Researches (E)

The following pioneering works extend the frontiers of material model research to support the creativity of designers and engineers.

Forming-limit prediction method based on bifurcation theory for improving the press formability of difficult-to-form metals

To achieve ultimate lightweighting of vehicles, the use of difficult-to-form metals such as high-strength steels and aluminum alloys has increased. As a result, the necessity of not only shape prediction techniques but also fracture prediction techniques has arisen for successful forming. We have been engaged in a fundamental study with the aim of constructing a ductile fracture prediction method that can be used for both sheet and bulk metals even under large strains. As part of this research theme, a novel material model for anisotropic metals, which play an essential role in the precision of forming simulations, has been investigated. In the proposed material model, a non-associated flow rule was adopted to separate the potential function from the yield function to distinguish deformation anisotropy and stress anisotropy. In addition, we have succeeded in defining both functions using arbitrary order functions to obtain sufficient physical resolution for various metals. Then, combined with a stress-rate-dependent constitutive equation, which is known as the non-normality rule, and the three-dimensional bifurcation theory, the above aim can be achieved.

Springback compensation system based on springback mechanism and a new shell element taking thickness-stretchability into account

Springback, which is an elastic recovery, occurs when panel-like parts of an automobile are stamped, causing significant problems. Since a large amount of springback results in difficulty in assembly, it should be suppressed to within a tolerance during the design of the forming process; however, it is difficult to apply an efficient countermeasure when high-strength steel is used or the shape is complex. Therefore, we have developed a CAE (computer-aided engineering) system for springback compensation based on the springback mechanism and FEM (finite element method) to establish an effective measure. Specifically, we propose a linear FEM solver that can simulate the springback process precisely, making use of forming analysis data as its starting point, and a novel shell element that can deal with the normal stress component in the thickness direction. This method allows the designer of a forming process to consider optimal conditions based not only on geometrical variables but also on mechanical treatment such as bottoming and draw beading.

Multiscale analysis of ductile property and a strategy for optimal layer design to improve the press formability of CFRP

To construct a sustainable society, it is necessary to increase the efficiency of the use of energy and resources. Lightweight materials undoubtedly play a significant role in achieving this goal, especially in energy-consuming transport such as automobiles, high-speed trains, and aircraft. As well as the use of high-strength steels and multi-materialization, the application of CFRP (carbon fiber reinforced plastic) is expected in contribution for lightweighting. Research on the press forming of CFRTP (carbon fiber reinforced thermoplastic) is currently under way; however, the press forming of CFRP is anticipated as a more promising future technology. Unfortunately, an effective simulation methodology for press forming with CFRP, which is an essential technology, has not yet been developed owing to the uncertainty of the plastic deformation mechanism of CFRP sheets. Therefore, it is difficult to conduct a trial-and-error simulation, which is a general method in the case of metal forming. To overcome this problem, mainly focusing on CFRP, we are investigating the mechanical factors affecting the ductility of CFRP subjected to press forming and conducting a theoretical approach to establish a phenomenological model for CFRP.

Optimal design methodology based on biomimetics

Inhomogeneity and anisotropy have been avoided in engineering materials owing to their unpredictable response; however, some materials such as DP (dual phase) steel and FGMs (functionally graded materials), which positively utilize inhomogeneity and anisotropy, have recently been developed. Moreover, akin to the multi-materialization trend seen in automotive design, optimization of the material choice and location is rapidly occupying a central role in structure design methodology. On the other hand, some materials in nature possess inhomogeneity and anisotropy, and some creatures have acquired ultimately optimized shapes and material distributions to survive. Namely, it can be claimed that artificial products are becoming similar to biological creatures. Biomimetics or bio-inspired design has the aim of constructing a methodology for mechanical design and manufacturing by the extraction and abstraction of biological characteristics of natural materials. We have been attempting to realize an integrated material-structure design based on bio-information.

Numerical material testing based on crystal plasticity

The demand for high-precision and high-level forming simulations has been increasing. To meet this demand, it is necessary to acquire sufficient material data without burdensome labor. However, some advanced materials exhibit strong anisotropy, which means that material testing, such as equi-biaixial tests which are not common, is essential to carry out forming simulations. In other words, if a simple and reliable way of acquiring material data is not available, the latest high-performance computers and cutting-edge material models cannot achieve their potential. Therefore, to construct a reliable alternative, we are developing a numerical material testing method that integrates some common methods of testing, simulation, and optimization. As a numerical procedure, the crystal plasticity finite element method, which is a type of material simulator, has been applied in our study.

Development of measurement system for material testing based on digital image correlation

When conducting material tests, it is ideal to capture the three-dimensional deformation field over as wide area as possible without contact. Various methods have been employed in the past, but owing to the increasing performance and decreasing cost of computers and image sensors, in recent years the research and development of measurement methods based on digital image correlation (DIC) have been popular. We hope to apply such a method not only to general material testing but also to observation of the local deformation and shear banding of metal specimens, the material testing of CFRP, and the material testing of biological materials with strong individual differences.