Anti-Penetration Performance of Honeycomb Steel Tube Concrete Target Plates

Literature Overview

This paper by Zhao Hongyuan et al., published in Explosion and Shock Waves (2023, Vol. 43, No. 5, pp. 44-54), presents an experimental investigation into the anti-penetration performance of honeycomb steel tube concrete (also referred to as honeycomb CFST) target plates. The research is conducted at the State Key Laboratory of Explosion Science and Technology, Beijing Institute of Technology, and funded by the National Natural Science Foundation of China (Grant 11390362). The study combines high-velocity impact experiments using a 125 mm caliber smoothbore gun with uniaxial compression tests on hexagonal steel tube concrete specimens.

Core Technical Content

The experimental program consisted of 6 impact tests using a 125 mm caliber smoothbore gun to penetrate honeycomb steel tube concrete target plates. The researchers obtained damage morphology and penetration depth data under different test conditions. They analyzed typical damage forms of the target plates, compared damage patterns at different projectile-to-target size ratios, and investigated the effects of impact point location and steel tube wall thickness on the anti-penetration capability.

Additionally, uniaxial compression tests were conducted on 7 groups of hexagonal steel tube concrete specimens with different wall thicknesses and 3 groups of hexagonal concrete specimens without steel tubes. These tests investigated the strength enhancement and ductility improvement effects of hexagonal steel tubes on the core concrete at different wall thicknesses. The researchers fitted the relationship between the core concrete strength enhancement factor and the confinement factor, and improved the empirical formula for ordinary concrete penetration depth to obtain a maximum penetration depth calculation formula applicable to honeycomb steel tube concrete.

Key Experimental Results

Structural Engineering Analysis

The honeycomb steel tube concrete configuration represents an innovative structural form where hexagonal steel tubes are arranged in a honeycomb pattern, with each tube filled with concrete. This configuration creates a composite structure that combines the energy absorption capacity of the steel tubes with the compressive strength of the concrete cores. The hexagonal geometry is particularly efficient for structural applications because it provides uniform confinement in all directions within the plane of the honeycomb, unlike rectangular or circular tubes which have directional anisotropy.

The strength enhancement effect observed in the uniaxial compression tests is consistent with the well-established confinement mechanism in CFST members. The steel tube provides lateral restraint to the concrete core, preventing the concrete from expanding laterally under compressive loading. This confinement transforms the concrete from a brittle material into a more ductile one, allowing it to sustain higher compressive stresses. The confinement factor, defined as the ratio of the steel tube's confinement pressure to the concrete's unconfined compressive strength, is the key parameter governing the degree of enhancement.

From a steel tube manufacturing perspective, the hexagonal tubes used in this study present unique fabrication challenges. Unlike circular tubes, hexagonal tubes require precise angular control during bending and welding operations. The welds at the hexagonal corners are particularly critical because they represent regions of geometric discontinuity where stress concentrations can develop. In high-velocity impact scenarios, these stress concentration points can initiate cracks that propagate through the steel tube, compromising the structural integrity of the target plate.

Impact Mechanics and Damage Analysis

The 125 mm caliber smoothbore gun experiments provide valuable data on the dynamic response of honeycomb CFST target plates under high-velocity projectile impact. The penetration process involves several distinct phases: initial contact and spalling, projectile deceleration and energy absorption, and potential perforation. The steel tubes play a crucial role in the energy absorption phase by deforming plastically and dissipating kinetic energy through work hardening.

The finding that impact point location has a complex influence with scatter on penetration depth is significant. This scatter likely arises from the interaction between the projectile trajectory and the honeycomb cell geometry. When the projectile strikes near a cell wall, the local structural response differs from when it strikes near a cell corner. This geometric sensitivity highlights the importance of understanding the cellular structure's role in damage distribution.

The modified penetration depth formula derived in this study represents a practical engineering tool for predicting the protective capacity of honeycomb CFST structures. By incorporating the confinement effect of the steel tubes into the penetration depth calculation, the formula extends the applicability of existing empirical models to this novel structural configuration.

Quality Control and Manufacturing Considerations

For the fabrication of honeycomb CFST target plates, several quality control measures are essential:

The welding of hexagonal steel tubes requires particular attention to the corner joints. Fillet welds at the corners should be designed to achieve full penetration where possible, and the weld metal should have mechanical properties compatible with the base steel. Post-weld heat treatment may be necessary to relieve residual stresses that could affect the target plate's performance under impact loading.

Study Insights and Reflections

This research bridges the gap between structural engineering and protective engineering, demonstrating that CFST technology can be adapted for ballistic protection applications. The honeycomb configuration offers advantages over monolithic concrete plates in terms of energy absorption efficiency and damage localization.

The scatter observed in the penetration depth data, particularly related to impact point location, suggests that the design of honeycomb CFST protective structures should account for the variability in impact response. In engineering practice, this means that the protective capacity should be evaluated based on the worst-case scenario rather than average performance.

The improvement of the empirical penetration depth formula for honeycomb CFST targets is a valuable contribution to the field. However, the formula's validity should be verified across a wider range of projectile types, impact velocities, and target thicknesses before being adopted for design purposes. Future research should also consider the effect of concrete strength grade, steel tube material properties, and honeycomb cell size on the anti-penetration performance.

The uniaxial compression test data on hexagonal steel tube concrete specimens provides fundamental material data that can be used for structural design of honeycomb CFST members in non-ballistic applications, such as energy-absorbing structures in transportation infrastructure or seismic isolation systems.