Zhuojin Pipe Fitting Co., LtdSHPB Impact Testing of Steel Tube Reactive Powder Concrete After High Temperature Exposure
Literature Overview
This paper by Guo Zhikun, Chen Wanxiang, Jiang Meng, and Zou Huihui from the State Key Laboratory of Explosion Protection for Civil Engineering, PLA University of Science and Technology, published in Journal of Vibration and Shock (2017, Vol. 36, No. 10, pp. 134-139), presents the results of Split Hopkinson Pressure Bar (SHPB) impact compression tests on steel tube reactive powder concrete (RPC) and plain RPC specimens after exposure to high temperatures of 200°C and 300°C. The study investigates the combined effects of strain rate and temperature on the dynamic mechanical properties, failure modes, toughness, and energy absorption characteristics of these composite materials.
Core Technical Content
The SHPB test is a widely used method for determining the dynamic mechanical properties of materials at high strain rates (typically 100-5000 s⁻¹). In this study, a 75 mm diameter SHPB system was used to conduct impact compression tests on cylindrical specimens of steel tube RPC and plain RPC at room temperature and after exposure to 200°C and 300°C. The specimens were subjected to controlled heating and then rapidly cooled before testing to simulate post-fire conditions.
The results reveal several important trends:
| Property | Effect of Temperature Increase | Effect of Strain Rate Increase |
|---|---|---|
| Toughness | Increases with temperature (200°C, 300°C) | Increases with strain rate |
| Relative toughness | Increases with temperature | Increases with strain rate |
| Energy absorption | Increases for steel tube RPC; relatively unchanged for plain RPC | Increases for both materials |
| Failure mode | Steel tube RPC: ductile; Plain RPC: brittle | Consistent with temperature effect |
A particularly noteworthy finding is that the toughness and relative toughness of steel tube RPC increase with both strain rate and temperature. This is counterintuitive for most materials, which typically exhibit reduced mechanical properties at elevated temperatures. The explanation lies in the composite action: the steel tube confinement provides lateral restraint that becomes more effective at higher temperatures as the concrete core softens, creating a more ductile failure mode.
Metallurgical Analysis and Welding Implications
The steel tube used in these composite specimens is typically made from structural steel such as Q235 or Q345, which undergoes significant microstructural changes at temperatures above 200°C. At 200°C, the steel is still in the elastic regime with minimal strength loss, but the yield strength begins to decrease noticeably at temperatures above 300°C. At 400-600°C, the steel experiences substantial strength degradation due to the transition from BCC to FCC crystal structure in carbon steels.
The welding quality of the steel tube is critical for the overall performance of the steel tube RPC composite under fire conditions. If the steel tube is fabricated from welded pipe (such as ERW, HFW, or LSAW), the weld HAZ is the weakest link in terms of high-temperature strength retention. The HAZ microstructure, which typically includes a mixture of martensite, bainite, and ferrite, may exhibit different thermal softening behavior compared to the base metal. This differential behavior can lead to premature failure at the weld location under combined thermal and impact loading.
Welding Process Considerations for High-Temperature Applications
| Welding Process | Suitability for High-Temperature CFT | Key Considerations |
|---|---|---|
| SAW (Submerged Arc) | Excellent for thick-walled tubes | Low hydrogen; good HAZ toughness |
| FCAW (Flux-Cored Arc) | Good for field welding | Shielding gas composition affects HAZ |
| GTAW (Tungsten Inert Gas) | Suitable for thin-walled tubes | Precise heat input control |
| ERW/HFW | Suitable for thin-walled seamless-like pipe | Consistent weld quality; limited thickness range |
The SHPB test results also have implications for the selection of welding consumables for CFT members in fire-prone environments. The filler metal must have adequate high-temperature strength retention and ductility to ensure that the weld does not become the weakest link in the composite member. Low-alloy steels with controlled carbon equivalent (CE ≤ 0.45) are preferred to ensure good weldability and HAZ toughness.
Energy Absorption and Structural Implications
The energy absorption characteristics measured in the SHPB tests are directly relevant to the blast and impact resistance of CFT structures. The steel tube RPC composite exhibits superior energy absorption compared to plain RPC, particularly at elevated temperatures. This is attributed to the ductile failure mode of the steel tube, which allows significant plastic deformation and energy dissipation before failure.
For structural applications in nuclear facilities, military structures, or industrial plants subject to blast loading, the selection of CFT materials with adequate high-temperature impact resistance is critical. The SHPB test provides a rapid and reliable method for characterizing the dynamic properties of candidate materials under simulated post-fire conditions.
Study Insights and Recommendations
The research demonstrates that the composite action between the steel tube and RPC core provides significant advantages in terms of toughness and energy absorption under combined high-temperature and impact loading. The ductile failure mode of steel tube RPC, compared to the brittle failure of plain RPC, is particularly important for progressive collapse prevention in post-fire scenarios.
However, the study is limited to temperatures up to 300°C, which is below the typical fire temperature range (600-1000°C). Future research should extend the temperature range to include severe fire conditions and investigate the effect of steel tube microstructure (such as thermomechanical controlled rolled steels) on the high-temperature composite performance. Additionally, the effect of welding residual stresses on the post-fire impact behavior should be investigated, as residual stresses can significantly influence crack initiation and propagation in the HAZ under dynamic loading.