With the aerospace industry soaring into the 21st century, aircraft engineers and manufacturers face exciting challenges. The newest aerospace structures and materials require close manufacturing evaluation, and aging in-service aircraft need careful inspection and monitoring. The traditional “find and fix” method – where damage is found and immediately repaired on aircraft – often involves costly, destructive testing. Thankfully, significant advances in non-destructive testing (NDT) techniques have changed how the aerospace industry approaches aircraft maintainability, particularly metal fatigue.
The extraordinary phenomenon of “metal fatigue” has catastrophic, real-world impacts. In the 1840s, railway engineers experienced the effects of stress reversals when broken axels caused countless rail accidents. The axels, made of rotating horizontal shafts with heavy vertical loads, suffered severe metal fatigue with each rotation.
Aircraft manufacturers met the same ill fate nearly a century later. By the 1950s, passenger planes started flying higher and needed to pressurize the cabins. In 1954, tragedy struck when two de Havilland Comet aircraft mysteriously broke up midair. While the disaster all but destroyed the industry, aviation engineers discovered critical information about how metal fatigue impacts flight, and most importantly, how to prevent stresses and tears. Find out how Bruker combats metal fatigue through aircraft non-destructive testing!
Driven by more recent airline catastrophes – like United Flight 232, where a defect went undetected in an engine disk – efforts to ensure airworthiness continue to evolve. Aviation engineers have discovered that even the smallest material impurities and hardness nuances affect a plane’s fatigue life.
Ideally, planes would be made of steel alloys. When subjected to cyclic stress levels below their endurance limit, steel rarely fails from fatigue. In other words, a steel plane can theoretically have an infinite life. However, steel metal alloy is too heavy for the job. Aluminum alloys, used for their strength and lightness in aircraft construction today, pose the opposite dilemma. They cannot live indefinitely, and all aluminum alloys will fail sooner or later from fatigue.
So, what should an aircraft engineer do to verify the purity of alloys? Most have turned to aircraft non-destructive testing (NDT) practices. From flaw detection to leak determination to dimensional measurements, NDT is a technique that allows components to be tested for serviceability, without impairing their usefulness. In short, NDT enables aviation specialists to inspect and measure without doing harm.
One of the earliest methods of aircraft NDT relied on visual inspection. In remote visual testing, flexible borescopes (which contain a bundle of optical fibers) examine inaccessible cavities, like air inlets, turbine blades, and the combustion chamber. Here, good image quality is critically important. More sophisticated visual NDT testing introduces video borescopes, where a display shows the camera view. Digital models have an integrated recorder so images and videos can be saved and reexamined.