Testing times for structural integrity: NDT to the rescue
Non-destructive testing is a technique employed in a number of different industries to verify structural integrity, but what requirements are specific to aviation, and what equipment and expertise are used to satisfy them? Joanne Perry went in search of the answers.
In April this year, Southwest Airlines flight 812 from Phoenix to Sacramento in the US came close to disaster when the skin of the 737-300 tore open at 34,000ft. The aircraft landed safely at Yuma International Airport, Arizona, with only two minor injuries sustained, but the incident sparked a major investigation which caused the cancellation of 300 Southwest flights. Seventy-nine Southwest 737 aircraft were subsequently checked for the lap-joint fatigue cracking which caused the depressurisation incident.
The method stipulated by Boeing’s service bulletin (SB), enforced by a Federal Aviation Administration (FAA) airworthiness directive (AD), was high-frequency eddy current testing. This classic non-destructive technique proved its usefulness by detecting cracks in five other aircraft, which were then removed from service for repairs.
Non-destructive testing (NDT) can be defined as the assessment of material integrity without compromising future use, for example by taking samples for analysis. It is a collection of processes used across a number of different industries, such as power generation and construction as well as transportation. The simplest form is a visual inspection, aided by remote visual inspection (RVI) equipment such as borescopes for areas that would be inaccessible without disassembly. However, this method is only useful for superficial problems and is heavily dependent upon the skill and dedication of the technician.
The American Society for Nondestructive Testing (ASNT) lists six basic methodological categories: mechanical and optical; penetrating radiation; electromagnetic and electronic; sonic and ultrasonic; thermal and infrared; and chemical and analytical testing. The ASNT supplements these with image generation and signal image analysis. Varieties of NDT too numerous to mention branch out from each main type.
In aviation, NDT is used not only during post-incident investigations as in the case of Southwest, but during component manufacture, to preclude flaws, and in the maintenance and repair of both airframes and engines to detect not only cracks but disbonding, corrosion, scratches and other problems or damage.
Steven Shepard, president of Thermal Wave Imaging (TWI), based in Michigan, US, explains some of the requirements of aerospace NDT: "Speed and economy are essential for NDT equipment in almost any industry in today's economy. However, aviation requires a higher degree of accuracy and reliability than most". The Southwest incident provides a timely example of the important role NDT has to play in verifying the airworthiness of aircraft and the safety of passengers. Mike Fortman, president of NDT service provider Aerotechnics in Minnesota, US, says that "nearly all aircraft components require some sort of special attention or detailed inspection". With a constant
drive to reduce aircraft weight because of its impact on fuel burn, parts are called upon to perform their respective functions at high load relative to material strength. They must furthermore withstand the stress created by repeated loading and unloading of the aircraft, temperature and pressure changes, and other atmospheric conditions such as lightning strikes.
The need for NDT does, however, vary across the aircraft. As Mark Ginn, chief inspector at Delta TechOps, explains, many components "require special attention when receiving NDT as a result of their criticality to safe flight", whether they are located in the engines, landing gear or airframe. Albrecht Maurer, senior product manager testing systems, at GE Sensing & Inspection Technologies agrees that the difference in safety requirements is the main factor in determining the intensity of the NDT which is conducted. "Primary structures receive more NDT attention than secondary or tertiary structures," he states.
Shepard adds that, in an additional complication for the aerospace industry, many aviation inspections "involve large areas in which the component construction may vary considerably". A central aspect of this variation stems from the material used in manufacture, but further considerations are the nature of the suspected flaw and the conditions of inspection, says Philippe Boiteux, COO managing director of NDT Expert, an NDT solutions provider based in France. The latter include the expertise of the company and technicians concerned, and whether it is a manufacturing or maintenance operation performing the NDT. Maurer points out that the choice of NDT technique is also governed by manufacturer and certifier approvals.
MRO provider Delta TechOps maintains a broad portfolio of NDT capabilities in order to balance out the strengths and weaknesses of the different methods: eddy current; magnetic particle; fluorescent penetrant; ultrasonic; radiographic; and infrared testing. As an example, Ginn explains that the magnetic particle method, which involves the dusting of a magnetised surface with iron particles to highlight anomalies, cannot be applied to non-ferrous materials. In addition, although magnetic particle NDT can be used to identify subsurface defects, its effectiveness decreases with depth. It also requires the removal of paint from the test surface. Meanwhile, eddy current testing, which involves the generation of electrical currents by a changing magnetic field and the noting of any flow disruption, cannot be performed on non-metallic materials such as composites.
NDT for composites
"Infrared (thermal imaging), ultrasonic testing (resonance, low frequency pitch-catch, pulse echo, through transmission), and radiography (digital or film x-ray) need to be used to find flaws in composite materials," according to Fortman. GE Sensing & Inspection’s Maurer believes that for such applications ultrasonic testing (UT) "is the most reliable method with least technical restriction, i.e. minimum/maximum thickness and complexity of structure".
In UT, both geometric surfaces and internal integrity can be analysed by the transmission of high-frequency sound waves into the test material. Resonance testing in its most rudimentary form consists of a "tap" test, which is what the name suggests, but is computerised at the highest levels. In through transmission, a transmitter is positioned on one side of the test material and a receiver on the other, while pulse echo is a single-sided technique for less accessible areas, and in the pitch-catch method the transmission occurs at an angle and is useful for the testing of non-linear objects. Linear array UT involves a single source of transmission, while a more complex and more commonly used version with multiple pulsing elements - phased array - creates a kind of steerable "searchlight" for inspection.
Newly launched NDT products in the UT category include the "Bondtracer" unveiled by GE in May this year in collaboration with Boeing. This is a portable composites inspection tool which is designed to enable mechanics to assess minor impact damage at an airport gate. The quick feedback produced by "Bondtracer" determines whether an aircraft is fit to fly or requires further investigation and repair.
In radiographic testing (RT), gamma or X-rays - which one depends upon the thickness of the material in question - are directed through the test object onto a film, which produces a shadowgraph depicting internal features. As in the well-known medical application, variations in density are represented by lighter and darker areas. Radiography has the advantage of removing disassembly requirements, but brings with it the disadvantage of safety concerns. Owing to the hazardous nature of X-rays and radioactive isotopes such as iridium 192, which produce gamma rays, extra precautions must be taken when using this technique, such as protective equipment and warning systems.
Shepard asserts that thermography is also a good option for composites, because "the cost of a large-scale thermography system is significantly less than the alternatives". As a result, "many companies have replaced C-scan [UT] systems with thermography". In thermography, the test material is heated so that the temperature decay can be observed over time, with structural anomalies disrupting normal cooling. However, Shepard notes that the unprocessed images from the standard infrared camera used to detect the thermal patterns "are not sufficient for many aviation NDT requirements".
TWI therefore uses a thermographic signal reconstruction (TSR) processing method which analyses the time evolution of each pixel, enabling not only the identification of anomalies but quantitative measurement of thickness, thermal diffusivity or porosity. He says this effectively allows a user to "drill down" into the test item. "This combination of fast area coverage and the ability to ‘self-validate’ image results using time response is a unique advantage of thermography," he states. Other advantages include a fair tolerance of non-planar geometries, surface characteristics and ambient conditions. "The most fundamental limitation is the one imposed by the physics of diffusion, which requires that the diameter of a subsurface feature is larger than its depth," states Shepard.
Maurer says that, in summary, the NDT methods which can be used for composites are "ideal to detect lack of bonding (delamination) as well as porosity over the whole area of each component without impacting its properties". This is because composites “rely strongly on the perfect bonding between individual layers and on absence of pores which may develop during the hot curing process".
Flaw characteristics and inspection conditions
The properties of material flaws which impact on NDT include size, depth and accessibility, says Fortman. He gives the example of rotating engine parts such as turbine blades and discs, which may contain very small defects that require the use of special penetrants and/or automated scanning. In penetrant testing (PT), a simple NDT method based on the capillary action of liquids, a solution of visible or fluorescent dye is applied to the test object, before the excess solution is removed to highlight any breaks in the surface. A developer is used to draw the penetrant out of the defects. Visible dyes rely on colour contrast between the penetrant and the developer, while fluorescent dyes are activated by ultraviolet light. However, as Ginn points out, a key drawback is that penetrants can only detect superficial discontinuities. The chemicals may also damage composite materials. Care must be taken in any case to properly clean off the penetrant, or risk misleading results. This method, like magnetic particle testing, also necessitates the removal of paint from the test material.
As regards the differences between NDT for manufacturing and maintenance applications, Maurer explains: "Parts with MRO-testing requirements demand instant image output rather than time consuming scanning processes, e.g. impact analysis through shearography [optical NDT], remote visual inspection and in special cases UT testing with portable scanners." TWI, for example, provides large-scale thermographic systems for manufacturing contexts and portable/ handheld products for in-service use. There is some crossover, however. GE's Phasor XS and DM phased array UT products can be used for volumetric inspection during both manufacture and maintenance, as can the company's range of RVI equipment.
In light of the pros and cons of different NDT methods, it can be necessary to use multiple techniques during inspection. For example, the emerging NDT technique of process compensated resonance testing (PCRT) offers a high degree of objectivity through the compilation of statistical data - but relies on a known sample set to establish basic parameters. In this method, samples of defective and defect-free parts identified by destructive analysis or another NDT technique are used to build a customised software algorithm, based on the contrast between a series of natural frequencies or resonant responses from the two groups. As Greg Weaver, director of operations at Vibrant Corporation in New Mexico, US, explains: "The software is defining not only the absolute response differences, but more importantly the relationship difference across multiple responses." The system can be “taught” to recognise acceptable and unacceptable differences between components of the same type, compensating for the unintentional variation generated by even the most modern manufacturing processes.
Like other NDT types, PCRT possesses a mixture of good and bad points. On the negative side, although PCRT can detect a decline in structural integrity, it does not specify the defect type or location as would magnetic particle testing and phased array UT. However, Weaver says the technique does challenge FP and X-ray NDT, and at a comparable cost. One of the key advantages, he explains, is that PCRT can detect more than one defect type, internally and externally, in a single inspection. Additionally, it is "the only NDT method that can detect metallurgical issues such as alloy overtemp and intergranular attack". The inspection time is also impressive - between four and six seconds for the resonance test itself. Importantly, PCRT can be applied to both metallic and non-metallic parts including composites, during either manufacture or maintenance processes. "A PCRT test at the front of an MRO receiving process could save companies an enormous amount of time and money," states Weaver. "The same goes for blade manufacturing, where PCRT can not only be used as an inspection tool, but also as a process control measurement.”
In aviation, PCRT is mainly used for turbine components, including solid and hollow turbine blades, silicon nitride bearing elements and forgings/castings. Weaver believes that PCRT "should become a dominant inspection in the turbine blade world” and that it might be included in most OEM standard practice manuals within five years, bearing in mind the difficulty of making predictions. Delta TechOps received FAA approval for PCRT in September 2010, and Ginn views the technique as "an important capability in years to come and an important part of the Delta TechOps NDT portfolio".
The latest on NDT
"Even conventional techniques are permanently moving on," Boiteux observes. One of the biggest sources of change in the NDT business is the trend in airframe manufacturing toward greater proportions of composite materials. Jeff Stetson, senior product manager, ultrasonics, at GE, says that on the UT side, “composite airframes are driving some changes in equipment”. According to Boiteux, the increase over the past decade has led to an expansion of UT, thermographic and shearographic capabilities. As the composite level rises, “NDT methods such as ultrasonics, radiography, and infrared inspections become more valid,” notes Fortman. “Established methods
such as penetrant testing, magnetic particle testing, and eddy current are nearly obsolete or unusable on composite materials”.
At Delta TechOps, Ginn has witnessed the growing importance of UT, “with ultrasound being the method of choice for many composites”, but also a similar trend in eddy current testing. He adds that eddy current NDT, UT, RT and infrared testing have all “benefited from technological advancements over the last few years”.
Referring to NDT as a whole rather than composites-focused NDT, Ginn says the use of mature methods such as magnetic particle testing and FP “has remained fairly constant”. Stetson adds that for engine inspections one of the most noteworthy technological advancements has been the “huge transition from film to digital RT”, for which GE has developed products such as the DXR 250P, a digital and portable RT solution for on-wing inspections.
There is clearly a wealth of NDT equipment and expertise on the market. If the near-disaster of flight 812 is anything to go by, it is more a question of ensuring that these services are called upon as appropriate. Although eddy current testing detected cracks in a number of Southwest aircraft during the fleet-wide investigation, prior to the incident only visual inspections were required for the 737 Classic serving the flight. Nor was this the first such incident involving a Southwest aircraft; something of a track record had even led to a $7.5m settlement in 2009 for missed fuselage inspections. It seems safe to say that a little more non-destructive testing on the ground could prevent a whole lot more destruction in the air.