Abera Tullu

Bio: Abera Tullu is an academic researcher from Pusan National University. The author has contributed to research in topics: Finite element method & Computer science. The author has an hindex of 4, co-authored 5 publications receiving 23 citations.

Elastic deformation of fiber-reinforced multi-layered composite conical shell of variable stiffness

Abstract: Tailoring composite materials by fibers of spatially varying orientation angles has been realized with the advent of automated tow-placing machine. In order to use these variable stiffness composite materials as structural components, their responses to multiple external loads should be investigated. In this study, a truncated composite conical shell structure subjected to inflating pressure and surface shear traction force is considered. A mathematical model that predicts strain and stress distributions on the conical shell is developed. Based on the model, numerical examples are given for various fiber paths defined on the truncated conical shell that is subjected to inflating pressure and spatially varying shear traction force on the inner and outer surfaces, respectively. Numerical examples show that, under these external loads, the meridional strain and stress components are very sensitive to the type of fiber path definitions and value of semi-vertex angle of the cone. Boundary conditions have, also, shown remarkable effects on strain and stress distributions. To verify the adequacy of the mathematical model, the truncated composite conical shell of variable stiffness is simulated using finite element based ABAQUS commercial software. The numerical results obtained through the developed mathematical model and ABAQUS simulations show good agreement.

Elastic deformation of fiber-reinforced multi-layered composite cylindrical shells of variable stiffness

Abstract: For better structural performance, composite materials are being tailored by fibers of spatially varying orientation angles. The spatial variation of reinforcing fiber orientation induces variable stiffness in composite materials. In order to take full advantage of these variable stiffness materials, their structural responses to various forms of external loads should be investigated. This work examines a response of composite cylindrical shell structure of variable stiffness subjected to inflating radial pressure and spatially varying surface shear traction force. A mathematical model that predicts strain and stress distributions on this cylindrical shell structure is developed. The developed model is not limited by neither the number of plies in the shell nor orientation angles of reinforcing fibers. Based on the model, numerical examples are given for various fiber paths defined on the cylindrical shell. The numerical examples show that a cylindrical shell of variable stiffness subject to the mentioned external loads suffers axial compression and circumferential stretch, irrespective of reinforcing fiber paths. However, high axial compression is observed in the region where the orientation angle of reinforcing fiber deviates much from the axial direction. To verify the adequacy of the model, composite cylindrical shell of variable stiffness is simulated using finite element based ABAQUS commercial software. The numerical results obtain through the developed mathematical model and ABAQUS simulation show good agreement.

Parameter optimization to avoid propeller-induced structural resonance of quadrotor type Unmanned Aerial Vehicle

Abstract: Structural resonance in Unmanned Aerial Vehicle (UAV) is one of the major challenges that impose malfunctioning of the UAV sensor thereby degrading maneuverability. Vibration in quadrotor UAV is induced, mainly, due to the rotation of its motor-driven propellers. The constructive interference between the excitation frequencies of the propellers and fundamental frequencies of the UAV structure causes structural resonance. To avoid this structural resonance, the lowest fundamental frequency of the whole UAV structure has to be higher than the maximum working excitation frequencies of the propellers. The fundamental frequency of the quadrotor UAV structure depends on the parameters such as dimensions and stiffness of motor-supporting arms and landing gears as well as the mount location of the landing gears on the arms. The surrogate model that approximates functional dependency of the first fundamental frequency of the quadrotor UAV structure on the mentioned parameters is developed. Using the developed model and a non-linear sequential quadratic programming algorithm, the optimal values of the parameters at which the first fundamental frequency of the structure can be maximized are obtained. The adequacy of the developed surrogate model to approximate the functional form of the first fundamental frequency is verified through various statistical and experimental methods.

Design and Implementation of Sensor Platform for UAV-Based Target Tracking and Obstacle Avoidance

Abstract: Small-scale unmanned aerial vehicles are being deployed in urban areas for missions such as ground target tracking, crime scene monitoring, and traffic management. Aerial vehicles deployed in such cluttered environments are required to have robust autonomous navigation with both target tracking and obstacle avoidance capabilities. To this end, this work presents a simple-to-design but effective steerable sensor platform and its implementation techniques for both obstacle avoidance and target tracking. The proposed platform is a 2-axis gimbal system capable of roll and pitch/yaw. The mathematical model that governs the dynamics of this platform is developed. The performance of the platform is validated through a software-in-the-loop simulation. The simulation results show that the platform can be effectively steered to all regions of interest except backward. With its design layout and mount location, the platform can engage sensors for obstacle avoidance and target tracking as per requirements. Moreover, steering the platform in any direction does not induce aerodynamic instability on the unmanned aerial vehicle in mission.

Theoretical and numerical predictions of stretch-bending deformation behavior in composite sheet

Abstract: Stretch-bending of fiber reinforced composite sheets is one of the thermo-mechanical forming techniques in composite materials manufacturing. Due to an anisotropic nature of composite materials, predicting their deformation behavior during combined stretch-bending is difficult. To analyze this behavior, a mathematical model for semi-cylinder forming of continuous and unidirectional fiber reinforced composite sheet is developed. Plies in the sheet are treated as continuum bodies that interact through weak viscous interfaces. The model depicts both spatial and temporal variations of stresses and strains in composite sheet. Based on this model, numerical examples on semi-cylinder forming of cross-ply and angle-ply symmetric composite sheets are given and the results show good agreement with the simulation results obtained by ABAQUS commercial software.

Formability analyses of uni-directional and textile reinforced thermoplastics Part A Applied science and manufacturing

Abstract: The formability of two different composite materials used in aerospace industry has been investigated for a representative product geometry. The deformations during forming of carbon UD/PEEK and glass 8HS/PPS blanks with a quasi-isotropic lay-up were analysed. The UD/PEEK product showed severe wrinkling in doubly curved areas, whereas the 8HS/PPS product showed better formability in those areas. This can be explained by the relatively high resistance against intra-ply shear for the UD/PEEK material. Moreover, the predictive capability of a finite element based simulation tool was shown. For both materials, the prediction of intra-ply shear and large wrinkles showed good agreement with those observed in the actual product. The smaller wrinkles in the products cannot be accurately represented with the element size used. However, predicted waviness at the corresponding locations could indicate critical areas in the product. The presented modelling approach shows great potential for application in the composite product design process.

Bending and wrinkling of composite fiber preforms and prepregs. A review and new developments in the draping simulations

Abstract: Bending properties play a significant role in the forming of textile composites reinforcements, particularly in determining the shape of wrinkles. The physics related to the bending of fibrous reinforcements is specific. Bending is due to slippage between the fibers and since the fibers are quasi-inextensible, standard plate and shell theories are irrelevant. To measure bending characteristics, three experimental tests (and their variants) have been developed in the last decades, and efforts are currently devoted to extending and improving these tests. From their results, simulations can be performed by introducing a flexural energy related to the bending moment and the curvature. In particular, wrinkles during forming can be simulated. In the case of 3D modeling of thick reinforcements, the use of generalized continuum mechanics model is necessary because of the bending stiffness of each fiber and the slippage between fibers. In order to simulate textile reinforcements with shells, some shell approaches, different of the standard theories, can correctly calculate the rotations of textile reinforcement normals.

An integrated framework of exact modeling, isogeometric analysis and optimization for variable-stiffness composite panels

Abstract: Isogeometric analysis (IGA) is particularly suitable for the prediction of buckling load and design optimization of variable-stiffness composite panels , since curvilinear fiber path can be described exactly to improve the analysis efficiency, moreover, analytical sensitivity can be derived to improve the optimization efficiency. In this study, an integrated framework of exact modeling, isogeometric analysis and optimization for variable-stiffness panels is developed for the global optimum. Due to the inherent feature of multiple local optima for this type of problems, a novel multi-start gradient-based strategy is developed to enhance the global optimization capacity , and multiple initial designs for gradient-based optimization are determined by space tailoring method, which can guarantee the convergence rate and efficiency. Once the constraint aggregation and parallel computing methods are employed, the computational efficiency will be further improved. For typical illustrative example, it can be demonstrated that the proposed method is able to provide a more efficient optimum design with significant less computational cost compared to other traditional methods, including FEA-based optimization, direction optimization using genetic algorithm , gradient-based optimization without K–S function, gradient-based optimization based on difference method.

The interface strength and delamination of fiber-reinforced composites using a continuum modeling approach

Abstract: A closed-form expression of the interface strength between a fiber and a matrix is obtained using a continuum approach based on van der Waals interactions. The explicit solution of the interface delamination of fiber-reinforced composites with functionally graded interphase under three-dimensional load is derived. The present analytical results indicate that the delamination behavior of the fiber-reinforced composites highly depends upon the interphase thickness, the fiber radius, the Young's moduli and Poisson's ratio of the fiber and the matrix. The present analytical solution provides a sufficient support for the later research on the mechanical behaviors of fiber-reinforced composites and finally for developing new micro-composites.

Manufacturable insight into modelling and design considerations in fibre-steered composite laminates: state of the art and perspective

Abstract: The advent of novel robot-assisted composite manufacturing techniques has enabled steering of fibre paths in the plane of the lamina, leading to the emergence of the so-called variable angle tow (VAT) composite laminates. These laminates, with spatially varying fibre angle orientations, provide the designer with the ability to tailor the point-wise stiffness properties of VAT composites with substantially more efficient structural performance over conventional straight fibre laminates. As the application of fibre-steered composite laminates has reached an unprecedented scale in both academia and industry in recent years, a reflection upon the state-of-the-art advancements in the modelling, design, and analysis of these advanced structures becomes vital for successfully shaping the future landscape. Motivated by the gap and shortcomings in the available review works, in the present paper, we first summarize and discuss underlying fibre placement technologies including tailored fibre placement (TFP), continuous tow shearing (CTS), and automated fibre placement (AFP). Afterwards, mathematical models of reference fibre path in fibre-steering technology will be reviewed, followed by a detailed discussion on the manufacturing limitations and constraints of the AFP process. Then, design considerations in constructing a ply with multiple courses are elaborated, and key techniques to fill the entire layer with several courses are reviewed. This review is then followed by an introduction to the continuity and smoothness of fibre paths. Furthermore, a description on the material and geometric uncertainties is elaborated. Last but not least, the plate and shell laminate theories, which serve as the fundamental core of the modelling and design of VAT composite structures, are discussed.