HIGH-STRAIN-RATE BEHAVIOR OF POLYCRYSTALLINE  -IRON D. Rittel, M. Vural, M. Tao, S. Mizrach, A. Bhattacharyya, G. Ravichandran SPECIMEN GEOMETRY and MATERIAL Goal: Investigate the thermo-mechanical behavior of pure  -iron (BCC), from quasi-static to dynamic strain rates: Techniques: Quasi-static compression, dynamic compression, non-contact, high speed infrared thermography, shear compression specimen (SCS). Material CompositionRectangular SCS geometry Typical Dimensions (mm) –– t 12.7D L Typical Dimensions (mm) 2.60–1.30– w 2.54t 12.7D L P, d h t h w L 45  D 99 w/o pure Fe EFFECTIVE STRESS-STRAIN DETERMINATION k i, k 2 and k 3 are material and geometry (w/t) dependent. They are determined from numerical simulations and verified by experiments. COMPARISON WITH PRIOR RESULTS Weston (1992) compared the dynamic behavior of as- received pure Fe to that of pre-shocked specimens. Our results indicate that pure Fe behaves as expected at low strain rates but reaches strength levels of pre- shocked Fe at high strain rates. STRAIN RATE SENSITIVITY OF PURE Fe Pure Fe exhibits a marked strain rate sensitivity, as shown in the figure below for  p =0.1 flow stress level. The transition is noticeable at Results were obtained, using both cylindrical and SCS specimens. SCS vs. TORSION EXPERIMENTS Comparing results obtained with SCS specimens to those obtained in pure shear (Klepaczko, 1969) further validates the effective stress strain reduction technique. Pure Fe softens noticeably at high strain rates, in accord with Weston’s (1992) observations for pre- shocked iron By contrast, Fe hardens continuously at large strains in the quasi-static regime. THERMOMECHANICAL BEHAVIOR Continuous recording of the temperature rise of the specimen allows for determination of the thermomechanical conversion of plastic work to heat. Define. Note that  int  1 while there is no such restriction for  diff. As shown below, the behavior at moderate strain rate of pure Fe shows no anomalies in terms of  By contrast, the high strain-rate response shows that  int > 1. Such an anomaly can be attributed to the operation of an additional heat source, such as the release of latent heat associated with a phase transformation. The  (BCC)  (HCP) martensitic phase transformation is well documented for pressure levels on the order of 13 GPa under shock loading conditions. It has not been reported for large strain, high strain-rate experiments. MECHANICAL BEHAVIOR T MICROSTRUCTURAL OBSERVATIONS Twinning observed in the heavily sheared gage section. WORK IN PROGRESS Elucidate a possible shear induced phase transition (TEM, X-ray). Microstructural and material characterization. Texture analysis using orientation imaging microscopy (OIM). Both  are >  m Hardness in the gage section for the material deformed at high strain rates is comparable to shock loaded iron. Validation studies of the multi-scale computational models for  -iron. length time mm nmµmµm ms µs ns Phase stability, elasticity Energy barriers, paths Phase-boundary mobility BCC HCP Martensite Plasticity Grains Polycrystals E. A. Carter R. E. Cohen M. Ortiz A.M. Cuitino R. A. Radovitzky