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The Case of the Cursed Chip
Kenneth Russell, Professor Emeritus, MIT, Cambridge, MA -- Design News, March 7, 2005
Eye injury cases give me the creeps, especially after I had to wear an eye patch for a few days because of a scratched eyeball. Operating on one eye was an intensely frustrating experience. The thought of permanent loss of sight is frightening.
A goodly fraction of my consulting cases involve eye injuries. In all cases the eye was struck by a high-velocity object and in no case was the injured wearing eye protection.
The Investigation
Some 30 years ago an attorney for the manufacturer presented me with a pair of 14-inch bolt cutters, a 0.25 × 2-inch steel bolt, and a small chip of metal. The chip had been removed from the plaintiff's eye and had allegedly come from the cutters as the bolt was being cut.
I needed to answer three questions in this case:
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Did the chip come from a cutter blade?
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Did the cutter blade have some defect that would cause it to chip?
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Did the chip result from the plaintiff cutting the 0.25-inch bolt?
The photo (top) shows the cutter jaws. The cutting blades have taken some hard treatment. There are two "nicks" where chips have been removed and two indentations.
I very carefully tried fitting the chip into each of the two nicks. The photo (bottom) shows that the chip fits perfectly into the upper nick.
Bolt cutters are designed to cut relatively soft material, but how soft is soft? A routine hardness test gives the answer to this question. There are numerous hardness testing techniques, each with its strengths and limitations. Metallurgists often use the so-called Rockwell C hardness test, in which a 150-kg load is used to press a conical diamond indenter into the metal surface. The less the penetration, the higher the hardness number. A reading of 70 indicates an extremely hard, brittle alloy, such as virgin (untempered), high carbon steel martensite. A reading of 20 is characteristic of cold rolled, low carbon (mild) steel. A reading of 40 is characteristic of martensite, which has been tempered to give enough ductility to be broadly useful while still retaining most of the strength of the virgin martensite.
I measured the hardness of the cutter blade, the bolt, and a 3/32 inch threading tap. The blade had a hardness of C-59, which is very hard, as would be expected of a cutter. The bolt had a hardness of C-18, characteristic of cold rolled steel, and the tap had a hardness of C-54, characteristic of a cutting tool and nearly as hard as the cutter blade.
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I cut the bolt, tap, and a 0.125-inch high speed drill bit with the subject cutters. The bolt and drill bit cut easily without deforming the cutter blades. The tap also cut easily, but left the indentations to the far right of the blades.
My study revealed no evidence of cracks, inclusions, or other defects on the surface of the chip or the mating nick. The cutting blade was thus not defective.
I concluded that cutting the soft bolt could not possibly have caused the blade to chip. Instead the chip resulted from cutting of an object nearly as hard as the cutter blade. I left it to the lawyers to determine whether cutting such an object came under the heading of foreseeable misuse, and whether or not using safety glasses constituted contributory negligence.
I never learned the outcome of the case but doubt that the plaintiff got much of a settlement.
| 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. |
| Columnist Ken Russell will be the keynote speaker at the upcoming Design News E2E Technical Conference on materials and fastening on March 30, 2005. He will recount some of his most famous cases, talk about material trends, and take questions from the audience. Register for this free event now at http://rbi.ims.ca/4388-534 . |
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