Finding Flaws Without Causing More: The Art and Science of Nondestructive Evaluation
Eric C. Johnson and Oscar Esquivel
A mixture of insight and innovation helps detect hidden defects in space system components, safely and reliably.
The launch process is severely taxing for the countless components and assemblies found aboard spacecraft and launch vehicles, and space is a difficult environment for many materials. Invisible flaws, such as impact damage or a partially bonded joint, can lead to expensive failures during launch or on orbit, where components generally can't be fixed. Detection of flaws is therefore essential. Moreover, flaws need to be discovered in a nondestructive manner—that is, the process used to test for them must not damage the potentially unflawed component being tested.
Eric Johnson performs an ultrasonic inspection to detect flawed regions in a Kevlar composite overwrapped pressure vessel. The reverberations of transmitted sound pulses are monitored on an instrument display screen; internal flaws are distinguished by intermediate echoes, scattering, or interruption of the acoustic signal. |
The process of flaw detection and characterization is known as nondestructive evaluation, and it's an important part of ensuring the flightworthiness of launch vehicles and spacecraft. It begins with inspection of raw materials, both to check material characteristics and to screen for incipient flaws. It continues through manufacture and assembly as a means of process control and quality assurance. It concludes with the final check of components for damage incurred during shipping, storage, and handling.
Various types of nondestructive evaluation can be applied, depending on the component and material. Under ideal circumstances, the failure modes have already been identified and destructive tests have been performed to establish inspection criteria (e.g., what type and size of defects must be found). Critical inspection parameters include the geometry and constitution of the part and the type of defect to be detected. Choosing the best approach for a given application often requires more familiarity and expertise than a typical contractor might possess. For instance, the task of searching for cracks in a material can be approached from various perspectives. If surface cracks are the primary concern, a penetrating dye can be applied to make the cracks more visible. To check for cracks just below the surface, an eddy-current probe can be used to look for the changes in conductivity associated with subsurface flaws (assuming the material is conductive). To look for even deeper cracks—in a weld line, for instance—the best approach might be to direct ultrasonic shear waves into the weld and look for characteristic echoes.
Aerospace maintains the equipment and expertise needed to conduct these and many other nondestructive evaluation techniques. In fact, some nondestructive tests now routinely used in the manufacture of launch vehicles and spacecraft originated at Aerospace.
Testing with Sound and Radiation
Many nondestructive evaluation techniques involve sending some form of energy into a specimen and analyzing the changes that result. In the aerospace industry, two of the most widely used techniques for detecting volumetric (subsurface) flaws are radiographic and ultrasonic testing.
In radiographic testing, x rays, gamma rays, or neutrons are directed at a part and detected after passing through it. Flaws give rise to shadows or bright spots in the detection field recorded on film or a detector array. Typical applications include detection of voids in solid propellants, casting ingots, and adhesives; assembly verification; detection of core anomalies in honeycomb panels; and inspection of welds. Aerospace is often asked to evaluate the quality and completeness of radiographic inspections. In one instance, Aerospace detected critical propellant cracks in a solid-rocket motor igniter that had been overlooked by prior inspectors. Left undetected, such a flaw could lead to the mistiming of motor ignition, or, when occurring in the motor propellant, cause premature failure of the case insulation and ultimate failure of the rocket motor. The suspect igniter was subsequently removed and replaced from a vehicle being readied on the launchpad. The mission was valued at $1.2 billion.
An acoustic transducer sends ultrasonic pulses into the part. Echoes received by the transducers can indicate the presence of internal cracks or other structural flaws. The scattering of sound by the material's microstructure can also provide important information. Aerospace developed an ultrasound technique to validate the proper grain orientation in small data recorder tape-head brackets. |
In ultrasonic testing, sound energy is transmitted into the part. Flaws can be found by checking for reductions in the amplitude of the transmitted sound or reflections that bounce back toward the transmitter (similar to sonar). Typical applications include inspecting for debonding of adhesives or laminates and verifying weld integrity. Standard practice is to use piezoelectric crystals, held in contact with the test article, for transmitting and receiving the ultrasound. Because many spacecraft components are fragile or sensitive to contamination, Aerospace has been investigating a number of noncontact methods for generating and receiving ultrasonic signals. These methods include the use of air-coupled transducers and ultrasound generated by laser pulse heating. Often, hardware requires inspection late in the assembly process because of mishandling, accidents, or unanticipated concerns (e.g., previously undetected potential manufacturing errors). In one instance, an ultrasonic method developed at Aerospace for an on-pad inspection revealed an unbonded thrust-chamber baffle in a launch vehicle engine. The flawed engine was subsequently removed and repaired prior to launch. In another case, Aerospace used ultrasound to distinguish bad parts in a nearly inaccessible region of a spacecraft prior to launch. A small tape-head positioning bracket used in a satellite data recorder cracked during a ground test. Analysis revealed that the bracket had been machined in a specific orientation with respect to the rolling direction of the original alloy plate stock, and this made it susceptible to cracks. Engineers needed assurance that none of the bad brackets had been installed in the multiple recorders in satellites that were still awaiting launch. Aerospace developed and demonstrated a procedure for inserting a fine ultrasonic probe into the satellites to confirm proper bracket grain orientation through acoustic backscatter. The use of this technique enabled mission planners to avoid the costly process of removing and dismantling the recorders and the associated schedule delays.
Testing with Heat, Light, and Microphones
In addition to working with radiographic and ultrasonic testing, Aerospace has been quite active in adapting other nondestructive evaluation techniques for use in the space industry. The most important of these include thermography, shearography, and acoustic emission monitoring.
In a typical thermographic test, the test article is exposed to a transient heat source such as a flash lamp or a blast of hot air. An infrared camera then images the surface temperature of the part. Subsurface defects impede or enhance the flow of heat through the part, resulting in localized hot or cold regions on the surface. Applications include detecting bond-line flaws beneath composite face sheets in honeycomb structures, detecting heat leaks, and isolating hot components on printed circuit boards. In one instance, Aerospace demonstrated that thermography was more effective and reliable for detecting face-sheet debonds in a medium launch vehicle's honeycomb-panel nacelles than the simple, conventional tapping test used by the contractor. In another case, Aerospace paved the way for the flash thermographic methods now being employed to inspect the bonds connecting solar cells to substrates on the large arrays used for power generation for a number of satellite systems. The cell bond layer plays an important role in the transfer of heat to maintain cell operating temperature and output performance. Thermography is used to identify cells with voids that exceed threshold criteria and require rework. Aerospace also demonstrated that the same method could be used to detect adhesive voids beneath satellite optical solar reflector tiles.
Oscar Esquivel evaluates an infrared image obtained during flash thermographic inspection of a solar cell array test coupon. Adhesive bond voids or other discontinuities to heat flow are revealed by viewing the surface with an infrared camera immediately after flash lamp heating. |
Shearography is a field-applicable holographic technique that can be used to identify regions of very fine relative displacement, or strain. A special camera takes pictures of the reflection of diffuse laser light from the test article surface before and after it is subjected to a mechanical stress, such as slight pressurization of a fuel tank. These pictures are processed to yield an image consisting of fringe patterns. Regions of concentrated surface strain under load result in a higher density of fringes. Fringe-pattern anomalies are indicative of subsurface flaws. In past work, Aerospace showed that this method could be used to detect impact damage in the spherical composite-overwrapped pressurization vessels used to contain compressed gases for a number of spacecraft. Efforts in this area were recently revisited and expanded when Aerospace was called on to support the NASA Composite Overwrapped Pressure Vessel Super Problem Resolution Team in the wake of the Columbia accident.
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Thermal vacuum testing created "dimples" in this optical solar reflector tile array, shown left. Aerospace applied thermographic inspection techniques to identify the severity of the problem. As in the case of solar cell bond inspection, areas with an underlying void or debond appear brighter than well-bonded areas because they retain heat longer. As shown here, the inspection revealed significant voids in the adhesive below several of the reflector tiles. |
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Acoustic emission monitoring can be performed during hardware proof tests. Small microphones, sensitive to sound at frequencies beyond the audible range, are affixed at strategic locations on the test article. The acoustic transducers are monitored for sound emissions emanating from the hardware throughout the loading cycle. Cracks and other flaws that grow during the proof test make noise that is picked up by these sensitive microphones, and the location of these flaws can be determined through triangulation. Aerospace has worked closely with contractors in employing acoustic emission testing in a number of programs, most notably during the hydroproof testing of some composite solid-rocket motor cases.
Beyond Testing
Aerospace work in nondestructive testing goes beyond independent review and verification. In many cases, Aerospace works in close association with the community of manufacturers, contractors, and academic institutions to develop expertise in areas that are often overlooked or neglected for earthbound applications but are critical for maintaining high reliability in advanced spacecraft components (see sidebar, Eddy Currents ).
Yong Kim applies a novel eddy-current evaluation technique developed at Aerospace to detect critical deviations in the lay-up of a filament-wound graphite epoxy composite specimen. |
For example, precision ball bearings are essential for the operation of diverse space vehicle mechanisms, such as deployment devices, solar-array drives, antenna gimbals, reaction and momentum wheels, and control-moment gyroscopes. Hybrid bearings made from silicon-nitride ceramic balls and steel rings are being adopted for use in space mechanisms because of their superior strength and tribological properties. Appropriate screening methods for material flaws are essential to prevent these rolling parts from failing prematurely through contact fatigue. Aerospace found that these components were getting relatively little attention within the industry for spacecraft applications. To address this shortcoming, Aerospace worked with steel suppliers to implement more effective ultrasonic and eddy-current inspection methods to detect and screen for material flaws such as nonmetallic inclusions. Aerospace has been working with ball suppliers to ensure that proper optical and microfocus x-ray inspections are employed to detect material defects, and has aided in the design and instrumentation of contractors' bearing-life test programs to verify high reliability. Aerospace has also been active in developing appropriate specimens or "flaw standards" to test the effectiveness of the various inspection techniques.
Conclusion
Independent research in the nondestructive evaluation laboratory allows Aerospace to develop methods and techniques for current and anticipated needs. This work covers a wide range of tasks, including verifying the sensitivity of nondestructive evaluation methods, developing advanced techniques to meet program-specific needs, demonstrating proof-of-concept, and researching new technology. Aerospace's capability for nondestructive evaluation research and support is a major asset in the drive to ensure total mission success.
