July 14, 2004
Method aims to improve aviation safety by monitoring rivets
WEST LAFAYETTE, Ind. Commercial airliners have up to a million rivets, some of which are unavoidably created with imperfections that make them more susceptible to corrosion, leading to cracks that could result in serious failures.
Riveting in new aircraft is done with automated machines, whereas repairs to older aircraft are performed manually by two workers, one wielding the rivet gun while the other holds a "bucking bar" on the opposite side. As the riveter pushes the rivet through the hole, the other worker positions the bucking bar on the opposite side of the hole so that the rivet hits the bar and is hammered snugly into place.
"This happens very quickly, roughly three seconds for each rivet, in rapid-fire succession," said Douglas E. Adams, an assistant professor of mechanical engineering at Purdue University. "Inevitably, some of the rivets are created with flaws."
Sometimes a worker holds the rivet gun at the wrong angle, causing the rivet to go in sideways. At other times, the riveter holds the gun's trigger a few seconds too long or pushes too hard on the gun, creating indentations around the rivet in the skin of the aircraft's fuselage, or the worker holding the bucking bar sometimes pulls it away prematurely, preventing the rivet from forming completely.
"Even the automated machines can introduce flaws that are not detected," Adams said.
Engineers and technicians have no way of knowing which rivets are inferior, but Adams is collaborating with the U.S. Air Force Research Laboratory/Materials and Manufacturing Directorate (AFRL/ML) and researchers in Purdue's School of Aviation Technology to develop a system that tracks the quality of every rivet. Sensors attached to the rivet gun record information that can later be interpreted by a computer to determine the rivets' quality.
Findings about the method were detailed in a paper presented July 7 during the Second European Workshop on Structural Health Monitoring in Munich, Germany. The paper was written by Adams; Kumar Jata, an engineer at the AFRL/ML; Ronald Sterkenburg, an assistant professor of aviation technology at Purdue; Timothy J. Johnson, Purdue mechanical engineering doctoral student; and undergraduate student Raymond Manning.
As larger commercial aircraft are being built, the ability to monitor the quality of rivets is becoming increasingly important, Adams said.
"One of the great challenges when you've got something with a million rivets is how in the world do you identify the bad ones?" Adams said. "Talk about a hard problem.
"One quick way to observe obvious surface corrosion is to walk around the aircraft and visually inspect it. You often see a pilot walking around an aircraft before takeoff. One thing pilots look for is signs of corrosion and other defects around rivet holes. To find hidden corrosion requires many labor hours and sophisticated non-destructive evaluation methods."
The likelihood of catching corrosion sites before they become dangerous should not be left up to visual inspections and other conventional methods, said Adams, who specializes in a field known as "structural health monitoring," or using sensors and mathematical techniques to assess the integrity of a structure.
The researchers have shown how to use a rivet gun equipped with sensors called accelerometers to monitor the riveting process. As each rivet is created, data about its quality are recorded and stored in a computer.
"We are trying to use a technique like this to make a map that shows where the sub-par rivets are," Adams said. "Such maps would help to guide the ground inspections by alerting technicians to locations that are more susceptible to corrosion."
Ground crews could focus on these areas when inspecting aircraft, he said.
The researchers showed how data from the sensors could be used to accurately identify good and inferior rivets.
"After some more development and commercialization of this technique, such a system could be used for military aircraft," said Jata, a technical adviser in the Metals, Ceramics and NDE Division of the Air Force lab's Materials and Manufacturing Directorate at Wright-Patterson Air Force Base in Ohio.
NDE stands for non-destructive evaluation, or using various technologies to analyze a structure without taking it apart.
"Corrosion and corrosion-fatigue cracks emanate from fastener holes and, therefore, are dominant problems in the aircraft industry," Jata said. "If the rivets are loose, for instance, your structure is going to start rattling. You don't want that because you are causing fatigue."
As cracks develop and grow, they eventually link up and can cause catastrophic accidents in which the fuselage loses its strength and gives way, he said.
Sources: Douglas Adams, (765) 496-6033, firstname.lastname@example.org
Kumar Jata, (937) 255-1304, email@example.com
Vibration-Based Structural Health Monitoring of Tool-Part Interactions During Riveting Operations on an Aircraft Fuselage Structure
Conventional structural health monitoring methodologies focus on loads identification, damage identification and damage prognosis of structural systems for the purpose of nondestructive evaluation after those systems are manufactured and deployed. However, it is well-known that many manufacturing processes involving, for example, cutting, grinding and polishing in metal structures and forming in composite structures involve processing dynamics and, as such, can affect material quality (i.e., defect densities). Riveting operations for assembling aero structures and their effects on local material/part quality in the neighborhood of the holes are examined here. Physics-based models and experimental data are utilized to extract features for assessing rivet fitness in situ using vibration signals along the pneumatic riveting gun. Environmental, materials and other practical issues are discussed as they relate to rivet quality in air vehicles in general. It is demonstrated using rivet quality assurance indicators based on spectral transmission through the tool-part interface and human operator specifications that such techniques can be utilized to identify riveting processes with skewed delivery (gun or bar), circular indentations and bucking bar slip to define susceptibility to future damage in fuselage structures.