A consistent modified Zerilli-Armstrong flow stress model for BCC and FCC metals for elevated temperatures F. H. Abed and G. Z. Voyiadjis, Baton Rouge, Louisiana Received July 29, 2004; revised October 27, 2004 Published online: February 24, 2005 Ó Springer-Verlag 2005 Summary. The Zerilli-Armstrong (Z-A) physical based relations that are used in polycrystalline metals at low and high strain rates and temperatures are investigated in this work. Despite the physical bases used in the derivation process, the Z-A model exhibits certain inconsistencies and predicts inaccurate results when applied to high temperatures-related problems. In the Z-A model, the thermal stress component vanishes only when T !1. This contradicts the thermal activation mechanism that imposes an athermal behavior for the flow stress at certain finite critical temperatures. These inconsistencies, in fact, are attributed to certain assumptions used in the Z-A model formulation that causes the model parameters to be inaccu- rately related to the microstructural physical quantities. New relations are, therefore, suggested and proposed in this work using the same physical bases after overcoming any inappropriate assumptions. The proposed modified relations along with the Z-A relations are evaluated using the experimental results for different bcc and fcc metals. Comparisons are also made with the available experimental results over a wide range of temperatures and strain rates. The proposed model simulations, in general, show better correlation than the Z-A model particularly at temperatures values above 300K  . Numerical identification for the physical quantities used in the definition of the proposed model parameters is also presented. 1 Introduction Large deformation problems, such as high speed machining, impact, and various primarily metal forming operations, require constitutive models that are widely applicable and capable of accounting for complex path of deformation, temperature, and strain rate. The degree of success of any model mainly depends on: (i) the physical basis used in the derivation process producing material parameters that are related directly to the nano-/micro-physical quantities; (ii) the flexibility and simplicity of determining material constants from a limited set of experimental data; (iii) capturing the important aspects of static and/or dynamic behavior besides being mathematically and computationally accurate. In dynamic problems that intro- duce high strain rates, the dynamic yield stress is considered the most important expression needed to characterize the material behavior and is also used in finite element codes. In this regard, the dislocation-mechanics-based constitutive relation for material dynamics calculations developed by Zerilli and Armstrong [1] is considered as one of the most widely used models that have been implemented in many finite element dynamic codes (ABAQUS, DYNA, and others) and used by many authors in different types of low and high strain rates and temperature-related applications (see, for example, [2], [3]). Other authors [4]–[6] reviewed and Acta Mechanica 175, 1–18 (2005) DOI 10.1007/s00707-004-0203-1 Acta Mechanica Printed in Austria