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The Case of the Misplaced Trust
True stories
Kenneth Russell, Professor Emeritus, MIT, Cambridge, MA -- Design News, April 20, 2003
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Many people fear heights. Some call this fear acrophobia, others call it common sense. A suburban Boston high school attempted to ease this fear in its students with a confidence-building scheme. A 2-inch-diameter rope was attached to a heavy eyehook which extended from the 30-ft-high gym ceiling. A seat was attached 20 ft up one gym wall, where the student could sit prior to grasping the rope and swinging by it. The student was to swing in a great arc, with the nadir some 10 ft above the gym floor The student was to swing back and forth, high in the air, prior to sliding down the rope to the floor.
The student in this case was induced to climb the wall and take his place in the seat. The nature of the inducement was never stated, but a severe acrophobe would have required a cattle prod. The student had just started his first arc when the eye bolt snapped, sending him plummeting 20 ft on to the gym floor, severely mangled his knees in the process. So much for confidence building.
The parents of the injured boy were understandably upset and decided to sue the parties responsible. They hired a lawyer, who in turn hired me to investigate the metallurgical aspects of the eye bolt fracture. The various potential defendants also hired lawyers, who in turn hired my fellow consulting metallurgists for the ensuing study.
We employed the usual tools: chemical analysis, impact testing, optical microscopy, and scanning electron microscopy. Our study showed the bolt to be of cast steel with about 0.4% carbon. Optical microscopy of a polished and etched section showed a sound microstructure with the distribution of iron crystals and iron carbide crystals expected of that composition steel.
Scanning electron microscopy told a different story. The bolt had broken in a threaded section where the roots of the threads were heavily cracked. The "rock candy" appearance of the fracture surface in an electron micrograph is characteristic of a brittle failure. Cracks were seen running into the fracture surface. A fracture of this sort is to be expected in a brittle material, such as ordinary gray cast iron. Steels are supposed to be ductile and should exhibit a rough, dimpled fracture surface resembling a fork-split English muffin, though on a much finer scale. Impact testing at room temperature also showed a brittle fracture mode with very little energy absorption.
One expert concluded that the brittle fracture was due to the bolt being a casting. This conclusion is erroneous. The bolt was sound, with none of the defects which may bedevil a poorly designed and manufactured casting. In fact, forming by casting was preferable to bending a drawn rod to the needed radius and welding it to complete the eye. The bending and welding could both cause problems of their own, in particular cracking.
Several factors contributed to the failure. The high carbon content gave the steel a tendency toward brittle fracture. The impact loading inherent in the application of the bolt gave high bending stresses at the roots of the threads.
What would constitute a proper installation? The impact loading and bending stresses were unavoidable. A different design of the bolt and anchoring mechanism might have eliminated threads from the highly stressed region.
The best solution would be to make the bolt out of a lower-carbon-content "mild" steel. Such a steel would have fractured in a ductile manner, even with the impact loading and sharp threads. The higher carbon steel is a bit stronger, and a slightly higher diameter mild steel piece might have been needed.
I prepared an utterly convincing, bullet-proof report for my client, the attorney for the injured boy. False modesty! It was one of the strongest, most definitive reports I have ever written. My efforts were for naught. The attorney missed a key filing deadline and could not take action against any of the parties responsible for the selection and installation of the unsuitable bolt.
