Browsing by Author "Vignjevic, Rade"
Now showing 1 - 20 of 31
Results Per Page
Sort Options
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 Derivation of SPH equations in a moving referential coordinate system(Elsevier Science B.V., Amsterdam., 2009-06-01T00:00:00Z) Vignjevic, Rade; Campbell, James C.; Jaric, J.; Powell, S.The conventional SPH method uses kernel interpolation to derive the spatial semi-discretisation of the governing equations. These equations, derived using a straight application of the kernel interpolation method, are not used in practice. Instead the equations, commonly used in SPH codes, are heuristically modified to enforce symmetry and local conservation properties. This paper revisits the process of deriving these semi-discrete SPH equations. It is shown that by using the assumption of a moving referential coordinate system and moving control volume, instead of the fixed referential coordinate system and fixed control volume used in the conventional SPH method, a set of new semi- discrete equations can be rigorously derived. The new forms of semi-discrete equations are similar to the SPH equations used in practice. It is shown through numerical examples that the new rigorously derived equations give similar results to those obtained using the conventional SPH equations.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 Design of a composite golf driver club head(Cranfield University, 2004-02) Colin de Verdiere, Mathieu; Vignjevic, RadeThe game of golf is enjoyed by a large number of people, from all over the world. According to the rules of golf, each player has to choose from a maximum of 14 clubs, before playing a shot. This makes the number of pieces of equipment required much higher than in any other ball related sport. This research project focuses on the design of a composite golf driver club head, the club head being the part of the golf club that strikes the ball. The driver club head is selected when a golfer wants to achieve a long distance shot. Modern driver heads are manufactured from metal, either steel or titanium and their performance can be defined as the accuracy and length of a shot. An investigation is made to compare the performance of composite and metal club heads. The project included a study of an existing composite golf driver and the design of a new driver. The new shape was analyzed using non linear finite element software (Ls Dyna), and some overall design in composites and metals were done. It was found that the front face structural properties and geometry were critical to club’s performances. So initially the front face only was considered (smaller model) using Ls Dyna and Nastran, before focusing again on the complete club head. Once the best design was done an attempt to manufacture a prototype was made in order to try to validate the computer model. Experiments revealed that the new driver shape had a higher moment of inertia, which improves the accuracy of a shot, than the existing Callaway C4 club, for the same mass. The ball speed after impact for the new driver was 0.2 m/s less, when using a high strength carbon epoxy composite material compared to a titanium one. For this reason, it is concluded that it is possible to achieve similar overall performances with composite and titanium heads. It is anticipated that with more research into stronger fibres, 3 D fabrics and nano-reinforcements, a carbon composite club head will offer a greater overall performance than metallic golf club heads.Item Open Access Development of a total Lagrangian SPH code for the simulation of solids under dynamioc loading(Cranfield University, 2007) Reveles, Juan R.; Vignjevic, RadeThis thesis makes use of an alternative SPH formulation, the Total Lagrangianf ormulation, to characterised ynamic eventsi n solids and to achieve the proposed objectives outlined in Chapter 1. The structure is as follows: Chapter 1, Introduction, describes the motivation for this research and outlines the objectives and the structure of this thesis. Chapter 2, SPH fundamentals, supplies the standard procedure to generate particle equations and provides a comprehensive summary of gradient approximation formulae in SPH. The discretised SPH form of the conservation laws is included here. Chapter 3, SPH drawbacks: describes the limitations of SPH such as particle deficiency, consistency, zero energy modes, treatment of boundaries and the tensile instability problem. A rigorous stability analysis of continua and SPH particle equations is also presented in this chapter. Chapter 4, Total Lagrangian SPH. Continuum Mechanics considerations are discussed here; detailed derivations of SPH equations in a total Lagrangian framework are given together with potential corrections to the total Lagrangian SPH equations. Chapter 5, Total Lagrangian SPH algorithms and their implementation using FORTRAN. This chapter gives a brief introduction to explicit codes. It also provides flow charts describing the Total Lagrangian algorithms and their integration into the MCM code. Chapter 6, Total Lagrangian SPH code validation. This chapter includes problems of varying degrees of complexity. Examples are provided to illustrate how the Total Lagrangian SPH code compares to a conventional collocational SPH code. Cases are supplied for which the analytical solution is known, and the results compared with the SPH approximations in order to show the accuracy of the approximation. Some examples are supplied which provide a direct comparison between SPH and non linear FE results and SPH and experimental results. Chapter 7, Alternative formulation of SPH equations and improvements to the standard MCM code: Various modifications to the standard SPH code are presented. These modifications include the implementation of subroutines that make use of an alternative approach to ensure the conservation of mass law is met locally at every particle. The introduction of XSPH to achieve further stabilisation of the code was also carried out and some examples are provided. The theory behind an alternative form of the conservation of mass equation as proposed by Belytschko [4] is explained and its implementation into the SPH code is assessed through examples. Also, an alternative formulation of SPH equations based on the general theory of mixed Lagrangian-Eulerian formulations [35] is presented: these equations could serve as the foundation for future research in this field. Chapter 8, Conclusions are presented in this chapter. A brief literature review is provided at the beginning of each chapter as a means of introduction to the topic and a concise summary outlines the main points discussed.Item Open Access Dynamic analysis and control system design of a deployable space robotic manipulator(2001-01) Romero, Ignacio; Vignjevic, RadeThis thesis presents a dynamic analysis and a control system for a flexible space manipulator, the Deployable Robotic Manipulator or DRM, which has a deployable/retractable link. The link extends (or retracts) from the containing slewing link of the manipulator to change the DRM's length and hence its workspace. This makes the system dynamics time varying and therefore any control strategy has to adapt to this fact. The aim of the control system developed is to slew the manipulator through a predetermined angle given a maximum angular acceleration, to reduce flexural vibrations of the manipulator and to have a certain degree of robustness, all of this while carrying a payload and while the length of the manipulator is changing. The control system consists of a slewing motor that rotates the manipulator using the open-loop assumed torque method and two reaction wheel actuators, one at the base and one at the tip of the manipulator, which are driven by a closed-loop damping control law. Two closed-loop control laws are developed, a linear control law and a Lyapunov based control law. The linear control law is based on collocated output feedback. The Lyapunov control law is developed for each of the actuators using Lyapunov stability theory to produce vibration control that can achieve the objectives stated above for different payloads, while the manipulator is rotating and deploying or retracting. The response of the system is investigated by computer simulation for two-dimensional vibrations of the deployable manipulator. Both the linear and Lyapunov based feedback control laws are found to eliminate vibrations for a range of payloads, and to increase the robustness of the slewing mechanism to deal with uncertain payload characteristics.Item Open Access Evaluation of the SPH method for the modelling of spall in anisotropic alloys(2005-06-27T00:00:00Z) De Vuyst, Tom; Vignjevic, Rade; Bourne, Neil K.; Campbell, James C.Spall caused by hypervelocity impacts at the lower range of velocities could result in significant damage to spacecraft. A number of polycrystalline alloys, used in spacecraft manufacturing, exhibit a pronounced anisotropy in their mechanical properties. The aluminium alloy AA 7010, whose orthotropy is a consequence of the meso-scale phase distribution or grain morphology, has been chosen for this investigation. The material failure observed in plate impact was simulated using an explicit finite element code and a smoothed particle hydrodynamics (SPH) code. A number of spall models where used, and the Hugoniot Elastic Limit (HEL) and spall strength have been studied as a function of orientation, and compared to experimental results.Item Open Access Experimental and numerical investigation on the bird impact resistance of novel composite sandwich panels(Cranfield University, 2015-04) Orlowski, Michal; Vignjevic, RadeBird strikes represent a major hazard to the aerospace composite structures, due to their low impact resistance. Accurate selection and lay up of the materials in the composite structure can significantly improve the out of plane properties of the composites. However, application of the complex hybrid sandwich composites into bird strike proof structures was not investigated yet. Therefore, this work was focused on the soft body impact resistance of a novel composite design for aerospace applications. The investigation was divided into experimental and modelling parts. In the beginning of this thesis, the numerical techniques for modelling of bird im¬pact and composite materials were studied. The theoretical background for the corresponding issue was provided, followed by the thorough validation of the exist¬ing numerical approaches. A Smooth Particle Hydrodynamic (SPH) method was chosen for the modelling of the soft body. This modelling technique was validated against experimental data for the rotating fan blade. Three parametric studies of bird impacting fan blades revealed strong influence of the bird impact location and timing on the final deformed shape of the blade. Moreover, it was proved that the SPH is capable of reproducing the exact load on the structure and is appropriate technique for modelling bird strikes. [...cont.]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 Fracture toughness characterization of thin Ti/SiC composites(Cranfield University, 2011-12) Ma, Wei; Vignjevic, RadeTitanium based alloys reinforced uniaxially with silicon carbide fibres (Ti/SiC) are advanced and innovative materials for aerospace vehicles. To avoid potential problems, these new materials should be extensively tested and analyzed before application. This research focuses on experimental fracture toughness study on 0.5 mm thick Ti/SiC composite materials for aerospace applications. The fracture toughness tests are mainly based on BS 7448 with some modifications for transversely isotropic behaviour of the composite materials. By loading on specimens in the direction perpendicular to the fibre axis, three critical values of fracture toughness parameters characterizing fracture resistance of material, plane strain fracture toughness [Plane strain fracture toughness }, critical crack tip opening displacement [Critical crack tip opening displacement ] and critical J-integral [Critical at the onset of brittle crack extension or pop-in when Δa is less than 0.2 mm. ]are measured for two kinds of titanium alloy specimens and three kinds of Ti/SiC composites specimens. The values of [Provisional value of Plane strain fracture toughness ] obtained from the fracture toughness tests are not valid [Plane strain fracture toughness ] for these materials, since the thickness of specimens is insufficient to satisfy the minimum thickness criterion; however, the results could be used as particular critical fracture toughness parameter for 0.5 mm thick structures of the materials. The valid values of [Critical J at the onset of brittle crack extension or pop-in when Δa is less than 0.2 mm. ] and [Critical crack tip opening displacement ] could be used as fracture toughness parameters for all thickness of structures of the materials. The results also show that: fracture toughness of the titanium alloys decreases dramatically after being unidirectional reinforced with SiC fibre, which is mainly triggered by poor fibre/matrix bonding condition. Moreover, Ti-Al3-V2.5 reinforced with 25% volume fraction SiC fibre performs better than the other two composites in fracture resistance.Item Open Access Hydrocode modelling of water impact(2003-03) De Vuyst, Tom; Vignjevic, RadeThis thesis addresses the problem of hydrocode modelling of water impact. Two facets that are of importance when numerically modelling the impact of metallic structures on water are metal anisotropy and water behaviour during impact. In order to be able to take account of these effects in a hydrocode simulation an SPH solver has been incorporated into LLNL-DYNA3D. The treatment of contact in meshless methods has been addressed through the development of a contact algorithm which does not require the construction of surfaces. The interaction of finite elements and SPH particles is accounted for by using a novel approach in treating the finite element nodes as particles in the contact treatment. The same contact algorithm developed for the treatment of contact in the SPH method has been used. In order to take account of metal anisotropy a material model that takes account of anisotropy in the elastic and plastic regimes, strain-rate dependency and non-linear behaviour at high pressures including spall failure was developed. The developed simulation tool is validated against experimental data for the case of water impact of rigid cylinders on water. Further validation is achieved by demonstrating that the simulation tool can be used to analyse the crash behaviour of subfloor designs on water. This was achieved by simulating the impact on water of a structure representative of an aircraft subfloor. The effect of material anisotropy, skin thickness and skin failure on the structural response was demonstrated. A first step in extending the coupled FE-SPH modelling beyond fluid-structure interaction problems has been the development and validation of an explicit time integration ID Lagrangian kernel SPH code which in combination with an algorithm to track crack propagation would make the simulation of dynamic brittle fracture problems possible.Item Open Access Lagrangian hydrocode modelling of hypervelocity impact on spacecraft(1998-05) Campbell, J.; Vignjevic, RadeThis thesis addresses the problem of modelling hypervelocity impact on spacecraft structures using a Lagrangian hydrocode. Lagrangian hydrocodes offer the advantages of computational efficiency and structural elements, but traditionally have been unable to model large material deformations. Analyses of impact on a thin plate were per¬formed using the finite element code DYNA3D. These analyses highlighted three areas where improvement was necessary: material modelling, element erosion criteria and modelling large deformations. To improve the material modelling the SESAME equa¬tion of state was implemented in the DYNA3D code. Two new element erosion criteria were then developed, one based on total element deformation, the second on element accuracy. The two criterion were then tested for modelling impacts onto semi-infinite and thin plate targets. The deformation criterion produced the best results for crater size and hole diameter, but can not be used to model the debris cloud. The element accuracy criterion allows a sufficient number of elements to survive to measure debris cloud velocities and spread angle. It was concluded that an alternative method for modelling the debris cloud is required. The Smoothed Particle Hydrodynamics (SPH) method was selected as it is a Lagrangian method, and allows modelling of large deformations as it does not use a computational mesh. The most recent SPH developments require the boundary conditions to be rigorously treated. A penalty contact algorithm for SPH was developed and tested in 1D and 2D. The tests revealed that for successful treatment of boundary conditions it was necessary to address the problem of zero-energy modes. A alternative discretisation method that calculates the velocity and stress at different points was proposed as a cure for the zero-energy mode problem. This method was tested in 1D, and was shown to be a solution to the zero-energy mode problem.Item Open Access Material characterisation, testing, and modelling of finite element analysis of impact structures(Cranfield University, 2012-10) Nichols, Rachel; Vignjevic, RadeFormula One race cars have to pass rigorous safety tests before they are allowed on track. This type of testing has been in place for years but the requirements for testing are continually increasing in order to reduce the amount of risk to the drivers’ safety during a race. The number of structures that need to be made and tested can quickly make this process an expensive one. Additionally, it is necessary to pass the mandated tests within a reasonable amount of time so as not to have an impact on the development on the rest of the car. There is a desire to reduce the number of structures needed for testing through finite element analysis (FEA), and as such, to reduce the time needed to pass the safety tests. FEA of laminated composites can be complex and is a balance between accuracy and the time it takes to find a solution. The current project looks into increasing understanding of the requirements for material characterisation, experimental impact testing, and explicit simulation of a carbon fibre fabric pre-impregnated with epoxy resin. Mercedes-Benz Grand Prix (MGP) Formula One Team has provided a pre-preg material for evaluation. Material experiments were performed per the American Society for Materials and Testing (ASTM) in order to find the tensile modulus, tensile strength, Poisson’s ratio, compressive strength, shear modulus, and shear strength of the material. Nine tubes were manufactured at MGP and tested in the drop tower at the Cranfield Impact Centre (CIC) ... [cont.].Item Open Access Modeling shock waves in orthotropic elastic materials(American Institute of Physics, 2008-08-01T00:00:00Z) Vignjevic, Rade; Campbell, James C.; Bourne, Neil K.; Djordjevic, NenadA constitutive relationship for modeling of shock wave propagation in orthotropic materials is proposed for nonlinear explicit transient large deformation computer codes (hydrocodes). A procedure for separation of material volumetric compression (compressibility effects equation of state) from deviatoric strain effects is formulated, which allows for the consistent calculation of stresses in the elastic regime as well as in the presence of shock waves. According to this procedure the pressure is defined as the state of stress that results in only volumetric deformation, and consequently is a diagonal second order tensor. As reported by Anderson et al. [Comput. Mech.15, 201 (1994)], the shock response of an orthotropic material cannot be accurately predicted using the conventional decomposition of the stress tensor into isotropic and deviatoric parts. This paper presents two different stress decompositions based on the assumption that the stress tensor is split into two components: one component is due to volumetric strain and the other is due to deviatoric strain. Both decompositions are rigorously derived. In order to test their ability to describe shock propagation in orthotropic materials, both algorithms were implemented in a hydrocode and their predictions were compared to experimental plate impact data. The material considered was a carbon fiber reinforced epoxy material, which was tested in both the through-thickness and longitudinal directions. The psi decomposition showed good agreement with the physical behavior of the considered material, while the zdeta decomposition significantly overestimated the longitudinal stresses.Item Open Access Modelling of behaviour of metals at high strain rates(Cranfield University, 2006) Panov, Vili; Vignjevic, RadeThe aim of the work presented in this thesis was to produce the improvement of the existing simulation tools used for the analysis of materials and structures, which are dynamically loaded and subjected to the different levels of temperatures and strain rates. The main objective of this work was development of tools for modelling of strain rate and temperature dependant behaviour of aluminium alloys, typical for aerospace structures with pronounced orthotropic properties, and their implementation in computer codes. Explicit finite element code DYNA3D has been chosen as numerical test-bed for implementation of new material models. Constitutive model with an orthotropic yield criterion, damage growth and failure mechanism has been developed and implemented into DYNA3D. Second important aspect of this work was development of relatively simple experimental methods for characterization of engineering materials, and extensive experimental work has been undertaken. Tensile test has been used for the characterisation of two aluminium alloys, at different levels of the strain rates and temperatures, and for three different orientations of materials. The results from these tests allowed derivation of material constants for constitutive models and lead to a better understanding of aluminium alloy behaviour. Procedures for derivation of parameters for temperature and strain rate dependant strength models were developed and parameters for constitutive relations were derived on the basis of uniaxial tensile tests. Taylor cylinder impact test was used as a validation experiment. This test was used to validate the implementation, and accuracy of material model in computer code. At the end of each incremental development, validation of the constitutive material model has been performed through numerical simulations of Taylor cylinder impact test, where simulation results have been compared with the experimental post-test geometries in terms of major and minor side profiles and impact-interface footprints. Plate impact test has been used to determine the material properties at high strain rate, and to investigate damage evolution in impact-loaded material. Initially the material model has been designed as a temperature and strain rate dependant strength model in a simple isotopic form, which then has been tested and verified against the experimental results. Coupling of the Hill’s orthotropic yield criterion with isotropic, temperature and strain rate dependant, hardening material model, has been chosen to suit the orthotropic behaviour. Method for calibration of orthotropic yield criterion has been developed and parameters have been identified for the orthotropic model under the associated flow rule assumption and in case of plane stress on the basis of tensile and cylinder impact tests. The complexity of the model has been further increased through coupling of hardening model with orthotropic yield criterion including damage evolution and failure criteria. The constitutive model was developed within the general framework of continuum thermodynamics for irreversible processes, and plate impact test and tensile tests have been used for determination of parameters for damage part of the new material model.Item Open Access Modelling of Damage in Orthotropic Materials: Including Strain-Softening Effects in Dynamic Problems(Cranfield University, 2013-10) Gemkow, Karla Simone; Vignjevic, RadeDamage models are developed within the continuum damage mechanics framework which allows the description of material degeneration with general constitutive equations. The difficulty in the description of damage behaviour increases with increasing complexity of the material behaviour. This is especially true when it comes to composite materials which have an orthotropic material behaviour. The conventional description of damage, i.e. the local continuum damage mechanics description, leads to strain-softening behaviour which is characterised by a decline in stress with simultaneously increasing strain. Due to strain-softening the tangent stiffness becomes negative which forces the wave speed to become imaginary in dynamic problems. Consequently the partial differential equations governing the dynamic problem change from hyperbolic to elliptic and, therefore, the initial boundary value problem no longer has a unique solution. Due to this the physical meaning becomes unrealistic. Strain-softening is limited to an infinitely small area in which waves are not able to propagate in a process called wave trapping. A displacement discontinuity in an area of width zero (localisation zone) develops. The strain becomes infinite in this zone and is accompanied with a zero stress. Areas outside the softening zone are not able to interact with the strain-softening domain. As a consequence the strain-softening domain acts similar to a free boundary at which waves reflect. The implementation of local continua with strain-softening behaviour in finite element codes leads to additional numerical problems. Strain-softening behaviour manifests itself in the smallest area possible which is a single point in analytical considerations. This area is defined by the element discretisation in finite element codes. Therefore, strain-softening leads to a pronounced mesh sensitivity of results in addition to mathematical and physical issues. This work aims to find a solution which removes problems associated to strain- softening. Its aim is to represent material behaviour due to damage realistically and enable numerical results to convergence to a unique solution. The strain-softening problem is the focus of this work. It was investigated using a 1D wave propagation problem described by Bažant and Belytschko [1]. This simple experiment allows for an easy comparison of analytical and numerical results and therefore gives an insight into the problems connected to strain-softening. Furthermore, regularisation methods, specifically nonlocal and viscous methods, were investigated. Regularisation methods add additional terms to constitutive equations which keep the initial boundary value problem well-posed and enable a unique solution independent of the element discretisation. It was found that these methods are indeed capable of regulating the softening problem; however, they add additional difficulties in the description of material behaviour. A new approach to the strain-softening issues, unique at this point of time, was developed in this work which implements damage as an equivalent damage force. This approach is able to keep the initial boundary value problem stable and converge to a unique solution without adding additional terms in the constitutive equations, such as regularisation methods. This new approach to strain-softening was implemented for an isotropic material with scalar damage variable in DYNA3D successfully. Numerical results converged to a unique solution and were physically reasonable. The concept of an equivalent damage force was further developed to orthotropic material behaviour. This made an advanced representation, using an 8th rank damage tensor, necessary. The 8th rank damage tensor is able to represent anisotropic damage and it is also the most general damage representation possible.Item Open Access Modelling of Dynamic Behaviour of Orthotropic Metals Including Damage and Failure(Elsevier Science B.V., Amsterdam., 2012-11-01T00:00:00Z) Vignjevic, Rade; Djordjevic, Nenad; Panov, ViliA physically based material model for metals, with elastic plastic and damage/failure orthotropy is proposed in this paper. The model is defined within the frameworks of irreversible thermodynamics and configurational continuum mechanics and integrated in the isoclinic configuration. The use of the multiplicative decomposition of deformation gradient makes the model applicable to arbitrary plastic and damage deformations. To account for the physical mechanisms of failure, the concept of thermally activated damage initially proposed by Klepaczko (Klepaczko, 1990) was adopted as the basis for the new damage evolution model. This makes the proposed damage/failure model compatible with the Mechanical Threshold Strength (MTS) model (Follansbee and Kocks, 1988; Chen and Gray, 1996; Goto et al., 2000; Gray et al., 1999; Chen et al., 1998) which was used to control evolution of flow stress during plastic deformation. In addition the constitutive model is coupled with a shock equation of state which allows for modelling of shock wave propagation in the material. The new model was implemented in DYNA3D and our in-house non-linear transient SPH code, MCM (Meshless Continuum Mechanics). Parameters for the new constitutive model for AA7010 (a polycrystalline aluminium alloy, whose orthotropy is a consequence of grain morphology), were derived on the basis of the tensile tests and Taylor anvil tests. The tensile tests were performed for the range of temperatures between 343.15K and 413.15K, and strain rates between and . The new model was validated in two stages. The first stage comprised a series of single element tests design to separately validate elasticity, plasticity and damage related parts of the model. The second stage comprised a series of numerical simulations of Taylor anvil and plate impact tests for AA7010 and comparison of the numerical results with the experimental data. The numerical results illustrate the ability of the new model to predict experimentally observed behaviour.Item Open Access Modelling of dynamic damage and failure in aluminium alloys(Elsevier Science B.V., Amsterdam., 2012-11-30T00:00:00Z) Vignjevic, Rade; Djordjevic, Nenad; Campbell, James C.; Panov, ViliA physically based damage and failure model, applicable to orthotropic metals is proposed in this paper. To account for the physical mechanisms of failure, the concept of thermally activated damage initially proposed by Klepaczko [1], has been adopted as the basis for the model. This assumption makes the proposed damage/failure model compatible with the Mechanical Threshold Strength (MTS) model [2-6], which was used within the overall constitutive model to describe material behaviour in the plastic regime. A shock equation of state [7] was coupled with the rest of the constitutive model to allow for modelling of shock wave propagation in the material. The new model was implemented in DYNA3D [8] and coupled with our in-house non-linear transient SPH code, MCM (Meshless Continuum Mechanics). Parameters for the new constitutive model, i.e. parameters for the plasticity model and the damage model, were derived on the basis of the uniaxial tensile tests and Taylor anvil tests. The subject of investigation is a polycrystalline aluminium alloy AA7010, whose orthotropy is a consequence of meso-scale phase distribution, or grain morphology. Tensile tests were performed for the range of temperatures between and , and strain rates between and . In order to validate the new damage model, a numerical simulation of Taylor anvil tests has been performed for AA7010, using a single stage gas gun at velocity of . The numerical analysis clearly demonstrates the ability of this new model to predict experimentally observed damage and failure.Item Open Access Modelling of impact on a fuel tank using smoothed particle hydrodynamics(2005-06-27T00:00:00Z) Vignjevic, Rade; De Vuyst, Tom; Campbell, James C.; Bourne, Neil K.This paper describes a modelling approach for the simulation of hypervelocity impact on fuel tanks using the Smoothed Particle Hydrodynamics (SPH) method. To determine a suitable particle density, three two-dimensional axi-symmetric models were analysed. Then three-dimensional simulations with cylindrical and cubic penetrators were performed. For each analysis the transient pressure values at locations corresponding to experimental transducer locations were recorded. The pressure time histories are shown for the axi-symmetric and 3D models. The simulation results are compared with the experimental results. The purpose of the research was to demonstrate the capability and potential of SPH for simulating this type of problem.