Experimental Investigation And Numerical Analysis Of Low-velocity Penetrating Responses Incorporating Macro And Micro Structures Of Targets

The projectile/target penetration is a transient contact problem, coupling with the propagation of stress waves, the deformation, damage and its evolution of the projectile and target. Finite element analysis is a useful method to solve such complex penetration problem. However, the existing macroscopic and mesoscopic constitutive models and failure criterions are difficult to completely describe the mechanical responses of concretes. Furthermore, there are no efficient methods to model the mesoscopic structure of concrete. In the present dissertation, an improved failure criterion for concrete and a new modeling method incorporating the microscopic structure of concrete are proposed. The penetration responses of simplified projectiles into layered targets are numerically investigated using the nonlinear explicit FEM software Ls-Dyna based on the single point integral. The conclusions are presented as follows.The testing system for dynamic response measurement is set up using the strain-gage technique, accelerometer technique and high-speed photography technique. The experiments for projectiles normal penetration into the concrete target and the earth target and the oblique penetration into the layered target are performed. The experimental results indicate that the concrete plate is destroyed by compressive load in radius direction and tensile load in circumference direction. Due to the effect of tension stress wave reflected at the back side of the target, the phenomenon of target splash, crack and fragment is observed during penetration process. The dynamic response process of the concrete plate can be divided into three stages, namely the initial wave-propagation stage, the penetration, wave-propagation and structure-vibration coupling stage and the final structure-vibration stage. The destruction pattern of soil subjected to the steel-ball penetration is like a funnel. The funnel size of soil and the penetration depth of steel ball increase with the increase of impact velocity. After the oblique penetration of the cone-shaped penetrator into the layered target, a cup tunnel with an elliptical fracture contour line is produced in the surface layer of the target and a straight tunnel in the base, compacted and natural base layers.Based on the triaxial test results of concretes, an improved failure criterion is proposed. The material parameters of concretes and soils are determined by the corresponding penetration tests. The finite element models are constructed to simulate the normal penetration into the earth target and concrete target and the oblique penetration into the layered target. The models are verified by test results. The effects of the impact velocity, obliquity angle, yaw angle of the simplified penetrator and the strength of the base layer on the penetration responses such as the acceleration, velocity and deflection angle of the projectile are numerically investigated. The numerical results indicate that the projectile deflects in the layered target and the maximum deflection emerges in the compacted earth layer and the natural earth base layer. The responses of projectile acceleration and velocity depend on the thickness and strength of layers. The values of acceleration decrease with penetration time as an obvious step pattern. The relation between the fuse depth and igniting time and the oblique angle, impact velocity and yaw angle of penetrator is revealed. The sequence of factors sorted by weight for depth setting fuze is obliquity, impact velocity and yaw angle. The factor sequence for delay fuse is impact velocity, obliquity and yaw angle.For grated numerical concrete model, a new method of random aggregate arrangements called "space meshed and taken-in" is developed to high-efficiently arrange number of aggregates with high volume fraction. A PCL program of Patran is developed to mesh the geometric model. The bonding strength of the transition band between the aggregate and the mortar is modeled using failure strength of tied-contact in LS-DYNA. Consequently, a numerical concrete model with aggregates and mortars is constructed and the validity of the model is verified by the concrete material tests. The effect of randomly distributed aggregates on the penetration responses of penetrators is numerically investigated. It can be found that the penetrator gesture is apparently affected by the randomly-distributed aggregates. The deflection angle decreases with increasing the projectile diameter; when the projectile penetrates from different points on the target, the aggregate effect on the projectile deflection is different due to the difference of the penetration trace.