The Three-dimensonal SPH Method For Discrete Problems And Application In Impact Dynamics

Investigation of impact dynamics,such as water entry impact of structure,high-velocity collision between solid body,explosion of high explosive and so on,has become more and more important in ship and ocean engineering,aerospace and engineering structure design fields,for its great theoretical and academic value and military engineering significance.Considering the economic efficiency and adaptability,numerical simulation has widely been applied in the investigation of impact dynamics and become one of the most import methods.However,in the typical impact dynamics problems mentioned above,the existence of free surface,high strain rate and material large deformation makes it difficult for numerical simulation.Recently,with the rapid development of numerical simulation technique,the smoothed particle hydrodynamics(SPH)method has been gradually introduced into the calculation of impact dynamics for its great advantage in treatment of material large deformation.While there are still several problems need to be solved,which restrict the maturely application of the SPH method especially in engineering fields.For example,treatment of the discrete interface,three-dimensional problems and so on.Aiming at problems mentioned above,the investigation progress of water entry impact,high-velocity collision,explosion impact and the discrete SPH method are summarized.During the search it is found that the former application of the SPH method mainly focuses on one-dimensional,two-dimensional or axisymmetric problems because of the limits of computational efficiency,while rare numerical simulation about impact dynamics based on the real three-dimensional SPH method has been conducted.The stress and motion state of homogeneous medium in the infinite field can be properly simulated by the SPH method.However,in the flow field with interaction among different mediums,especially for the impact dynamics problems with strong discontinuity and severe interaction among particles,some obstacles will appear in the process of numerical calculation.Pressure oscillation may occur on the contact discontinuity surface.Particles of two different materials contact each other at the beginning,while they may separate with moving and even no long interact with each other as neighbor particles.Then numerical errors will appear on the discrete interface and cumulate in the process of time integration,which will cause the distortion of numerical calculation results.In the former numerical simulation by the three-dimensional SPH method,the calculation domain size is reduced or the particle distance is enlarged to ensure the successful proceeding of calculation.However,the restrict of calculation domain size may influence the comprehensiveness of the studied problem,and the enlarged particle distance may cause the decrease of accuracy.For all the reasons mentioned above,the three-dimensional SPH method has not been effectively applied in the engineering fields so far.In order to solve these problems,the three-dimensional discrete SPH method is studied and successfully applied to simulate some typical impact dynamics problems in this work,such as water entry impact,armor-piercing bullet penetration,cumulative jet impact,explosion and so on.Besides,for the effective application of this method in the engineering fields,the three-dimensional SPH-FEM coupling algorithm is also developed in the work.The dynamic response of the three-dimensional complex structure under explosion impact is analyzed to verify the validity of this algorithm.The main reason causing the numerical error on the discrete interface in the traditional SPH method is analyzed.Based on it,the three-dimensional SPH contact algorithm for treating with the discrete problems on the material interface is derived.What's more,considering the interaction on the material interface in the typical impact dynamics problems,different forms of contact force on the interface is presented.Then the effective three-dimensional SPH numerical model for treating with the discrete interface in the impact dynamics problems is built in the end.Water entry impact of structure is a common impact problem in the ship and ocean engineering fields.Taking it as the investigation background,the three-dimensional numerical model of cylinder structure water entry impact is built to study the mechanics of water entry impact,using the three-dimensional discrete SPH method developed in this work.Besides,a series of work conditions are set by changing the initial entering velocity,the entering angle and so on.Influences of different parameters on the process of water entry impact are analyzed based on the conclusion from the results.It aims at providing references for the investigation of water entry impact in the ship and ocean engineering fields.What's more,experiments of water entry impact are conducted to verify the validity and accuracy of the three-dimensional numerical model built in this work.A set of mechanical device with good serviceability and flexibility is designed and a high-velocity camera system is used to record the process of experiments.From the comparison between the numerical calculation results and the experimental results,it can be concluded that the three-dimensional SPH numerical model built in this work can effectively simulate the process of structure water entry impact.Armor-piercing penetration is an important form of impact for ships in war.In order to study the mechanics of the armor-piercing bullet damaging the target and the dynamic response characteristics of the target under collision impact,the three-dimensional numerical model of the armor-piercing bullet penetrating the target is built by the three-dimensional discrete SPH method,introducing the constitutive models and equations of state for different metal materials.Based on it,a series of work conditions are set by changing some key parameters,such as the touching velocity,touching angle,mass distribution(slenderness ratio)and head shape of the bullet.The influences of different parameters on the process of armor-piercing bullet penetrating the target are analyzed based on the conclusion of calculation results.It aims at providing references for structure optimum design of the armor-piercing bullet and the protective design of ships.The shaped charge bullet damage is another form of impact for ships in war except the armor-piercing bullet penetration.In order to study the cumulative effect of the shaped charge bullet and its damage mechanics on the target,the three-dimensional numerical model of cumulative jet forming and attacking the target is built using the three-dimensional discrete SPH method.Besides,the influences of some key parameters on the cumulative effect and damage form are analyzed,such as the charge mass,cone angle of the metal covering,height of burst and so on.Explosion impact is another important aspect in the impact dynamics problems.It is also one of the problems properly displaying the advantage of the SPH method in numerical simulations of the problems with instantaneity,high frequency and large deformation.In order to study the operating principle of the explosion bolt with typical explosion impact,the three-dimensional numerical model of the explosion bolt is built by the three-dimensional discrete SPH method to simulate its unlocking process.Besides,for verifying the validity of the calculation results,the same simplified model of the explosion bolt is also simulated by the non-linear finite element software LS-DYNA.From the comparison of the bolt structure deformation under the explosion impact and the impact pressure peak at different measuring points,it can be found that the calculation results obtained by the two methods meet well,which verifying the accuracy of the numerical model of the explosion bolt built by the three-dimensional discrete SPH method.Based on the successful application of the three-dimensional discrete SPH method in the typical impact dynamics problems,the effective application of this method in the engineering fields is studied.The advantage of the SPH method in dealing with material large deformation and discrete interface and the mature technique of the FEM method in complex structure model building and structural dynamics analysis are combined to develop a three-dimensional SPH-FEM coupling algorithm for engineering application.Then the three-dimensional numerical model of the explosion bolt unlocking process is built by this coupling algorithm to study its operating principle and the dynamic response characteristics of the three-dimensional complex structure under explosion impact,which verifying the validity of the three-dimensional SPH-FEM coupling algorithm developed in this work.