Simulation Of Ice Layer Failure Under Low Velocity Impact Based On Cohesive Element Method

Polar region has an important strategic position with a variety of extremely rich mineral resources,and also takes an important role in affecting the earth's climate.In particular,polar region is an important military center,with the shortest path connecting Asia,Europe and North America.It has the shortest strike distance from the Arctic to all three surronding continent.Moreover,the perennial ice layer is a natural shielding field,which can provide good concealment for underwater activities.In recent years,more and more attention has been paid to the important military strategic position of the polar region.Due to the special environment of the polar region,it is of great significance to study the failure behavior of polar ice layer under the low velocity impact of structures.Ice materials have the characteristics of randomness,anisotropy and initial defects.The failure modes of ice materials vary greatly with the strain rate,presenting ductile failure at low strain rate and brittle failure at high strain rate.These features bring challenges to the failure simulation of ice structures.The finite element method and the discrete element method can simulate certain damage characteristics of ice layer under quasi-static and high velocity loading well respectively,but it is difficult to simulate the failure characteristics of ice layer under low velocity impact.The cohesive element method is based on the finite element method and combining the idea of the discrete element method.By inserting cohesive elements between the finite elements to simulate the failure and crack propagation,it has been gradually used in the failure analysis of brittle materials in recent years.Therefore,based on the cohesive element method,the low velocity impact damage behavior of ice layer is studied in this paper.Firstly,based on the cohesive element method,the ice layer model is established according to the characteristics of ice layer,and the simulation results are compared with those of finite element method and discrete element method.The ice layer has the characteristics of anisotropy and preferential failure of grain boundary,and contains random initial defects.The grain boundary failure characteristics of ice crystals are simulated by pentahedral modeling and strengthened lateral cohesive element;the randomness and initial defects of ice layer are simulated by randomly deleting vertical cohesive elements.Compared with the finite element method and the discrete element method,in the low velocity ice damage simulation,the cohesive element method not only has the accuracy of the finite element in stress analysis which obtains a better crack propagation path,but also has the characteristics of the discrete element simulation which can simulate the phenomenon of ice breaking and splashing.Generally speaking,the cohesive element method is more suitable for the damage simulation of ice layer under low velocity impact.Secondly,the influence of various parameters of cohesive element method is further discussed,the artificial compliance is analyzed,and the applicable range of parameters of cohesive element method is obtained.Through the discussion of the influence of TSL(Traction Separation Law)curve shape and fracture energy on the simulation results,it is found that the fracture energy is the most important parameter affecting the structural failure,and the TSL curve affects the mode of failure.Analyzing the influence of different element shapes on the simulation results,it is shown that the triangular element constitutes more structural boundaries and can restore the failure path of the structure better.Therefore,the pentahedral element is selected as the main element of ice modeling.The artificial compliance is analyzed to avoid the influence on the structural stiffness while simulating the damage behaivor,and the appropriate cohesive element stiffness and element size are found.Finally,the failure mode and velocity change of ice layer under different impact inclination angles are analyzed,and the corresponding impact conditions of the structure and ice layer are obtained.According to different conditions,it can be divided into three cases:small angle,large angle and critical angle,which provides a reference for practical application.