When metal cutting stress-strain data are compared with data from other materials tests, poor agreement is observed. The several factors that could influence the flow stress in cutting are considered and it is found that the presence of a normal stress on the shear plane, the temperature on the shear plane, and the higher rates of strain in cutting are of negligible importance. The interrelationship between temperature and strain rate is treated theoretically and the two effects are found to tend to cancel. It is shown experimentally that a material cut does not behave as an ideal plastic, but that the modulus of strain hardening is essentially the same at the high rates of strain encountered in cutting as in other materials tests carried out at low rates of strain. The principal discrepancy between cutting and torsion test data is seen to be due to the smallness of specimen size in cutting. The size effect that is observed in materials tests (tension and torsion) is shown to be in good quantitative agreement with the size effect observed when metals are cut at different feed rates. The presence of flow ahead of the conventional shear plane near the free surface is demonstrated and the importance of this preflow discussed. Part of the discrepancy between cutting and torsion test data is found to be due to use of chip-length measurements in obtaining the shear area rather than measuring this value directly from photomicrographs. This flow ahead of the shear plane also is found to account for part of the discrepancy between computed and measured values of the shear angle.

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