orbital replacement unit) enclosures. This paper examines the probabilistic NDE process in detail, including: analysis of the manufacturing methodology, expected flaw types, construction of the disbond flaw data base, and the subsequent evaluations and results using laser shearography. The experimental data is then reduced utilizing the statistical methodology outlined in a proposed military standard for NDE reliability demonstrations, and contrasted against conventional through transmission ultrasonic inspection. The effort revealed that substantial gains in system reliability and flaw type discrimination are possible with laser shearography, along with a nearly order of magnitude reduction in inspection time.
Shearography is equivalent to a full-field strain gage which reveals flaws in materials by looking at flaw-induced strain anomalies which are translated into anomalies in the fringe pattern. This paper presents a technique that allows the phase distribution in shearographic fringe patterns to be automatically and precisely determined, thus eliminating the human interpretation of fringe patterns. Applications of the technique for automated nondestructive evaluation and strain measurement are demonstrated.
Controlled heating of a test specimen with a laser source provides several advantages for flaw detection using shearographic detection. This stressing method is non-contacting, can be localized, and allows defect information to be obtained while heating. In addition, the beam profile can be tailored to aid in the detection of different defect types. This paper presents results of simultaneous observations of material response to an applied thermal load using both TRIR and shearographic detection methods. Of particular importance is the demonstration that the depth of a defect can be determined by measuring the time-dependence of the shearographic fringe development during heating.
The primary inspection of composite structures is usually performed on composite components prior to assembly into a full structure. Inspection of these structures after assembly verifies the quality of the structure before it is placed in service, and can provide a baseline for future in- service inspections. This paper addresses the concerns, problems, and solutions that have been addressed for post-assembly inspections at McDonnell Douglas.
Pulsed infrared imaging (PII) has become a prominent technique for large area, non-contact inspection for many types of advanced materials and structures. This paper discusses applications relating to the evaluation of repaired composite materials.
Substantial growth potential for composite materials exists in the private sector, where high volume, low cost production is required. Post processing inspection can represent a significant percentage of the cost of composite products. Alternate ways of assuring quality must be examined. In-process inspection can be easily adapted to continuous composite manufacturing techniques such as the pultrusion process, enabling 100% inspection. Recent research efforts at the Center for Composite Materials has focused on ways of evaluating pultruded composites on-line by using ultrasonic non-destructive evaluation (NDE) techniques. The most accurate method to date is based upon Lamb wave velocity measurements. This inspection technique uses two transducers positioned at normal incidence to the composite in a through-transmission mode to generate and receive ultrasonic waves which propagate through the sample. Careful processing of the waveform data reveals the degree of porosity in the composite samples. A series of tests were performed in-situ on an actual laboratory scale pultrusion process which accurately predicted the porosity in a 6.4 mm multiplied by 3.2 mm (0.25' multiplied by 0.125') cross-section pultruded rod over a range of 0.5% to 12% void volume fraction.
An alternative to conventional computed tomography (CT) inspection of solid rocket motor (SRM) component bondlines provides high radial resolution at critical bondlines for large SRM components. By concentrating measurement, computation, and radial resolution at the periphery of the test specimen, this new technique promises to improve bondline anomaly detection by approximately a factor of ten. This technology and a project to demonstrate it in a full-scale implementation using a variety of real-time radiography (RTR) acquisition devices and test articles are discussed.
It is well known that the fiber matrix interface plays the dominant role in the mechanical behavior of advanced composite materials. Engineering and control of the interface is paramount to producing the properties for desired mechanical performance of the ultimate composite structure. This is particularly critical for advanced composites (metal matrix composites, ceramic matrix composites) which are currently being investigated for structural applications in aerospace and other structures. In this work, we describe a technique for in situ interfacial characterization and determination, which is demonstrated for in-process analysis and mechanical behavior studies of composites. The methodology described herein is based on an optical analysis of a guided ultrasonic signal response, but with appropriate technology, the fundamental parameters used in this study can be effectively applied to other conventional and non-conventional detection systems. The approach for analyzing the process cycle of composite materials, in-line utilizes the multi-functional use of fibers as reinforcement and sensors. The fact that typical reinforcing fibers can be effectively used as ultrasonic wave guides implies a mechanism for transfer for ultrasonic information along the length of the fiber-matrix interface. Ultimately, this information can be fed back to the processing line to update the process for optimization and affordability of manufacture.
Computed tomography (CT) systems have the ability to rapidly and nondestructively scan parts and extract part contours, independent of material type and/or surface condition. Under the correct conditions, these part contours are dimensionally correct and can be used to extract metrological information suitable for determining dimensional conformance or creating files readable by computer-aided-design (CAD) systems. Important CT-assisted reverse engineering and part characterization capabilities are being developed by ARACOR for the Advanced Research Projects Administration (ARPA) to support its solid freeform fabrication (SFF) initiative. SFF refers to the machine capability to convert an electronic master of a part into a solid object of near-net shape without part-specific tooling or other specialized operator intervention. Application software specific to advanced composite materials in general, and ceramic materials in particular, is being developed for the SFF program. The new software will run on a variety of common workstation platforms, accept data from different CT scanners, and output results in various formats to support a variety of engineering and manufacturing needs. An overview of CT-to-CAD technology is presented, and work in progress relevant to emerging aerospace composites is reported.
The U.S. Air Force has over 100 airfields located in various climates around the world. Many of the pavements at these bases are nearing the end of or have exceeded their design life. The Air Force implemented a pavements management program, primarily using the falling weight or heavy weight system to insure the pavements are properly maintained and can support existing and projected missions. The management system includes structural evaluations, friction characteristics testing, conditions surveys and MICRO-PAVER, a computerized program to manage the pavements. This paper only discusses structural evaluations.
This paper discusses the equipment and methodologies currently used for nondestructive testing (NDT) and nondestructive evaluation (NDE) of the structural capacity of military and civil airport pavements, including: (1) commonly used equipment and test methods for measuring pavement response to dynamic loads; (2) qualitative and quantitative evaluation of NDT data; (3) methods for back-calculating layer properties from NDT data; (4) layered elastic methods for evaluating pavement performance using processed NDT data; and (5) application of analytical results for developing pavement rehabilitation and management strategies.
Current design criteria for rigid pavements for commercial and military airfields assume that 25% of the load applied to an edge of a slab is transferred through the joint to an adjacent unloaded slab. A nondestructive testing technique using a falling weight deflectometer (FWD) was used to conduct field testing at a number of sites. A transfer function, developed from an analytical study, was used to estimate load transfer from the measured joint efficiency as a function of the loaded area and the radius of relative stiffness of the pavement. This procedure, although analytically sound, lacks actual field verification at an instrumented pavement site. This procedure was used to estimate load transfer at a number of commercial and military airfields for a variety of joint types, climate conditions, and pavement structures. The results of these tests indicate that the assumption of load transfer as a constant value of 25% appears to be unconservative, especially during the winter months.
A review is presented of past and current technology used to measure pavement deflection. The configuration for an improved system for deflection measurement which is currently under development is presented. The rolling wheel deflectometer (RWD) based on scanning Lidar profiling is described and rationale and proposed procedures for RWD use and data analysis are presented. Also included are projected calculation procedures for backcalculation of layer moduli from a rolling wheel load.
A laser based deflection tester is being developed by the Swedish National Road Administration. Forty sensors are mounted on a heavy truck to determine two transverse profiles. One profile constitutes an unloaded case. The other profile just behind the rear wheels of the vehicle constitutes the loaded case. The high sampling rate is adequate for filtering the macro texture of the pavement.
A prototype continuous deflection device, referred to as a rolling weight deflectometer (RWD), has been developed as a nondestructive evaluation tool for airfield pavements. The system consists of a rigid trailer equipped with specially designed optical triangulation pavement sensors, a high-speed data acquisition system, and a high-pressure tire/load platform assembly. Pavement sensors are mounted on a rigid box beam equipped with an internal sensor system that corrects, in real time, the relative pavement height position measurements for displacements induced in the beam by mechanical vibrations, changes in temperature, or nonuniform dynamic loads at points where the beam attaches to the frame. The device produces continuous deflection profiles that show pavement response to a moving loaded wheel along the path of travel. These deflection profiles, combined with multiple passes along a lane, provide a far more detailed picture of the pavement structural integrity than has ever before been possible, because existing evaluation tools only produce response information at discrete points. Preliminary results show deflections measured by the RWD are in general agreement with the expected pavement response for various loads. A discussion of the device configuration, preliminary data, and potential as a pavement management tool is presented.
A ground penetrating radar (GPR) system has been developed and implemented for the U.S. Air Force's pavement evaluation teams. This tool can be used to rapidly, continuously, and non-destructively perform feature delineation, anomaly detection, and layer thickness measurements for airfield pavements. The layer thickness function has been automated. The basic GPR hardware, a Geophysical Survey Systems Inc. (GSSI) SIR-8 with three sets of antennas (500 MHz, 900 MHz, and 2.5 GHz), is mounted in a full-size van and has been extensively modified. An additional SIR-8 controller was installed to allow simultaneous operation of two antenna sets. Direct data capture, on-board digital data storage, and real-time analysis and display capability has been provided by installing a 486 computer and color printer, then connecting them directly to the antenna data lines. Distance markers are also embedded directly into the data stream. Layer thickness measurement is accomplished by combining the dielectric constant obtained from the 2.5 GHz antennas (using the surface reflectivity ratio method), with the 2-way layer travel time measured by the 900 MHz antennas. Automated layer thickness determination techniques have been developed for analyzing data from the ground-coupled antennas that provide the deeper penetration necessary on thick airfield pavements. Automatic data processing software has been developed that performs the direct data capture and calculations necessary to provide real-time output of dielectric constant and layer thickness versus distance travelled. The Air Force GPR device has been successfully used on several airfields to date, demonstrating it can be used to differentiate between pavement features of significantly different thickness, detect anomalies such as voids under pavement and water intrusion, as well as accurately measure the surface layer thickness. Several comparisons of core-measured versus radar-predicted thicknesses have been made, on both PCC and AC, with good correlation.
Based on laser optics, speckle interferometry is sensitive to very small changes in displacement or deformation. Electronic speckle pattern interferometry (ESPI) combines optics and electronics together to provide full-field observation and analysis with accuracy and sensitivity in a sub-micron range. As a state-of-the-art non-destructive testing/evaluation technology (NDT/NDE), electronic speckle pattern interferometry (ESPI) was successfully used to detect very small cracks in concrete. The fast image processing made it possible to conduct real-time testing on concrete structures. The laboratory experiments have also shown that it is possible to bring the ESPI technology to field applications, such as NDE of aging airport pavements, bridges, and other aging infrastructure.
An overview of important nondestructive evaluation considerations necessary for safe life qualification of aerospace propellant tanks is presented. Options for tank qualification without performing NDE are discussed. The concept of initial flaw size limits and their relationship to fracture mechanics analysis are compared to the capability of various NDE methods. It is concluded that it may not always be possible nor desirable to demonstrate safe life using NDE.
Standard nondestructive (NDE) methods, borescope, x ray, dye penetrant, ultrasonic, and eddy current are used to detect flaws generated from the extreme environments that the Space Shuttles encounter. Each of the four space shuttles, Columbia, Discovery, Atlantis, and Endeavour, are required to have a structural inspection performed at the Rockwell Space System Division, Palmdale facility. This paper describes the NDE methods used at Palmdale during the orbiter maintenance down period (OMDP) of the Space Shuttle Atlantis.
Sleeve bolts, which can provide an interference fit in a cylindrical hole by expanding during assembly, were used on an aerospace structural joint. Ultrasound was used to take measurements during preload application and monitoring. Ultrasonic bolt gages have commonly been used on regular bolts and studs, but not sleeve bolts. Ultrasound preload measurement on a sleeve bolt is different in many ways from that on a regular bolt. The pressure and friction on the sleeve bolt shank are absent or insignificant for regular bolts. Pressure and friction change depending upon initial conditions and applied load. The two factors affect the stress distribution inside the bolt and the ultrasound measurements. No published material was available for ultrasonic measurement of preload in sleeve bolts. The manufacturers of both the SLEEVboltR (PB Fasteners, Inc.) and the ultrasonic bolt gage were asked for information about this application. Since they had no experience in this application and were unable to help, the author developed the application descried in this paper.
A time-dependent term is introduced into the Arrhenius rate equation to refine the lifetime projections for materials that do not satisfy the linearity assumptions of the theory. This new term describes those materials where properties decay exponentially during the early phase of their exposure to elevated temperatures and subsequently equilibrate to almost constant values. The method is applied to the acoustic-absorber polyimide foam, `Solimide,' resulting in lifetime predictions for specific strength that are believed to be more accurate and reliable than predictions with the linear theory. Furthermore, the entire data set was included in a single nonlinear regression method which assured an orderly temperature dependence and smoothed out uncertainties in the measurements.
A controversy exists within the aerospace community as to whether etching of components before dye penetrant testing to remove metal smear is always necessary. A test program was initiated to detect the degree of metal smear present in aluminum alloy 2024-T851 after machining by various techniques. Use of the scanning electron microscope confirmed the presence of metal smear. In addition, dye penetrant tests were performed before and after a chemical etching process that removed approximately 0.0005 in. It was determined that etching always improves flaw detectability, even for cracks that have not been smeared.
Performance of the International Space Station Alpha (ISSA) United States On-Orbit Segment (USOS) Electric Power System (EPS) will be degraded through the mission life of the station. The power generation photovoltaic array and thermal control radiator will be directly exposed to the natural environment and the environment induced after the station is built. These environmental effects result in lower array current and voltage output as well as lower radiator heat rejection capability. Aging is the major cause for the energy storage nickel-hydrogen (NiH2) battery performance degradation. Over time, there is an increase in the internal impedance, which results in a decreased efficiency as the battery ages. Design of the ISSA EPS takes into consideration the various equipment degradation modes, to make it compatible with the environments and to meet power, lifetime, and performance requirements.
In the early 1970s the National Aeronautics and Space Administration sponsored a series of studies designed to quantify the reliability of the common nondestructive inspection methods. The results of those studies became the basis for the assumed minimum initial crack sizes contained in NASA fracture control requirements and the fracture analysis software, NASA FLAGRO. The origin of the assumed initial crack sizes is described along with the current program for verifying inspection reliability. The suitability of the current program is assessed and options for improving the program are discussed.
NASA has recently initiated a reassessment of requirements for the performance of in-space nondestructive evaluation (NDE) of the International Space Station Alpha (ISSA) while on- orbit. given the on-orbit operating environment, there is a powerful motivation for avoiding inspection requirements. For example the ISSA maintenance philosophy includes the use of orbital replacement units (ORUs); hardware that is designed to fail without impact on mission assurance or safety. Identification of on-orbit inspection requirements involves review of a complex set of disciplines and considerations such as fracture control, contamination, safety, mission assurance, electrical power, and cost. This paper presents background discussion concerning on-orbit NDE and a technical approach for separating baseline requirements from opportunities.
Spacecraft hardware such as radiators requires the maintenance of solar absorptance within tight bounds for their design life. Such hardware is sized in part based on the beginning- and end-of-life absorptance. It has been difficult to make accurate end-of-life determinations based on either ground based data or flight data. The synergistic effect of atomic oxygen, ultraviolet radiation, and contamination has made it difficult to duplicate space exposures in the laboratory. The absorptance of flight exposed samples brought back to earth are not representative of the conditions in space because of changes brought about by exposure to air. This paper proposes to augment the current in-space monitoring techniques with periodic, in- space, direct measurements of the solar absorptance on operational hardware. NASA funded AZ Technology to develop a portable, space-rated device similar to the LPSR-200 portable spectroreflectometer, a space portable spectroreflectometer (SPSR). This instrument is robotically compatible and can be run using spacecraft power or batteries. The instrument also has measurement storage capacity for later retrieval and evaluation. Although extensive development work has already been completed, authorization to build a unit for a flight experiment has not been received. The Russians have expressed an interest in having absorptance measurements made on their MIR I Space Station as part of the NASA/MIR flight experiments. Proposals are currently being made to obtain authorization for the construction and use of SPSR on NASA/MIR flight experiments, to help mitigate potential problems for the International Space Station Alpha (ISSA).
In response to a need for a better understanding of aging systems, the Nondestructive Testing Information Analysis Center (NTIAC) is conducting an analysis and evaluation of relevant scientific and technical information on the role of NDE in accomplishing life management of aging systems. Information is being identified and acquired through literature searches of appropriate bibliographic databases and through personal contact. This information is being organized into a computer searchable database on life management of aging systems that will enable engineers, scientists and mangers to readily locate information most useful to their area of interest, to characterize relationships among various factors affecting life management of aging systems, and to identify technology gaps that need to be filled.
Two distributed fiber optic sensors for use in the prevention and monitoring of corrosion in aircraft are described. These sensors, based on optical fibers that are intrinsically sensitive to either water or changes in pH, will alert maintenance personnel to the presence of water in lap joints and other inaccessible critical areas. Furthermore, the sensors can also locate precisely where the moisture infiltration has occurred. In a typical application, a sensor fiber would be embedded in a lap joint along the bottom panel of an aircraft's body, or on a wing, where water is likely to collect. Changes in the optical transmission through the fiber can be monitored either periodically or continuously to determine the extent of water penetration.
Shearography inspection techniques have been developed and implemented for the inspection of aluminum honeycomb turbofan aircraft engine fan cases for the JT15D-5D. Shearography has yielded improved sensitivity to unbonds and throughput over ultrasonic techniques formerly used in the production inspection. This paper discusses vacuum stress shearography, test method verification on the JT15D-5D fan case and shearography data correlation with destructive evaluation of test parts.
I am pleased to welcome you to this timely and informative conference organized by The International Society for Optical Engineering (SPIE). As Conference Chair, I am very pleased, but not at all surprised, by your interest and enthusiastic response: Over 300 participants are contributing over 210 technical papers to six topical conferences
The infrastructure in the United States and the world is aging. There is an increasing awareness o the need to assess the severity of the damage occurring to our infrastructure. Limited resources preclude the replacement of all structures that need repairs or have exceeded their lifetimes. Methods to assess the amount and severity of damage are crucial to implementing a systematic, cost effective approach to repair and/or replace the damaged structures. The challenges of inspecting aging structures without impairing their usefulness rely on a variety of technologies and techniques for nondestructive evaluation. This paper will briefly describe several nondestructive evaluation technologies that re required for inspecting a variety of systems and structures.
the role of the National Laboratory in technology transfer is to help industry implement the latest technology. The National laboratory must have a unique capability that does not compete with tax paying companies. Typically a National Laboratory will demonstrate the benefit of applying NDE to a company's production line. The company then contracts with a vendor to supply a production inspection system based on the laboratory's demonstration. Both the company and the vendor benefit from the participation with the National Laboratory. The company improves the quality of its products and the vendor has a new product to market. The purpose of this paper is twofold: (1) to present a brief overview of NDE capabilities and application activities within the National Laboratory System and (2) to identify NDE point-of- contacts at each laboratory.
A number of NDT technology transfer programs now exist within the aviation industry. The Aging Aircraft Research Program was initiated to enhance airline inspection capability. This paper addresses NFT technology integration for commercial aircraft operators, and critical issues such as research subject identification and project management.