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The Case of the Lamed Lothario
Kenneth Russell, Professor Emeritus, MIT, Cambridge, MA -- Design News, February 6, 2005
Prior to the Salk vaccine, the word "polio" struck terror into the hearts of parents. Children, especially, became horribly ill from polio, but none knew why or how. Mothers tried to keep their children from places, such as public swimming pools, where they suspected polio might be spread. The mothers were right in this case as the disease turned out to be spread largely by infants fouling the water in public pools.
This case involved a knee that had been crippled by polio. The brace was belted to the thigh and hinged at the knee, and had tabs extending under the user's shoe. The brace had a slip lock to join the upper and lower portions to allow walking. The lock was released to allow the leg to bend during sitting.
The user complained that the lock came undone while walking on sand and brought the brace back to the dealer several times for modification. Finally the brace snapped at the knee joint, causing a nasty fall. The user sued: I was retained as a metallurgical expert by his attorney.
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The figure on the left is a view of the front of the brace hinge. The lock (not visible) slid down from the top and ended just above the head of the hinge pin, seen to the right. The lower part of the brace was cantilevered from this lock. The fracture was through the lower part of the brace and through the hinge-pin hole, as would be expected from the nature of the loading.
The right figure is a close-up photo of the front part of the fracture surface. The dark region near the top is part of the hinge-pin hole. Fatigue beach marks are seen to emanate from the lower right corner of the fracture surface. The brace failed from a fatigue crack that started at this corner.
But why was the brace subject to early fatigue failure? In a similar consulting case, the user was a 300-lb woman, also a polio victim, whose brace failed on both sides. The user was simply too heavy for the brace to support. The plaintiff in this case was of ordinary weight and the brace was relatively new.
Study of the left photo shows the culprit. Someone had done a hack job of modifying the brace. Most of the markings appear to be from a file, presumably made in an attempt to get the lock to work properly. There are also some notches, as if the brace were whacked with the edge of the file. One such notch is clearly seen at the right side of the fracture surface and is visible both above and below the break. This notch is what is known as a "stress riser," which multiplied the nominal stress several fold and greatly aided the formation of a fatigue crack.
The fatigue crack further extended the notch, so the stress just got higher and higher and the crack propagated faster and faster with each loading cycle. Ultimately the strength of the brace was so reduced that the remaining metal failed suddenly by ductile overload. The roughly 20 percent of the fracture to the top left in the right figure was by overload. Once the front part of the brace broke, the material to the rear of the hinge-pin hole quickly fractured.
But who would do such a dreadful job of filing? I suspect that the user took modifications into his own hands, perhaps out of frustration with the dealer's efforts. I find it hard to believe that the dealer did such an amateurish job. The case was settled without trial, but I do not know the terms of the settlement.
Whence the title of this column? A (then) junior colleague was retained as metallurgical expert by the defense. The attorney remarked that the plaintiff had a reputation of being a bit of a bedroom athlete and asked my colleague if such activity might have led to premature failure of the brace. The answer, of course, was "no." My colleague passed the exchange on to me after the case settled.
I rather suspect that such braces are removed before retiring, whether for sleep or other reasons.
