Tae-Uk Kim

Optimal design of composite laminated plates with the discreteness in ply angles and uncertainty in material properties considered

Abstract An algorithm is developed for optimizing laminated plate stacking sequences and determining thicknesses, which incorporates discrete ply angles and considers the uncertainties of material properties in a two-step optimization process. The branch and bound method was modified to handle discrete variables; and convex modeling was used to allow the consideration of variable material properties. The numerical results obtained show that the optimal stacking sequences are determined with fewer evaluations of the objective function than might be expected from considerations of the size of the design space. Our results also show that the optimal thickness increases when elastic moduli uncertainties are considered, which indicates that such uncertainties should not be ignored at the design stage.

Functionally graded piezoelectric strip with eccentric crack under anti-plane shear

In this paper, we examine the singular stresses and electric fields in a functionally graded piezoelectric ceramic strip containing an eccentric crack off the center line under anti-plane shear loading with the theory of linear piezoelectricity. It is assumed that the properties of the functionally graded piezoelectric ceramic strip vary continuously along the thickness. Fourier transforms are used to reduce the problem to the solution of two pairs of dual integral equations, which are then expressed to a Fredholm integral equation of the second kind. Numerical values on the stress intensity factor and the energy release rate are obtained.

Dynamic characteristics of an eccentric crack in a functionally graded piezoelectric ceramic strip

The dynamic response of an eccentric Griffith crack in functionally graded piezoelectric ceramic strip under anti-plane shear impact loading is analysed using integral transform method. Laplace transform and Fourier transform are used to reduce the problem to two pairs of dual integral equations, which are then expressed to Fredholm integral equations of the second kind. We assume that the properties of the functionally graded piezoelectric material vary continuously along the thickness. The impermeable crack boundary condition is adopted. Numerical values on the dynamic stress intensity factors are presented for the functionally graded piezoelectric material to show the dependence of the gradient of material properties and electric loadings.

Crack Analysis of Piezoelectric Material Considering Bounded Uncertain Material Properties

Piezoelectric materials are widely used to construct smart or adaptive structures. Although extensive efforts have been devoted to the analysis of piezoelectric materials in recent years, most researches have been conducted by assuming that the material properties are fixed and have no uncertainties. Intrinsically, material properties have a certain amount of scatter and such uncertainties can affect the performance of component. In this paper, the convex modeling is used to consider such uncertainties in calculating the crack extension force of piezoelectric material and the results are compared with the one obtained via the Monte Carlo simulation. Numerical results show that crack extension forces increase when uncertainties considered, which indicates that such uncertainties should not be ignored for reliable lifetime prediction. Also, the results obtained by the convex modeling and the Monte Carlo simulation show good agreement, which demonstrates the effectiveness of the convex modeling.

Structural analysis of flexible wing using linear equivalent model

Aircraft needs high lift-to-drag ratio and weight reduction of the structure for long endurance flight with a small power. Generally high aspect ratio wing is applied to HALE(High Altitude Long Endurance) aircraft. Also high modulus, and high strength CFRP(Carbon Fiber Reinforced Plastic) has been used in primary structures. and thin mylar(membrane material) film has been applied to skin of wing. As a result, wing is more flexible than the other structures. and the stiffness of thin mylar film has an affect on dynamic stability. In this study, the membrane characteristic of mylar film has been simulated using nonlinear gap elements. And equivalent modeling method using shell elements is presented using the nonlinear simulation result. The linear equivalent model has verified using the results of nonlinear membrane method. Proposed linear equivalent shell model has applied to mode analysis for estimate the effect of mylar mechanical properties on natural frequency.

Crashworthy Landing Gear Design Using a Composite Tube by Extra Energy Absorber

Landing gear is the one of the key components for improving aircraft crashworthiness because its primary function is the energy absorption. But, in general, the shock absorbers are designed to have best efficiency for normal landing cases and can be ineffective when faced with very high sink speed. Thus special design and implementation are necessary for landing gear to have crashworthiness. For this purpose, various concepts have been studied and put to practical use such as structural pin, pressure relief valve and additional energy absorbing devices, etc. In this paper, the composite tube is investigated as an extra energy absorber and adopted to landing gear to increase shock absorbing performance in case of crash. To do this, first the quasi-static and impact test of composite tubes are conducted and the analysis model is tuned to explain the test results. During the correlation process, the failure modes and the specific energy absorption of the composite tubes are analyzed and the optimal configurations are searched.The overall performance of landing gear including the composite tube is analyzed by developing a simplified dynamic model. Each force-stroke relation of oleo-pneumatic shock absorber, tire and composite tube are modeled as spring and damper, then the equation of motion is solved to obtain the crash responses. In this model, after the bottoming of shock absorber, the crushing of composite tube is activated for additional energy absorption. Numerical solutions show that the enhanced shock absorbing capability in case of crash when the composite tube adopted. For practical use, the landing gear performance should be verified by drop tests and this is author’s future research project.Copyright

Fatigue Test and Evaluation of Landing Gear

S-N . 피로설계 요구조건 충족 여부를 최종 검증한다 본 논문에서는 항공기 착륙장치의 피로수명 해석 및 . 시험절차를 실제 적용 사례를 통해 제시하였다. Abstract: For the fatigue design of aircraft landing gear, the safe-life approach is applied. Structural defects such as cracks or detrimental deformations should not occur under the fatigue load spectrum depicting the entire lifetime usage of the aircraft. In the design phase, the fatigue life of the landing gear is estimated analytically by adopting the stress-based approach because the fatigue of aircraft landing gear is generally high-cycle fatigue. This utilizes S-N curves that are factored to produce design curves that account for the scatter and surface finish of the material. In the test and evaluation phases, a fatigue test should be conducted for full-scale landing gear to substantiate the fatigue design requirement in the end. In this study, the procedure for the fatigue test and evaluation of aircraft landing gear is presented with real application cases.

Matrix Structure Reliability Analysis using AFORM

The wing is a framework composed chiefly of skin, spars, ribs and can be simplified by matrix structure. In this paper, a displacement reliability of matrix structure is analysed by AFORM(Advanced First Order Reliability Method) and applicability is assessed.

Crashworthy Design and Test of Landing Gear

The main function of a landing gear is to absorb the impact energy during touchdown. It it occasionally required for landing gear to have crashworthiness for improving survivability and safety in case of emergency landing. This paper introduces the design concept, performance analysis and drop test procedures for the development of the crashworthy landing gear. The shock absorbing ability and the crash behavior are proved by analyzing various sensor data and video clips from high speed camera recording during drop tests.

Drop Test of an Oleo-pneumatic Landing Gear

The main function of a landing gear is to absorb the impact energy during touchdown. Most landing gear use an oleo-pneumatic shock absorber which essentially consists of an oil damper and a gas spring. The performance of a shock absorber can be estimated by analysis but it should be verified by drop test, which is required by MIL Spec. and FAR. In the drop test, various data such as ground loads, shock absorber pressure, stroke and mass travel are analyzed to validate the shock absorbing efficiency and the mathematical model for analysis. This paper presents the introduction of drop test facility, the test procedure and data evaluation method with real drop test example.