A micromechanical approach to determine flow stress of work materials experiencing complex deformation histories in manufacturing processes

Work materials experience a broad range of strains, strain rates, and temperatures in many manufacturing processes such as machining, forming, etc. Strain rate has an important effect on the yield and flow stress of work materials, especially metals, since at higher strain rates there is less time for thermally-activated events; consequently, it is equivalent to a lowering of the temperature of the materials. On the other hand, it is also true that, for high strain rate deformations such as metal cutting, adiabatic plastic flow may produce significant temperature changes in the materials. Flow stress is significantly affected by the strain rate history; hence, mechanical behavior may not be fully described in terms of a mechanical equation of state relating the instantaneous stress, strain, strain rate, and temperature. Based on the concept of dislocation mechanics, a micromechanical approach has been explored to determine flow stress at high strain rates by combining athermal stress (the long-range barriers to dislocation motion) and thermal stress (the short-range temperature and strain rate-dependent barriers to dislocation motion). The SHPB (split Hopkinson pressure bar) compression test data of Aluminum 6061-T6 and titanium Ti-6A1-4V in literature over a temperature range of 77K-1000K was used for this purpose. Based on the baseline test data, the constitutive model describing the flow stress was developed. The constitutive model was further modified and extended to predict flow stress above the critical temperature. The corresponding model predictions were compared with the experimental data, attaining good agreement.