Ercan Gürses

Analytical and Numerical Methods for Finite-Strain Elastoplasticity

An important class of finite-strain elastoplasticity is based on the multiplicative decomposition of the strain tensor F = F el F pl and hence leads to complex geometric nonlinearities. This survey describes recent advances in the analytical treatment of time-incremental minimization problems with or without regularizing terms involving strain gradients. For a regularization controlling all of ∇F pl we provide an existence theory for the time-continuous rate-independent evolution problem, which is based on a recently developed energetic formulation for rateindependent systems in abstract topological spaces.

A Hybrid Morphing Trailing Edge Designed for Camber Change of the Control Surface

In this study, the design and analyses of a novel morphing trailing edge control surface is presented. The developed control surface is intended to be utilized on an Unmanned Aerial Vehicle (UAV). The morphing features of the control surface was obtained by using different compliant materials, which are able to undergo large in-plane deformations. The design also includes the utilization of the composite materials together with conventional aluminum material hence the design is called a hybrid one. The actuation was applied by using various number of small servo actuators located inside the control surface at different locations. During the design, CATIA V5-6R2012 package program was utilized and the structural analyses were conducted with Finite Element Method by using ANSYS® WorkbenchTM v14.0 package program. First, the design and analyses were done for in-vacuo condition and the relevant aerodynamic loading was later considered. The required aerodynamic loads, which were representing the flight conditions of the UAV, were calculated by Computational Fluid Dynamics analyses. The aerodynamic mesh used was generated by Pointwise® V17.2 R2 package program. The SU2 (Stanford University Unstructured) V3.2.1 open source software was also used in the study as the flow solver. The UAV had a baseline wing with NACA6510 airfoil. The required camber and de-camber characteristics were tried to be achieved for various NACA airfoil targets. By conducting a non-linear Finite Element Analysis it was shown that the control surface can successfully undergo both camber and de-camber morphing, both in-vacuo condition and under aerodynamic loading.

Structural and aerodynamic analyses of a hybrid trailing edge control surface of a fully morphing wing

In this article, the design and analysis of a hybrid trailing edge control surface of an unmanned aerial vehicle are presented. The structural design was performed to increase and decrease the camber of the control surface to match selected airfoil profiles. The design was first analyzed with the help of finite element method to assess the morphing capability. The morphed control surface was then analyzed aerodynamically and comparisons with the original target profiles were made. According to the aerodynamic analyses, it was concluded that the control surface can successfully morph into target profiles with very minor changes in the target aerodynamic values while still ensuring the structural integrity and the safety of the control surface.

Development of Artificial Neural Network Based Design Tool for Aircraft Engine Bolted Flange Connection Subject to Combined Axial and Moment Load

................................................................................................................ v ÖZ .............................................................................................................................. vii ACKNOWLEDGMENTS............................................................................................ x TABLE OF CONTENTS ............................................................................................ xi LIST OF TABLES .................................................................................................... xiii LIST OF FIGURES ................................................................................................... xv CHAPTERS ................................................................................................................. 1 1.INTRODUCTION ........................................................................................ 1 1.1. Scope of the Thesis ........................................................................ 6 2.FINITE ELEMENT ANALYSIS OF BOLTED FLANGE CONNECTIONS ............................................................................................. 9 2.1. Finite Element Modeling and Geometry ........................................ 9 2.1.1. The analysis geometry ................................................ 9 2.1.2. Finite element modeling ........................................... 14 2.1.3. Loads and boundary conditions ................................ 20 2.2. The Application of Bending Moment .......................................... 22 2.3. Application of the Shear Force .................................................... 30 2.4. The Effect of the Shear Force ...................................................... 36 3.PARAMETRICAL ANALYSES AND DATABASE CONSTRUCTION 45 3.1. The Selection of Input Parameters ............................................... 45 3.2. Database Generation .................................................................... 47 3.3. The Parametric Finite Element Analysis Results ......................... 54 4.SETUP OF ARTIFICIAL NEURAL NETWORK AND RESULTS ......... 59 4.1. The Modeling of Artificial Neural Network ................................ 59 4.2. The Training Results of the Artificial Neural Network ............... 61 xii 4.3.Comparison of Artificial Neural Network and Finite Element Analysis Results ................................................................................. 65 4.4.The Graphical User Interface of the Bolted Flange Design Tool . 70 5.CONCLUSION ........................................................................................... 73 5.1.Future Work .................................................................................. 76 REFERENCES ........................................................................................................... 77 APPENDICES ............................................................................................................ 81 A. JUSTIFICATION OF EQUATION FOR AXIAL FORCE DUE TO MOMENT ..................................................................................................... 81 B. ARTIFICIAL NEURAL NETWORK TRAINING CODE ..................... 83 C. CREATING THE GRAPHICAL USER INTERFACE ........................... 85 D. BOLTED FLANGE DESIGN TOOL GRAPHICAL USER INTERFACE MAIN CODE ................................................................................................ 91 E. BOLTED FLANGE DESIGN TOOL GRAPHICAL USER INTERFACE USER MANUEL ........................................................................................ 101 F. THEORETICAL BOLT STRESS CALCULATION CODE ESDU ... 103

Experimental and Computational Investigation of Out-of-Plane Low Velocity Impact Behavior of CFRP Composite Plates

Strength of composite materials under transverse loading has remained a major weakness despite numerous advancements in composite technologies. Most frequent and critical result of this characteristic is internal delamination damage, which is undetectable and lead to major strength reduction in the structure. This condition is usually encountered in low-velocity impact situations which frequently occur during the maintenance of aircraft. Past studies have successfully developed experimental and analysis methods for accurately predicting impact force history and damage footprint based on the comparison with post-impact results. However, there is almost no experimental work on the progression sequence of damage during impact in the literature. This paper focuses on experimental and computational investigation of the damage initiation and growth process during low-velocity impact of [07/904] s and [907/04] s cross-ply CFRP laminates. In the experiments, through-the-thickness direction is tracked using ultra-high speed camera and DIC technique to record damage progression and dynamic strain fields. In the numerical part of the study 3-D explicit, finite element analysis is conducted to model matrix crack initiation and propagation. The finite element results are then compared with experiments in terms of failure modes and sequence.

Modeling of a Three-Dimensional Spherulite Microstructure in Semicrystalline Polymers

A finite element (FE) model, that explicitly discretizes a single 3D spherulite is proposed. A spherulite is a two-phase microstructure consisting of amorphous and crystalline regions. Crystalline regions, that grow from a central nucleus in the form of lamellae, have particular lattice orientations. In the FE analyses, 8-chain and crystal viscoplasticity constitutive models are employed. Stress-strain distributions and slip system activities in the spherulite microstructure are studied and found to be in good agreement with the literature. Influences of the crystallinity ratio on the yield stress and the initial Young’s modulus are also investigated.

Development of Bolted Flange Design Tool Based on Finite Element Analysis and Artificial Neural Network

In bolted flange connections, commonly utilized in aircraft engine designs, structural integrity and minimization of the weight are achieved by the optimum combination of the design parameters utilizing the outcome of many structural analyses. Bolt size, the number of bolts, bolt locations, casing thickness, flange thickness, bolt preload, and axial external force are some of the critical design parameters in bolted flange connections. Theoretical analysis and finite element analysis (FEA) are two main approaches to perform structural analysis of bolted flange connections. Theoretical approaches require the simplification of the geometry and are generally oversafe. In contrast, finite element analysis is more reliable but at the cost of high computational power. In this paper, a methodology is developed for iterative analyses of bolted flange that utilizes artificial neural network approximation of a database formed with more than ten thousand non-linear analyses with contact algorithm. In the design tool, a structural analysis database is created by taking permutations of the parametric variables. The number of intervals for each variable in the upper and lower range of the variables is determined with the parameters correlation study in which the significance of parameters are evaluated. The prediction of the ANN based design tool is then compared with FEA results and the theoretical approach of ESDU. The results show excellent agreement of the ANN based design tool with the actual non-linear finite element analysis results within the training limits of the ANN.Copyright

Constitutive modeling of stress-driven grain growth in nanocrystalline metals

In this work, we present a variational multiscale model for grain growth in face-centered cubic nanocrystalline (nc) metals. In particular, grain-growth-induced stress softening and the resulting relaxation phenomena are addressed. The behavior of the polycrystal is described by a conventional Taylor-type averaging scheme in which the grains are treated as two-phase composites consisting of a grain interior phase and a grain boundary-affected zone. Furthermore, a grain-growth law that captures the experimentally observed characteristics of the grain coarsening phenomena is proposed. To this end, the grain size is not taken as constant and varies according to the proposed stress-driven growth law. Several parametric studies are conducted to emphasize the influence of the grain-growth rule on the overall macroscopic response. Finally, the model is shown to provide a good description of the experimentally observed grain-growth-induced relaxation in nc-copper.