Zhuojin Pipe Fitting Co., LtdDynamic Mechanical Properties of Hollow Sandwich Steel Tube Concrete Under Axial Impact Loading
Literature Overview
This paper, published in the Journal of Southeast University (Natural Science Edition) in 2022 by Zong Zhouhong and colleagues from the Ministry of Education Engineering Research Center for Blast Protection at Southeast University, investigates the dynamic compressive behavior of hollow sandwich steel tube concrete (HSCTC) specimens under axial impact loading using the Split Hopkinson Pressure Bar (SHPB) technique. The research is supported by the National Natural Science Foundation of China (NSFC Grant No. 52178462) and the National Key R&D Program (2021YFC3100700), reflecting its significance in blast protection and structural dynamics.
Core Technical Content
Test Configuration and Methodology
The experimental setup employed a 75 mm diameter SHPB apparatus, which is a standard configuration for medium-scale dynamic compression testing of composite structural members. The SHPB system operates on the principle of one-dimensional stress wave propagation, where a striker bar impacts an incident bar transmitting a compressive pulse through the specimen. For HSCTC specimens, the key challenge lies in ensuring proper impedance matching between the bars and the composite specimen to minimize wave reflections and dispersion effects.
The specimens investigated were hollow sandwich steel tube concrete, which consists of an outer steel tube, an inner steel tube, and concrete infill between them. This configuration creates a sandwich-like structure where the outer and inner steel tubes provide confinement and load-bearing capacity, while the concrete core contributes to overall stiffness and energy absorption. The hollow core region between the inner tube and the concrete layer introduces additional complexity in terms of load distribution and failure modes.
Dynamic Compressive Behavior
The study examined the influence of impact velocity on the dynamic compressive strength of HSCTC specimens. The results demonstrate a strain rate sensitivity effect consistent with the Dynamic Increase Factor (DIF) concept established in the Concrete Reinforced with Steel Fiber (CSM-F410) and International Code for Seismic Design of Concrete Structures (ACI 318). As impact velocity increases, the dynamic compressive strength of HSCTC specimens increases, but the rate of increase diminishes at higher velocities. This observation is consistent with the Johnson-Cook constitutive model behavior, where strain rate hardening effects become progressively less pronounced at very high strain rates.
The integrity of HSCTC specimens under axial impact loading was found to be relatively good, suggesting that the sandwich configuration provides effective confinement and damage tolerance. The outer steel tube prevents concrete spalling and fragmentation, while the inner tube provides additional confinement to the concrete layer.
Parametric Study Findings
| Parameter | Effect on Dynamic Compressive Strength | Mechanism |
|---|---|---|
| Concrete strength grade | Positive correlation | Higher strength concrete provides greater load-bearing capacity |
| Outer steel tube thickness | Positive correlation | Thicker tube provides greater confinement and load sharing |
| Hollow ratio | Significant influence | Changes load distribution between steel tubes and concrete |
The hollow ratio, defined as the ratio of the hollow core diameter to the outer tube diameter, emerges as a critical design parameter. A larger hollow ratio reduces the concrete volume and shifts the load-bearing responsibility toward the steel tubes, while a smaller hollow ratio increases the concrete contribution. The optimal hollow ratio represents a balance between weight efficiency and structural performance.
Proposed Dynamic Constitutive Model
The authors developed a dynamic stress-strain constitutive model for HSCTC that captures the strain rate sensitivity and nonlinear hardening behavior observed in the SHPB tests. This model is intended to be applicable in engineering design and numerical analysis of HSCTC structures subjected to impact and blast loading. The model likely incorporates the DIF concept for concrete and the Johnson-Cook or Cowper-Symonds model for the steel tubes, with appropriate interaction terms to account for the composite behavior.
Engineering Practice Implications
From a steel pipe manufacturing and structural engineering perspective, this research has several practical implications:
- Material selection: The outer and inner steel tubes should be selected with consideration of their dynamic properties, not merely their quasi-static strength. Materials with favorable strain rate sensitivity, such as certain low-carbon steels and some high-strength low-alloy (HSLA) steels, would be advantageous.
- Welding quality: In practical HSCTC construction, the connection between the steel tubes and any additional components must be carefully controlled. Welding defects, particularly in the Heat-Affected Zone (HAZ), could significantly reduce the dynamic performance of the composite member.
- Design optimization: The hollow ratio provides a design freedom that can be exploited for weight optimization in blast-resistant structures. Engineers should consider the trade-off between structural weight and dynamic performance when selecting the hollow ratio.
Key Questions and Reflections
The study raises several important questions for further investigation. First, the long-term durability of HSCTC under cyclic impact loading has not been addressed. Second, the effect of temperature on the dynamic properties of HSCTC is not considered, which is relevant for fire resistance applications. Third, the numerical model validation is primarily based on quasi-static and low-velocity impact tests, and further validation at higher velocities would strengthen the findings.
The proposed constitutive model should be benchmarked against established models such as the Concrete Damage Plasticity model in ABAQUS and the Concrete Damage model in LS-DYNA to assess its predictive capability for engineering applications. Engineers should also consider the anisotropy effects introduced by the manufacturing process of the steel tubes, particularly for ERW or HFW welded pipes where the weld seam orientation relative to the loading direction could influence the dynamic response.
Study Insights and Implications
This research contributes valuable experimental data and a constitutive model for the design of hollow sandwich steel tube concrete structures under dynamic loading. The findings confirm that the sandwich configuration provides effective confinement and damage tolerance, making HSCTC a promising structural system for blast-resistant and impact-resistant applications. Engineers working on protective structures, such as blast walls, impact barriers, and protective enclosures, should consider HSCTC as a viable structural option. The proposed constitutive model can be incorporated into finite element analysis software for the design and assessment of HSCTC structures. However, further research is needed to address the limitations identified above, particularly regarding cyclic loading, temperature effects, and high-velocity impact scenarios. The work represents a solid contribution to the field of dynamic structural engineering and provides a foundation for future research on advanced composite structural systems.