Published papers

The effect of specimen shape on the fracture angle in tensile testing of copper alloy sheets was investigated. Specimens with a width-to-thickness ratio of more than 30 were investigated, and it was found that relatively thick and narrow copper alloy sheets exhibited a fracture angle oblique to the loading direction. In contrast, the fracture angle was perpendicular to the loading direction in the thinner and wider specimens. To understand the mechanism of the change in fracture direction, digital image correlation (DIC) analysis was performed. The DIC analysis showed that the oblique localizations of the strain rate appeared in two directions in any thickness and width after diffuse necking. In thick specimens, one of the localizations was selected by the oblique fracture. In contrast, two localizations remained until the perpendicular fracture in thin and wide specimens. The analysis of the ratio of local strain rate revealed that the length strain over width strain was basically unchanged until fracture. However, in a few cases, the length strain over width strain drastically changed just before the fracture. These results suggest that the remaining two localized strain rates and the change in the strain state lead to the perpendicular fracture in thin and wide specimen. Regarding the edge of the specimen, the fracture angle tends to be oblique. This is presumably due to the less constrained condition of the edge, which is the possible reason for the oblique fracture of the narrow specimen.

In formability evaluation, it is important to accurately predict the onset timing and morphology of fracture in a workpiece from a physical point of view. The authors have developed a material model based on the stress rate direction-dependent constitutive equation, which is a non-normality law model, and applied it to forming analyses. The authors have also developed a forming limit prediction method based on the three-dimensional local bifurcation theory, which can theoretically predict the forming limits of arbitrary sheet and bulk metals. In this study, these methods are applied to evaluate the formability of thin and ultra-thin metallic sheets. The proposed model was applied to a square cylinder drawing analysis, and the accurate evaluation of the local necking allowed us to capture the transition of the fracture zone in response to changes in the amount of corner cut. In addition, appropriate forming parameters for ultra-thin sheets were also investigated. These analyses demonstrated the validity and usefulness of the proposed stress rate direction-dependent constitutive model.

This study was undertaken to attempt to establish the press-forming of carbon fiber-reinforced plastic (CFRP) sheets through a forming simulation. Since press-forming induces large plastic deformation of a material, a reliable simulation requires accurate modeling of the plastic deformation of resin. However, there has been little research on the deformation mechanism of thermosetting resins under large plastic forming, which is targeted in this study, and therefore no appropriate plastic deformation model of a resin has been established. Thus, a new model was constructed for a thermosetting resin on the basis of plastic deformation characteristics considering the microscopic mechanism. In the proposed model, the breakage of molecular chains of a thermosetting resin is considered. A molecular dynamics simulation of epoxy resin determined the effect of this molecular chain breakage. It was confirmed that the proposed model can appropriately represent the mechanical characteristics of an actual thermosetting resin. Furthermore, using this model, the numerical analysis of a CFRP sheet through its representative volume element was carried out, and the result explained the improved formability of CFRP at high temperatures, as observed in related experiments.

In this study, an anisotropic constitutive model based on the non-associated flow rule was developed for anisotropic sheet metals. This model was defined in the quadratic form of the Hill’s anisotropic function under a general three-dimensional stress condition. The anisotropic parameters for the yield function were identified using the directional planar yield stresses, bulge yield stress and shear yield stress, while those for the plastic potential function were identified using the directional r-values. A full expression related to the non-associated flow rule was applied and the model was implemented into the finite element code ABAQUS. A static-implicit analysis and the solid element were applied. Capability of the developed model for predicting the anisotropic behavior of sheet metal was investigated by considering two different sheet metal forming processes: cylindrical cup drawing of AA2090-T3, A6061P-T6 and SPCE; and hole expansion forming test of A6016-O. Cup heights and through-thickness strain distributions obtained from the simulations were compared with the experimental data. Results demonstrate that the developed material model considering 3D condition can improve accuracy of predicting the anisotropic behaviors. Furthermore, the simple formulations are efficient and user-friendly for computational analyses and solving the common industrial sheet metal forming problems.

In this paper, a novel computational procedure for numerical material testing method based on a strain rate-independent crystal plasticity model and an optimization scheme is presented. In order to simulate deformation of materials that exhibit mechanical anisotropy with high accuracy, it is necessary to apply material models that include many material parameters. Therefore, if difficult-to-perform material tests such as biaxial tests can be replaced by numerical material tests, the cost and time required for experiments can be significantly reduced. The proposed method was developed using a crystal plasticity-based finite element model, known as finite element polycrystal model (FEPM), and an optimization scheme based on a genetic algorithm (GA) for microscopic parameters. FEPM is a model that performs analysis while determining the activity of all slip systems without using the strain rate-dependent constitutive law that is commonly used and is suitable for numerical material testing because it has less physical ambiguity. In the optimization process, deformation anisotropy is related to the grain orientation expressed by the Euler angle, and yield stress anisotropy is related to the work-hardening coefficients of the slip system. In addition, by generating pseudo-anisotropic crystal orientation by numerical rolling, the need for crystal orientation observation is eliminated. In this paper, after giving a description of the proposed method, experimental verifications on aluminum alloys to prove the effectiveness of the proposed method are presented.

Magnesium and its alloys are attractive structural light materials because they have a high spe-cific strength and high specific stiffness. That is why their usage in high performance automotive and aerospace applications has been increased. However, their high anisotropy and low ductility could bring to forming failure, to avoid these a better accuracy for metal forming simulation is re-quired. The identification of macroscopic parameters is expensive due to the experiments needed to find them, the use of multi-scale approach allows to find parameters for the model through easier experiments. A finite element polycrystal method, based on a rate independent polycrystal plasticity model, is implemented in order to perform the material testing of a cast magnesium alloy AZ31B. The parameters of the model are adjusted to better fit the experimental data through a trial and error process.

In the case of complex forming process that receives a secondary loading after the primary loading, it is necessary to appropriately represent the work-hardening state according to the forming history. Namely, it is necessary to accurately model complex forming process including reversal loading (Bauschinger effect) and orthogonal loading (cross-hardening). In this study, a composite anisotropic hardening expression based on the crystallographic structure using the finite element polycrystal model is proposed. Numerical investigations are conducted to consider the effectiveness of the proposed model. In which, an alternation rate is introduced to capture the activity of the slip systems to support the idea behind the proposed model. In addition, flow curves of the secondary loading are produced by the proposed model, which is compared with the conventional model.

The press formability of cured CFRP (carbon fiber reinforced plastic) sheets composed of a thermoset polymer and continuous fibers was examined in this study. Press forming requires large plastic deformation, but CFRP is not sufficiently ductile in general. Therefore, it has been considered that CFRP is not suitable for press forming. However, recent experimental research has shown that the press formability of CFRP sheets is improved at 100 °C compared with that at room temperature. This finding indicates the possibility of the press forming of CFRP, but the temperature dependence of its formability has yet to be shown. To establish a press forming method for CFRP, an analytical study of its plastic deformation is essential. To determine the temperature dependence of the formability of CFRP, the strain-softening behavior of epoxy at warm temperatures was focused on in this study. This is because the mechanical characteristics of the polymer have a strong effect on the plastic deformation of CFRP. An approximated curve based on the temperature parameters was constructed to express the stress–strain relationship at different forming temperatures. Then, a model for press forming simulation was constructed to consider the effect of the mechanical characteristics of epoxy in CFRP. The approximated stress–strain curves at room temperature and 100 °C were applied to the model. Using this model, the stress distribution in CFRP sheets formed at each temperature has been shown, and the results clearly demonstrate that the formability of CFRP sheets is improved at higher temperatures.

© 2018 Institute of Physics Publishing. All rights reserved. Establishment of a press forming method of carbon fiber reinforced plastic (CFRP) is desired. However, fundamental research on a ductility improving mechanism of CFRP is still insufficient. Since unidirectional CFRP shows different characteristics in compression and tensile deformation, accurate expression in both characteristics is necessary to investigate factors that affect formability. To express fiber kinking and ductile behavior of resin, a microscale model that separated fiber and resin was made. The fiber part in FEM model was tilted as initial misalignment and Gurson-Tvergaard-Needleman (GTN) model was applied to the resin part. To investigate the influence of design parameters such as temperature and initial void fraction on formability, this study performed tensile, compression and bending analyses by changing the resin temperature and initial void fraction. Results of compression analysis showed that the higher the temperature and initial void fraction, the earlier fiber kinking occurrence. Bending analysis showed a similar tendency. These results are physically reasonable. Therefore, these numerical experiments confirmed that the model used in this research is valid for studying factors that affect formability.

© 2018 Institute of Physics Publishing. All rights reserved. In this paper, the proposed framework for fracture prediction during sheet metal forming is explained and some numerical investigations are presented to demonstrate the effectiveness of this method. In particular, the role of parameters that affects the forming limit curves, which are represented by the initiation of local bifurcation in each stress rate, are examined; namely, anisotropic parameters and the KC value, which means a stress-rate dependence, and other material properties. The newly proposed concept of forming limit diagram is used to evaluate the formability of the concerned material and working process.

For advanced plasticity problems such as fracture prediction, the use of a non-associated flow rule as a constitutive model is effective to represent bifurcation phenomenon. In addition to this, to represent strong anisotropy observed in cold rolled sheets, the use of a non-associated flow rule model is essential because this can separate the deformation anisotropy and stress anisotropy. However, a sufficiently practical model has not been developed. In this study, experimental and analytical investigations on a function for the non-associated flow rule model proposed by the authors are presented.

In this paper, a new criterion for evaluating speckle pattern in DIC method is proposed. In our study, the authors assumed that the identification of unknown parameters by inverse analysis using strain field measurement based on digital image correlation (DIC) is promising because there are many possible applications: in-process monitoring of metal working, extraction of implicit know-how of manufacturing process, and identification of mechanical parameters not only in industrial materials but also in inhomogeneous biological materials. To establish this method, it is difficult to construct a useful physical model, an efficient reverse analysis process, and the measurement quality evaluation criteria. As a first step, this paper focuses on evaluating the quality of transitional speckle patterns on the surface of objects during large deformation. Although the quality of the speckle pattern greatly affects the number of search errors, it is difficult to evaluate the quality reliably by a single criterion such as the mean subset fluctuation or Shannon entropy proposed as the quality evaluation criterion. Therefore, a new criterion FE is proposed and its characteristic is investigated through experiments.

Recent rapid progress in metal forming has brought difficult problems when it comes to accurate prediction in plastic deformation in which high-strength advanced metals are used. If a metal that exhibits strong anisotropy is applied, the use of some advanced material model is recommended. Usually, advanced material models require large number of material parameters to be determined by experiments. To avoid this situation, construction of numerical material testing could be beneficial if such methodology can be an alternative for some difficult-to-conduct experiments. Therefore, we have developed a numerical material testing using finite element polycrystalline model based on successive integration method. The proposed method consists of crystal plasticity-based model and a deep neural network to capture the microstructural behavior of polycrystalline metals. In this study, a description of the proposed method that is based on the concept of material learning, and some verification with experimental data are presented. In the learning phase, experimental data obtained from in-plane tensile tests are provided as teaching data, and after the multiscale material learning, the virtual material will acquire generality to non-learned out-of-plane mechanical characteristics. In this work, prediction for equi-biaxial stress-strain relation resulted in acceptable agreement with experimental data from a literature.