• Transactions of Materials Processing
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• v.19 no.7
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• pp.393-398
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• 2010

This paper investigates the effect of material properties on the formability of sheet metals based on the Marciniak-Kuczynski model (M-K model). The hardening behavior of the material is modeled as the Hollomon model with the strain rate effect. The yield surfaces are constructed with Hosford79 yield function. The material properties considered in this study include the R-value, the strain hardening exponent, the strain rate hardening exponent, and the crystal structure of the material. The effect of the crystal structure on formability is roughly expressed as the change of the yield surface by varying the value of the exponent in Hosford79 yield function. Results show that the R-value affects neither the magnitude nor the shape of right hand side of forming limit diagrams (FLDs). Higher strain hardening exponent and higher strain rate hardening exponent improve the formability of sheet metals because they stabilize the forming processes.

• Transactions of Materials Processing
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• v.28 no.1
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• pp.27-33
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• 2019

The effect of hardening methods and process parameters on surface hardening of a Ti-6Al-4V Alloy has been investigated in this study. To characterize the effectiveness of the respective surface hardening methods, samples of a Ti-6Al-4V alloy were self-quenched, laser-nitrided, laser-carburized, laser-carbonitrided at the same laser irradiation conditions. This experimental procedure was followed by comparing the microstructural evolutions and mechanical properties of the respective samples after the laser surface hardenings. The hardening characteristics of the respective laser surface hardenings were well defined in this study, and the hardness was significantly influenced by the reaction compounds and laser energy density.

• Nuclear Engineering and Technology
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• v.54 no.3
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• pp.1098-1108
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• 2022

The effect of the material hardening model of the canister on a finite element vertical cask drop simulation is investigated for the strain-based acceptance evaluation. Three different hardening models are considered in this paper: the isotropic hardening model, the strain rate-dependent Johnson-Cook (J-C) hardening model, and the modified J-C model which are believed to be the most accurate. By comparing the results using the modified J-C model, it is found that the use of the J-C model provides similar or larger stresses and strains depending on the magnitudes of the strain and strain rate. The use of the isotropic hardening model always yields larger stresses and strains. For the strain-based acceptance evaluation, the use of the isotropic hardening model can produce highly conservative assessment results. The use of the J-C model, however, produces satisfactory results.

• Structural Engineering and Mechanics
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• v.56 no.3
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• pp.369-383
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• 2015

In this study, effect of various material hardening models based on Holloman's isotropic, Ziegler's linear kinematic, non-linear kinematic and mixture of the isotropic and nonlinear kinematic hardening laws on springback prediction of titanium alloy (Ti-3Al-2.5V) in a tube rotary draw bending (RDB) process was investigated with presenting the keynotes for a comprehensive step by step ABAQUS simulation. Influence of mandrel on quality of the final product including springback, wall-thinning and cross-section deformation of the tube was investigated, too. Material parameters of the hardening models were obtained based on information of a uniaxial test. In particular, in the case of combined iso-nonlinear kinematic hardening the material constants were calibrated by a simple approach based on half-cycle data instead of several stabilized cycles ones. Moreover, effect of some material and geometrical parameters on springback was carried out. The results showed that using the various hardening laws separately cannot describe the material hardening behavior correctly. Therefore, it is concluded that combining the hardening laws is a good idea to have accurate springback prediction. Totally the results are useful for predicting and controlling springback and cross-section deformation in metal forming processes.

• Nuclear Engineering and Technology
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• v.53 no.9
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• pp.2960-2967
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• 2021

In this work, a new hardening model is proposed for the depth-dependent hardness of ion-irradiated polycrystals with obvious grain size effect. Dominant hardening mechanisms are addressed in the model, including the contribution of dislocations, irradiation-induced defects and grain boundaries. Two versions of the hardening model are compared, including the linear and square superposition models. A succinct parameter calibration method is modified to parametrize the models based on experimentally obtained hardness vs. indentation depth curves. It is noticed that both models can well characterize the experimental data of unirradiated polycrystals; whereas, the square superposition model performs better for ion-irradiated materials, therefore, the square superposition model is recommended. In addition, the new model separates the grain size effect from the dislocation hardening contribution, which makes the physical meaning of fitted parameters more rational when compared with existing hardness analysis models.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.37 no.10
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• pp.1239-1249
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• 2013

In this study we establish a process to predict hardening behavior considering the Bauschinger effect for zircaloy-4 sheets. When a metal is compressed after tension in forming, the yield strength decreases. For this reason, the Bauschinger effect should be considered in FE simulations of spring-back. We suggested a suitable specimen size and a method for determining the optimum tightening torque for simple shear tests. Shear stress-strain curves are obtained for five materials. We developed a method to convert the shear load-displacement curve to the effective stress-strain curve with FEA. We simulated the simple shear forward/reverse test using the combined isotropic/kinematic hardening model. We also investigated the change of the load-displacement curve by varying the hardening coefficients. We determined the hardening coefficients so that they follow the hardening behavior of zircaloy-4 in experiments.

• Transactions of the Korean Society of Mechanical Engineers
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• v.14 no.5
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• pp.1193-1199
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• 1990

Effects of strain hardening exponents on the behavior of fatigue crack propagation are experimentally investigated. The retardation effect of fatigue crack propagation after single overloading is investigated in relation to strain hardening exponent and crack closure. A relationship between crack opening ratio and strain hardening exponents is inspected through an examination of the crack closure behavior. An empirical equation relating retardation effect of fatigue crack propagation after single overloading, percent peak load and strain hardening exponent of materials is proposed.

• Korean Journal of Metals and Materials
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• v.49 no.6
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• pp.448-453
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• 2011

Pre-aging heat treatment after solution heat treatment (SHT) of Al-0.4 wt%Mg-1.2 wt%Si-0.1 wt%Mn alloy sheets for auto-bodies was carried out to investigate the effect of pre-aging and its conditions on the bake-hardening response. Mechanical properties were evaluated by a tensile and Vickers hardness test. Microstructural observation was also performed using a transmission electron microscope (TEM). It was revealed that pre-aging treatments play a great role in the bake-hardening response. In addition, it was found that the sphere-shaped nanosized clusters that can directly transit to the needle-shaped ${\beta}$" phase during the paint-bake process, not being dissolved into the matrix, are formed at 343 K. The result, reveals that the dominant factor of the bake-hardening response is the pre-aging temperature rather than the pre-aging time.

• Journal of the Korean Society for Heat Treatment
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• v.34 no.4
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• pp.179-184
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• 2021

Two types of secondary hardening martensitic steels, 10Co-14Ni and 6Co-5Ni, were produced by vacuum induction melting to investigate the effect of ausforming process on mechanical properties. According to the results of present study, the alloy samples ausformed at low temperature indicated a rather low hardness level in overall aging time despite the refinement of martensite lath width. As the result can closely be related with the presence of primary carbides precipitated within the initial austenite matrix, we confirmed that, in ultrahigh strength secondary hardening martensitic alloy steels, the ausforming process can rather limit the degree of secondary hardening during the subsequent aging treatment.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.32 no.6
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• pp.488-495
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• 2008

In order to achieve long fatigue lifetimes for cyclically pressurized thick cylinders, multi-layered compound cylinder has been proposed. Such compound cylinder involves a shrink-fit procedure incorporating a monobloc tube which has previously undergone autofrettage. The basic autofrettage theory assumes elastic-perfectly plastic behaviour. Because of the Bauschinger effect and strain-hardening, most materials do not display elastic-perfectly plastic properties and consequently various autofrettage mo dels are based on different simplified material strain-hardening models, which is assumed that combination of linear strain-hardenig and power strain-hardening model. This approach gives a more accurate prediction than the elastic-perfectly plastic model and is suitable for different strain-hardening materials. In this paper, a general autofrettage model that incorporates the material strain-hardening relationship and the Bauschinger effect, based upon the actual tensile-compressive stress-strain curve of a material was proposed. The model was obtained using the von Mises yield criterion and plane strain condition. The tensile-compressive stress-strain curve was obtained by experiment. The parameters needed in the model were determined by fitting the actual tensile-compressive curve of the material. Finally, strain- hardening model was compared with elastic-perfectly plastic model.