Zhuojin Pipe Fitting Co., LtdPenetration Resistance of Honeycomb Concrete-Filled Steel Tubes Under High-Velocity Projectile Impact
Literature Overview
The paper by Zhao Hongyuan et al. (2023), published in Explosion and Shock Waves (Vol. 43, No. 5, pp. 44-54), presents experimental and analytical investigations into the penetration resistance of honeycomb concrete-filled steel tube (HCFST) targets under high-velocity projectile impact. The research was conducted at the State Key Laboratory of Explosion Science and Technology, Beijing Institute of Technology, supported by the National Natural Science Foundation of China (Grant No. 11390362). The study employed a 125 mm bore smoothbore gun to conduct penetration tests and uniaxial compression tests on hexagonal steel tube specimens with varying wall thicknesses.
Technical Background
Honeycomb concrete-filled steel tubes represent an advanced composite structure where multiple hexagonal steel tubes are arranged in a honeycomb pattern and filled with concrete. This configuration offers enhanced energy absorption and penetration resistance compared to monolithic concrete or single-tube CFST targets. The application of HCFST structures is primarily in protective engineering, including military shelters, blast-resistant buildings, and critical infrastructure protection. The unique geometry of the honeycomb configuration introduces complex failure mechanisms that differ significantly from conventional CFST targets.
Experimental Program
Penetration Tests
A total of 6 penetration tests were conducted using a 125 mm bore smoothbore gun. The projectile characteristics, target configurations, and impact velocities were varied to investigate the effects of different project-target size ratios and impact locations on the penetration depth and failure patterns. The tests provided comprehensive data on the damage morphology, penetration depth, and residual projectile velocity under various impact conditions.
Uniaxial Compression Tests
Uniaxial compression tests were performed on 7 groups of hexagonal steel tube specimens with different wall thicknesses and 3 groups of hexagonal specimens without steel tubes (concrete only). These tests were designed to quantify the strength enhancement and ductility improvement provided by the hexagonal steel tubes to the concrete core under confinement.
Key Findings and Analysis
Wall Thickness Effect on Penetration Depth
The wall thickness of the steel tube was identified as a critical factor influencing the penetration depth. As the wall thickness increases, the penetration depth decreases, indicating improved penetration resistance. This finding is consistent with the general understanding that thicker steel tubes provide greater confinement to the concrete core and also contribute directly to the energy absorption through plastic deformation of the steel tube walls.
Impact Location Effect
The position of the impact point on the target surface was found to have a complex and somewhat scattered effect on the penetration depth. The impact location significantly influences the surface damage pattern of the target. When the projectile impacts near the junction of two hexagonal cells, the failure mechanism differs from that when the impact occurs at the center of a cell face. This geometric sensitivity has important implications for the design of HCFST protective structures, where the impact location cannot be predicted with certainty.
Strength Enhancement Effect
The hexagonal steel tubes effectively increased both the strength and ductility of the core concrete. The confinement effect of the hexagonal geometry was quantified through the relationship between the concrete strength enhancement factor and the confining factor. The confinement provided by the hexagonal steel tubes was found to be more effective than that provided by circular tubes of equivalent wall thickness, owing to the flat faces of the hexagonal cells that provide more uniform lateral pressure distribution to the concrete core.
| Parameter | Effect on Penetration Resistance |
|---|---|
| Steel tube wall thickness | Larger thickness reduces penetration depth |
| Impact location | Complex effect with significant scatter |
| Project-target size ratio | Influences failure pattern and penetration depth |
| Hexagonal geometry | More effective confinement than circular tubes |
Improved Penetration Depth Formula
An empirical formula for the maximum penetration depth of projectiles into HCFST targets was developed by modifying the conventional penetration depth formula for plain concrete targets. The modified formula incorporates the strength enhancement effect of the steel tube confinement and the energy absorption characteristics of the honeycomb configuration. The formula was validated against the experimental data and was found to provide reasonable predictions of the maximum penetration depth.
Engineering Practice and Quality Control Implications
From a steel pipe manufacturing and welding perspective, the HCFST structure imposes stringent requirements on the fabrication quality of the hexagonal steel tubes. The hexagonal geometry requires precise forming of the steel tubes, which may involve cold bending or rolling processes that can introduce residual stresses and affect the mechanical properties of the steel. The welding of hexagonal steel tubes, if required, must be performed with careful attention to the joint geometry and the potential for stress concentration at the hexagonal corners.
Quality control for HCFST structures should include: (1) dimensional inspection of the hexagonal steel tubes to ensure geometric accuracy; (2) NDT of all welds, particularly at the hexagonal corners where stress concentration is highest; (3) mechanical property testing of the steel tube material to verify compliance with the specified grade; and (4) concrete quality control including slump testing, compressive strength testing, and void detection after placement.
The forming process for hexagonal steel tubes may involve the use of hydraulic forming machines or roll-forming equipment. The forming process parameters, including the forming speed, the lubrication condition, and the die geometry, must be carefully controlled to prevent material thinning, cracking, or excessive work hardening. For high-strength steel tubes used in HCFST applications, the forming process may require intermediate annealing to restore ductility and prevent cracking.
Study Insights and Reflections
The finding that the impact location has a complex and scattered effect on penetration depth is of significant practical importance. In real-world scenarios, the impact location on a protective structure is unpredictable, and the design must account for the worst-case scenario. The scatter in penetration depth data suggests that the current understanding of the failure mechanisms in HCFST targets is incomplete, and further research is needed to develop more reliable design methods.
The improved penetration depth formula provides a useful tool for preliminary design, but its accuracy should be verified for specific applications. The formula is based on empirical fitting of experimental data, and its applicability may be limited to the range of parameters tested. Engineers should exercise caution when extrapolating the formula to configurations outside the tested range, such as different steel grades, different concrete strengths, or different projectile types.
Summary
This study provides valuable experimental data and analytical tools for the design of honeycomb concrete-filled steel tube protective structures. The wall thickness emerges as the primary design variable for controlling penetration depth, while the impact location sensitivity highlights the need for robust design practices that account for geometric uncertainty. The improved penetration depth formula offers a practical design tool, and the emphasis on hexagonal geometry highlights the advantages of this configuration over conventional circular CFST targets for penetration resistance applications.