Keeping planes flying longer
By Tim Kunkel
Three decades ago, manufacturers used expiration dates to determine when a part needed to be replaced. If a spar had more than 25,000 operating hours, it was probably discarded - even if there were no defects in the part. This had to be done to ensure safety. The growth of non-destructive testing (NDT) techniques, such as eddy current, liquid penetrant, and magnetic particle inspection, has given the industry another option - one that has saved aircraft owners considerable amounts of money, while maintaining the highest possible safety level.
In testing a part, an NDT inspector has many tools at his or her disposal. The most obvious is a simple visual inspection of a part. Obvious cracks, corrosion, and distortions don't require more advanced testing. Those parts are replaced immediately. Flaws deep in the metal, fatigue cracks, or very light surface corrosion, however, often require more advanced techniques to identify and measure. Today, there are five basic NDT methods used to identify these flaws - liquid penetrant, magnetic particle, eddy current, ultrasonic, and radiographic. Among these, the most commonly used in testing aviation parts are eddy current, liquid penetrant, and magnetic particle.
Eddy current can be the most effective at identifying and measuring the severity of flaws at or near the surface of a metal part. A small electromagnetic pulse (EMP) is injected into a part. Defects are measured when the path of the electricity meets a defect and causes a needle to jump or a graph to spike on a monitor.
However, to effectively test any component, the NDT inspector may use a variety of test methods - even if no defects are found using eddy current. NDT inspectors determine which tests to use based on a variety of factors, including the material being tested and the accessibility of the part.
Liquid penetrant testing
Liquid penetrant is an effective way to test ferrous and nonferrous materials for cracks, laps, cold sheets, porosity, corrosion, and other defects open to the surface. It is important that the part be thoroughly cleaned before the penetrant is applied. Cleaning methods include vapor degreasing, solvent wipe, detergent, or alkaline cleaners. Paints, varnishes, scale, carbon, and other contaminants not removed by these cleaning methods should be removed with chemical cleaners.
In conducting the test, the inspector applies a fluorescent liquid to a surface using a combination of spray and brush. The liquid is allowed to "dwell" for a set time. Depending on the substance used, this time can vary. Warm water is used to remove excess penetrant, and the part is dried with circulating warm air dryers. A developer is then added and allowed to dwell.
Once the part has been prepared using a liquid penetrant and the appropriate developer, it is examined in a darkened area using a ultraviolet (black) light capable of producing a minimum intensity of 1,200 microwatts per centimeter squared and a wave length between 3,200 and 3,800 angstrom units.
Under the ultraviolet light, surface defects are exaggerated and made visible. An experienced NDT inspector or technician can then determine whether any damage has occurred and compare it to the parameters set forth by the manufacturer to determine if a part needs to be discarded or replaced.
Magnetic particle inspection
In testing ferromagnetic materials, another effective method for identifying defects on or just under the surface is magnetic particle inspection. This test uses a longitudinal, circular, or multidirectional magnetic technique to gauge deformities or defects in castings, forgings, weldments, and wrought materials.
As with liquid penetrant testing, the first step in magnetic particle inspection is to prepare the surface being tested. This includes thorough cleaning and paint removal, but can also include grinding or bead blasting any surface irregularities that may mask test results.
Magnetic particle testing should always consist of at least two tests. During the second, the lines of the magnetic flux should be perpendicular to those used during the first examination.
Once an examination has begun, the magnetic pulse is passed continuously through a multiturn fixed coil that is wrapped around the part or section being examined. Three different particle types can be used - dry, wet, or fluorescent. Dry and wet particles are colored to provide an easily visible contrast with the part being tested. Fluorescent particles are examined under an ultraviolet light, much like a liquid penetrant test.
Either of these methods produces easily visible indications of defects on or near the surface through the clustering of the particles. An experienced NDT manager can interpret this information and compare it to manufacturer guidelines to make recommendations about specific parts.
Improving safety and product life
In the past 30 years, nondestructive testing has saved the aviation industry millions by keeping viable components from an early death on the scrap heap. At the same time, it has undoubtedly saved many lives by catching defects deep within a part that were previously undetectable. Using multiple tests, such as eddy current, liquid penetrant, magnetic particle, and others, we are saving money and ensuring a much higher safety level by identifying defects before they can become a problem.
About the author
Tim Kunkel is a Level III nondestructive testing manager with Stevens Aviation in Greenville, South Carolina. He has been with Stevens for more than 25 years and has been involved with nondestructive testing since 1985.
One of the most critical steps in the penetrant inspection process is preparing the part for inspection. All coatings, such as paints, varnishes, plating, and heavy oxides must be removed to ensure that defects are open on the surface of the part. If the parts have been machined, sanded, or blasted prior to the penetrant inspection, it is possible that a thin layer of metal may have smeared across the surface and closed off defects. It is even possible for metal smearing to occur as a result of cleaning operations such as grit or vapor blasting. This layer of metal smearing must be removed before inspection.
NDT Resource Center www.ndt-ed.org
Common nondestructive testing techniques used in the aviation industry
Eddy Current Testing
Uses an electromagnetic pulse to identify and measure flaws deep within a part.
Uses a fluorescent or colored dye to reveal surface flaws such as breaks, scratches, and corrosion.
Uses metal particles and magnetic current to detect discontinuities on or near the surface of a metal part.
Early MPI testing
The earliest known use of magnetism to inspect an object took place as early as 1868. Cannon barrels were checked for defects by magnetizing the barrel then sliding a magnetic compass along the barrel's length. These early inspectors were able to locate flaws in the barrels by monitoring the needle of the compass. This was a form of nondestructive testing but the term was not really used until some time after World War I.
NDT Resource Center, www.ndt-ed.org