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[en] We prove a static shakedown theorem for viscoplastic and plastic workhardening materials which satisfy a constitutive inequality similar to the principle of maximum plastic work for perfectly plastic materials. Then we are allowed to define a static and a kinematic shakedown safety factor for a loading domain. It is shown that for a certain class of workhardening plastic and viscoplastic materials, including most of well-known models, the two shakedown safety factors are equal and define a true safety factor. Finally we prove a partial shakedown theorem, which can be used to analyse the shakedown possibilities of a structure of which the elastic response contains singularities. (orig.)
[en] A methodology for stress-based forming limit analysis has been developed for advanced high strength steel (AHSS). It was proposed that localized necking occurs when a critical normal stress condition is met. Using a basic, isotropic material model (von Mises, power law hardening), the criterion was applied to various 980 Class AHSS. In most cases the simplified criterion adequately described the experimental strain-based forming limit curve (FLC). For AHSS with substantial volume fractions of metastable austenite, a more sophisticated material model and/or an adapted failure criterion will be required. A strong linear relationship was found between the critical normal stress and the measured true stress at maximum load in tension. This empirical functionality applies over a large range of strength levels and may form the basis for a methodology by which FLCs may be estimated from standard tension tests. Finally, in the context of the proposed failure criterion, the effects of work hardening behavior on the “shape” of the strain-based FLC are explored. (paper)
[en] This work aims to characterize the 36 NiCrMo 6 steel in monotic traction. The tested samples were subjected to mechanical surface treatment (MST) by diamond ball burnishing. This process allow it possible to modify the physical and geometrical properties of the material. The stress-strain diagram obtained were processed and digitalised. The hardening domain was modelled and rational curves were established, taking into account the stresses triaxiality. Thus, the work hardening exponent of material considered was evaluated. The effect of ball burnishing on this exponent has studied in proceeding a comparative manner between burnished and machined surface. The results obtained show that by applying the optimal parameters for burnishing, the work hardening exponent of pre-machined surface layer can be increased by 10%, where nB = 0.407 (nT = 0.312 machined surface). (paper)
[en] This paper reviews the work hardening, recovery and recrystallization mechanisms in alloys containing dispersed precipitates. In the section on work hardening, the influence od spacing, particle size and shape on the density and distribution of dislocations have been discussed. They represent a large part of the energy stored in the material following drformation, which in turn is driving force for recrystallization. Next, the role of precipitates on recovery, on the formation and the growth of recrystallized regions has been discussed in detail. The competition between recovery and recrystallization and recrystallization of supersaturated solid solutions have also been mentioned. Finally, the technological relevance of the aspects treated in this paper has been discussed. (author)
[pt]Este trabalho faz uma revisao dos mecanismos de encruamento, recuperacao e recristalizacao de ligas contendo dispersao de precipitados. No topico referente ao encruamento sao discutidos os efeitos do espacamento, tamanho e forma das particulas de densidade e distribuicao das discordancias, as quais representam a parcela principal da energia armazenada no material apos a deformacao, que por sua vez e o potencial termodinamico para a recristalizacao. Em seguida o papel dos precipitados na recuperacao, na formacao e no crescimento das regioes recristalizadas e discutido em detalhes. A competicao entre recuperacao e recristalizacao e a recristalizacao de solucoes solidas supersaturadas sao tambem abordadas. Finalmente e discutida a relevancia tecnologica dos fenomenos tratados. (autor)
[en] The hot deformation behavior of as-cast hypoeutectic Al-Si-Mg alloy has been investigated through hot compression tests at temperatures between 573 and 773 K and the strain rate of 0.001-1 s−1. A modified Hansel-Spittel constitutive model is proposed, which takes the effect of strain rate on strain hardening into account. The processing maps are established based on the dynamic material model and the Murty criterion. Microstructure observations show that dynamic recovery dominates the dynamic softening behavior, and recrystallized grains are found in the sample tested at 773 K with strain rate of 0.01 and 0.001 s−1. The size of Si particles decreases by about 64.73% with the effective strain increasing from 0 to 1.2. The optimal hot processing parameters of as-cast hypoeutectic Al-Si-Mg alloy are established based on the processing maps.
[en] Highlights: • The rate-dependent behavior of 42CrMo steel exists at a wide range of strain rates. • Due to the adiabatic temperature increase, the thermal softening effect exists. • An improved Z-A model taking account of the thermal softening is proposed. 42CrMo steel used in high-speed train axles withstands complex loading conditions. To study the mechanical properties and the stress-strain relationship of 42CrMo steel, it was subjected to quasi-static and dynamic compression experiments. Experiments at eight different strain rates, ranging from 10− 3 to 4500 s− 1, demonstrate that it exhibits rate-dependent plastic behavior, thermal softening, and work-hardening behaviors. The dislocation theory explains the deformation mechanism of 42CrMo steel. Furthermore, a new constitutive model, which includes the thermal softening effect, based on the Zerilli–Armstrong constitutive model, is proposed to describe the dynamic mechanical behavior of 42CrMo steel. The model results are in good agreement with the experimental data, demonstrating that the proposed constitutive model describes the mechanical behavior of 42CrMo steel at various strain rates very well.
[en] In order to clarify the competition between work hardening (WH) caused by dislocation movements and the dynamic softening result from dynamic recovery (DRV) and dynamic recrystallization (DRX), a new two-stage flow stress model of X12CrMoWVNbN10-1-1 (X12) ferrite heat-resistant steel was established to describe the whole hot deformation behavior. And the parameters were determined by the experimental data operated on a Gleeble-3800 thermo- mechanical simulation. In this constitutive model, a single internal variable dislocation density evolution model is used to describe the influence of WH and DRV to flow stress. The DRX kinetic dynamic model can express accurately the contribution of DRX to the decline of flow stress, which was established on the Avrami equation. Furthermore, The established new model was compared with Fields-Bachofen (F-B) model and experimental data. The results indicate the new two-stage flow stress model can more accurately represent the hot deformation behavior of X12 ferrite heat-resistant steel, and the average error is only 0.0995. (paper)
[en] A microfriction model is proposed to delineate the linear and nonlinear properties of engineering materials. The model may be used to disclose the physical meaning of some empirical formulas. Thus, the elasto-plastic behavior of engineering materials may be clarified with physical terms, furthermore, certain clues about isotropic and kinematic hardening might also be available through this channel. The validity of this model was checked by working out the finite element analysis with some examples and the results were found well conformed with the experimental data. Some common used models were compared and contrasted numerically and it was shown that the present model is able to be used to estimate the elasto-plastic behaviour of structures and components. (orig./RF)