Kwansoo Chung

Non-orthogonal constitutive equation for woven fabric reinforced thermoplastic composites

One of the ultimate objectives of this study was to investigate the feasibility of shaping preconsolidated woven FRT (fabric reinforced thermoplastics) using stamp thermo-hydroforming, a new forming method for composite manufacturing. A new constitutive model has been developed based on a homogenization method by considering the microstructures of composites including both the mechanical and structural properties of fabric reinforcement. In particular, the current model aims to account for the effect of the fiber strength difference and orientation on anisotropy and also to simulate shear deformation without significant length change, common in FRT composite forming. For validation purposes, the model was implemented in an explicit dynamic finite element code and tested for in-plane simple shear, pure shear, uniaxial tension, and draping behavior of woven composites.

Springback prediction for sheet metal forming process using a 3D hybrid membrane/shell method

To reduce the computational time of finite element analyses for sheet forming, a 3D hybrid membrane/shell method has been developed and applied to study the springback of anisotropic sheet metals. In the hybrid method, the bending strains and stresses were calculated as post-processing, considering the incremental change of the sheet geometry obtained from the membrane finite element analysis beforehand. To calculate the springback, a shell finite element model was used to unload the sheet. For verification purposes, the hybrid method was applied for a 2036-T4 aluminum alloy square blank formed into a cylindrical cup, in which stretching is dominant. Also, as a bending-dominant problem, unconstraint cylindrical bending of a 6111-T4 aluminum alloy sheet was considered. The predicted springback showed good agreement with experiments for both cases.

Fabric Surface Roughness Evaluation Using Wavelet-Fractal Method Part I: Wrinkle, Smoothness and Seam Pucker

A wavelet-fractal method to calculate the fractal dimension is proposed to objectively evaluate the surface roughness of fabric wrinkle, smoothness appearance and seam pucker. The proposed method was validated using the fractal surfaces produced from the mathematical functions and compared with the box and cube counting methods. The more accurate three-dimensional mesh grid data points of wrinkle replicas, smoothness appearance replicas and seam pucker samples were obtained using a three-dimensional, noncontact scanning system. As a supplementary reference the standard roughness parameters, which differentiate the degree of fabric surface roughness, were also investigated. The results show that the fractal dimension measured by the wavelet-fractal method as well as the surface average mean curvature show the power to clearly discern the grades of wrinkle, smoothness appearance as well as seam pucker, and thus can evaluate the fabric surface roughness objectively and quantitatively

Sheet hydroforming of woven FRT composites: non-orthogonal constitutive equation considering shear stiffness and undulation of woven structure

For the simulation of sheet hydroforming for the shaping of woven fabric reinforced thermo-plastic (FRT) composites, a non-orthogonal constitutive model was developed based on a homogenization method by considering the microstructures of composites including mechanical and structural properties of the fabric reinforcement. This model is modified to capture the wrinkling behavior due to the undulation geometry of the woven structure and shear stiffness at the crossover of the warp and weft yarns of woven FRT composites. The model was implemented in an explicit dynamic finite element code to analyze the forming behavior of woven FRT during the stamp thermo-hydroforming process. Wrinkling behavior was investigated based on the application of a counteracting fluid pressure and changes to the initial blank shape.

A Hybrid Membrane/Shell Method for Rapid Estimation of Springback in Anisotropic Sheet Metals

A semi-analytical method to predict springback in sheet metal forming processes has been developed for the case of plane strain. In the proposed hybrid method, for each deformation increment, bending, and unbending stretches are analytically superposed on membrane stretches which are numerically obtained in advance from a membrane finite element code. Springback is then obtained by the unloading of a force and a bending moment at the boundary of each element treated as a shell. Hills 1948 yield criterion with normal anisotropy is used in this theory along with kinematic and isotropic hardening laws during reverse loading. The method has been applied for the springback prediction of a 2008-T4 aluminum alloy in plane-strain draw-bending tests. The results indicate the necessity of including anisotropic hardening (especially Bauschinger effects) and elastoplastic unloading in order to achieve good agreement with experimental results.

Influence of initial back stress on the earing prediction of drawn cups for planar anisotropic aluminum sheets

Abstract Anisotropy is closely related to the formability of sheet metal and should be considered carefully for more realistic analysis of actual sheet metal forming operations. In order to better describe anisotropic plastic properties of aluminum alloy sheets, a planar anisotropic yield function which accounts for the anisotropy of uniaxial yield stresses and strain rate ratios simultaneously was proposed recently. This yield function was used in the finite element simulations of cup drawing tests for an aluminum alloy 2008-T4. Isotropic hardening with a fixed initial back stress based on experimental tensile and compressive test results was assumed in the simulation. The computation results were in very good agreement with the experimental results. It was shown that the initial back stress as well as the yield surface shape have a large influence on the prediction of the cup height profile.

Drape Simulation of Woven Fabrics by Using Explicit Dynamic Analysis

For the three-dimensional drape simulation of large samples of woven fabric, an explicit-dynamic analysis code is developed to predict the shape of woven fabric as it deforms into a natural shape under gravity. A contact algorithm is incorporated into the code, which is based on explicit algorithms for the non-linear dynamics of shells with a simple damping effect. For validation purposes, various simulations are performed, and the results are compared with those obtained in experiments.

Texture evolution of FCC sheet metals during deep drawing process

Abstract The stability of ideal orientations and texture evolution was investigated for FCC sheet metals during deep drawing. Lattice rotation fields around ideal orientations were numerically predicted using a rate-sensitive polycrystal model with full constraint boundary conditions. In order to evaluate the strain path during deep drawing of an AA1050, simulations using a finite element analysis were carried out. The stability of orientations and texture formation was examined at sequential paths such as flange deformation, transition and wall deformation. Depending on the initial location in the blank, the deviation from the plane strain state in the flange deformation path decreased the orientation density around P {0 1 1}〈8 11 11 〉 and shifted the final stable end orientation from P to Yf near {1 1 1}〈 1 1 2〉 . The texture evolution in AA1050 sheet metals during deep drawing was experimentally investigated. The change of orientation density around ideal orientations in the RD and TD samples was in good agreement with the rate-sensitive polycrystal model.

Incorporation of sheet-forming effects in crash simulations using ideal forming theory and hybrid membrane and shell method

In order to achieve reliable but cost-effective crash simulations of stamped parts, sheet-forming process effects were incorporated in simulations using the ideal forming theory mixed with the three-dimensional hybrid membrane and shell method, while the subsequent crash simulations were carried out using a dynamic explicit finite element code. Example solutions performed for forming and crash simulations of I- and S-shaped rails verified that the proposed approach is cost effective without sacrificing accuracy. The method required a significantly small amount of additional computation time, less than 3% for the specific examples, to incorporate sheet-forming effects into crash simulations. As for the constitutive equation, the combined isotropic-kinematic hardening law and the nonquadratic anisotropic yield stress potential as well as its conjugate strain-rate potential were used to describe the anisotropy of AA6111-T4 aluminum alloy sheets.

Measurements of anisotropic yielding, bauschinger and transient behavior of automotive dual-phase steel sheets

In order to present better prediction capability in computational analysis, mechanical properties of the dualphase high strength steel have been characterized especially for anisotropy as well as the Bauschinger and transient behavior. As for the anisotropy, the non-quadratic anisotropic yield function Yld2000-2d has been utilized and its material parameters have been obtained using the uni-axial tension tests as well as the hydraulic bulge test. To measure the hardening behavior including the Bauschinger and transient behavior, a newly developed test method has been applied for the uni-axial tension/compression and compression/tension tests, in which solid blocks along the both sides of the sheet specimen prevent buckling. From the tension/compression curves, the equations to describe isotropic and kinematic hardening behavior have been obtained. The modified Chaboche model has been confirmed to well represent the hardening behavior including the Bauschinger and transient behavior.