Browsing by Author "Theotokoglou, Efstathios E."
Now showing 1 - 13 of 13
Results Per Page
Sort Options
Item Open Access Bird strike virtual testing for preliminary airframe design(Emerald, 2021-02-25) Perdikoulis, Petros V.; Giannopoulos, Ioannis K.; Theotokoglou, Efstathios E.Abstract Purpose – The purpose of this paper is to use numerical methods early in the airframe design process and access the structural performance of wing leading edge devices made of different materials and design details, under bird strike events. Design/methodology/approach – Explicit finite element analysis was used to numerically model bird strike events. Findings – Structural performance charts related to materials and general design details were drawn to explore the design space dictated by the current applicable airworthiness requirements. Practical implications – This paper makes use of the current capability in the numerical tools available for structural simulations and exposes the existing limitations in the terms of material modelling, material properties and fracture simulation using continuum damage mechanics. Such results will always be in the need of fine-tuning with experimental testing, yet the tools can shed some light very early in the design process in a relative inexpensive manner, especially for design details down selection like materials to use, structural thicknesses and even design arrangements. Originality/value – Bird strike simulations have been successfully used on aircraft design, mainly at the manufactured articles design validation, testing and certification. This paper presents a hypothetical early design case study of leading edge devices for appropriate material and skin thickness down selectionItem Open Access Bird strike virtual testing simulations and results, for preliminary airframe design structural optimization(International Centre for Numerical Methods in Engineering (CIMNE), 2021-01-15) Perdikoulis, P. V.; Theotokoglou, Efstathios E.; Giannopoulos, Ioannis K.External airframe structural components facing the aircraft flight direction, are prone to bird collisions. Aircraft manufacturers meet the bird strike airworthiness requirements through physical bird strike testing. Mainly due to the high costs involved in the certification process, recent studies have highlighted the capabilities and benefits of hybrid simulation-experiment techniques that reduce certification costs. The numerical investigation presented herein, studied the bird-strike simulation methodologies implemented to support airframe manufacturers to partially fulfill the current certification airworthiness requirements. The methodology can be also applied during preliminary aircraft parametric design stages. In the current study, the method was applied onto an aircraft wing leading edge preliminary design, which led to design exploration by correlating the leading edge skin materials and thicknesses with the rib pitch positioning. The bird-strike impact model was simulated using the Smoothed Particle Hydrodynamics numerical method using ABAQUS® Explicit finite element package. The materials benchmarked were aluminum alloy 2024-T3, carbon fiber reinforced epoxy IM7/8552 and S2 glass Fiber Metal Laminate GLARE®. The design goal of the case study was to provide with preliminary evidence for impact resistance, quantified as residual permanent structural deformation of the critical structural components for which design charts were drawn and presented hereinItem Open Access Crashworthiness behaviour of a composite fuselage section with cargo door(Erasmus Conferences and Events, 2021-07-07) Milan, Carlos; Giannopoulos, Ioannis K.; Theotokoglou, Efstathios E.This paper present the crashworthiness assessment of a composite fuselage section with a cargo door by means of the Finite Element (FE) software ABAQUS/Explicit. In crashworthiness research, no analysis, either experimental or numerical, of a composite fuselage section with a cargo door has ever been carried out. Therefore, the numerical analysis of the vertical drop test of three models of composite fuselage sections, representative of a regional aircraft fuselage, will be performed: a typical fuselage section without cargo door, a section with the cargo door but not the appropriate reinforced structure and a section with the surrounding door structure reinforced as in commercial aircraft sections with such cut-outs. In order to guarantee the integrity of the passengers as well as the structure, the crash kinematics of each model as well as the accelerations experienced by the passengers have been compared and examined in detail. The comparison between the three models allowed to identify the penalty that a duly reinforced cargo door structure induces on the crashworthiness of a composite fuselage.Item Open Access Design and numerical modeling of a pressurized airframe bulkhead joint(American Institute of Aeronautics and Astronautics, 2015-11-01) Giannopoulos, Ioannis K.; Theotokoglou, Efstathios E.; Zhang, XiangThe structural loading on a conceptual lap joint in the empennage of a civil aircraft has been investigated. The lap joint interfaces the end-pressure part-hemispherical bulkhead to the cylindrical fuselage. The pressure bulkhead is made of carbon fiber reinforced plastic materials. The aim of the study is to present numerical results of the induced structural loading from the fuselage positive internal pressure differential and the localized high stress intensity field at the lap joint location. A methodology for the appropriate numerical approach to analyze the domed pressure bulkhead is presented. The results of the numerical investigation showed that the laminate loading levels calculated by the use of either initial sizing analytical formulas for pressurized domes or by the use of equilibrium nodal loading from finite element models of low fidelity compared to refined finite element analysis can be significantly underestimated. Some of the implications on carbon fiber reinforced plastic structural sizing at the specified location are developed.Item Open Access Experimental testing correlation with numerical meso-scale modelling of CFRP structures and the significance to virtual certification of airframes(International Centre for Numerical Methods in Engineering (CIMNE), 2021-01) Moulkiotis, C. N.; Giannopoulos, Ioannis K.; Theotokoglou, Efstathios E.The design of structural components has altered fundamentally since laminated composites were proved excellent candidate materials in aerospace applications. The key aspects rendering CFRPs preferable to metals, are mostly their significantly higher specific mechanical properties, and the design flexibility through the stacking sequence selection. However, the currently in use limit and polynomial failure criteria, are inadequate to accurately predict all experimentally observed failure modes and damage specificities of the lamina individual constituents, imposing difficulties in the numerical certification of airframe composites. Thus, component and lamina-level testing are sometimes inevitable, requiring industrial resources which are expensive as well as environmentally costly. For that reason, virtual testing could be more promising in substituting real experimental testing, if conducted under advanced failure criteria which better describe the nature of failure. In this study, the open hole tensile (OHT) test has been simulated under the LaRC05 phenomenological failure criterion, with embedded strain-based progressive damage material behavior. A relatively common composite material in aerospace structures has been selected, IM7 8552 of Hexcel, to compare the numerical strength predictions with its corresponding experimental values. The simulations carried out are based on a standard test method by ASTM international, which address the standardisation of strength tests of polymer matrix composite laminates. The, model was created in ABAQUS/Explicit under the VUMAT user subroutine. The resulted predictions have been found to well – correlate with the testing data, irrespective the specimen stacking sequence.Item Open Access Impact damage and CAI strength of a woven CFRP material with fire retardant properties(Elsevier, 2016-02-03) Giannopoulos, Ioannis K.; Theotokoglou, Efstathios E.; Zhang, XiangThis paper presents the interrogation of low velocity impact and compression after impact test results on a woven fibre composite having a fire retardant, syntactic core, two phase epoxy matrix. The results of the study were to be utilized in a decision making process regarding the appropriateness of the material usage in question for a certain aerospace application. The epoxy matrix of the material system had dispersed black-pigmented particles with flame-retarding properties. Impact tests were performed at five impact energy levels. Two different laminate layup configurations were tested. Visual and C-Scan inspection were conducted, in order to observe the extent of the damage in the composite material. Compression tests were performed to study the residual strength after impact. Analytical formulation correlations with the test results presented opportunities for quantifying the interfacial fracture toughness resistance. Micro-graphs of the specimen's cross section were also produced in an effort to observe the fractured sections and characterise the various fracture mechanisms involved. The results exploitation in terms of design decision making are presented.Item Open Access Micromechanical modelling and interfacial strength prediction of multidirectional laminated fibre reinforced polymers(2017-06-30) Bolyky, Ákos; Giannopoulos, Ioannis K.; Theotokoglou, Efstathios E.Delamination initiation and propagation is a common failure mode in laminated composites that must be considered when assessing damage in composite structures. Delamination usually propagates at the interface between laminas. Current approved testing procedures address the inter-laminar strength in fracture modes I and II for interfaces of unidirectional laminas oriented in the same direction. The aim of this study was to investigate the interlaminar fracture initiation strength in multi-layer lamina interfaces by the use of micromechanical numerical analysis. Representative volumetric elements with randomly distributed fibres and the ability of numerically modelling fibre-matrix interfacial debonding were generated with different ply interfacial orientations. Failure initiation and damage sequences were captured and the global stresses where failure initiated were determined for the studied configurations. Insights on the variations in the strength observed due to the different lamina orientations were provided.Item Open Access Numerical analysis failure prediction of CFRP stiffened panels in the context of optimal airframe structural performance(Hellenic Society for Theoretical and Applied Mechanics, 2019-09-25) Giannopoulos, Ioannis K.; Bleoju, Nicolae A.; Tourlomousis, Ilias; Theotokoglou, Efstathios E.This paper presents the effect of stiffener damage on Carbon Fibre Reinforced Composite (CFRP) stiffened panels subjected to compression, for various stiffener design configurations. Nonlinear finite element progressive damage numerical simulations were used for the analysis. The investigation targeted the percentage decrease of the panel compression strength between the pristine (undamaged) and damaged stiffened panel states. The three designed cases sought, were assuming stiffened panels of the same weight but of different stiffener design. The study aimed at displaying that for CFRP stiffened panels used in aircraft structures and designed to carry loads where material strength could be the driver for the maximum compression loading capacity and not the structure’s resistance to buckling, the stiffener geometry and material damage propagation are some of the major parameters for optimal stiffened panel design. In that regards and for cost saving from expensive testing surveys, nonlinear finite element analysis is a valuable tool for preliminary design studies and optimal design down-selection.Item Open Access Numerical FEA parametric analysis of CAI behaviour of CFRP stiffened panels(Elsevier, 2019-06-10) Gaitanelis, Dimitrios G.; Giannopoulos, Ioannis K.; Theotokoglou, Efstathios E.This paper examined the effect of numerical modelling parameters on the accuracy and computational efficiency of Carbon Fibre Reinforced Polymer (CFRP) stiffened panels under Compression After Impact (CAI). Pristine and damaged CFRP stiffened panels were subjected to compression in Abaqus® software using Cohesive Zone Model (CZM) method. Various case studies were examined and the effect of the stiffness parameters of the cohesive elements was critically assessed. Moreover, the required number of cohesive zones to fully capture the damage mechanisms of the impacted and pristine panels under compressive loading was examined. The results showed that a wrong set of parameters can even lead to neglecting the induced damage and can cause severe convergence problems in the numerical model. The importance of the Overall Meshing Factor (OMF) was highlighted and a user-defined subroutine (USDFLD) was applied to capture the decrease in the load bearing capability of an impacted panel prior to the compressive loading, since CZM was found insufficient for this scope. The above-mentioned remarks illustrated the process of investigating the optimum numerical parameters set to achieve an accurate and efficient finite element modelling of the stiffened panels structural performance and maximum load-carrying capability, when subjected to CAI loading.Item Open Access Numerical simulation of bolted joints pull through failure(Erasmus Conferences and Events, 2021-07-07) Miao, Tijian; Giannopoulos, Ioannis K.; Theotokoglou, Efstathios E.Finite Element Analysis numerical models were generated to simulate and investigate the pull-through damage of bolted joints on composite laminates. Three-dimensional elements were used along with a user material subroutine incorporating the material failure criterion in Abaqus® software. Simplified Discrete Ply Modelling (DPM) and Cohesive Zone Modelling (CZM) were also employed. The numerical model predictive capability was assessed and the parameters influencing the pullthrough failure process were investigated. The residual bearing strength of bolted joints following pull-through damage suggested a qualitative agreement between the numerical model and the experimental testing results.Item Open Access Parametric analysis of cohesive zone model method for CFRP stiffened panel CAI behaviour(The Hellenic Society of Theoretical & Applied Mechanics, 2019-09-25) Gaitanelis, D. G.; Giannopoulos, Ioannis K.; Theotokoglou, Efstathios E.This paper examined the effect of the stiffness of the cohesive elements on the accuracy and the computational efficiency of Carbon Fibre Reinforced Polymer (CFRP) stiffened panels under ,Compression After Impact (CAI). Abaqus® software was used and the Cohesive Zone Model (CZM) method was applied to capture the damage initiation and propagation of the panels. Various case studies were examined and the effect of the stiffness parameters of the cohesive elements wascritically assessed. Moreover, the required number of cohesive zones to fully capture the damage mechanisms of the impacted and pristine panels under compressive loading was examined. The results showed that a wrong set of parameters can even lead to neglecting the induced damage and can cause severe convergence problems in the numerical modelItem Open Access Preliminary analysis method for FRP laminate impact damage size prediction(Technical University of Crete Press, 2018-12-31) Deskiewicz, Adam W.; Giannopoulos, Ioannis K.; Theotokoglou, Efstathios E.Low velocity impact damage on carbon reinforced polymer laminate composites has been identified as a key threat to airframe structural integrity since it reduces the strength under compressive loading. Airworthiness certification specifications dictate that the airframe structural components up to the full scale subassemblies have to adhere to the strength and fatigue airworthiness requirements imposed whilst being damaged. The study presented herein combines a set of numerical tools for generating an approach to numerically quantify the damage size after low velocity impact on FRP laminates.Item Open Access Sensitivity of composite scarf joints to manufacturing deviation and disbond under tensile load(2017-06-30) Larkin, David J.; Giannopoulos, Ioannis K.; Theotokoglou, Efstathios E.Scarf joints are an effective method of bonding thick composite laminates for applications such as the repair of composite aircraft structures. However, concerns remain about their damage tolerance characteristics. Typically composite scarf repairs to aircraft structures require use of hand tools or rudimentary jigs. If the scarf is incorrectly prepared, this may cause a profile deviation to the joint, affecting the bond line stresses and in turn, reducing the residual strength of the joint or repair. The subject of this work examined the sensitivity of composite scarf joints to machining profile deviation and artificial disbond, when subject to static tensile load. Tensile test specimens were prepared with two different configurations of scarf for representing an undercut or imprecise scarf typical of a machining error. In addition, sensitivity of the scarf joints in the presence of an artificial disbond was also tested. Results indicated that for the specimens tested, the scarf is relatively insensitive to minor profile deviation, but highly sensitive to an artificial disbond. Experimental results were also compared with finite element analysis.