US-1779 Aircraft Tie Bolt Inspection System

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.

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 pre-flight 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 failure-prone 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.

Magnetic particle testing reveals surface defects by means of fine ferromagnetic particles applied to a magnetized tie bolt. Because defects in the bolt surface interrupt magnetic field flux lines and reverse field polarity across the interruption, particles around a defect break the linear pattern of the particles on undamaged sections of the bolt resulting in irregularities in the particle pattern surrounding the defect. Like LP testing, MT requires visual inspection, which makes quantification and recording of results less efficient than automated systems.

Because magnetic particle and liquid penetrant testing indicate defect length and width, but not the depth, they tend to indicate shallow defects that may not affect bolt integrity, resulting in the rejection of serviceable bolts. This makes the process somewhat less cost efficient than other methods. Bolts rejected by MP and PT may pass eddy current or ultrasound testing, because those methods account for flaw depth in addition to length and width.

Ultrasound testing detects variations in the velocity of returning signals from very short bursts of ultrasonic waves propagated into the body of the bolt to identify flaws at the surface and within the body. Being a volumetric NDT method, UT is better at differentiating deep flaws from shallow ones. However, it cannot sense cracks as small as those detected using eddy current testing.

Eddy current testing
Within the past decade, advances in materials science and computer technology enabled improving eddy current testing to a level competitive with other NDT methods as a leading test method of choice. Eddy current testing uses an alternating electric current in a coil positioned near a target metal surface to generate a pulsating magnetic field in the metal. This, in turn, modulates the electrical frequency of the original current in a consistent way until a flaw in the metal surface or body disrupts the magnetic domain and distorts the eddy current. The distorted eddy current is transduced to a digital signal and displayed as a geometric eccentricity on the readout screen.

Unlike NDT systems that require additive materials to visualize defects, eddy current testing uses the material under inspection (the metal itself) as an integral part of the electronic testing circuitry. Unlike liquid penetrant and magnetic particle testing, which require direct visual inspection, application of image-enhancing media, intensive pretest cleaning, post inspection cleaning, and waste disposal, eddy current testing requires minimal pre-cleaning, no detection media, no clean up, and no waste disposal.
Although eddy current testing instruments can be used on any metallic material, in the aerospace industry, alloy steel and Inconel superalloys are the most widely used metals for tie bolts. Because the magnetic and electrical characteristics of these materials are different, eddy current systems must be adaptable to both materials.

Reliable test performance  
UniWest calibrates maximum acceptable flaw values on its eddy current testing instruments by correlating a software null point with a 0.03 x 0.03-in. electrodischarge machined (EDM) notch cut to a depth of 0.060 in. for steel bolts. The null point is set to 0.02 x 0.02 in. for Inconel bolts because the material is not ferromagnetic and because cracks propagate at different angles and more rapidly than in steel. Bolts containing cracks larger than the null points are subject to rejection, either by automatic signal or operator judgment. Once the signal processing unit is properly calibrated, software filters allow modulation of parameters for different metals and user preferences.

UniWest’s eddy current Tie Bolt Inspection System, which includes an eddy current instrument (such as our EddyView II), the US-1779 Tie Bolt Scanner, and US-1839 specialty probe, is used to perform eddy current inspection of the threaded area of the tie bolt and the underside of the bolt head. “As of now, the Tie Bolt System is in demand in commercial and military aircraft industries, as well as wheel and brake shops,” noted Kurt Oldson, president of UniWest, “but this system certainly can be used for other applications requiring testing of bolt threads.”