Investigation On The Anti-penetration Mechanisms Of Polygonal Steel-tube-confined Concrete Targets

The steel-tube-confined concrete has extensively application in protective engineering because of its excellent anti-penetration performance.Based on the existing research of performance against penetration and mechanisms of circular steel-tube-confined concrete targets,the performance and mechanisms against penetration for structural unit of polygonal steel-tube-confined concrete targets is studied with penetration experiments,numerical simulation and engineering modeling in this paper.The mechanisms against penetration of polygonal steel tube confined concrete targets are revealed and an engineering model to predict the depth of penetration for confined concrete target is developed.The results in this paper can provide important instruction for engineering application of steel-tube-confined concrete in protective structure.The mainly works can be summarized as follows:1.Experiments of 12.7mm hard core bullets penetrating into hexagonal,square and circular steel-tube-confined concrete targets are conducted,and the failure modes and mainly damage parameters are obtained.The results show that: the hexagonal steel-tube-confined concrete targets have the superior anti-penetration performance than circular and square steel-tube-confined concrete targets.The cracks are uniformly distributed at surface of funnel and there are governing cracks at the sides of concrete for circular steel-tube-confined concrete targets.While the cracks are mainly distributed nearby diagonal at surface of funnel and there are a lots of fine cracks at the sides of concrete for polygonal steel-tube-confined concrete targets.Compared with circular steel-tube-confined concrete targets,the eccentricity has little effect on depth of penetration as to polygonal steel tube confined concrete targets when the ratio of eccentricity is less than 0.35.2.Mechanisms against penetration of steel-tube-confined concrete targets are revealed with ANSYS/LS-DYNA at the basic of penetration experiments.The influence of hexagonal steel tube's thickness and length on performance against penetration is chiefly analyzed.Furthermore,distribution regularities of stress and factors affecting anti-penetration performance of hexagonal steel-tube-confined concrete targets are obtained.The simulation results show that: there are double confined effects of steel-tube-confinement and outer concrete self-confinement for steel-tube-confined concrete targets.Steel-tube confinement consists of stress wave effect and displacement constraint of concrete during cavity expansion stage,giving priority to the latter.The circular steel tube is in the simple tensile state under radially uniform pressure;while there are tensile deformation in-plane and bending deformation out-plane for polygonal steel tube under nonuniform inner pressure.The stress is nearly uniformly distributed along the circumference in circular steel-tube-confined concrete targets,while there are high-stress regions near the diagonal in polygonal steel-tube-confined concrete targets.It is the high-stress regions of short distance that enhances the confinement of hexagonal steel-tube-confined concrete targets.Both thickness and length have effect on the anti-penetration performance of hexagonal steel-tube-confined concrete targets.Optimizing the matching of thickness and length can get good anti-penetration performance and the good matching is length with 47.5mm and thickness with 3.5mm for the projectiles in this paper.3.Finite dynamic cavity expansion models and corresponding engineering model are developed firstly for confined concrete thick targets based on the modified Griffith criterion to describe the strength feature of concrete in comminuted region.The influence of confinement stiffness and cavity expansion velocity on cavity expansion process,radial stress at cavity surface and response mode is analyzed.The results show that increasing confined stiffness can improve penetration resistance efficiently.The depth of penetration obtained with engineering model is in good agreement with experiments of hard core bullets penetrating into steel-tube-confined concrete targets with the maximum error of 16%.