Mechanisms-Based Transitional Viscoplasticity

Abstract

When metal is subjected to extreme strain rates, the conversation of energy to plastic power, the subsequent heat production and the growth of damages may lag behind the rate of loading. The imbalance alters deformation pathways and activates micro-dynamic excitations. The excitations immobilize dislocation, are responsible for the stress upturn and magnify plasticity-induced heating. The main conclusion of this study is that dynamic strengthening, plasticity-induced heating, grain size strengthening and the processes of microstructural relaxation are inseparable phenomena. Here, the phenomena are discussed in semi-independent sections, and then, are assembled into a unified constitutive model. The model is first tested under simple loading conditions and, later, is validated in a numerical analysis of the plate impact problem, where a copper flyer strikes a copper target with a velocity of 308 m/s. It should be stated that the simulations are performed with the use of the deformable discrete element method, which is designed for monitoring translations and rotations of deformable particles. relevant material information, such as sensitivities to strain rate, temperature, size effect, etc. In a typical scenario, the models adapt the strain rate-insensitive solving scheme, where radial return seems to be the favorite procedure. In consequence, plastic strain is merely a byproduct of the requirement that stress must adhere to the yield surface. An interesting technique suitable for generating random dislocation arrangements is proposed by Gurrutxanga-Lerma [23]. The main concern of the study is focused on how a given population of dislocation structures influences the dominant features of the stress field. The stochastic analysis provides justification for Taylor's equation and the Hall-Petch relation. The study, however, is not concerned with how the dislocation structures are formed in the first place. Another approach to crystal plasticity is proposed by Langer [24]. His thermodynamics dislocation theory (TDT) is based on the observation that dislocation experience complex chaotic motions and the motion is responsible for the field fluctuations. One should expect the frequency of the fluctuations to be many orders of magnitude lower in comparison with the frequency of thermal excitations. According to Langer, the non-deterministic nature of dislocation dynamics makes the behaviors suitable for statistical and thermodynamics treatment. Brown [25] offered an interesting idea that plastic flow is responsible for the formation of dislocation structures in the state of self-organized criticality. In this interpretation, the dislocation structures evolve until reaching the state of maximum structural adaptiveness such that every site of the structure has a statistically equal chance to act as a nucleation site of further deformation. At the initial stages of plastic deformation, flow is initiated at points of weakness. However, as the process proceeds, the initiation sites become inactive and are replaced by newly cre...

Наноиндентирование И Механические Свойства Материалов В Субмикро- И Наношкале. Недавние Результаты И Достижения (О Б З О Р)

“…4, а). [52]. Поэтому и в наномасштабных структурах и объектах требуются определение m в широком диапазонеε.…”

“…Однако в большинстве случаев этого все же недостаточно. Как уже упоминалось выше, из опытов на макрообразцах известно, что скоростные зависимости механических свойств при большихε нельзя получить простой экстраполяцией данных, полученных при малых и среднихε, поскольку скоростная чувствительность приε > 10 2 −10 3 s −1 зачастую выше, чем при квазистатическом нагружении [52,55]. Для достижения больших значенийε в макрообразцах требуются более высокие абсолютные скорости нагружающего инструмента.…”

“…Влияние скорости деформирования на параметры пластического течения Cu и Al с различной исходной микроструктурой. а) -Зависимость напряжений течения поликристаллической меди при комнатной температуре и ε = 20% от скорости относительной деформацииε для двух размеров зерен d: 1 µm и 80 µm[52]. b) Скоростная чувствительность m напряжений течения ультрамелкозернистого Al (светлые символы) и Сu (темные символы), приготовленных методом равноканального углового прессования, при индентировании сферическими инденторами разного радиуса R[53].…”

Holistic Approach on the Research of Yielding, Creep and Fatigue Crack Growth Rate of Metals Based on Simplified Model of Dislocation Group Dynamics

“…The latter closely relates to connect problems of the strength and fracture of materials with the macro scale [2][3][4][5].…”

Crystal Plasticity

“…In terms of more theoretical works in which crystal plasticity model attempts were evaluated to reproduce the complex deformation processes of polycrystalline metals, Zubelewicz [1] has provided an extensive review on mechanisms-based transitional viscoplasticity and Nguyen et al [2] have made a short review of computational modeling of dislocation slip mechanisms. Masood Chaudry et al [3] have compared the results of Electron Backscatter Diffraction measurements and viscoplastic self-consistent calculations upon analysis of Ca-induced plasticity in Mg alloys, while a similar approach was also applied by Galán-López and Hidalgo [4] to stainless steel.…”