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various forms such as strands, tows, roving, tape, woven cloth, mats, knits, and braids. Honeycomb comes in different sizes and materials that also effect results, from aluminum to treated paper. All of these variations effect the NDT techniques in different ways and the technician has to know how all these various material forms effect the interrogating energy in order to properly interpret the indications his equipment produces.


What’s Needed Now The unique world of in-service aviation inspection requires skills and knowledge that are not common to infrastructure, petrochemical, or other industries. Both the performance of tests and the interpretation of results also require a knowledge of aging aircraft issues and maintenance concepts. In response to all these issues, the aviation portion of any initial or refresher training course should include:


 FAA regulations  Personnel qualification issues  Human Factors and its effects on Probability of Detection (PoD)


 Aerodynamics, design, stress, including the concepts of fail safe, safe life, damage tolerance and how they fit into the determination of inspection intervals


 The origin and detection of in-service flaws such as fatigue, and corrosion


Because there are many technicians that


are already certified that never received any specific aviation training, Dassault Falcon developed refresher courses in cooperation with Sandia at AANC for conventional inspections, and teamed with Advanced Composites Technology for the NDT of Composites. Regardless of the approach taken, an effective program should include:


 Instructors that are a combination of experienced, active technicians and engineers


 Overview of applicable NDT methods, techniques and reference standards


 Extensive testing of actual flawed on-aircraft structures


 Student fabrication and testing of composite specimens


 Hands-on comparison of various NDT methods and techniques on actual composite components


 Review of less frequently encountered theory and instrument issues


 Hands-on practice and/or practical exams on actually flawed, retired aircraft


It is critical that technicians understand composite material forms and structural concepts. Prior to inspecting a composite structure it is critical to know its construction such as material and material form, thickness, thickness variations, and the location and nature of bonded substructure. For example, a laminate of unidirectional carbon tape will look very different to ultrasound than a woven carbon fabric. In addition, carbon reinforcing fibers come in different diameters which may effect the inspection. Also, fiberglass plies are generally twice as thick as carbon plies and some structures may contain both. Composites, by definition contain


a combination of materials, and some of these materials may not be evident at first, such as fiberglass or primer between carbon and aluminum (corrosion protection) or a layer of aluminum or brass mesh just under the first layer or two of a laminate for lightning protection. Therefore, in order to understand and evaluate the signals obtained it is necessary to understand the particular structure and what techniques and reference standards are appropriate. It is critical that drawings be available for this information. It can be a big mistake to simply calibrate on what is assumed to be a local good area as is often done with metal structures. Metal structures have a limited number


of flaw situations. They can contain fatigue cracks, corrosion, and physical damage, all of which almost always occur at the surface. Damage in composites is frequently not visible and can be at different layers that don’t exist in metals. Figure 5a illustrates two carbon laminates bonded to a piece of aluminum such as flange. Porosity, voids, or microcracks in the laminates or adhesive might be acceptable in manufacturing, but act as stress risers and may lead to cracks in service. In service defects include the possibility of delaminations in the laminate, and disbonds between the laminates and the aluminum structure. Actual structures are even more complicated than this with laminate thickness changes and carbon stiffeners than could be either co-cured or secondarily bonded, each of which look different to ultrasound. A honeycomb sandwich structure offers even more flaw possibilities as shown n Figure 5b. For example, in addition to those typical of laminates, there may be water entrapment (which can cause corrosion, or damage if it freezes and expands), core splice failures, crushed core (in many cases, the skin will spring back after impact and leave no visible external evidence) and sheared


34 Aviation Maintenance | avmain-mag.com | October / November 2011


Figure 5. Composite Defects


Figure 6. Students fabricating composite panels


(torn) core. Complicating things even further is the fact that, due to the nature of fiber reinforced composites, actual damage may extend much farther than what is evident visually near an impact site. Inspection of composite structures


employs methods and techniques that will not be familiar to a technician who simply has the typical Level II in ultrasonics. Some of the ultrasonic techniques include through transmission, resonance, ring pattern, mechanical impedance, phased array, and the use of the full RF waveform which few technicians are familiar with, much less comfortable with. Other methods include radiography, many different forms of infrared, and shearography (a modified form of holography). Technicians who might use these methods must have additional training in their capabilities and limitations, and maintain proficiency in their use. Hands-on exercises give the students


the opportunity to see first hand not just how different NDT methods responded to different flaws and flaw locations, but also how fabrication issues can also effect the results. The students are allowed to design and fabricate their own specimens so they have a better understanding of the relationships between the material, fabrications, defects and NDT capabilities (Figure 6). For example, Figure 7 illustrates


a specimen made by students that consists of a carbon laminate bonded to


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