Analysis And Evaluation Of The Structural Crashworthiness Of Typical Civil Aircraft Fuselaege Sections

Crashworthiness is the ability of an aircraft structure and its internal systems to protect occupants from injury in an event of crash.The crashworthiness of civil aircraft is one of the most important indicators in aircraft safety;it plays a significant role to determine the survival rate of the occupants when crashes occur in large degree.In order to ensure the survival of occupants in crash accidents,it must be considered comprehensively at the beginning of civil aircraft design.Therefore,taking the typical fuselage structure of civil aircraft as the research object,this paper systematically studies the crashworthiness analysis and evaluation method of civil aircraft fuselage structure.The main research work is as follows:In view of the fact that it is difficult to simulate the luggage material model and its parameters in the cargo compartment,a material model which characterizes the stiffness and damping characteristics of the luggage is constructed based on the luggage compression test,and considering the influence of the luggage gaps in the actual loading,the parameters of the luggage material model were further modified.Using LS-Dyna nonlinear transient dynamics module,the crash model of fuselage section including cargo compartment luggage is constructed.The effects of luggage loading on the failure mode,energy absorption characteristics and acceleration response of fuselage section are analyzed.The factors of luggage loading dispersion are considered,the influence of luggage loading under different stiffness,damping characteristics and worst-case conditions on the fuselage section are analyzed.According to the analysis results,the luggage played as a cushioning role during the crash event,because the presence of the luggage prevented the broken cargo floor beam and fuselage frame from crashing into the cabin floor,which can better guarantee the integrity of the cabin floor structure while reducing the impact force of the cabin floor.In the vertical crash environment,the metal fuselage frame is mainly characterized by plastic failure under the action of ultimate bending moment,which dominates the failure mode and energy absorption characteristics of the fuselage structure.In order to study the bending moment distribution of the fuselage frame under impact loading,a simplified crash model of the fuselage section dominated by curved beam elements is constructed based on the stiffness equivalent principle by using LS-Dyna,and compared with the traditional crash model,the calculation efficiency is obviously improved.The bending moment formula of the plane curved beam subjected to impact loading is deduced,and the rationality of the simplified crash model is verified by analytical solution and traditional crash model in terms of bending moment distribution and fuselage failure mode.Based on the ultimate bending moment plastic failure theory of the fuselage frame,the ultimate bending moment distribution of the fuselage frame and its influencing factors are studied by using the simplified crash model of the fuselage section.The concept of controllable failure energy absorption design of the fuselage structure is proposed.The failure load,failure location and failure sequence of the fuselage frame can be controlled by the arrangement of components and the combination of different stiffness,so that the structure can be deformed and destroyed in a controllable or predictable manner.The cargo barrier net that protects the occupants in the transport aircraft is a soft structure,which is extremely unstable under the initial impact loading.The influence of the geometric nonlinearity and belt material nonlinearity must be considered in load transfer and deformation analysis which is difficulty to solve and converge.Based on Newton-Raphson algorithm,this paper proposes a strategy of gradual loading and combination convergence to alleviate the difficulty of convergence in solving the barrier net.At the same time,the influence of the material nonlinearity of the barrier net is also considered by the iterative simulation of the net webbing stiffness.The finite element model of the cargo barrier net of transport aircraft was constructed by using MSC.Nastran nonlinear module and the influence of different initial shape and deflection of barrier net on fitting reaction and net deformation are analyzed.Considering the diversity of cargo transported by the aircraft,the barrier net under three different load conditions: uniform distributed loads,concentrated load,uniform and concentrated combination load are analyzed.The results show that the concentrated load is the most dangerous load condition.In order to investigate the influences of different impact velocity on the occupant's head injury under horizontal impact conditions,an effective and reliable dynamic model of occupant restraint system was constructed using LS-Dyna platform based on restraint system crash test,and the dynamic response and injury of occupants during horizontal impact were analyzed.In order to quantitatively evaluate the severity of occupant injury during crash,a comprehensive evaluation method based on occupant's Abbreviated Injury Scale and occupant's injury risk probability is proposed.Taking the typical civil aircraft triple seat as an example,the relationship between the occupant's head injury,occupant casualty rate versus horizontal impact velocity is given.The occupant restraint system model combined with occupant injury assessment method can be more intuitive and visualized to evaluate and predict the possible injury location and degree of occupant injury,which has a certain reference value for occupant safety protection.Based on the specific requirements of airworthiness clause for structural response and human injury,the relevant airworthiness clause requirements are divided into five categories according to living space,retention strength,occupant injury,evacuation channel and post-crash environment.The concept of comprehensive crashworthiness evaluation of civil aircraft is proposed and the crashworthiness evaluation system is established,and the specific evaluation process is given.The crash simulation model of fuselage section including dummy and seat restraint system is constructed by LS-Dyna,and the dynamic response of the structure and occupants is analyzed in detail when the vertical impact velocity is 9.14 m/s.The credibility of the crash simulation model was verified by comparing with crash test in the aspects of structure failure mode,energy absorption duration,and rebound velocity and occupant lumbar injury.The Crashworthiness of the fuselage structure is systematically evaluated by the comprehensive evaluation method proposed in this paper,which shows that the fuselage structure has a good crashworthiness under the airworthy crash conditions.