Boeing AUSS Tower, used for NDE of large composite structure during manufacturing.
Nondestructive Evaluation (NDE) is a critical technology area for Boeing that has grown and developed along with the company throughout its 100 years. The future will continue to rely on innovation in the non-destructive inspection of our aircraft. This is a brief overview of NDE development at Boeing.
Nondestructive Evaluation has paralleled and sometimes enabled many other technologies. At times this core competency is referred to NDI (Nondestructive Inspection) or NDT (Nondestructive Testing), depending upon its particular application or industry of use.
NDE can generally be defined as the evaluation of a structure without harming or affecting its purpose. This definition sets NDE apart from destructive or mechanical testing of subscale or full-scale structures, which allows the determination of properties or flaws, but which makes the part unusable afterwards. NDE for aerospace covers two distinct but related application areas: NDE during production and NDE during in-service usage.
The specific goals of NDE (and NDI or NDT) were expressed very well by Robert McMaster in the 1959 version of the American Society of Nondestructive Testing Handbook:
Ensuring reliability of the product and preventing accidents and saving lives are important and obvious goals for NDE. The goal of “making a profit for the user” is often underappreciated, yet is essential to the effective use of NDE for a manufacturer like Boeing. As aerospace manufacturing platforms have grown more competitive in the recent decades, NDE development as a cost and flow-time-reducer has become more critical.
The early history of NDE emergence into aerospace is a fascinating one.
The first commercial Boeing airplane structures were visually inspected during manufacturing to verify proper wood frame assembly, fabric attachment and adhesive application during fabrication. No instruments beyond the human eye were used, except possibly lighting aids or magnification to improve defect detectability.
Visual inspection was exclusively used in the early years of aircraft up to the early 1930s, when the first all metal airplane, the Boeing Model 247, was introduced. Industrial radiographic inspection processes for metals and the first magnetic induction/magnetic particle inspection approach were introduced in the 1920s. These were applied on a limited basis for inspection of Model 247 components, as well as the more mass-produced Douglas DC-3 that came along in 1936.
World War II saw the development of the first eddy current instruments, as well as the first ultrasonic testing method. These became the crux of aerospace NDE as Boeing entered the jet age in the mid-1950s. As the space program came along in the 1960s, McDonnell had a hand in the development of the first Sondicator to support inspection of heat shield bonds on the Gemini spacecraft, probably the most critical element of the vehicle. The early Sondicator led to development of more advanced low-frequency bond testing methods still used today for inspection of adhesive bonds.
While visual inspection continued to be the primary NDE method, visual inspection could no longer address the defect and damage detection needs, especially in-service. Metal fatigue caused by the cyclic stresses of aircraft flight produces small cracks that must be identified before they grow to the point of structural failure.
These fatigue cracks, and cracks generated by excessive loads or corrosion could be identified using an NDE method called dye penetrant, which relies on a dye wicking into surface cracks. Structural parts made with most steels could be inspected with magnetic particle inspection. Both these methods essentially enhanced visual inspection. Visual inspections require human reckoning that require high skill interpretation and judgment. As the need for inspection increased, new instrumented methods had to be developed to allow discovery with less judgment.
Several major air catastrophes drove the need for better NDE. The F-111 crashes of the late 1960s and early 1970s led to the introduction of the fail-safe/damage-tolerant design philosophy. The first aircraft designed in the damage tolerance era was the Boeing (McDonnell Douglas) F-15. Boeing, along with Pratt & Whitney from the engine side, took the lead in addressing inspection reliability in conjunction with all NDE processes used to support manufacturing.
Boeing also was first to use structural analysis and NDE reliability assessments to define in-service inspection intervals. The 1988 Aloha Airlines fuselage peeling led to an “aging aircraft” monitoring approach. This explosive decompression incident was caused by widespread fatigue damage. The incident, along with the United Airlines DC-10 crash in 1989 (engine) and corrosion failures associated with the KC-135 in the early 1990s, led the FAA to join with the Department of Defense and NASA to cooperatively address aging issues. This resulted in significant funding going to aging aircraft research, including NDE.
As the complexity and design criticality have increased, composites, as a percentage of an airplane structure, have increased as well. Ultrasonic inspection of composites has benefited from improvements over the years with electronics, automation and computing power, so that 2D and 3D imaging and analysis of UT data is now commonplace.
Boeing has been a world leader in the development of automated ultrasonic scanning systems, for production and in-service inspection of composite structure. Two examples of Boeing Automated Ultrasonic Scanning System (AUSS) options are shown in Figures 5a and 5b.
To date, over 70 AUSS gantry systems (such as shown in Figure 5a) are used across the aerospace industry, and more than 130 mobile systems (Figure 5b) have been sold in support of manufacturing, maintenance and research and development. In total, the influence of Boeing’s advances in automated systems have resulted in over $100 million dollars in equipment sales and billions of dollars in cost savings, through reduced inspection time and improvements in quality.
Boeing has taken the initiative to develop various automated tools for NDE that can extend or supplement the important AUSS product line, particularly for in-service inspections, and provide for greater personnel safety by eliminating the requirement to be on or adjacent to the aircraft under inspection. Two recent innovations are the ROVER (Remotely Operated Vacuum Enabled Robot) for aircraft exterior structural inspection, and the Boeing “Blade Crawler” for rotorcraft rotorblade NDE.
More about the ROVER and Blade Crawler is described in the full journal paper online.
Other Boeing developed NDE technology innovations are changing the way the industry assesses aerospace structures. One example is the Boeing-developed X-ray Backscatter system that creates an image of the interior of a structure by scanning an x-ray pencil beam across it and collecting the x-rays that scatter back.
X-ray Backscatter was originally proposed by Boeing as an NDE method in the 1970s, but then set aside due to the cost and speed limitations of legacy technology. Because of terrorism and border security concerns of the 1990s, the security industry rapidly evolved the technology to decrease the cost and size and increase its capability, therefore, making it more attractive to develop as an NDE tool.
X-ray Backscatter does not require access to both sides of a structure in order to do an inspection, which is an advantage for NDE of both large and in-service structures. It also selectively scatters from and discriminates between materials. The method is particularly sensitive to adhesives, moisture ingress, density changes, voids, and foreign object debris. It has recently been shown to be able to characterize composite heat damage and detect wrinkles in composites. More research is needed to quantify these new capabilities.
It is worth noting that the NDE technologies developed and matured by the Boeing team have found valuable applications beyond characterizing defects during inspection. Imaging technology and methodologies like photogrammetry and profilometry that were originally developed to characterize defects in the shape or finish of a surface are now being used for reverse engineering applications to support 3D modeling of legacy aircraft during modification and upgrade efforts.
Past NDE-related radio frequency research has now been spun to create RFID technology used to track products and inventories in our factories.
So, what are key NDE development areas that we can expect to see within Boeing as we look into the future?
NDE in the future will include automated data analysis that increases throughput by reducing time-consuming human-based data analysis. Lower cost pedestal and modular NDE robots will replace the current higher cost, large footprint, stationary scanning systems.
Factory flow will be optimized for speed and cost with automated crawling NDE platforms. NDE sensors will see significant technology innovation. Waterless stand-off NDE sensors, such as Laser Ultrasound Arrays, will inspect complex shapes and edges faster, without having to touch the part or deal with water collection and recirculation issues. Thinner laminates, like the 787 barrel skin, could soon be inspected with faster, large area NDE methods, such as Infrared Thermography (IRT), with UT used only for characterization of flaws.
The value of utilizing NDE and other measurement data as a process control tool is only now being fully appreciated. The goal is to move inspection (such as UT, IRT, CT, etc.) back up the manufacturing chain so it becomes transparent to the fabrication process. This approach will drive quality improvements through trend analysis, and reductions in process variations. In-process sensor feedback during manufacturing will be expanded to newer manufacturing methods, like additive manufacturing.
Ultimately, this approach will tie the NDE and measurement data into a “digital thread” that supports cost-effective implementation and maintenance during the entire life cycle of the aircraft. Better NDE planning during the design process reduces the uncertainty of new manufacturing capability and allows design teams to have confidence to optimize the design and not add costly overdesign to account for uncertainty. Better NDE during development helps optimize the production process, which results in fewer requirements for NDE in perpetuity.
For in-service NDE, some new capabilities that are likely to be implemented to reduce NDE costs include nanotechnology-based self-sensing structures/surfaces, robotic surgical NDE, and fully networked remote expert (tele-operational) NDE that extends the reach of the expert to virtually any place in the world. Advances in radiographic methods, including Computed Tomography, X-ray Backscatter, and Neutron Radiography are also part of Boeing’s long-term research and development plan to provide the best possible NDE tools when new critical, difficult or time-sensitive challenges arise.
Of course, the future is impossible to fully predict. Many factors will determine the direction of technology development in any field. However, we can point to the fact that Boeing has been a leader in NDE innovation this past century.
Gary Georgeson is Boeing Senior Technical Fellow for nondestructive evaluation, whose expertise has supported commercial, defense and advanced research programs.
By Gary Georgeson
EDDY CURRENT (EC) TESTING
An electric current-based method developed first in the industries making and inspecting pipes and tubes. It was the first significantly used instrumented method of NDE in aerospace. With this method, a changing electromagnetic field is generated by a coil containing alternating or pulsed electric current. The field produces corresponding electric (eddy) currents in the metal, whose paths are modified by cracks. The same coil or a separate receive coil senses the field change that the eddy currents produce, thereby allowing detection of cracks using electronic circuitry and (first) analog then (later) digital display.
MAGNETO-OPTICAL IMAGING (MOI)
Developed in the 1980s to enable 2D EC-based imaging of cracks around fasteners in fuselage lap joints and other structures. Linear EC arrays have more recently been developed that can be swept along a lap joint for in-service inspection for cracks around fasteners.
The first radiographic NDE method used for aerospace structure to find cracks, voids and foreign material. It was also very effective with moisture detection in metal honeycomb used for flight control surfaces, like flaps and trim tabs. Boeing was involved in the development and implementation of advanced radiographic inspection processes in the 1990s, including DR and CT.
DIGITAL RADIOGRAPHY (DR)
Digital radiography and other digital forms of x-ray have replaced film x-ray for many aerospace applications in recent decades, due also to the development and advancement of x-ray detector panels. The film-to-digital transition was driven by the advantages of digital data sets, as well as the cost reductions and environmental benefits of eliminating film, processing chemicals and disposal.
X-RAY COMPUTED TOMOGRAPHY (CT)
Standards were established in the early to mid-1990s for aerospace components and the resulting reports are still referenced today. CT provides 2D and 3D imaging of material density, voids, porosity and geometry with very high resolution capability for smaller parts. Today, CT is a key technology in supporting qualification and certification of metallic parts fabricated using additive manufacturing processes.
ULTRASONIC TESTING (UT)
Another important NDE method for aerospace structure. UT uses high frequency stress waves generated at the surface of a structure to interrogate a structure for defects that reflect or attenuate the signal. UT can be performed from one side of a part with a single transducer that sends and receives an ultrasonic signal, or in a through-transmission mode, with a sensing transducer listening for losses in transmission that is caused by flaws. UT has the benefit of being able to see deeper flaws than EC, and will work in non-electrically conductive media.
PHASED ARRAY ULTRASOUND (PAUT)
Originally developed in the medical field for looking into the human body. With this technology, a linear set of transducers can be activated in various time-phased or simultaneous options that dramatically increase ultrasounds’ speed and capabilities over traditional single transducer inspections. Boeing researchers have developed and implemented many end effector innovations using ultrasonic PAUT technology.