Hong Gun Kim

Theoretical Assessment of Stress Analysis in Short Fiber Composites

An investigation of composite mechanics to investigate stress transfer mechanism accurately, a modification of the conventional shear lag model was attempted by taking fiber end effects into account in discontinuous composite materials. It was found that the major shortcoming of conventional shear lag theory is not being able to provide sufficiently accurate strengthening predictions in elastic regime when the fiber aspect ratio is very small. The reason is due to its neglect of stress transfer across the fiber ends and the stress concentrations that exist in the matrix regions near the fiber ends. To overcome this shortcoming, a more simplified shear lag model introducing the stress concentration factor which is a function of several variables, such as the modulus ratio, the fiber volume fraction, the fiber aspect ratio, is proposed. It is found that the modulus ratio is the most essential parameter among them. Thus, the stress concentration factor is expressed as a function of modulus ratio in the derivation. It is also found that the proposed model gives a good agreement with finite element results and has the capability to correctly predict the variations of the internal quanitities.

Molding Characteristics of Etched Surface Pattern in Polymers

Molding characteristics are presented for the surface pattern of polymeric materials in injection molding. They are related to the polymer properties and surface roughness of the etched mold. The polymer structure and the mold roughness can affect roughness deviation and surface duplication quality on polymers. Experiments are performed with respect to the surface roughness analysis. It is expected that the duplication properties can contribute to molding process of surface pattern in injection molding.

Assessment of Fiber Stress Based on Elastoplastic Analysis in Discontinuous Composite Materials

An elastopalstic analysis of the micromechanical approach is performed to investigate the stress transfer mechanism in a short fiber reinforced composites. The model is based on the New Shear Lag Theory (NSLT) which was developed by considering the stress concentration effects that exist in the matrix region near fiber ends. The unit cell model is selected as the Representative Volume Element (RVE) for the investigation of longitudinal elastoplastic behavior in discontinuous composites. Thus far, it is focused on the detailed description to predict fiber stresses in case of the behavior of elastoplastic matrix as well as elastic matrix. Slip mechanisms between fiber and matrix which normally take place at the interface are considered for the accurate prediction of fiber stresses. Consequently, onset of Slip points is determined analytically and it showed a moving direction to the fiber center region from the fiber tip as the applied load increases. It is found that the proposed model gives the more reasonable prediction compared with the results of the conventional model (SLT).

Shape Design Optimization of the Boom System in High Ladder Vehicle

A shape design optimization of the boom system in high ladder vehicle modeled by 3-dimensional finite elements is carried out. The structural analysis is performed using 3-D FEA (Finite Element Analysis) giving the results of displacements, stresses and natural frequencies. The section height and width of the boom is controlled by the FEA result data. The subproblem approximation method is implemented for the optimal shape based on displacement and principal stress. In the meantime, Lanzcos algorithm method is implemented in order to find the natural frequency of the system. The optimal models obtained by three different optimization processes are compared with the initial model and evaluated for economic conditions respectively. It is found that the different shape optimization processes give the different shape optimization results, which suggest their unique optimal shapes under their necessary design condition.

Analysis of Residual Stress in Thin Wall Injection Molding

A relationship of residual stress distribution and surface molding states on polymeric materials is presented in thin-walled injection molding. The residual stress is computed by computational numerical analysis, observed with stress viewer and birefringence. The residual stress on polymeric parts can allude the surface quality as well as flow paths. The residual stress distribution on polymeric parts is related with thickness, gate layout, and polymer types. Molecular orientation on polymeric parts is also important in thin wall injection molding. The residual stress and molecular orientation are related to the surface molding states intimately. Analysis of the residual stress is validated through photo-elastic method and surface molding states..

Prediction of Elastic Properties in Discontinuous Composite Materials

The shortcoming of conventional SLT (Shear Lag Theory) is due to the neglect of stress transfer across the fiber ends, which results in the inaccurate stress variation for the fiber when the fiber aspect ratio is small in elastic loading. Thus a new model called NSLT (New Shear Lag Theory) is developed considering the stress concentration effects that exists in the matrix regions near fiber ends. In this paper the prediction of elastic composite modulus is presented to evaluate the stress transfer mechanism using NSLT. A micromechanical FEA (Finite Element Analysis) model with axisymmetry is implemented to verify the results of fiber stresses and interfacial shear stresses. It is found that the proposed model gives a reasonable prediction compared with the results based on other models.

A Study on the Thermoelastic Analysis in Shear Deformable Discontinuous Composites

A stress analysis has been performed to evaluate the thermally induced elastic stresses which can develop in a short fiber composite due to coefficient of thermal expansion (CTE) mismatch. An axisymmetric finite element model with the constraint between cells has implemented to find the magnitude of thermoelastic stresses in the fiber and the matrix as a function of volume fraction, CTE ratio, modulus ratio, and fiber aspect ratio. It was found that the matrix end regions fall under significant thermal stresses that have the same sign as that of the fibers themselves. Furthermore, it was found that the stresses vary along the fiber and fiber end gap in the same manner as that obtained in a shear-lag model during non-thermal mechanical loading.

Stress Analysis and Design Strategy for Lightweight Car Seat Frame

A seat frame structure in automotive vehicles made of polymer matrix composite(PMC) with reinforced by X-shape steel frame was developed to obtain weight reduction at low cost. The frame structure was designed and analysed using finite element analysis(FEA) and was compared with experimental impact test to verify the structural safety after fabricated. The design model based on safety was analysed with appropriate boundary conditions and loading conditions. Each result was utilized to modify the actual shape to obtain a lighter, safer and stabler design. It was found that the substitution of PMC material reinforced by an X-shaped steel frame resulted in a weight reduction effect with equivalent strength, impact characteristics and fracture property.