Zhuojin Pipe Fitting Co., LtdDynamic Mechanical Properties of Hollow Sandwiched Steel Tube Concrete Under Axial Impact Loading
Literature Overview
This paper published in the Journal of Southeast University (Natural Science Edition) in 2022, authored by Zong Zhouhong and colleagues from the Ministry of Education Engineering Research Center for Explosion Protection and Safety at Southeast University, presents a systematic investigation into the dynamic compressive behavior of hollow sandwiched steel tube concrete (HSCC) specimens under axial impact loading. The research was supported by the National Natural Science Foundation of China (Grant No. 52178462) and the National Key R&D Program of China (Grant No. 2021YFC3100700). The study employs a 75 mm diameter Split Hopkinson Pressure Bar (SHPB) apparatus to conduct dynamic compression tests at varying impact velocities, followed by numerical simulation and development of a dynamic constitutive model.
Core Technical Content
The researchers investigated how material strength, outer steel tube thickness, and hollow ratio influence the dynamic compressive strength of HSCC specimens. The key findings reveal that under axial impact loading, HSCC specimens maintain relatively good structural integrity, with dynamic compressive strength increasing as impact velocity rises—though the rate of increase diminishes at higher velocities. This diminishing return effect is consistent with the strain rate sensitivity behavior observed in steel and concrete composites under dynamic loading.
Key Experimental Parameters and Findings
| Parameter | Influence on Dynamic Compressive Strength | Engineering Significance |
|---|---|---|
| Material strength (concrete grade) | Significant positive correlation | Higher grade concrete provides better dynamic resistance but increases cost |
| Outer steel tube thickness | Significant positive correlation | Thicker tubes improve confinement effect under impact |
| Hollow ratio | Significant negative correlation | Higher hollow ratio reduces dynamic capacity but saves weight |
| Impact velocity | Positive but diminishing return | Strain rate effect saturates at higher velocities |
Dynamic Constitutive Model
The proposed dynamic stress-strain constitutive model for HSCC captures the strain rate sensitivity inherent in both the steel tube and concrete components. The model accounts for the confinement effect provided by the outer steel tube on the sandwiched concrete layer, which is particularly critical under impact conditions where the steel tube's lateral restraint prevents premature concrete failure. The model is suitable for engineering design and analysis of HSCC structures subjected to blast or impact loading.
Process and Material Analysis from a Welding and Steel Pipe Perspective
From the standpoint of steel pipe manufacturing and welding engineering, the HSCC composite structure raises several important considerations:
- Steel tube fabrication quality: The outer steel tube in HSCC applications typically uses welded pipes (HFW or LSAW). The weld quality directly affects the tube's ability to provide uniform confinement under dynamic loading. Any weld defects—such as lack of fusion, undercut, or residual stress concentration at the weld toe—could initiate failure under impact.
- Hollow ratio and wall thickness: The hollow ratio design requires careful consideration of minimum wall thickness to prevent local buckling during impact. For dynamic applications, the outer tube wall thickness should generally not be less than 6 mm to ensure adequate confinement and avoid thin-wall instability.
- Surface preparation and coating: The interface between the steel tube and concrete is critical for load transfer. Surface roughening (shot blasting to Sa 2.5 per ISO 8501-1) improves bond strength and is essential for composite action under impact.
Engineering Practice Integration
In practical applications such as blast-resistant structures, military facilities, and critical infrastructure protection, HSCC members offer a favorable strength-to-weight ratio compared to solid steel tube concrete (STC). The hollow core reduces material consumption while the sandwiched concrete layer maintains structural integrity. Engineers should note that the dynamic constitutive model presented in this paper can be directly incorporated into finite element software for impact analysis of HSCC columns and beams.
Key Questions and Reflections
A significant question that arises from this research is: how does the welding residual stress in the outer steel tube interact with the dynamic impact stress field? In welded steel tubes, residual tensile stresses at the weld zone can reach 200-300 MPa, which may reduce the effective yield strength under superimposed dynamic loading. Future research should investigate the interaction between pre-existing weld residual stresses and impact-induced stress waves in HSCC members.
Another consideration is the long-term durability of the hollow core interface. Under cyclic or repeated impact loading, the concrete-steel interface may experience progressive degradation, which is not captured in single-impact SHPB tests. For engineering applications involving repeated seismic or blast events, fatigue assessment of the composite interface is essential.
Study Insights and Implications
This research provides valuable data for the design of lightweight blast-resistant structures using HSCC. The dynamic constitutive model is a practical tool that can be directly applied in engineering design. For welding engineers involved in fabricating the steel tube components, the study reinforces the importance of achieving high-quality welds with low residual stress, as the dynamic performance of the composite system is directly dependent on the integrity of the steel tube. The diminishing return of dynamic strength at higher impact velocities suggests that design for extreme impact events requires conservative safety factors beyond what static analysis would suggest. This work represents a meaningful contribution to the field of protective engineering and composite steel-concrete structures.