Impact Resistance of Circular CFST Composite Members Under Lateral Drop-Weight Loading

Literature Overview

The study by Hu Changming and Han Linhai, published in the China Civil Engineering Journal in 2016 (Vol. 49, No. 10, pp. 11-17), investigates the impact resistance of circular CFST composite members under lateral drop-weight loading. The research examines the failure modes, overall and local deformation characteristics, and the influence of impact energy, axial force, and steel tube thickness on mechanical performance. The specimens are composite members consisting of an outer reinforced concrete (RC) component and an inner CFST core, which work together to provide enhanced impact resistance. This work is highly relevant to engineers designing protective structures, such as nuclear facilities, military installations, and infrastructure subject to blast or impact loading.

Specimen Configuration and Test Methodology

The test specimens are circular CFST composite members, where an outer RC shell surrounds a central CFST core. This composite configuration combines the high compressive strength and ductility of CFST with the mass and stiffness of RC, creating a synergistic system for impact resistance. The drop-weight impact test involves releasing a heavy weight from a controlled height onto the mid-span of the simply supported specimen, generating a lateral impact load.

Parameter	Variation
Impact energy	Multiple levels (increasing drop height)
Axial preload	With and without pre-applied axial compression
Steel tube thickness	Multiple thicknesses
Specimen type	CFST composite, plain CFST, plain RC

The comparison between CFST composite specimens, plain CFST specimens, and plain RC specimens provides a clear understanding of the contribution of each component to the overall impact resistance. The test setup should include high-speed data acquisition to capture the force-time and displacement-time histories, which are essential for understanding the dynamic response of the specimens.

Failure Modes and Deformation Characteristics

The study identifies distinct failure modes for the different specimen types:

Specimen Type	Failure Mode	Key Observation
CFST composite	Progressive deformation with coordinated action	RC outer shell and CFST core work together
Plain CFST	Local buckling followed by concrete crushing	Steel tube buckling initiates failure
Plain RC	Brittle crushing with spalling	Concrete crushing without ductile deformation

The CFST composite specimens exhibit a failure mode characterized by progressive, coordinated deformation of both the outer RC shell and the inner CFST core. The RC shell provides initial stiffness and distributes the impact load over a wider area, while the CFST core absorbs energy through ductile deformation of the steel tube and confinement of the concrete. This coordinated action results in a more gradual energy absorption process compared to plain CFST or plain RC specimens.

From a welding perspective, the steel tube in the CFST composite member must be fabricated with high quality to ensure that the weld zones do not become weak links under impact loading. Impact loading is a high-strain-rate event, and the HAZ of the weld may exhibit different mechanical properties compared to the base metal. The dynamic fracture toughness of the HAZ should be considered in weld design to prevent brittle fracture under impact conditions.

Force-Time and Displacement-Time Characteristics

The force-time curves and displacement-time curves provide critical information about the dynamic response of the specimens:

Parameter	Trend with Increasing Impact Energy
Impact force peak	Increases
Mid-span deflection peak	Increases
Impact duration	Increases

The impact force-time curve typically exhibits a characteristic shape with an initial peak, followed by a plateau region, and then a gradual decay. The plateau region represents the quasi-static phase of deformation, where the specimen absorbs energy through plastic deformation. The impact duration increases with impact energy because higher energy inputs result in larger deformations and longer interaction times.

The influence of pre-applied axial compression on the impact response is also significant:

Parameter	Effect of Pre-applied Axial Compression
Impact force peak	Increases
Impact force plateau	Decreases
Energy absorption	Enhanced through axial confinement

The pre-applied axial compression increases the initial stiffness of the specimen, resulting in a higher impact force peak. However, the plateau value decreases because the axial compression reduces the available deformation capacity. This trade-off between peak force and energy absorption is an important consideration in the design of impact-resistant structures.

The effect of steel tube thickness on impact performance is also well-characterized:

Parameter	Effect of Increasing Steel Tube Thickness
Impact force peak	Increases
Impact force plateau	Increases
Mid-span deflection peak	Decreases
Impact duration	Decreases

Thicker steel tubes provide greater stiffness and strength, resulting in higher impact force peaks and plateaus. However, they also reduce the mid-span deflection and impact duration, indicating a stiffer response with less deformation. The optimal steel tube thickness for impact resistance depends on the specific design objectives, balancing force reduction, energy absorption, and deformation control.

Composite Action Mechanism

The study highlights the coordinated complementary action between the outer RC component and the inner CFST core:

Initial impact phase: The RC outer shell absorbs the initial impact energy through elastic deformation and cracking, distributing the load to the CFST core.
Intermediate phase: The CFST core begins to yield and deform plastically, while the RC shell continues to crack and spall. The concrete in the CFST core is confined by the steel tube, enhancing its compressive strength and ductility.
Final phase: The steel tube undergoes significant plastic deformation, and the RC shell may partially detach. The CFST core continues to absorb energy through large plastic deformations, providing ductile failure behavior.

This composite action mechanism is essential for achieving high impact resistance. The RC shell provides mass and initial stiffness, while the CFST core provides ductility and energy absorption capacity. The interface between the RC shell and the CFST core should be designed to ensure proper load transfer, potentially using mechanical connectors, dowels, or chemical bonding agents.

Welding and Fabrication Considerations for Impact Applications

Impact loading imposes special requirements on the welding and fabrication of CFST composite members:

Requirement	Specification
Weld metal toughness	High Charpy V-notch (CVN) impact energy at service temperature
HAZ toughness	Controlled heat input to minimize HAZ coarsening
Residual stress	Post-weld heat treatment (PWHT) to reduce residual stresses
Surface quality	Smooth weld finish to prevent stress concentration
NDE inspection	UT and MT for all welds, with strict acceptance criteria

The dynamic loading conditions associated with impact events require that the weld metal and HAZ possess adequate toughness to prevent brittle fracture. The Charpy V-notch impact test should be performed on weld metal and HAZ samples at the expected service temperature to verify that the materials meet the required toughness levels. The welding procedure should be qualified to produce welds with consistent toughness properties.

Engineering Practice Implications

The findings of this study have several important implications for engineering practice:

Design of impact-resistant structures: CFST composite members are an effective choice for structures subject to impact loading, such as blast walls, protective barriers, and critical infrastructure components.
Optimization of steel tube thickness: The steel tube thickness should be optimized based on the expected impact energy and design objectives, balancing force reduction, energy absorption, and deformation control.
Pre-applied axial load consideration: In structures where axial loads are present (such as columns), the interaction between axial load and impact load should be considered in the design.
Quality control for impact applications: Welding quality requirements for impact applications should be more stringent than for static loading applications, with particular emphasis on toughness and defect acceptance criteria.

Study Insights and Implications

This study provides comprehensive experimental data on the impact resistance of CFST composite members, demonstrating the effectiveness of the composite configuration for energy absorption and force reduction. The coordinated action between the RC outer shell and the CFST core creates a synergistic system that outperforms either component alone. Engineers should consider CFST composite members for applications requiring high impact resistance, such as protective structures, military installations, and critical infrastructure. The welding quality requirements for impact applications should be clearly defined and enforced, with particular emphasis on weld metal and HAZ toughness. Future research should extend to larger-scale specimens, include multiple impact events, and investigate the residual capacity after impact damage. The study also highlights the importance of dynamic material characterization for accurate prediction of impact response, which should be incorporated into design methodologies for impact-resistant structures.