4/13 Eddy Current Testing

As Seen in ASM International

 

4/13 Eddy Current Testing - Higher throughput, smaller flaw detection, greater precision, and more cost-efficient NDT

by UniWest

 

In the world of nondestructive testing (NDT), few areas of failure prediction hold greater consequence than the commercial aerospace industry. Airplanes weighing over 200,000 lb accelerate from 0-150 mph in 90 seconds, fly in hostile weather conditions, and decelerate from 150-0 in another 90 seconds, and repeat the cycle until the landing gear tire treads are worn to their safe limit.

UniWest’s Eddy Current Tie-Bolt System including US-454A eddy current instrument (left), US- 1779 scanner (right), and US-1839 Specialty Bolt and Screw Probe mounted on scanner. Bolts are cradled between a pair of mandrels, commonly referred to as “hot dog rollers.” The probe contains an AC coil that generates a magnetic field, creating a secondary electrical wave, or eddy current, at the surface and near subsurface of the bolt under inspection. As the bolt turns on the rollers, the probe detects disturbances to the eddy current caused by discontinuities in the metal. Disturbances are processed and displayed as geometric eccentricities on the instrument. Inset photo shows close up of specialty probe contacting tie-bolt at screw threads.

Although landing gears are engineered to handle about five times the impact of a landing, much of the force of those stresses is transferred to the wheel tie bolts that hold two halves of an aircraft wheel together. Typically made of alloy steel or Inconel superalloy, wheel tie bolts are points of loading during aircraft taxiing, take off, and landing. As with all metals subjected to stress points, wheel-tie bolts are subject to fatigue, fracture, and possible failure. Failed bolts are frequently discovered during a preflight walk-around inspection, and the wheel is replaced. The number of tie bolts in a typical wheel is sufficiently large to ensure safety by means of redundancy, but flight delays cost airlines profits, both in terms of unscheduled repair costs and lost flight time.

 

Therefore, accurately detecting failureprone tie bolts before they fail is a matter of high importance to airlines aiming to maximize punctuality and profits. Because tie-bolt failures typically originate as small cracks either in the root of a bolt thread or where the bolt head joins the shank, early detection of cracks in those areas is predictive of future bolt failure. However, the total volume of a crack is a better predictor of failure than a simple linear measure. Long surface cracks may not affect bolt serviceability, whereas a much shorter crack that extends deeper into the body of the bolt is a more accurate predictor of bolt failure. The problem is how to distinguish between the discontinuities most likely to lead to bolt failure from those that will not affect performance.

 

Inspection methods

Because a typical commercial aircraft tire is changed at intervals of between 30 and 60 days, depending on OEM recommendations, airline policies, and seasonal tire wear, ample opportunity exists to inspect tie bolts for evidence of cracks while an aircraft is out of service. Because the vast majority of tie bolts inspected for cracks are returned to service, testing is limited to nondestructive test methods. Until recently, three NDT methods reigned in the tie-bolt inspection field: Liquid penetrant testing (PT), magnetic particle testing (MT) and; ultrasound testing (UT).

 

Liquid penetrant testing infuses a surface with a light-sensitive liquid. After allowing sufficient dwell-time for penetration into surface discontinuities, the penetrant is chemically developed and visually inspected using white or fluorescent light. Because dirt and/or corrosion tend to obstruct penetration, LP testing requires extensive bolt cleaning before testing, as well as after testing to remove residual penetrant chemicals. It also requires waste disposal of penetrant, developer, and cleaning solutions. Because LP testing depends on visual inspection and evaluation, permanent records of quantified data can be cumbersome.

 

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