Browsing by Author "Hughes, Kevin"
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Item Open Access Characterising the level of crashworthiness for impacts on hard ground and water surfaces for a metallic helicopter under floor structure: What lessons can be learned?(2008-05-07T00:00:00Z) Hughes, KevinHelicopters are seen by the petroleum industry as the only viable way of transportation between on and offshore platforms. At present, there exists no certification requirement to ensure a high level of survivability in the event of a water impact. Within the literature, there exists a body of information related to the post crash analysis of accident data, which supports the finding that a conventional metallic under floor design performs poorly during a water impact, in relation to the transmission of water pressure and the absorption of energy. In order to characterise this behaviour, this paper concerns the crashworthiness of helicopters to two extremes in loading, namely hard ground and water surfaces, for an impact speed of 8ms-1, for a simple box-beam construction common to metallic helicopters. The experimental findings were used to validate finite element simulations, with a view for assessing the level of crashworthiness currently offered, together with identifying potential design improvements. To improve the level of crashworthiness, careful redesign of frames, joints and skin is required, together with developing a passive next generation floor that can cater for both hard surface and water impacts, by being able to degrade its localised strength, depending upon the type of surface encountered.Item Open Access Comparative assessment of implicit and explicit finite element solution schemes for static and dynamic civilian aircraft seat certification (CS25.561 and CS25.562)(Cranfield University, 2013-03) Gulavani, Omkar Vitthal; Hughes, Kevin; Vignjevic, RadeDue to the competitive nature of airline industry and the desire to minimise aircraft weight, there is a continual drive to develop lightweight, reliable and more comfortable seating solutions, in particular, a new generation slim economy seat. The key design challenge is to maximise the “living space” for the passenger, with strict adherence to the ‘Crash Safety Regulations’. Cranfield University is addressing the needs of airliners, seat manufactures and safety regulating bodies by designing a completely novel seat structure coined as “Sleep Seat”. A generous angle of recline (40 degree), movement of “Seat Pan” along the gradient, fixed outer shell of the backrest, and a unique single “Forward Beam” design distinguishes “Sleep Seat” form current generation seats. It is an ultra-lightweight design weighing 8kg (typical seat weight is 11kg). It has to sustain the static (CS 25.561) and dynamic (CS25.562) “Emergency landing” loads as specified by “Certification Specifications (CS). Apart from maintaining structural integrity; a seat-structure must not deform, which would impede evacuation, should absorb energy so that the loads transferred to Occupants are within human tolerance limits and should always maintain survivable space around the Occupant. All these parameters, which increase a life-expectancy in a ‘survivable’ crash, can be estimated using either experimental testing or virtual simulation tools such as “Finite Element Analysis (FEA). Design of the “Sleep Seat” is still in its conceptual phase and therefore experimental testing for all the design iterations involved is unrealistic, given a measure of the costs and timescales involved. Therefore focus of research is to develop practical and robust FE methodologies to assess static and dynamic performances of a seat-structure so as to compare different design concepts based on their strength, seat interface loads (a limit defined by strength of aircraft-floor), maximum deformations and cross-sectional forces ... [cont.].Item Open Access Constitutive model for fibre reinforced composites with progressive damage based on the spectral decomposition of material stiffness tensor(Elsevier, 2022-05-11) Vignjevic, Rade; Djordjevic, Nenad; Galka, Agata; Appleby-Thomas, Gareth J.; Hughes, KevinComplex nature of the fibre reinforced composites, their non-homogeneity and anisotropy make their modelling a challenging task. Although the linear – elastic behaviour of the composites is well understood, there is still a significant uncertainty regarding prediction of damage initiation, damage evolution and material failure especially for a general loading case characterised with triaxial state of stress or strain. Consequently, simplifying assumptions are often unavoidable in development of constitutive models capable of accurately predicting damage. The approach used in this work uses decomposition of the strain energy based on spectral decomposition of the material stiffness tensor and an assumption that each strain energy component represent free energy for a characteristic deformation mode. The criteria for damage initiation are based on an assumption that the damage corresponding to a deformation mode is triggered when the strain energy for that mode exceeds a specified critical limit. In the proposed model the deformation modes are not interacting at continuum scale due to orthogonality of the eigenvectors, i.e. the stiffness tensor symmetry. Damage and its evolution are modelled by reduction of the principal material stiffness based on the effective stress concept and the hypothesis of strain energy equivalence. The constitutive model was implemented into Lawrence Livermore National Laboratory (LLNL) Dyna3d explicit hydrocode and coupled with a vector shock Equation of State. The modelling approach was verified and validated in a series of single element tests, plate impact test and high velocity impact of hard projectile impact on an aerospace grade carbon fibre reinforced plastic. The model accurately predicted material response to impact loading including the test cases characterised by presence of shock waves, e.g. the plate impact test. It was also demonstrated that the model was capable of predicting damage and delamination development in the simulation of the high velocity impact tests, where the numerical results were within 5% of the post impact experimental measurements.Item Open Access Design exploration methodology for ultra thick laminated composites(Cranfield University, 2012-01) Carrasco-Munoz Y Guerra, Jacinto; Vignjevic, Rade; Hughes, KevinExisting test and analytical methods (theoretical and numerical) are normally restricted to thin laminate components, which cannot accurately represent the 3D stress state behaviour of the so called Ultra Thick Laminates (UTL) structures. Thus, it is necessary to expand the scope of application of the current numerical methods to accurately predict the out-of-plane delamination failure associated with these types of structures (mainly due to the transverse shear stresses and interlaminar stresses). The overall objective of this work is to address the following research objectives: • To assess the functionality, advantages and limitations of different solid element formulations, including layered solid elements that are available in commercial Finite Element codes, applied to the mechanical response prediction of UTL composite components (thicknesses up to 30 mm are considered). • To perform a design exploration and optimisation of constant thickness UTL composite component in terms of the orientation of a varying and repeatable stacking sequence of an eight ply Non-Crimped Fabric, in order to assess the design implications on performance. In order to achieve the above stated objectives a standard, flexible and expandable FE based design exploration methodology (at a ply level) for UTL composite components is proposed, which considers a commercial FE tool (ANSYS), and a data management system and optimisation tool (ISIGHT), through the use of layered solid elements (SOLID186 and SOLID191, 20-node layered solid elements). Application of manufacturing design rules (for reducing the number of feasible stacking sequences to be evaluated) is also considered, in order to reduce the computational cost of such a study, as well as to present a practical solution from the manufacturing point of view. Initially, in-plane and out-of-plane capabilities of various layered element formulations and modelling strategies where evaluated for thin and thick laminate applications against known analytic solutions (CLT, etc), in order to understand the key parameters and the accuracy limitations of each formulation. This led to practical recommendations for pre and post processing of thick laminate FE models, such as for the number of layered solid elements required as a function of the thickness of the UTL component to effectively predict the magnitude and variation in transverse shear stress across the thickness. The application of this research was demonstrated on the design exploration and performance optimisation of a UTL composite specimen (with constant thickness) under a 3-point bending test (linear static analysis), for which experimental results were available. The individual ply orientations are the design variables considered, and the performance was assessed through the vertical displacement of the component and the maximum transverse shear stress value. This exploration of the design space did identify other possible configurations that may have a better performance than the baseline (Biax), considering only the maximum transverse shear stress values as directly responsible for the delamination failure. However, these improved designs may present a higher number of plies failed or a higher failure index (Tsai-Wu failure criteria). Further experimental studies are required to further explore the design space, but this work represents the starting point and possible approaches for development of robustness and weight optimisation of UTL composites are proposed.Item Open Access Experimental observations of an 8ms-1 drop test of a metallic helicopter underfloor structure onto a hard surface: part 1(Professional Engineering Publishing, 2007-06-12T00:00:00Z) Hughes, Kevin; Vignjevic, Rade; Campbell, James C.Abstract: This is the first part of a two-part paper that describes the experimental observations for two similar sections of floor that were dropped onto both hard and water surfaces at 8 m/s, as a part of one experimental campaign. The current paper provides an assessment of a simple box-beam underfloor structure typically found in metallic helicopters and provides an overview of the failure modes and the collapse mechanism observed when dropped onto a hard surface. All findings are supported by quantitative measurements and extensive photographic evidence. The current paper identifies two limitations with the existing design, which are based upon the observations of the failure modes for different frame types and the performance of the intersection joints. In order to increase the level of crashworthiness currently offered, significant frame and joint redesign is required in order to provide a more progressive collapse. The simple buckling modes currently observed should be avoided, as the existing stroke is not fully utilized in the event of a crash, resulting in an inefficient structure. The current paper also discusses the sensitivity to impact angle, as slight variations from a normal impact may result in a detrimental response.Item Open Access Extension of Skopinski's approach to reduce (strain gauge) flight load measurement uncertainties.(2017-05) Chedid, Marwan Maurizio; Hughes, KevinStructural Health Monitoring (SHM) is an essential technique for assessment of the integrity of ageing structures or certification of new aircraft. The SHM approach developed by Skopinski, based on conventional strain gauges, is one of the most reliable experimental methods for assessment of the structural loads experienced by lifting surfaces. This thesis concerns the extension of Skopinski’s approach to low aspect ratio wings with multiple spars. This is challenging problem, as the redundant load paths increase the complexity and difficulty in defining load equations and fitness functions. Similarly, locating, calibrating and selecting relevant strain gauges becomes more difficult. One of the new developments is a methodology for load measurement based on physical metrics and load data that is independent from the ground calibration used to generate the load equations. The concepts of Influence Coefficient Plots of Strain Gauge and of Loads Equations, were used to identify the predominant loads. The developed fitness function equations are all driven exclusively by these physical quantity parameters. Another key development was reduction of the effort required during the experimental phase achieved using distributed load data, obtained by numerical superposition of individual load cases to develop loads equations. In addition, this approach reduces the risk of damage to the flying test article and increasing the accuracy of the final results as the magnitude of the loads introduced does not need to be limited. This new framework was coded in Python including a new automated load equation computation technique, Linearised Physical Properties (LPP) and environment Loads Equation Technique Evaluator (LETE). Different search engines, including Exhaustive Search, Strain Gauge Reduction, and Genetic Algorithm, were made available for load assessment. Code validation (and performance) was performed through root mean square error evaluation in shear, bending and torsion loads using CIRA Test Data for two identical unmanned Space Vehicles, Castore and Polluce. The validation process demonstrated that loads equations developed on Castore can be applied to Polluce, as long as the SHM system design and instrumentations are the same.Item Open Access Finite element analysis of a novel aircraft seat against static certification requirements (CS25.561)(Heriot-Watt University, 2011-04-06) Gulavani, Omkar Vitthal; Hughes, KevinDue to the competitive nature of the airline industry and the desire to minimise aircraft weight, there is a continual drive to develop lightweight, reliable and more comfortable seating solutions, in particular, the development of a new generation slim economy seat. The key design challenge is to maximise the “living space” for the passenger, with strict adherence / compliance to Safety Regulations. This paper presents the analysis led design using finite element analysis of an innovative seat concept developed by BlueSKy Designers Limited, which has been acclaimed as “The most exciting development in aviation in over 30 years” and has won the company numerous awards. A generous angle of recline (40 degrees), movement of the “Seat Pan” along different gradients, and unique single “Forward Beam” design, distinguishes “Sleep Seat” from current generation seats. Compliance against Static Strength requirements (CS25.561) through a sequential model development approach was performed, in order to predict the stress induced in the primary seat structure, against static certification requirements. A critical design parameter is ensuring seat interface loads are below airline limits, which resulted in the inclusion of seat stud and track details in the finite element model. This stepwise and validated analysis framework, which includes mesh sensitivity studies, modelling of bolt-preload, representing bolted joints in FEA and obtaining a converged solution for non-linear FEA was essential in order to allow different concepts to be assessed virtually, thereby reducing development cycle time. The findings from this paper demonstrate that the seat is safe against CS 25.561.Item Open Access Helicopter crashworthiness: a chronological review of research related to water impact from 1982 to 2006(American Helicopter Society Inc, 2008-06-02T00:00:00Z) Hughes, Kevin; Campbell, James C.This paper discusses in detail the contributions made in the field of water impact from 1982 to 2006 and provides a summaryof the major theoretical, experimental, and numerical accomplishments, up to and including the latest present-day Europeanprograms on helicopter water crashworthiness. A summary of the major findings is presented, and their importance to thedirection of water crashworthiness development is discussed, together with recommendations for future research.Item Open Access Non-linear finite element analysis led design of a novel aircraft seat against certification specifications (CS 25.561)(Cranfield University, 2011-01) Gulavani, Omkar Vitthal; Hughes, Kevin; Vignjevic, RadeSeeking to quench airliners’ unending thirst for lightweight, reliable and more comfortable seating solutions, designers are developing a new generation of slim economy – class seats. Challenge in front of the designers is to carve out additional “living space”, as well as to give a “lie – flat” experience to air travellers with strict adherence to safety regulations. Present research tries to address all these industry needs through an innovative and novel “Sleep Seat”. A generous angle of recline (40 degree), movement of “Seat Pan” along the gradient, fixed outer shell of backrest, and unique single “Forward Beam” design distinguishes “Sleep Seat” form current generation seats. It is an ultralightweight design weighing 8kg (typical seat weight is 11kg). It satisfies “Generic Requirements (GR2)” which ensures “Comfort in Air”. It will be a “16g” seat, means it can sustain the “Emergency landing” loads as specified by “Certification Specifications (CS 25.561 and CS 25.562)”. For present research, only CS 25.561 has been considered. Since, the design of “Sleep Seat” is still in its conceptual phase, it is not possible to build the prototypes and their physical testing, due to costs and time involved. “Finite Element Analysis (FEA)” is a useful tool to predict the response of the structure when subjected to real life loads. Hence, the aim of research being undertaken is to develop a detailed FE model of the complete seat structure, which will help designers to identify potential weak areas and to compare different design concepts virtually, thereby reducing the development cycle time. In order to avoid handling of large number of design variables; major load carrying members (called Primary Load Path) i.e. Forward beam and leg; are designed for the most critical “Forward 9g” loads; using FEA results as a basis. A robust framework to verify the FEA results is developed. “Sequential Model Development Approach”; which builds the final, detailed FE model starting from preliminary model (by continuously updating the FE model by addition of details that are backed up by pilot studies); resulted in a FE model which could predict the stress induced in each of the components for applied CS 25.561 loads along with “Seat Interface Loads”. The “Interface Load” is the force exerted by the seat design on the floor and is one of the main contributing factors in seat design. “Optistruct” is used as a solver for linear static FEA, whereas “Abaqus / Standard” is used for non-linear FEA. Stepwise methodologies for mesh sensitivity study, modelling of bolt-preload, representing bolted joint in FEA, preventing rigid body motion, and obtaining a converged solution for non-linear FEA are developed during this research. Free-Shape Optimisation is used to arrive at a final design of Seat-leg. All the findings and steps taken during this are well documented in this report. Finally, a detailed FE model (involving all the three non-linearities : Contact, material and geometric) of the complete seat structure was analysed for the loads taken from CS 25.561, and it was found that design of “Forward beam” and leg are safe against CS 25.561. Therefore, all the aims and objectives outlined for this research were accomplished. For future work, first area to look for, would be validation of present FEA results by experimental testing. FE model to simulate dynamic loads CS 25.562 can be developed followed by design improvements and optimisation.Item Open Access A numerical study on the influence of internal corrugated reinforcements on the biaxial bending collapse of thin-walled beams(Elsevier, 2019-07-25) Vignjevic, Rade; Liang, Ce; Hughes, Kevin; Brown, Jason C.; De Vuyst, Tom; Djordjevic, Nenad; Campbell, James C.The Heat Treatment Forming and in-die Quench (HFQ) process allows for manufacturing of more complex geometries from Aluminium sheets than ever before, which can be exploited in lightweight automotive and aerospace structures. One possible application is manufacturing thin walled beams with corrugated internal reinforcements for complex geometries. This work considers different internal reinforcements (C-section and corrugated) to improve the energy absorption properties of thin walled rectangular beams under uniaxial and biaxial deep bending collapse, for loading angles ranging from 0 to 90 deg, in 15° increments. Using LS-DYNA simulations experimentally validated through unreinforced metallic tubes under quasi-static bending collapse, the finite element results demonstrate the stabilising effect of the reinforcements and an increase in the buckling strength of the cross section. Corrugated reinforcements showed a greater potential for increasing specific energy absorption (SEA), which was supported by investigating key geometric parameters, including corrugation angle, depth and number. This favourable response is due to an increased amount of material undergoing plastic deformation, which consequently improves performance of the beam undergoing post buckling and deep collapse. This concept is applicable to vehicle and aircraft passive safety, with the requirement that the considered geometries are manufacturable from Aluminium Alloys sheet only, using the HFQ processItem Open Access Optimisation of composite materials using a multilevel decomposition approach(Cranfield University, 2009) Chedid, Marwan Maurizio; Hughes, Kevin; Vignjevic, RadeThe design optimisation of thick composite components requires dealing with large number of design variables, highly non-linear equations and huge computational demand. A multi-level decomposition process has been developed to optimise these elements, investigating the potentialities of pre-existing approaches of the bioengineering field of hip and femoral implants. The strength of exploiting a multi-level decomposition process is related to the flexibility of choosing at each level specific optimisation methods which modulate according to the design variables appointed and to the difficulty level of the application. Specifically the proposed framework is developed through two main applications, Basic and Intermediate Example, with increasing difficulty. The Basic Example uses Graphical Optimisation tools selecting the typical design variables such as thicknesses and orientation angles. Then, the Intermediate Example considers numerical optimisation conditions with Kuhn- Tucker and Lagrange Multipliers and again Graphical Methods, but using lamination parameters. The optimisation analyses and classical lamination theory calculations are performed through a Matlab code linked with an external solver, Nastran, to perform the numerical structural analysis. These two applications, designed as 2-level decomposition approach, do not consider any inter-laminar effect and out-of-plane loading that it will be taken into account in the proposed final frame work application, Advanced Example. A single level optimisation analysis is performed by commercial software optimiser, Nastran Sol 200, as alternative mean to validate the two numerical applications. Results show the potentiality of this technique, mainly related to the capability to control each design cycle, though the full understanding will come after a numerical application of the advanced example. In fact, adding within the current Matlab code genetic algorithms, sequential linear programming, and gradient based methods capable to deal with out-of-plane loadings, quadratic failure criteria taking into account the through-thickness stress effects is still a complex direction of enhancement recommended to be explored.