The Research On Constitutive Modeling And Mechanical Behavior Of Ultrafine Grained Materials

A mixed hardening constitutive model of ultrafine grained materials was put forward in this paper, which not only contained isotropic-kinematic hardening, but also considered grain size effect and cyclic softening. The mixed hardening constitutive model introducing Hall-Petch relation based on Johnson-Cook model superimposed cyclic softening term which showed the attenuation of initial yield radius. The back stress consisted of three parts for the kinematic part of the hardening, of which each part followed the kinematic hardening rule of the Armstrong-Frederick type, hence the mixed hardening constitutive model could better describe the Basinger effect. The yield radius was the common-effect of kinematic hardening, strain hardening and strain softening.According to the mixed hardening constitutive model, stress integration algorithm was deduced through the discrete constitutive equation, newton iteration method and radial return; UMAT based on ABAQUS was compiled with FORTRAN language to define the mixed hardening constitutive model; A FE model was established to simulate the mechanical behavior of ultrafine grained aluminium and ultrafine grained Al-Mg alloy under the condition of monotonic and cyclic loading. The results indicated that the model could better be used to describe the stress-strain relationship of ultrafine grained materials under the above condition. Mechanical behavior of ultrafine grained materials with different grain size was simulated through the mixed hardening constitutive model in different conditions which included different strain rate, mean strain and strain amplitude. Besides, mechanical behavior of ultrafine grained materials was simulated in the condition that strain amplitude went through a process of an increase in this paper. The following results can be obtained through the simulation results:(1)grain size and strain rate affected yield strength and peak stress at the stable state for ultrafine grained materials, namely the decrease of grain size and increase of strain rate made yield strength and peak stress at the stable state increase, or the yield strength and peak stress at the stable state would decrease, furthermore, the relation between hysteresis energy and grain size obeyed exponential function, while the relation between hysteresis energy and strain rate could be expressed by logarithmic function;(2) the relation between hysteresis energy and strain amplitude was linear, and hysteresis energy increased with the increased strain amplitude;(3)the mean strain was smaller, the hysteresis energy larger, the relation between hysteresis energy and mean strain was linear;(4)if ultrafine-grain materials subjected to cyclic loading with small strain amplitude at first, then subjected to cyclic loading with large strain amplitude under cyclic loading, peak stress at the stable state decreased lightly compared with that of ultrafine grained materials which only subjected to cyclic loading with large strain amplitude, moreover, there is a exponential function relationship between the cyclic hysteresis energy and strain range at low strain level.