CERES :: Browsing by Author "Giannopoulos, Ioannis K."

Browsing by Author "Giannopoulos, Ioannis K."

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Open Access
3D-printed thermoplastic composite fasteners for single lap joint reinforcement
(Elsevier, 2021-12-10) Li, Wenhao; Guo, Shijun; Giannopoulos, Ioannis K.; Lin, Minxiao; Xiong, Yi; Liu, Yiding; Shen, Zhengquan
This study presents findings for the strength and failure mechanism of a 3D-printed Continuous Carbon Fibre reinforced Onyx (CCF/Onyx) Thermo-Plastic Composite Fastener (TPCF) and a single lap-joint (SLJ) made of fibre/polymer composite reinforced by the TPCF. The study was carried out by numerical analysis and experiment methods including test sample design, manufacturing process and mechanical test. The 3D-printed fasteners were manufactured and tested in shear mode for two types of joining arrangement: fastened and hybrid bonded/fastened joints. Firstly, experiment was carried out for the TPCF fastened SLJ and the results show that addition of CCF in the Onyx matrix and post heat-treatment process could significant enhance the TPCF strength. The results was then benchmarked against a SLJ with steel fastening. The shear failure load of the SLJ reinforced by heat-treated CCF/Onyx TPCF of 8mm diameter was 36% lower than a SLJ reinforced by a steel bolt of the same size. Numerical model for progressive damage simulation was also created based on the failure theory from Puck and Schürmann achieving good correlation with the experimental data. Secondly, the TPCF fasteners were manufactured with two types of heat-treated countersunk head and pan head forming and used to reinforce bonded SLJ. The test results show that the bonded SLJ reinforced by the TPCF fastener of countersunk head is of 11.7% higher strength and an increase in ultimate deformation by 9.1% compared to a bonded SLJ reinforced by steel fastener of 5mm diameter. From the numerical and experimental study, it was noted that this was attributed to countersunk configuration to reduce out-out-plane bending and provide better crack arresting for the joint bonding.
Open Access
The Airbus A320 family fan cowl door safety modification: a human factors scenario analysis
(Emerald, 2018-08-18) Kourousis, Kyriakos I.; Chatzi, Anna V.; Giannopoulos, Ioannis K.
Purpose The Airbus A320 family engine fan cowl doors (FCDs) safety issue is known to the industry for almost 18 years; however, it has not been addressed adequately by the aircraft manufacturer and the various operators and regulating authorities. The purpose of this paper is to examine in a systematic way the possible operational and safety implications of a new modification on the engine FCDs. Design/methodology/approach An array of error-prone scenarios is presented and analysed under the prism of human factors in a non-exhaustive qualitative scenario analysis. Findings All examined scenarios are considered more or less probable. A number of accident prevention solutions are proposed for each of the scenario examined, in view of the acceptance and implementation of this modification by operators. Research limitations/implications As these scenarios are neither exhaustive nor have been tested/validated in actual aircraft maintenance practice, the further analysis is necessary. A substantial follow-up survey should take place, which should include a wider array of scenarios. This would allow obtaining the necessary data for a quantitative (statistical) analysis. Practical implications This case study identifies issues in relation to this modification, introduced by Airbus and the European Aviation Safety Agency (EASA), which may prove problematic from the point of view of safety effectiveness and disruption of operations. Originality/value This case study examines a long-standing aviation safety issue and the implications of a solution proposed by the aircraft manufacturer and adopted by EASA. This can be useful in increasing the awareness around these issues and highlight the importance of a human-centric and scenario-based design of engineering modifications towards minimising error in aircraft technical operations.
Open Access
Ballistic impact and virtual testing of woven FRP laminates
(MDPI, 2021-04-22) Giannopoulos, Ioannis K.; Yasaee, Mehdi; Maropakis, Nikolaos
The aim of the work was to investigate the numerical simulations correlation with the experimental behaviour of steel ball high velocity impact onto a 2 × 2 twill woven carbon composite laminate. The experimental set up consisted of a pressurised gas-gun able to shot steel ball projectiles onto two different composite plate layup configurations of plates made of the same composite material fabric. Subsequently, the experiments were replicated using the LSDYNA explicit finite element analysis software package. Progressive failure numerical models of two different fidelity levels were constructed. The higher fidelity model was simulating each of the plys of the composite panels separately, tied together using cohesive zone modelling properties. The lower fidelity model consisted of a single layer plate with artificial integration points for each ply. The simulation results came out to be in satisfactory agreement with the experimental ones. While the delamination extent was moderately under predicted by the higher fidelity model, the general behaviour was complying with the experimental results. The lower fidelity model was consistent in representing the damage of the panel during the impact and better predicted the impactor residual velocities due to the better matching of the pane stiffness. Despite the competency of the higher fidelity model to capture the damage of the laminate in a more detailed level, the computational cost was 80% higher than the lower fidelity case, which rendered that model impractical against the lower fidelity one, to use in larger models representing more substantial or more complex structures.
Open Access
Bioinspired genetic-algorithm optimized ground-effect wing design: flight performance benefits and aircraft stability effects
(World Scientific and Engineering Academy and Society (WSEAS), 2024-05-02) Zammit, Karl; Smith, Howard; Lobo, Noel Sierra; Giannopoulos, Ioannis K.
This paper presents a bioinspired, genetic-algorithm evolutionary process for Ground-Effect vehicle wing design. The study made use of a rapid aerodynamic model generation and results evaluation computational fluid dynamics vortex lattice method software, supervised by a genetic algorithm optimization Python script. The design space for the aircraft wing parametric features drew inspiration from seabirds, under the assumption of their wings being naturally evolved and partially optimized for proximity flight over water surfaces. A case study was based on the A-90 Orlyonok Russian Ekranoplan, where alternative bioinspired wing variations were proposed. The study objective was to investigate the possible increased flight aircraft performance when using bioinspired wings, as well as verify the static and dynamic aircraft stability compliance for Ground-Effect flight. The methodology presented herein along with the study results, provided an incremental step towards advancing Ground-Effect aircraft conceptual designs using computational fluid dynamics.
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 selection
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 herein
Open Access
Composites joints reinforced by composite rivets
(Asranet, Glasgow, 2018-05-08) Li, Wenhao; Guo, Shijun; Giannopoulos, Ioannis K.
This paper presents an investigation into the mechanical behaviour of composite joints reinforced by using a novel composite rivet made of rolled laminates. Two typical joints have been modelled using three-dimensional solid finite element model in the study. The first type is a composites single lap joint bonded and reinforced by a composite rivet compared with the joint reinforced by a titanium bolt subjected to tensile load. The results are also compared with an adhesive bonded joint as reference. The second type of joint model is a wing box section with skin-rib joint reinforced by composite rivet subjected to a pulling load. A range of adhesive damage was modelled up to 50% (undamaged WBDM, WBDM I 16%, WBDM II 33% and WBDM III 50% respectively) of the bonding area. The results show that the rivets located in the regions where the adhesive bonding failed will carry higher stress and make more contribution to the structure integrity. Although the titanium rivets provide better mechanical performance to carry more load, composite rivets offer an alternative adequate reinforcement to delay the bonding failure and safeguard the structure.
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.
Open Access
Damage tolerance of CFRP airframe bolted joints in bearing, following bolt pull-through failure
(Elsevier, 2020-01-15) Giannopoulos, Ioannis K.; Grafton, Kaelan; Guo, Shijun; Smith, Howard
The experimental study presented herein, investigated the residual strength of bolted joints on Carbon Fiber Reinforced Polymer (CFRP) airframe structures within the context of structural damage tolerance and airworthiness regulations. The damage scenario assumed, subjected a series of bolted joint CFRP laminate specimens to quasi-static bearing loading, following bolt pull-through failure events of different magnitude. Representative CFRP laminate specimens manufactured from AS7/8552 carbon fiber/epoxy matrix system were artificially damaged under bolt pull-through loading, following the herein proposed modifications to the current pull-through ASTM testing procedure. The specimens were subsequently tested in static bearing loading for examining the specimen residual bearing strength. The residual joint bearing strength was related to the displacement travelled passed the initial failure stage in pull-through mode and was measured up to a maximum of a 13% decrease for the tested samples and the maximum damage imposed. The study explored the safe utilization of bolted joints at higher operating loading levels, within the context of the current airworthiness regulations. The inherent damage arrest features of the joints were highlighted. The study concluded with comments and suggestions on the expansion of the current utilization spectrum of damaged bolted joints from pull-through loading in airframe design, bound by the current airworthiness certification requirements.
Open Access
Design and numerical modeling of a pressurized airframe bulkhead joint
(AIAA, 2015-11-01) Giannopoulos, Ioannis K.; Theotokoglou, Efstathios E.; Zhang, Xiang
The 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.
Open Access
Effects of bolt torque tightening on the strength and fatigue life of airframe FRP laminate bolted joints
(Elsevier, 2017-05-23) Giannopoulos, Ioannis K.; Doroni-Dawes, Damian; Kourousis, Kyriakos I.; Yasaee, Mehdi
The experimental study presented herein, investigated the effects of bolt torque tightening on the strength and fatigue design of bolted AS7/8552 fibre reinforced polymer laminates. Damage initiation and final failure manifestation on the joints was investigated and presented using optical microscopy. Subsequent experimental result analysis explored the application domain of bolted joints within the airframe design sector, bound by the current airworthiness certification requirements and expected airframe design life. The reasons for the static strength of the joint laminates or the fatigue failure of the bolt being the main design drivers for the tested joints were highlighted. The study concluded with comments and suggestions on the application of bolt torque tightening in relation to the strength, fatigue life and damage tolerance characteristics of joints on similar fibre reinforced polymer laminate composite material systems.
Open Access
Effects of contact friction and ply blocking on the crush behaviour of thin-walled composite structures: a numerical study
(Elsevier, 2022-10-18) Liu, Haibao; Zhou, Jin; Zhang, Di; Li, Shipeng; Giannopoulos, Ioannis K.
The present paper presents a three-dimensional composite damage model for predicting the crush response of thin-walled structures, i.e., cylindrical tubes, manufactured employing fibre-reinforced polymer–matrix composites. This computational model is based upon a Continuum Damage Mechanics (CDM) approach and accounts for both the intralaminar and interlaminar damage as well as nonlinear behaviour that occur in the composite materials. Experimental data, obtained from published literature, are employed to validate the proposed composite damage model. A comparison between the experimental and computationally predicted results, including the load response, energy absorption and damage morphology, shows good agreement. Subsequently, the validated computational model is employed to investigate the effects of contact friction and ply blocking on the crush response of thin-walled composite structures. The results reveal that the friction between the tube/platen surfaces has a positive effect on the crushing performance of the composite structures and the ply blocking can somehow inhibit the crushing performance of the investigated composite structures.
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.
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, Xiang
This 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.
Open Access
Liquid hydrogen storage tank loading generation for civil aircraft damage tolerance analysis
(AIAA, 2023-06-23) Giannopoulos, Ioannis K.; Theotokoglou, E. E.
The study presented is a preliminary approach and a proposal to the derivation of a loading spectrum for fatigue and damage tolerance analysis for civil aviation Liquid Hydrogen storage tanks. It is anticipated for the first generation of LH2 storage tanks for aviation to utilize metallic lightweight materials. Existing solutions are either too structurally heavy or with a short life span, both constraints making them unsuitable for aircraft vehicles were less mass and longevity is of paramount importance. The objective of the work was to provide suggestions for the generation of representative loading spectra for storage tank fatigue and damage tolerance preliminary design analysis and sizing.
Open Access
Liquid hydrogen storage tank virtual crashworthiness design exploration for civil aircraft
(IOP Publishing, 2023-06-23) Gallois, A.; Giannopoulos, Ioannis K.; Theotokoglou, E. E.
Civil aviation industry is researching for alternative fuel energy sources to substitute current hydrocarbon-based aviation fuels. Carbon free emissions flights could be achieved with fuels like Hydrogen either through combustion or via electricity producing fuel cells. It is of great importance to explore the airframe designs to house Hydrogen in its cryogenic liquified state. The objective of the study herein was to provide a conceptual qualitative analysis related to the crashworthiness behaviour of civil aircraft carrying liquid Hydrogen fuel storage tanks. The design parameters of interest were the storage tank location in the airframe, the structural energy absorption following crash landing scenarios and the structural deformation of the structure surrounding the tanks, penetrating the survival space of the occupants. Several structural design arrangements were proposed and compared. Simulation results indicated that the optimal location for the fuel storage greatly depends on the actual aircraft layout as well as on the future civil aircraft airworthiness requirements that are still under development for that type of fuel energy source.
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.
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.
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.
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.