It is shown that a number of stress-strain devices control the rate of increase of the total load applied to the specimen, but that none takes account of the diminishing cross section as extension proceeds. It is shown how this may be done, thus providing a constant rate of stress increase, by means of a cam which introduces a variable lever arm through which the load is applied.
In the case of extension-versus-time curves, it is important to run the experiment at constant stress rather than at constant total load. A simple means of loading a specimen to accomplish this is described.
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