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Solid Mechanics & Materials Engineering Group |
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Dr Nik Petrinic's Research Projects
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CRAHVI - Crashworthiness of Aircraft for High Velocity ImpactNew experimental techniques are being developed that will enable quantification of the residual deformation and corresponding stresses in the close vicinity of impact induced edge notches on aeroengine components. The methodology relies upon the high speed photography, image processing and numerical simulation of the conducted experiments based upon optimal material properties obtained from material tests at high rates of strain. The results of this research are used for subsequent fatigue life predictions. A significant percentage of passenger fatalities and injuries in high velocity impact scenarios are caused by fire as a result of fuel tank rupture on impact. The design of aircraft structures to reduce the incidence of such events poses major challenges to manufacturers. The pressure to meet these challenges will increase with the predicted growth of air traffic and the foreseeable use of large aircraft carrying a great number of passengers. Therefore, research activities related to crashworthiness of aircraft subjected to survivable high velocity impact scenarios are essential in order to maintain a high standard of safety and reduce accident rates - thus CRAHVI. The overall objective of CRAHVI is to develop methods and tools to predict the behaviour of aircraft structures subjected to high velocity impacts. Implementation of such methods will promote enhanced safety through damage tolerant aircraft design and the development of crashworthy aircraft concepts. The main innovations, which go beyond the state-of-the art, will be: development of substitute material for the bird, simulation of bird strike on a composite leading edge, stochastic analysis approach applied to aircraft structures, FE prediction of metallic joint behaviour under high speed impact and innovative energy absorption design for composite leading edge structure.
IMPACT - Improved Failure Prediction for Advanced Crashworthiness of Transportation VehiclesCrashworthiness simulation has been a major factor that has enabled automotive manufacturers to achieve a 30 to 50% reduction in development time and costs over the past five years. Today this technology is considered a mature and proven design tool for the development of conventional 'ductile' steel automotives. Only minimal prototype testing is needed, usually at the end of the design phase, for the purpose of confirming the 'simulation based design'. However, demand for greater weight saving and crashworthiness protection has only been possible using new design concepts and employing lightweight materials that have limited ductility and a complex failure. The present crashworthiness codes are inadequate to predict failure in materials, or jointing systems, which has raised serious uncertainties over their results. In order to avoid a return to a 'prototype based design' and to maintain the high level of safety achieved in recent years there is an urgent need to improve the failure prediction capabilities of crashworthiness simulation codes. Various advanced metals including aluminium, high strength steel and magnesium; and plastic trim that must absorb occupant impact, will be used to develop new generic material failure models. Initial failure will be predicted using state-of-the-art 'void growth' and 'damage mechanics' concepts and subsequent crack propagation will be described using 'fracture mechanics' approaches. For jointing systems spotwelds, rivets and weldlines will all be studied and failure models developed. The partnership includes five major European automotive manufacturers and the leading European crashworthiness software developer in this field. The theoretical work will be undertaken by experienced research partners and supported by an automotive testing facility and a materials manufacturer. The consortium therefore represents the complete chain from material supplier to software developers and end-user. The main tasks in the project include:
Mathematical And Computational Modelling Of Shaped Charge MechanicsThe emphasis of the research is on macroscopic (continuum) modelling of the various mechanical processes involved in jet flow, impact and penetration. Nevertheless, microscopic considerations govern the development of constitutive relations for both the jet and the target. Axisymmetric flow and perturbations are emphasised as well as two and three dimensional models of shaped charges that emanate from wedge-shaped liners rather than conical ones.
Aspects Of Aeroengine's Lightweight Fan Systems Design Involving BirdstrikeThe objective of this research is to determine the best materials and methodology for use in the new design of lightweight aeroengine fan systems thus enabling the reduction of noise and fuel consumption. The emphasis is on the use of damping materials in various cavities within the hollow components of aeroengines. New techniques for material testing at elevated temperatures are being developed and inverse modelling technology is being used to fully characterise the behaviour of the newly manufactured materials.
Constitutive Modelling Of Deformation And Failure In Titanium Alloys Subjected To Impact LoadingThe objective of this research is to enable improved predictive modelling of failure of titanium alloys subjected to impact loading in which the transition from continuum to discontinuum is simulated by accurately representing the strain softening and localisation leading to crack initiation and propagation. Experimental methods for observation and quantification of the material behaviour at a range of strain rates are being refined to enable more accurate measurements. In order to model the experimentally observed phenomena realistically, a new physically based material constitutive model is being developed that is capable of describing the material behaviour at different rates of strain and temperatures.
Development Of Inverse Modelling Methodology For Identification Of Non-measurable Constitutive ParametersThe application of constitutive models to real materials requires the identification of material parameters to allow for quantitative representation of experimentally observed material behaviour. For some parameters it is possible to design suitable experiments to enable their measurement directly or indirectly. For others it is not, or else the experimental method inherently introduces too much uncertainty to allow sufficient confidence in the results. In such cases, the parameters can be identified by solving the corresponding inverse problem. The inverse modelling approach explicitly tries to match the outcomes of laboratory experiments and numerical simulations. The virtue of a given set of parameters is described by the use of an objective function, which measures the difference between experimental results and the corresponding numerical results. The problem of finding the optimal parameters may then be cast as an inverse problem, where the objective function is to be minimised. Evaluation of a single point of the objective function (a single solution of the direct problem) requires the comparison of experimental and numerical results. The desired parameter values for the chosen material constitutive model are found at the objective function's global minimum. A novel approach implemented to solve this problem is adopted in this research employing a heuristic gradient-based optimisation method.
Development Of Predictive Modelling Techniques For Evaluation Of Residual Stressed In Aeroengine Components Following Foreign Object DamageNew experimental techniques are being developed that will enable quantification of the residual deformation and corresponding stresses in the close vicinity of impact induced edge notches on aeroengine components. The methodology relies upon the high speed photography, image processing and numerical simulation of the conducted experiments based upon optimal material properties obtained from material tests at high rates of strain. The results of this research are used for subsequent fatigue life predictions.
Development of experimental techniques for observation of response of metallic honeycomb materials to impact loadingThe kinetic energy of rotating components in large aeroengines is sufficient to cause catastrophic structural damage in case of failure. These failures occur either as a result of just fatigue or are induced by impact of foreign bodies ingested into the engine. As a result, the incorporation of energy absorbing features within large aeroengine containment casing structures is of vital importance for the safety of aircraft and passengers. In this research programme the energy absorbing capabilities of honeycomb materials are being investigated. Titanium alloy based specimens are being subjected to impact by instrumented projectiles at velocities comparable to those in blade release events. The data is being used for improving the predictive modelling capabilities of presently used software based upon the finite element method.
Development of predictive modelling methodology for hybrid (metallic/composite) materials subjected to impact loadingBirdstrike poses a major threat to large aeroengines at take-off and landing. Following an increase in the recorded number of birdstrike events the Civil Aviation Authorities require that an engine which has suffered a birdstrike should provide thrust sufficient to fly the aircraft for 30 minutes after impact, thus enabling safe emergency landing. Experimental methods for evaluation of component resistance to soft body impact are being developed. The developed technology is being applied in the provision of data on new hybrid materials for aeroengine applications thus enabling accurate predictive modelling of aeroengine behaviour during and following a birdstrike event.
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