Specimen Geometry and Friction in Dynamic Compression

Abstract

A series of compression tests is reported in which the height/diameter ratio was changed by mans of both height variation and diameter variation. Also, with this ratio constant, the specimen size was systematically varied. The specimen material and diameter was a medium carbon cold worked steel and Teflon was used as the lubricant. Compression was conducted without interruption for relubrication between smooth hardened steel dies. The effects of the above process variables on the static (strain rate: 10 −3 s −1 ) and dynamic (strain rate: 10 3 s −1 ) stress strain characteristics and a barrel parameter are described. In addition the extent of peripheral lubrication breakdown was measured and a correlation with the barrel parameter made. An attempt to measure the coefficient of friction at the specimen-die interface is reported. It was found that a size effect was operating and that maintenance of an equal height/diameter ratio was not a sufficient means for ensuring equal frictional effects. This result is not in agreement with earlier work reported in the literature. It was also shown that specimen diameter was more influential in controlling lubricant failure and barrelling than was specimen height. It was noted that Amonton's Law does not necessarily breakdown under conditions of plastic flow, pro-viding a continuous lubricant film can be maintained over a large fraction of the specimen-die area. It is suqgested that the continuous testing mode nay provide useful and valid stress-strain data after correction for frictional effects.