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Wednesday, August 1, 2012

Seeing Beneath the Surface: NDT for Helicopters   

Nondestructive testing (NDT) of helicopters via ultrasonics is an essential facet of any maintenance and manufacturing program. NDT is a cost-effective way to quality check rotorcraft components and structures for defects, thickness, cracks, flaws and other problems without tearing apart your helicopter to access them. The helicopter industry is dependent on NDT. 

(Top) NDT plays a vital role in keeping air travel safe. (Middle) An immersion ultrasonic machine ensures that joints have not deteriorated. (Bottom) NDT allows inspections without disassembling internal areas.

“Without NDT, the cost of maintaining and flying helicopters would increase dramatically, while the safety of flying would decrease,” says Arthur C., marketing specialist at Qualitest Inc. of Plantation, Fla. “When people ride in helicopters they trust it will get them to their destination with as little turbulence as possible. NDT plays a vital role in keeping air travel one of the safest modes of transportation.”

NDT allows helicopter inspection that otherwise would not be possible without disassembling to gain access to internal areas. “The engine, rotors hub, link assemblies, blades assembly, transmission and major structure areas in the engine deck are frequently tested with ultrasonics because they have the highest amount of wear and tear, and moving parts,” says Wayne Weisner, aerospace director and NDT global government sales for Olympus NDT in Kennewick, Wash. “Also, the high amount of heat generated by the engine can cause heat-affect damage in the nearby structure and over lifetime can cause in-service failures.”

Damage such as tiny cracks too small to see visually can be detected via ultrasonic testing. Ultrasonics can measure helicopter skin thickness from the outside and corrosion-caused metal thinning on skins’ inside surfaces too. “When a rotorcraft lands and the door is opened, it fills with warm moist air,” says Arthur C. “When it takes flight and reaches altitude, the skin becomes very cold due to the outside air temperature. Water will collect at low areas and serve as the electrolyte needed for corrosion to occur.”

Ultrasonic testing can inspect installed attachment fasteners. “You can detect subsurface defects within these installation fasteners if you have access to the nut or collar side of a fastener, main rotor head or fear box,” says James Bittner, senior sales engineer at Olympus NDT. “This would reduce removal time and save labor, service time, and cost.” Ultrasonic testing also can detect a lack of fusion in a helicopter’s electron-beam welding geometries and inspect porosity, inclusions and lack of fusion in aluminum friction-stir welding.

How it Works

Ultrasonics use high-frequency sound energy sent straight or angled into helicopter parts to detect subsurface flaws. A typical ultrasonic inspection system consists of several functional units like a pulser/receiver, transducer and display devices. Driven by the pulser, the transducer generates high-frequency ultrasonic energy. The sound energy is introduced and propagates through the helicopter material being tested in the form of waves. When there is a discontinuity (such as a crack) in the wave path, part of the energy will be reflected back from the flaw surface.

A reflection phenomenon occurs when an ultrasonic beam meets an interface with a different acoustical impedance enabling defect detection such as delamination, cracking and disbonding. From this impedance, information about a defect’s location, size, orientation and other features can be obtained. Because of their recording function, ultrasonic mappings ensure traceability of manufactured parts.

Four Main Methods

There are four main ultrasonic NDT methods for helicopters: pulse-echo, through-transmission, pitch-catch and lasers. With the pulse-echo method, a piezoelectric transducer with a longitudinal axis located perpendicular to and mounted on or near the test material surface transmits and receives ultrasonic energy. The transducer is typically separated from the test object by a couplant (such as oil) or by water, as in immersion testing. The couplant increases process efficiency by reducing the losses in the ultrasonic wave energy due to separation between the surfaces. The ultrasonic waves are reflected by the material’s opposite face or by discontinuities, layers, voids or inclusions in the material. The waves are then received by the same transducer where the reflected energy is converted into an electrical signal.

With the through-transmission method, an ultrasonic transmitter is used on one side of the material, while a separate receiver is placed on the opposite side. This method will locate defects, flaws and inclusions in the X-Y plane of multi-layed and multicomponent materials like helicopter insulation, and composite materials and other attenuative materials.

The pitch-catch method transmits ultrasonic energy at any surface angle in the tested material and receives reflected energy returning at the reflected angle. It is used primarily for cylindrical and other nonlinear parallel-sided helicopter surfaces. It can determine flaw depths and locations in the X-Y plane.

Laser ultrasonic testing is a remote, noncontact extension of conventional, contact or near-contact ultrasonic testing. A laser pulse interacts at the surface to induce an ultrasonic pulse that propogates into the sample. This ultrasonic pulse interrogates the sample then returns to the surface. A separate laser receiver detects the small displacement generated when the pulse reaches the surface. The electronic signal from the receiver is then processed to provide the desired measurement.

Compared to conventional transducer-based ultrasonic testing, laser ultrasonic testing generates and detects the full complement of ultrasonic waves: bulk (compressional and shear), surface and plate. The non-contact nature of laser ultrasound is its prime advantage over conventional ultrasound. Also, no coupling is needed. Most laser ultrasound systems do not require particular knowledge of a component’s shape prior to inspection.

One of major challenges laser ultrasonic technology is price. Industrial laser ultrasonic systems range from $500,000 to $2 million, although R&D systems are less expensive. This can be a setback for end-users with limited budgets.

Close-up View of Composites

Composite materials are increasingly being used in helicopter construction. “The Bell-Boeing V-22 and Bell 409 are composed of approximately 50 percent composite structures, including the airframe, wing and rotor system,” according to Jerry Nissen, engineer specialist at Bell Helicopter in Fort Worth, Texas. “Composite components are being chosen because they are lighter and stronger than their aluminum equivalents, allowing the aircraft to fly faster, farther and carry heavier loads while costing less to maintain.”

As composite use has increased, so has the need to test its bond integrity. Virtually all composite parts of a helicopter will be ultrasonically inspected at least once during its life, and some fuselage and empennage components will be inspected many times, some as often as every 25 flight hours. Eddy current testing, another NDT methodology which involves generating electrical currents by a changing magnetic field and noting flow disruption, cannot be used on composites.

“Ultrasonic inspection is used at Bell for both primary and secondary structures from composite filler blocks to tiltrotor spars,” says Nissen. “It can assess bond quality or composite integrity for thin and thick structures. Ultrasonic inspection can detect and characterized processing and fielded defects so inspectors can verify component airworthiness.”

Once a rotorcraft has been put in service, ultrasonics can ensure flight safety. Rotorcraft can be damaged by something as simple as a dropped tool by a mechanic, to more severe impacts from service vehicles and hail storms.

“Traditional aluminum aircraft structures deform, dent and crack when impacted, thus making damage assessment and subsequent repair more straightforward,” says Brooks Longley, product manager at Imperium of Beltsville, Md. “Ultrasonic testing can assess barely visible impact damage that might occur on composite structures. Composite structures, when impacted, may not show any visible effect of the impact, and yet may have been weakened enough to fail at some time in the future. Advanced composite materials are more tolerant to impact damage than traditional aluminum structures.”

Testing & Training

Despite its many advantages, ultrasonic testing is not applicable for all helicopter parts and does have some limitations. “Very absorbent materials or ‘multi-materials’ (with multiple acoustical impedances) can’t be easily or even tested at all with ultrasound,” says Caroline Korosec, NDT Responsible in the Quality Materials Laboratory for Eurocopter in Marignane, France.

Helicopter surfaces must be accessible to transmit ultrasound. Ultrasonics cannot successfully inspect any material where it cannot penetrate or provide resolution. Components that are rough, irregular in shape, very small, exceptionally thin or not homogeneous are difficult to inspect. “There are all kinds of applications that are not suited for ultrasonic inspection,” says Nissen. “It is important as an OEM to develop a reliable inspection process that validates the design intent and certifies aircraft airworthiness. Having nondestructive design influence during the initial stages of a program or product development ensures that high quality is maintained without adding additional cost to new products.”

Ultrasonic testing must always be carried out by trained personnel and its training is more extensive than other testing methodologies. The ability to interpret and understand results, and use the tools to analyze and report the data will always have to be taught. “Ultrasonic operators are subject to certifications and follow training courses dedicated to ultrasonic methods,” says Korosec. “They need to pass theoretical and practical examinations every five years. Certification is acknowledged worldwide through the application of EN4179/NAS410 standards.” Following these standards, and implementing the right training and nondestructive technology on the ground will mean a lot less trouble in the air.

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