Essential NDT

NDT expert Joseph Stump describes everything you need to know to manage non-destructive testing/inspection processes in your shop or for when sub-contractors are supplying NDT services.

Non-destructive inspection (NDI) has been an integral part of aviation maintenance safety for more than 60 years. NDI encompasses a wide spectrum of inspection disciplines and procedures that range from the conventional to the truly unique. NDI has experienced unparalleled advancement over the last quarter century in both application and innovation. The contribution non-destructive evaluation has made to aviation safety surpasses any existing technology to date.

Non-destructive inspection implies a method that will detect defective or unserviceable items while avoiding damage to useful parts and assemblies. This article will focus on the five most widely employed testing disciplines that represent the mainstay of the aviation maintenance and manufacturing industries.

Magnetic particle inspection

In the early 1920s, William Hoke discovered a process that entailed magnetizing machined steel parts while applying colored ferromagnetic powders to the metal's surface. The powders gathered around cracks and other surface flaws, making them readily visible. By the 1930s, this method of inspection was adopted by the railroad industry where it was used extensively to inspect boilers, wheels, axles, and track. Aviation was soon to follow.

Today's magnetic particle inspection (MPI) is a bit more sophisticated than William Hoke's machine shop of yesteryear, but the principles remain unchanged. The concepts behind magnetic particle testing are straightforward. The part under test is magnetized with low voltage, high amperage equipment to a specified value. A mixture of base oil and fluorescent iron-oxide particles is then applied over the entire surface of the test article. If a flaw is present, such as a stress-related fracture, a magnetic leakage field forms around the defect. The leakage field establishes its own pair of north and south poles. It is here the magnetic lines of flux are the strongest. The fluorescent iron-oxide particles migrate to the defect by way of the leakage field. They congregate at the site where the break in the alloy's crystalline structure occurs. Visual examination by ultraviolet light readily detects the flaw.

For the inspection to be thorough, the part must be inspected in two directions. That is to say, a magnetic field must be established circumferentially as well as along the longitudinal axis of the specimen. Reliable observation of discontinuities can only be accomplished if the defect intersects the magnetic lines of flux at a favorable angle. This usually ranges from 45 to 90 degrees.

In aviation, MPI has been traditionally used to inspect such fracture-critical items as crankshafts, connecting rods, camshafts, engine mounts, and landing gear components. On jet engines, turbine shafts, entire gearbox assemblies, gear-reduction components, and compressor disks are just a few of the items that may be inspected. Components will vary with the type of engine and alloy composition.

MPI is appropriate for ferromagnetic materials such as iron, nickel, and cobalt alloys, but it is not suited for alloys of copper, aluminum, magnesium, titanium, and austenitic stainless steels. MPI is economical and adaptable to field work as well as the laboratory environment.

Eddy current inspection

In 1832, English scientist Michael Faraday was credited with the discovery of electromagnetic induction. While this did not immediately lead to the invention of the first eddy current phase analysis test system, there is no question we could not have done it without him.

The idea for an eddy current test method had been around for some time, but it wasn't until the 1940s that it became crudely functional. By the 1960s, the technology had sufficiently advanced to adapt to a wide range of industrial applications, including aviation. Eddy current evaluation introduced new possibilities in testing methodology that could not be accomplished by other disciplines.

Eddy currents are electromagnetically induced currents; therefore, the method is limited to materials that are good conductors. A coil is mounted into a probe that is connected to the eddy current unit. The coil's magnetic field fluctuates into the test material at high frequency. This generates the circular flow of eddy currents, which produce a fluctuating magnetic field of their own. This field is in direct opposition to the field of the test coil and creates impedance. Any factor affecting the flow of eddy currents causes a change in the impedance of the test coil. This registers on the test unit's display screen, meter, or CRT.

In aviation, eddy current evaluation has three primary functions: crack detection, conductivity testing, and coating thickness measurement.

Crack detection is the most widely used mode of test. Eddy current is an efficient and cost-effective method of finding stress-related fractures; it easily reads through paint and other organic coatings without the need to remove them.

Conductivity testing is one of eddy current's most outstanding applications. The eddy current unit measures the conductivity of nonferrous metals based on a system known as the international annealed copper standard. The ability to detect changes in conductivity allows the NDI technician to detect changes in heat treatment, work- hardening, annealing, and other detrimental metallurgical deviations before they become a serious concern.

Eddy current is principally a surface inspection method. Depth of penetration seldom exceeds a quarter of an inch. Eddy current density diminishes rapidly with depth, making it questionable for subsurface applications. However, there are specialized low frequency techniques that give this method some subsurface latitude.

Fluorescent penetrant inspection

In July 1942, the first successful fluorescent penetrant made its commercial debut under the trade name Zyglo. Manufactured by Magnaflux Corporation, this new industrial test system was quickly adopted by the Air Arms division of the U.S. Army, responsible for overseeing aircraft production. Some early applications included the following: bearings, supercharger components, tooling, exhaust valves, propellers, cylinder heads, and crankcases.

Fluorescent penetrant inspection (FPI) was developed to find discontinuities open to the surface in solid materials that are essentially nonporous. Penetrants are low viscosity liquids specifically designed to seep into the tightest hairline fractures by way of capillary action. The penetrant contains a fluorescent compound that illuminates when exposed to ultraviolet light. This gives FPI a significant edge over visible dyes that are often viewed under ambient light conditions. Visible dyes are for the most part unsuitable for aviation use. In most cases, these penetrants lack the sensitivity required to maintain a good margin of safety for flight-related hardware. They are forbidden in most NDI manuals and aircraft specifications.

To conduct a successful FPI evaluation, the surface of the part must be clean and free from grease, dirt, oil, and paint as these inhibit the penetrant's ability to enter open discontinuities. The penetrant is applied and allowed to dwell for a period of time dictated by the applicable procedure. The excess surface penetrant is then removed and a developer applied to visually enhance any indications that may appear.

Before the inspection can progress, black (ultraviolet) light and white light values must be monitored and recorded. Radiometers are used to measure black light intensity and ensure proper output. If the readings fall below specified values, the black light will be weak and fail to provide proper illumination of the dye. Photometers measure white light. If ambient white light conditions are excessive, the fluorescence of the dye will again become compromised. This can be overcome by tenting the inspection area.

Penetrant is a cost-effective and inexpensive method. The major drawback to this process lies in its inherent inability to located discontinuities below the surface. It may also be incompatible with some materials.

Radiographic inspection

In the 1920s, radiography was practically the only nondestructive testing method that was capable of supplying critical and accurate information about the soundness of a part. While still in its infancy, it was used to examine the soundness of welds, castings, and forgings. Steel products were difficult to inspect due to the low power of the machines of that era. Great technical strides in application and innovation were yet to come.

Today, X-ray evaluation of aging and contemporary aircraft has proven to be enormously beneficial. It is employed to detect cracks, corrosion, assess internal damage, and detect foreign objects in airframe structures and powerplants.

Before an inspection can be performed, the aircraft and surrounding area must be evacuated to avoid exposing maintenance personnel to the effects of ionizing radiation. The importance of safety practices and procedures cannot be overstated. However, the risk to maintenance personnel and the general public is negligible provided all warning signs and established safety barriers are heeded.

The function of the X-ray tube is to convert electrical energy into X-rays. The output of the tube is rated in kilovolts or (KV), the higher the KV, the more powerful the tube. Most aircraft tubes run approximately 150 KV. This is a relatively modest energy level.

The exposed film or "radiograph" is the heart of the inspection. The X-ray film is composed of a sheet of clear cellulose or triacetate that is treated on both sides with an emulsion of gelatin and silver halide compounds. When exposed to X radiation, gamma rays, or light, these compounds undergo a chemical change. When the exposed film is treated in a chemical solution (developer), further reaction takes place. The silver halide compounds form tiny crystals of black metallic silver. It is this silver, suspended in gelatin on both sides of the triacetate base, that form the radiographic image.

The radiograph is about "density" (light and dark) as well as image. The film resembles a photographic negative. Thinner sections of material will appear darker than thicker ones. Thicker cross sections absorb radiation more readily for any given material. This diminishes the X-rays' ability to expose the film. For example, a badly corroded section of aircraft aluminum loses mass due to oxidation and the build-up of corrosion products. On a radiograph, the areas most severely pitted, exfoliated, or affected by intergranular attack will appear darker than the rest of the test specimen. As the material thins due to the corrosive interaction of alkalis, acids, and salts, more radiation will naturally come in contact with the film. Areas largely unaffected by corrosive elements will appear lighter on the film as its mass is retained.

Ultrasonic inspection��������

Ultrasonic principles have been used since the 1930s when its potential was first researched in pre-war Germany.

Ultrasonic testing introduces high frequency sound waves into the test material to detect subsurface discontinuities. Transducers are used both to transmit and receive sound energy in the test article. Test frequencies between 1 and 25 MHz are normally employed. Cracks, laminations, shrinkage cavities, forging bursts, porosity, bonding faults, and other discontinuities with established metal-gas interfaces can easily be detected. Common aircraft applications include thickness testing, crack detection, and corrosion evaluation.

Pulse echo type test equipment is by far the most commonly used in aviation. In pulse-echo inspection, short bursts of ultrasonic energy are introduced into the test item at regular intervals. If the pulse encounters a reflecting surface (flaw), all or part of the energy is reflected. The amount of energy reflected is a function of the size of the flaw in relation to the size of the incident beam. The direction of the reflected beam (echo) depends on the orientation of the flaw. The reflected sound wave is monitored on the equipment's display. The amount of energy reflected, and the time delay between transmission of the initial pulse and arrival of the echo are measured. Some advantages of the ultrasonic testing method include high penetrating capability, high sensitivity and resolution, portability, single surface accessibility, and the immediate interpretation of test results.

There are drawbacks. The successful operation of the test equipment requires experienced technicians. Extensive knowledge and reference standards are needed to calibrate the equipment and simulate defects. Parts that are roughly finished or geometrically irregular are difficult to inspect. Shallow discontinuities lying immediately beneath the surface may not be detected due to anomalies in sound wave intensity.