Treading new ground

Mechanical engineers at PurdueUniversityhave developed a system that uses sensors and mathematical models to detect defects in newly manufactured tyres better than conventional inspections.

The diagnostic technique works by analyzing vibration waves passing through a tyre to detect damage that leads to cracks in the bead area, where the tyre connects to the steel rim of the wheel. A crack will sometimes form in the bead area and spread entirely around the tyre, causing the tyre to lose air or otherwise fail, said Douglas E. Adams, an associate professor of mechanical engineering who developed the system with doctoral student Timothy J. Johnson.

The US National Highway Traffic Safety Administration issued the new tyre-durability standards covering aspects including the detection of bead-area damage after Congress in 2000 passed the TREAD Act, for Transportation Recall Enhancement, Accountability and Documentation. The law was prompted by a series of SUV accidents in the 1990s, some fatal, which resulted in the recall of 6.5 million tyres.

“The fatigue endurance testing needed to ensure that all automotive tyres meet the new durability requirements is time consuming and costly,” Adams said. “And because the testing is carried out by technicians conducting manual inspections, the results can vary based on a technician’s skill and other factors.

“The bottom line is that there hasn’t been any objective way to determine when the tyre has bead-area damage.”

Johnson will present a research paper about the diagnostic technique on April 6 during the Society of Automotive Engineers’ 2006 World Congress in Detroit. Adams will give a talk on April 7 focusing on developing better mathematical models and simulations.

“When you build a mathematical model of a tyre, part of the purpose is to predict how a crack will spread once it starts,” said Adams. “But another big part of the model is to determine how the crack gets started in the first place. If industry can better understand how cracks initiate, manufacturers can build a better tyre.”

Such models promise to help industry improve the tyre-testing process. Manufacturers now randomly test new tyres using rollers to create punishing forces and conditions designed to push tyres to their limits and speed up the wear caused by actual driving.

“Periodically, the technicians will stop the test and manually inspect a tyre by passing their hands over the tyre bead area to feel for little air bubbles,” Adams said.

The bubbles form when rubber in the bead area separates from underlying layers.

“These tests will go on for several days on a single tyre to determine the tyre’s durability, and once the bubbles appear, the test is stopped,” Adams said. “You have to have somebody in the plant 24 hours a day doing this testing. Manufacturers would rather automate the testing process, which would enable them to detect bead-area damage at its very earliest stages, saving time and money.”

The Purdue researchers compared their diagnostic system’s performance to a technician’s manual inspection at a tyre-manufacturing plant. The technician discovered bubbles in a tyre after two and a half days of mechanical endurance testing, and Purdue’s system detected damage in the same tyre about 12 hours into the test, or one-fifth the time it took for the manual inspection.

“Detecting damage a day and a half earlier than the technician found it means this kind of system should save a lot of time and money,” Adams said. “We are able to do this because we can detect damage before the bubble actually forms.”

The Purdue engineers developed their diagnostic system using an SUV parked over a testing apparatus so that the left front tyre sat on rollers spun by an electric motor at 40 mph, while a hydraulic device exerted downward force on the tyre to simulate the weight of passengers. The researchers also heated the tyre with a portable furnace to mimic worst-case driving conditions and compared the difference between damaged and undamaged tyres.

“Damage causes a decrease in tyre stiffness because a tyre that has a crack in the bead area doesn’t resist as much when you push down on it,” Adams said. “That’s significant because the more the tyre squishes, the less able it is to resist the growth of the crack.”

As the test tyre rotated, the engineers used sensors called tri-axial accelerometers attached to the axle to pick up vibration waves passing through the tyre. The data were fed to a computer, where software containing algorithms interpreted the information.

Damage to the bead area causes the tyre to rotate unevenly, and data from vibration waves reveal this slight wobble. The data were displayed on a computer monitor in a “wavelet map” that enabled the engineers to not only detect damage but also possibly pinpoint its location on the tyre.

The technique might lead to diagnostic systems in cars and military vehicles that would constantly analyze vibration passing through tyres and alert a driver when the bead area has been damaged. The same general method also offers promise in various applications unrelated to tyres, including quality assurance in engine manufacturing and detecting damage in jet engines and electrical-generating turbines in hydroelectric and nuclear power plants.

“The method is generic enough that it could be used to detect damage in any mechanical system that rotates,” Adams said.