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The Case of the Ruptured Rim
Kenneth Russell, Contributing Editor -- Design News, February 5, 2007
America moves on many millions of rubber tires, each mated to a metallic rim that makes perhaps a billion rotations over its lifetime. During each rotation, stresses in the rim will oscillate between tension and compression, which may lead to a progressive failure phenomenon known as metal fatigue.
The Scene of the Crime
In the present case, a truck rim split at the angle where the outer flange joins the central portion of the rim. There was one 21-inch-long split, plus a number of smaller cracks. The inner tube had bulged out through the long crack and ruptured, leading to an accident. There was no evidence of accident damage or other trauma that could have contributed to the rim splitting. I was never told just what sort of damage the blowout caused, but I suspect it was only property loss.
The Investigation
I was retained by the rim manufacturer to determine whether defects in materials or workmanship led to premature rim failure. I used sections of the rim for spectrographic analysis, hardness testing, optical metallography and scanning electron microscopy. (All these tests have been described in past columns.) The testing showed the rim to be made of ordinary, mild steel that had been cold-formed from strip into the rim configuration. All this was in accordance with usual rim industry practice.
Scanning the electron microscope study of the fracture surface showed striations characteristic of fatigue failure. The striations are very fine, only about 10 millionths of an inch apart, so failure occurred only after many millions of stress cycles. The fracture lay generally on a circumference where the rim changes section from the flange to the central rim portion. One would expect failure at this point where the stress, due to the bending moment on the flange, is at maximum. The fracture surface zigs and zags back and forth, showing there were multiple, probably dozens of origins of the fatigue crack.
Fatigue starts from a so-called stress raiser, which is often a surface notch or crack introduced during manufacturing. A crack extends from the stress raiser and propagates little by little during cyclic loading. Finally, the piece is too weak to support the applied stress and sudden failure occurs.
The Smoking Gun
In the present case, there was no “smoking gun.” There was no machining gouge or manufacturing crack that would have led to early failure. The rim had been so heavily loaded, cracks started from a dozen or more tiny machine marks or other irregularities on the surface. Such irregularities are inevitable in the real world.
My study thus gave the rim manufacturer a clean bill of health. The rim was manufactured according to industry standards and was defect-free. Failure was due to overloading — the fault of the people who used the rim. It should have been easy for me to get up in court and tell my story. It should have been, but it wasn't.
Lawyers and engineers think very differently, which makes detailed preparation absolutely essential in presenting expert testimony. Usually, my lawyer-client and I spend hours or even days going back and forth over my studies and what they mean. I am then called as a direct witness and the attorney questions me about my work and what conclusions I draw from it. My work comes out ONLY in response to questions.
I spent several frustrating hours on the witness stand, not being asked the questions that would enable me to tell my story. The attorney simply did not know what questions to ask. He and I should have spent the previous evening going over my investigation, but did not. Instead, the divorced lawyer had spent the previous evening with his adolescent sons, who he rarely ever got to see.
The court decided against my client, not surprisingly. The president of the company didn't blame me for the court's decision, however — he was enough of an engineer to appreciate my helplessness on the witness stand.
| Author Information |
| Ken Russell (kenruss@mit.edu) is Professor Emeritus of Metallurgy and Nuclear Engineering at MIT. He specializes in physical metallurgy, forensic metallurgy and failure analysis. Cases presented here are drawn from his actual forensic files. |
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