In 2018, after a fractured fan blade led to the failure of a CFM56-7B engine on a Southwest Airlines 737, resulting in the death of a passenger, the U.S. Federal Aviation Administration directed a one-time ultrasonic inspection of all 24 fan blades on engines with more than 30,000 flight cycles.
A similar incident two years prior prompted the engine’s manufacturer, CFM International, to recom-mend inspections of fan blades on CFM56-7B engines (Fig. 1) with 20,000 cycles. All told, the FAA and CFM bulletins covered approximately 3,160 engines, or 76,320 separate fan blades.
The notices emphasize the importance of nondestructive testing (NDT) as a way of detecting fatigue, manufacturing flaws, heat-related damage and other issues in aviation production, maintenance, repair and overhaul.
NDT is a broad category of inspection methods ranging from sophisticated digital technologies that produce detailed images of cracks, corrosion, metallurgical changes and manufacturing flaws to strictly manual and visual techniques.
Whether you contract NDT services as part of a quality-assurance program or your organization han-dles its own inspections, it’s vital to understand the pros, cons and latest developments in tools and techniques for aerospace and aviation applications.
Compared to liquid penetrants, phased-array ultrasonics are on the other end of the sophistication scale. Instruments and software can render clear 3-D images of defects, such as corrosion in a pipe (courtesy Zetec).
PT and MT
Penetrant testing (PT) and magnetic-particle testing (MT) are two common methods for finding sur-face-breaking defects in metal and other nonporous materials.
PT involves applying colored liquid to the test material and allowing capillary action to pull the fluid into cracks and other openings on the surface. The liquid remains in these discontinuities after the excess is removed and becomes visible under ultraviolet light or by the contrasting color of the dye being used.
With MT, the technician applies very fine ferromagnetic particles to the surface and then uses a mag-netic current to draw the particles into cracks and other flaws.
Each method can be used to test components and large surface areas with complex shapes, and the inspection steps are straightforward. Both have limitations, however.
PT and MT require a multi-step process, including surface prep and cleanup. PT can detect only sur-face cracks and requires the technician to remove paint and other coatings from the test area. Once the inspection is complete, someone has to reapply any paint or coatings before the part goes back into service.
MT is only effective on surface and near-surface crack indications in ferromagnetic material. The test is also sensitive to the amount of current amperage, orientations of magnetization and concentration of ferromagnetic particles.
The quality of the inspection with either method depends on the skill of the technician, especially when the work environment is hazardous, uncomfortable or hard to reach.
Eddy-current testing is a cost-efficient way to test skins, stringers, frames, rivet holes, tubing and other ferrous and nonferrous components in aerospace and aviation. The latest generation of test instruments are portable and easy to handle, with a color touchscreen that allows technicians to analyze more areas faster with minimal fatigue (courtesy Zetec).
In terms of sophistication, ultrasonic testing is on the other end of the spectrum compared to PT and MT.
Ultrasonic testing (UT) instruments and probes use high-frequency sound energy to indicate flaws both on and beneath the surface. UT is effective on a wide range of materials, including lightweight compo-sites and anisotropics, where porosities, de-lamination and foreign bodies can occur within a layer of fiber or resin. These defects can be introduced during the materials-manufacturing process, which makes UT especially well-suited to quality control.
There are two UT methods used in aerospace inspections today.
Conventional or Standard UT
In simple terms, a conventional or standard UT probe has an electronic element with a fixed focal point and angle that’s capable of generating and receiving a single ultrasonic beam.
Phased-Array UT (PA UT)
Phased-array UT is an advanced inspection method that uses multiple individual elements (typically from 16 to 64) in a single probe. By exciting each element in a controlled manner, a PA UT instrument can produce a precise beam shape and generate views of a flaw with greater speed and accuracy (Fig. 2).
The latest generation of UT instruments and probes gives technicians a range of methods they can use to conduct inspections in the field quickly and thoroughly. The inspection results are recorded in a digital format, which means they can apply different processing algorithms immediately or in the future. Because of advancements in software, portability and battery life, technicians can bring this computing power virtually anywhere.
Eddy-current testing requires close contact with the material under test. Flexible-array probes conform to curves and complex geometries common to aerospace and aviation (courtesy Zetec).
Eddy-current testing is a fast, accurate, chemical-free method for detecting a variety of surface and sub-surface defects, including cracks, corrosion and heat damage. It’s perhaps the most effective and cost-efficient NDT technology for skins, stringers, frames, rivet holes, tubing and other ferrous and nonferrous components in aerospace and aviation (Fig. 3).
In simple terms, eddy-current testing involves placing a probe or coil to a metal surface. The probe generates a changing electromagnetic field that induces electrons to flow in the material. Any cracks or changes in metallurgical structure will distort the flow like eddies in a river. These distortions are cap-tured, analyzed by an instrument and displayed in a digital graphic format for the technician to review.
In aviation, where efficiency is critical and inspection points can be physically difficult to reach, ed-dy-current tools need to be portable and easy to handle without compromising data-acquisition speed and performance.
An ergonomic design, small form factor and light weight allow technicians to inspect more areas faster with minimal fatigue. One test of an instrument’s ergonomics and portability is how easy it is to operate with only one hand — indeed, one gloved hand.
A large, colorful, sunlight-readable C-scan display can present a digital “big picture” that helps the technician find more flaw indications in less time. Taking cues from consumer electronics, a touchscreen interface allows technicians to further increase their efficiency.
Battery life is important. A 10-hour charge allows technicians to work an entire shift without stopping to change batteries, and hot-swappable batteries mean the instrument does not need to be powered down first.
Scanners make it easier to run test probes over large surface areas and complex geometries, like riv-eted pieces on a wing. This scanner has a 24-inch motorized actuator arm moving on a flexible 54-inch track affixed to the surface by non-marring suction cups, each with an independent Venturi system that optimizes the vacuum level. The actuator arm and track allow for automated raster scanning in multiple directions and parallel or perpendicular probe orientation (courtesy Zetec).
Multi-coil Array Probes
The more sophisticated eddy-current instruments are compatible with multi-coil array probes. These probes have multiple coils in one assembly, positioned at longitudinal, transverse or off-axis orientations and firing at coordinated times. They allow technicians to capture more information in a single pass and dramatically increase the speed, accuracy and repeatability of nondestructive tests, especially on large inspection areas like fuselage skin panels.
Today, there are multi-coil array probes developed for specific inspection applications.
One example is Zetec’s Surf-X family of array probes, which has a unique circuit and coil technology that allows the probe to flex and conform to different surfaces and shapes (Fig. 4). It has inter-changeable array-coil sets so technicians can swap in precisely the right type of coil for the test application. For example, there is a coil set that can curve around tubing or on the surface of a wing or fuselage; another for skins that use multi-layer materials; and yet another for components with complex geometries like turbine dovetails. These array probes can reduce inspection time in aerospace applications by up to 95% versus pencil probes.
As maintenance requirements become more rigorous, NDT plays a vital role in identifying problems before they affect uptime and the safe operation of the aircraft. The latest tools, software and probes can help manufacturers, MRO managers and NDT technicians save time and money, inspect more areas faster and keep assets in the air.
Author Jesse Herrin is mechanical design engineer and software developer for Zetec Inc. 8226 Bracken Pl. S.E., Suite 100, Snoqualmie, WA 98065 USA. He may be contacted at 425-974-2700 or firstname.lastname@example.org. For additional information, visit www.Zetec.com.