Jin Yeon Cho

Direct Numerical Simulation of Composite Structures

Fiber-reinforced composites are not chemical compounds but physical mixtures of fiber and matrix, and the constituents are bonded together. Therefore, it is natural and essential to adopt a full microscopic model and directly analyze the model with no assumptions for local deformation or local loading conditions, in order to understand and predict not only the averaged or homogenized behaviors but also the detailed microscopic behaviors of composite structures. However, in spite of the necessity, full microscopic models of composite structures have rarely been dealt with, mainly because of their difficulties arising in the actual computation of the finite element model with immense degree of freedoms. In this work, to overcome the difficulties and analyze full microscopic models of composite structures, an efficient parallel multifrontal solver, which can utilize distributed computing resources unlimitedly, is developed and applied to the Direct Numerical Simulation (DNS) of composite structures. Using the developed code, feasibility studies are carried out to observe whether or not the proposed solver is adequate for the DNS of composite structures, such as in parallel performance and others. As an example of DNS, virtual experiments are carried out, where material constants are directly obtained through DNS. Comparisons with previous experimental and theoretical results show the promising possibility that the present virtual experiments by the DNS of composite structures can be an alternative way to obtain material constants without expensive real experiments. Additionally, a microscopic model with defects is also analyzed and compared with a perfect model to validate the potential of the DNS in dealing with irregularity of microscopic models of composite structures coming from imperfection. The examples show the future direction of analysis and design of composite structures as well as the usefulness of the proposed DNS methodology.

Role of matrix in viscoplastic behavior of thermoplastic composites at elevated temperature

from the piston valve to accelerate the formation of the shockwave front. In previously suggested valve designs, values of L/D from 20 to 40 for Ms = 1.61 were quoted. It is apparent that the present valve system exhibits good performance even with a heavy piston valve while having a relatively simple structure. Obtained shock-wave Mach numbers are almost the same values as the analytical ones. In previous shock-wave valve designs, the obtained shock-wave Mach number was between 60 and 90% of the analytical value.

Viscoelastic Model of Finitely Deforming Rubber and Its Finite Element Analysis

A viscoelastic model of finitely deforming rubber is proposed and its nonlinear finite element approximation and numerical simulation are carried out. This viscoelastic model based on continuum mechanics is an extended model of Johnson and Quigleys one-dimensional model. In the extended model, the kinematic configurations and measures based on continuum mechanics are rigorously defined and by using these kinematic measures, constitutive relations are introduced. The obtained highly nonlinear equations are approximated by the nonlinear finite element method, where a mixture of the total and updated Lagrangian descriptions is used. To verify the theory and the computer code, uniaxial stretch tests are simulated for various stretch rates and compared with actual experiments. As a practical example, an axisymmetric rubber plate under various time-dependent pressure loading conditions is analyzed.

Penalized Weighted Residual Method for the Initial Value Problems

To satisfy the need for network parallel computing in the whole time domain, a penalized weighted residual formulation for the second-order initial value problems is developed and its time finite element approximation is presented. To impose properly both the initial displacement and velocity, the initial velocity is imposed to the weighted residual equation as a penalized form that is constructed such that the approximated initial velocity can satisfy the initial condition in the average sense. By this procedure, the penalized weighted residual formulation makes it possible to handle the whole time domain of investigation and to use the conventional-displacement-based finite element technique without any other artificial routine. Through several numerical tests, it is confirmed that the present method gives very accurate solutions for various systems arising in the second-order initial value problems and presents promising characteristics for network parallel computation in the whole time domain of investigation.

AN EXPLICIT SOLUTION PROCEDURE OF DISCONTINUOUS TIME INTEGRATION METHOD FOR DYNAMIC-CONTACT/IMPACT PROBLEMS

In this work, an explicit solution procedure for the recently developed discontinuous time integration method is proposed in -order to reduce the computational cost as well as maintain the desirable numerical characteristics of -the discontinuous time integration method. In the present explicit solution procedure, a two-stage correction algorithm is devised to obtain the solution at the next time step without any matrix factorization. To observe the numerical characteristics of the proposed explicit solution procedure, stability and convergence analyses are performed. From the stability analysis, it is observed that the proposed algorithm gives a larger critical time step than the central difference method. From the convergence analysis, it is identified that the present method with linear approximation in time gives the third order convergence which is higher than that of central difference methods. To check the performance of the proposed method in simulating impact problems, several numerical tests are carried out, and some of the results are compared with those from central difference method.

Hard-landing Simulation by a Hierarchical Aircraft Landing Model and an Extended Inertia Relief Technique

In this work, an efficient aircraft landing simulation strategy is proposed to develop an efficient and reliable hard-landing monitoring procedure. Landing stage is the most dangerous moment during operation cycle of aircraft and it may cause structural damage when hard-landing occurs. Therefore, the occurrence of hard-landing should be reported accurately to guarantee the structural integrity of aircraft. In order to accurately determine whether hard-landing occurs or not from given landing conditions, full nonlinear structural dynamic simulation can be performed, but this approach is highly timeconsuming. Thus, a more efficient approach for aircraft landing simulation which uses a hierarchical aircraft landing model and an extended inertia relief technique is proposed. The proposed aircraft landing model is composed of a multi-body dynamics model equipped with landing gear and tire models to extract the impact force and inertia force at touch-down and a linear dynamic structural model with an extended inertia relief method to analyze the structural response subject to the prescribed rigid body motion and the forces extracted from the multi-body dynamics model. The numerical examples show the efficiency and practical advantages of the proposed landing model as an essential component of aircraft hard-landing monitoring procedure.

Design Optimization of a Wing Structure under Multi Load Spectra using PSO algorithm

In this paper, development of optimal design tools for wing structure is described including multi load spectra condition and fatigue analysis. Two dimensional CFD result are used for calculating aerodynamic force. Design variables are composed of a number of rib and spar, positions, and thickness of each structural member. The mission profile for fatigue analysis is composed based upon the results of CFD analysis, the flight-by-flight spectra method, the excessive curves for gust loads. Minor`s rule was used to deal with multi-load condition. Stress analysis and fatigue analysis are performed to calculate objective functions. Particle Swarm Optimization(PSO) algorithm was used to apply to problems which have dozens of design variables.

Advancements of Aerospace Computational Structure Technology in Korea

Over the past decades, various research groups in Korea have made considerable research efforts in the field of aerospace computational structure technology. Nowadays, the main research efforts include high performance computing, pre/post-processor, novel computational methodologies, and application of computational structure technology to real design and development of aircraft and satellite. In this paper, some of the representative current research activities in Korea are presented.

PARALLEL 3D SIMULATION OF STRUCTURAL DYNAMIC PROBLEM BY USING EXPLICIT DISCONTINUOUS TIME INTEGRATION METHOD

In this work, an explicit solution procedure for the recently developed discontinuous time integration method is proposed in order to reduce the computational cost as well as maintain the desirable numerical characteristics of the discontinuous time integration method. Also by the implementation to the parallel algorithm, a quite efficient parallel efficiency is obtained. In the present explicit solution procedure, a two-stage correction algorithm is devised to obtain the solution at the next time step without any matrix factorization. To observe the numerical characteristics of the proposed explicit solution procedure, stability and convergence analyses are performed. To check the parallel performance of the proposed method, speed-up check is conducted for the various numerical examples.