Jesse Herrin, is a mechanical design engineer and software developer who has spent 10 years working in product management and engineering roles at Zetec. Here, he takes a closer look at the role of NDT in aerospace applications.
Last year, when a fractured fan blade led to the mid-air failure of a CFM56-7B engine, resulting in the death of a passenger, the US Federal Aviation Administration issued an emergency airworthiness directive requiring operators of engines with more than 30,000 flight cycles to perform a one-time ultrasonic inspection of all 24 fan blades.
A similar failure two years prior prompted manufacturer, CFM International, to recommend ultrasonic inspections of fan blades on CFM56-7B engines with 20,000 cycles. All told, the FAA and CFM bulletins covered approximately 3,160 engines - 76,320 separate fan blades. The notices emphasised the importance of non-destructive testing (NDT) in aerospace manufacturing and aviation MRO.
NDT is a broad category of inspection methods ranging from sophisticated technologies that render 3D images of cracks, corrosion, and flaws in metal and composite materials to strictly manual techniques like liquid penetrant and magnetic particle testing.
Whether you contract NDT services as part of a quality assurance programme or your company handles its own inspections, it’s vital to understand the pros, cons, and latest developments in tools and techniques for aerospace and aviation applications.
Inspect and detect
Penetrant testing (PT) and magnetic particle testing (MT) have been used for decades as NDT techniques for finding surface-breaking defects in metal and other nonporous materials. These methods are common and affordable but each has its limitations. PT can detect only surface cracks and requires the technician to handle and dispose of chemicals, while MT is effective only on ferromagnetic materials.
Both techniques - including surface preparation and clean-up - are time-consuming and the results can vary depending on the skill of the inspector, especially when the work environment is hazardous, uncomfortable, or hard to reach. Neither one produces a detailed digital record of the inspection results.
In terms of sophistication, ultrasonic testing is at the other end of the spectrum compared to PT and MT. Ultrasonic instruments and probes use high-frequency sound energy to find indications in a variety of materials including composites and anisotropics, where porosities, delamination, and foreign bodies can occur on the surface or within a particular layer of fibre or resin. In some cases, these defects can be introduced during the materials-manufacturing process, which makes ultrasonic testing well-suited to quality control. Results are immediate, precise, and in a digital format that technicians can share, store, process, and compare at any time.
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. Today, it’s perhaps the most effective and cost-efficient non-destructive testing technology for inspecting skins, stringers, frames, rivet holes, tubing, and other ferrous and non-ferrous components in aerospace and aviation.
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 captured, analysed by an instrument, and displayed in a digital graphic format for the technician to review.
Multi-coil array 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 non-destructive 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. It has interchangeable array-coil sets so technicians can swap in precisely the right type of coil for the test application: for example, there’s 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. Surf-X array probes can reduce inspection times by up to 95% versus pencil probes.
The evolution of tools
In aviation, where efficiency is critical and inspection points can be physically difficult to reach, NDT tools need to be portable and easy to handle yet not compromise data-acquisition speed and performance.
Today, eddy current instruments are evolving in several important ways; Ergonomics: a focus on ergonomics has produced eddy current tools that are lighter and less fatiguing to use. One test of an instrument’s ergonomics and portability is how easy it is to operate with only one hand; Display: a handheld tool with a C-scan display can present a digital ‘big picture’ that helps inspectors find more flaws in less time. For example, the Zetec MIZ-21C handheld eddy current instrument has a display that’s large enough to handle a range of modes, including two signals side by side, or a reference signal and a live test signal simultaneously. A touch screen can provide additional functionality and convenience; Connectivity: the more advanced portable eddy current instruments today have USB, Wi-Fi, and/or Bluetooth connectivity as well as multiple channels to support different probe types; Software: the ability to store different eddy current test configurations within the instrument and to manipulate signals and views for each test application are keys to a fast, efficient inspection. The signal-to-noise ratio (SNR) of many eddy current instruments limits their ability to detect very small flaws and the loss of material that can occur in friction stir welds and multi-layer structures. Software that supports single- and dual-frequency eddy current, rotating scanners, and conductivity with the viewing tools of Impedance, Sweep, Waterfall, and C-Scans can help maximise the instrument’s ability to manage SNR and deliver an accurate, detailed inspection result.
As maintenance requirements become more rigorous, NDT plays a vital role in identifying problems before they affect uptime and the safe operation of 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.