Hard machining involves large strain, high strain rate, high temperatures, strain rate/temperature coupling, and potential loading history effects. The accuracy of characterizing the dynamic mechanical behavior in hard machining using any constitutive models is strongly affected by materials testing data in which a constitutive model is fitted. Tension or compression tests have been widely used to approximate material properties in various manufacturing processes. However, it has been a critical question whether tension or compression test should be utilized for capturing the true nature of material deformations in a hard machining process. In this study, the influences of two material testing modes on mechanical behavior of AISI52100 steel (62 HRc) were investigated using the internal state variable (ISV) plasticity model. Twenty material constants have been found by nonlinear fitting the ISV plasticity model to the base line test data obtained from each deformation mode. To understand the true nature of hard turning mechanics, a numerical model that incorporate the internal state variable plasticity model via a material user subroutine has been developed with the material constants from the compression and tension tests. A global material failure/damage evolution model was implemented to simulate chip formation which solely depends on the material deformation state. Orthogonal hard turning experiments have been performed to validate the numerical model. It has shown that the material testing modes have profound effects on some materials constants of the ISV model. The stress sensitivity study to ISV model parameters has identified the critical material constants for reflecting the nature of material deformation. The different testing modes have significant influence on the material constants associated with isotropic hardening rather than kinematic hardening. The numerical and experimental results have shown that the material constants from the compression test capture the true nature of a hard machining process. The compression mode of material deformation prevails in hard machining.