Fatigue Cracks Cited as Source on Tire Failures
An examination of recalled Firestone tires and the factories where they were made shows that the tires developed internal cracks that eventually caused them to lose their treads, according to an independent expert hired by Bridgestone/Firestone Inc. to investigate the series of tire failures.
The cracks spread to the area between the tires’ two steel belts, said Sanjay Govindjee, an associate professor in the Department of Civil and Environmental Engineering at the UC Berkeley, in a memo to Bridgestone/Firestone that the tire maker released Monday.
“My present focus is on the inter-belt materials and their fatigue properties,” Govindjee wrote. “All evidence to date points to a slowly developing fatigue crack that propagates through the belt wedge material and then subsequently into the belt skim between the steel belts. At some stage the cracks reach a critical size and the tires subsequently fail.”
Belt wedges are circular wedges placed around the circumference of both sidewalls between the two steel belts to stiffen and strengthen the tire. Belt skim refers to the rubber that encases the two steel belts.
Govindjee said he was basing his investigation on three separate visits to Bridgestone/Firestone’s technical center in Akron, Ohio, where he examined failed tires, and inspections of Firestone factories in Decatur, Ill., and Wilson, N.C.
Govindjee’s status report, requested by Bridgestone/Firestone, was the first information released by the company about the possible cause of the tire failures, which are being investigated by the government in connection with at least 101 deaths in accidents mostly involving the Ford Explorer sport-utility vehicle.
The comments also were the first to emerge from a detailed analysis of Firestone tires and the plants that made them. Bad tires have been traced in particular to the Decatur plant. Firestone has not given an explanation for that factory’s comparatively high production rate of failed tires, but some plaintiff attorneys and consumer activist groups say the plant’s use of less-skilled, temporary replacement workers during a strike in the mid-1990s could have played a role.
Fatigue cracks could be caused by defects in design and/or manufacturing, but they are not the only explanation, said Harold Herzlich, a former tire compounder at Goodyear and Pirelli and now an independent tire consultant.
“This is a very common type of crack propagation that is due to underinflation operation,” Herzlich said in a telephone interview. “When a tire is underinflated, that is the exact area where the tire goes through strains that exceed the ability of the compound to absorb it; it starts tearing the compound in that area.”
Govindjee’s study “doesn’t exclude defect possibilities but also it doesn’t exclude the other possibility that it’s underinflation causing an overstressing of that area. It’s really not identifying the cause of the failures yet,” he said.
Ford and Firestone have maintained that low tire pressure, either deliberate or from neglect, probably contributed to the failure of the 14.4 million Firestone ATX, ATX II and Decatur-made Wilderness AT tires that were recalled in August. About 6.5 million were estimated to be on the road at that time, and more than 4 million have been replaced so far.
“The big question there is the age of the tires and what happens over a period of time,” said Max Nonnamaker, a former Firestone tire designer and now also an independent tire consultant.
“We’ve been saying for some time that you should never place a steel-belted radial tire in service that’s over 5 years old, or you should remove it from service after five years,” he said from his home in Ohio. Steel, though robust, nevertheless rusts and weakens over time, he said.
Herzlich agreed that the tires’ age could be an issue because “rubber, when overheated, will age more quickly and will go into its fatigue failure a lot more quickly.”
Bridgestone/Firestone spokeswoman Anitra Budd said the tire maker could not respond to questions about Govindjee’s letter. Govindjee did not return a phone call.
But he said in his letter that “the issue at hand is a quite complex interaction of the effects of tire design, the manufacturability of the tire, and loading conditions (dynamic loads associated with the Explorer, running inflation pressures and temperatures).”
Govindjee said he expected to have a preliminary technical report at the end of this month or in early November, and a final report near the beginning of the year. He said he was confident he would find an explanation for the tires’ tread loss.
