Introduction

As a composite structure, foam material-filled metal tubes offer unique advantages over traditional metallic tubular energy absorbers1,2, including higher specific energy absorption, more stable deformation patterns, and superior lightweight energy absorption characteristics. These properties render them suitable for various transportation vehicles’ collision energy dissipation systems3,4. Aala Fardad et al.5,6,7 demonstrated that the interaction between lightweight foam materials and metal tubes significantly enhances energy absorption capabilities compared to individual components. Subsequent studies further explored foam-filled geometries: Zhou Hongyuan et al.8,9 optimized aluminum tubes with high-density foams, while Zhang Guangcheng et al.10 revealed that foam steel-filled tubes exhibit 2.91 times higher energy absorption per unit volume than foam aluminum-filled counterparts. Auxetic structures, such as aluminum foam-filled circular and square tubes11 and double-arrow auxetic designs filled with foam concrete12 achieved enhanced stability and load efficiency through controlled negative Poisson’s ratio behavior. Functionally graded re-entrant honeycombs13 and multi-cell aluminum foam-filled components14 further extended these principles, demonstrating that foam-filled structures universally outperform monolithic designs in energy absorption (E) and specific energy absorption (SEA).

Emerging frontiers: from bioinspired design to hybrid systems

These advancements transcend conventional foam-metal composites through structural innovation and multi-material synergy, encompassing four pivotal research directions:

  1. (1)

    Bioinspired geometries: Nautilus shell-inspired corrugated grooves15 reduced peak crushing forces by 17.9% while improving SEA, whereas origami-inspired tubes16 utilized controlled folding patterns to achieve over 30% higher energy absorption. Fibonacci spiral-based metamaterials17 exhibited tunable stiffness and Poisson’s ratio sign-switching, enabling adaptive energy dissipation.

  2. (2)

    Topological optimization: Defect channels mimicking natural hexagonal lattices18 improved SEA by 56.5% via strain delocalization, while concave curved supports in re-entrant honeycombs19 boosted specific energy absorption by 297%.

  3. (3)

    Hybrid material systems: Carbon fiber/metal composites20,21 synergized high strength and lightweight advantages, with CFRP/AL hybrids showing 42% SEA improvement. Additive manufacturing enabled complex architectures such as stiffened bioinspired tubular metamaterials and TPMS/plate-lattice-filled tubes22. These designs optimized core-tube interactions, achieving a specific energy absorption of 22.7 J/g.

  4. (4)

    Dynamic performance enhancement: Foam-filled sandwich panels23 increased ballistic resistance through plastic deformation and friction, while multi-tube configurations24 suppressed load fluctuations by 33.3%, highlighting the critical role of hierarchical designs. Recent studies further demonstrate that thin-walled metallic tubes with optimized cellular topologies (e.g., circular honeycombs) significantly enhance blast resistance under extreme loading. Patel et al.25 revealed that circular tube honeycombs reduce face deflection by 37% compared to square counterparts under high-intensity air blasts, while hybrid designs combining foam filling and fiber metal laminates (FML) improved specific energy absorption by up to 45%26,27,28. Notably, foam-filled circular honeycomb cores with FML skins achieved 28% lower plastic dissipation energy than conventional designs29.

Cost-effectiveness versus performance: the foam concrete paradigm

Despite these advances, most studies rely on expensive metallic foams such as aluminum foam30, limiting scalability. In contrast, foam concrete-a low-cost, lightweight, and porous material-has gained traction in insulation31,32, military protection33, and seismic engineering34,35. Zhou Hongyuan et al.12 pioneered its use in auxetic structures, yet critical gaps persist:

  • Existing designs predominantly focus on quasi-static loading12,36, neglecting dynamic interactions in rigid-flexible coupled systems under extreme conditions (e.g., blast loads)25,26,27,28.

  • Foam concrete’s potential remains underexplored in synergy with advanced geometries such as auxetic patterns11 and bioinspired topologies derived from natural systems15,17.

  • Cost-efficient manufacturing methods for hybrid systems, including foam concrete combined with carbon fiber/metal interfaces20,21, lack systematic validation.

This research group is dedicated to the study of burst-resistant support systems for coal mine roadways. Existing research on energy-absorbing components includes pre-corrugated metal energy-absorbing boxes, corrugated plates, straight-walled tubes, and expanding-diameter energy-absorbing tubes. However, studies on energy-absorbing structures filled with foamed concrete (a type of cellular material) remain limited. This study bridges these gaps by investigating the energy absorption characteristics of EFFC-filled thin-walled metallic square tubes. A rigid-flexible coupled body integrating foam concrete’s economic viability is proposed. The absorption characteristics are validated through compression experiments and finite element analysis, utilizing methodologies established in prior studies. This component demonstrates remarkable blast-resistant properties and cost-effectiveness, making it suitable even for impact energy absorption environments in gas-containing mines. This work provides a reference for the design and improvement of composite energy-absorbing members in anti-impact support equipment.

Mechanical experimental study on the energy absorption characteristics of EFFC

The influencing factors, including underground working conditions in coal mines and material costs, were comprehensively evaluated, leading to the selection of foamed concrete as the filling material. Compared with conventional concrete, foamed concrete exhibited a unique microstructure characterized by uniformly distributed closed micropores and fine air bubbles, which provided enhanced energy absorption capabilities absent in ordinary concrete. To further improve its mechanical performance, polypropylene fibers were strategically incorporated during the foamed concrete preparation process, thereby forming a fiber-enriched foamed concrete (EFFC, Fiber-Enriched Foamed Concrete). The correlation between density variations of EFFC and its energy absorption characteristics was systematically investigated in this chapter, aiming to establish a quantitative model for energy absorption optimization.

Preparation of EFFC and axial loading experiment

In this study, following the Chinese national industry production standards for foamed concrete37,38, the air compressor foaming method was adopted to prepare foamed concrete specimens. Polypropylene fibers were incorporated during the preparation process. Material composition of EFFC is detailed in Table 1. After curing, cubic blocks with an initial edge length of 100 mm were cut into 50 mm × 50 mm × 100 mm test specimens. The fabrication process is illustrated in Fig. 1a, and the fabricated samples are illustrated in Fig. 1b.

Table 1 Ingredients list of foamed concrete with different densities.
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Fig. 1
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Preparation flow chart of foamed concrete test block.

To study the effect of polypropylene fibers on the performance parameters of foamed concrete and the mechanical properties of EFFC under different density parameters, loading tests were conducted on standard EFFC specimens using a SUNS universal testing machine. The loading device for the specimens is shown in Fig. 2. During the loading test, the specimen was placed at the center of the testing machine’s loading head, and the loading method was displacement control with a loading rate of 20 mm/s, corresponding to a nominal strain rate of 0.2 s-1. The deformation mode of the EFFC test blocks with different densities and the load–displacement curve data were recorded.

Fig. 2
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Loading test of EFFC trial block.

Results and analysis of EFFC loading experiments

Deformation and failure modes of EFFC

This study conducted failure tests on EFFC specimens of different densities under quasi-static compression conditions. The experimental results indicate that the deformation and failure modes of EFFC specimens with different densities are basically the same. Taking the EFFC specimen with a bulk density of 800 kg/m3 as an example, its failure process is shown in Fig. 3. As seen in the figure, during the initial loading stage, the specimen remains intact; as the compression displacement increases, longitudinal cracks dominate on the surface of the specimen, gradually expanding into multiple through cracks; as the compression displacement continues to increase, block detachment occurs on the exterior of the specimen, ultimately leading to the compaction and densification of the test block. The uniform internal bubble distribution in EFFC eliminated explosive cracking and debris scattering during crack propagation, while promoting stable splitting behavior. Additionally, polypropylene fibers retained the integrity of fragmented surface blocks through interfacial bonding, preventing their detachment from the main matrix.

Fig. 3
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Quasi-static compression failure process of 800 kg/m3 EFFC test block.

Stress–strain relationship of EFFC

In this study, three sets of compression experiments were conducted for each density of EFFC specimens to obtain force–displacement curves during the loading process. To analyze the compressive mechanical properties of EFFC, the experimental data were processed as follows:

  • The average loading force per unit displacement was calculated based on the compressive displacement, generating average force–displacement curves.

  • Stress–strain curves were plotted according to the dimensions of the compressed specimens.

Taking EFFC with a density of 400 kg/m3 as an example, the measured stress, average stress, and standard deviation of the data are presented in Figs. 4 and 5. As shown in Fig. 5, the standard deviation of the data remained within 10%, indicating good consistency in the compression test results.

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The experimental curves and processed data curves of 400 kg/m3 EFFC.

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Standard deviation of the data.

Based on the experimental results, the nominal stress–strain curves of EFFC specimens are plotted, as shown in Fig. 6. All curves include three stages: elastic stage, platform stage, and densification stage. As shown in the figure, the peak stress and platform stress of EFFC increase with increasing density. For specimens with densities of 800 kg/m3, 900 kg/m3, and 1200 kg/m3, after reaching the peak stress, the stress load drops rapidly, and the stress decrease rate increases with increasing density. The stress stability during the platform stress stage is relatively low, with higher stress fluctuations observed as density increases. This indicates that high-density specimens exhibit more pronounced brittle characteristics and have a stronger tendency to experience impact-induced failure. In contrast, for low-density specimens with densities of 300 kg/m3, 400 kg/m3, and 500 kg/m3, a decrease in stress is also observed after reaching the peak stress. However, the stress reduction is significantly less compared to high-density specimens. The transition into the platform stress stage is smoother, and the stability during this stage is superior, exhibiting ideal “rectangular stress–strain” characteristics. These properties make low-density specimens an ideal material for filling composite energy-absorbing components. According to the research on the stress–strain relationship of ordinary foamed concrete39, it has been observed that upon reaching the ultimate failure load, the stress drops rapidly, with a decrease amounting to 50% of the ultimate failure load.

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Nominal stress–strain curves of each foamed concrete test block.

The mechanical behavior of EFFC under compression is significantly influenced by its density, which correlates with internal porosity. The incorporation of polypropylene fibers establishes interconnections between solid structures within pores, restricting deformation during specimen failure.

Initial Elastic Stage:

  • High-density EFFC: Smaller internal pores and thicker load-bearing walls between pores result in higher peak loads.

  • Low-density EFFC: Larger pores and thinner load-bearing walls lead to lower peak loads.

Post-Peak Failure Mechanisms:

  • High-density EFFC: Bulk spalling dominates the failure mode. The abrupt collapse of large fragments causes shear-induced rupture of polypropylene fibers, leading to a sharp post-peak load drop. Subsequent compression maintains high but fluctuating plateau stresses due to continued blocky fragmentation.

  • Low-density EFFC: Failure primarily involves pore compaction and microcracking. Polypropylene fibers remain intact, bridging fragmented particles and stabilizing deformation. This results in a gradual post-peak stress decline with minimal fluctuations.

Consequently, the addition of polypropylene fibers can effectively enhance the internal structure of ordinary foamed concrete, resulting in improved energy absorption characteristics. For low-density foamed concrete, the incorporation of polypropylene fibers significantly reduces the stress load decrease, with a lower tendency for impact-induced failure. In the case of high-density foamed concrete, however, the addition of polypropylene fibers has a relatively minor influence on the stress characteristics.

Analysis of EFFC energy absorption characteristics

The energy absorption characteristics of materials can be evaluated using parameters such as specific energy absorption (SEA)40,41, which is the energy absorbed per unit mass. The higher the SEA, the better the energy absorption performance. The total energy absorbed by EFFC is calculated based on the stress–strain curve shown in Fig. 6. To determine the total energy absorbed by the material, the compacted strain of the material under compression must be identified. The compacted strain can be determined through the energy absorption efficiency. The energy absorption efficiency Ef is defined as8

$$E_{f} \left( {\varepsilon_{a} } \right) = \frac{1}{{\sigma_{a} }}\int_{0}^{{\varepsilon_{a} }} {\sigma \left( \varepsilon \right)} d\varepsilon$$
(1)

In the above equation, σ(ε) represents the stress–strain curve of the material under quasi-static compression conditions, and σa is the stress value corresponding to strain εa(0 ≤ εa ≤ 1) . The compacted strain εd corresponds to the strain value at which the energy absorption efficiency Ef reaches its maximum value, and it can be determined using the following Eq. 8:

$$\left. {\frac{{dE_{f} \left( {\varepsilon_{a} } \right)}}{{d\varepsilon_{a} }}} \right|_{{\varepsilon_{a} = \varepsilon_{d} }} = 0$$
(2)

Figure 7 illustrates the relationship between the compacted strain and energy absorption efficiency for a 300 kg/m3 density EFFC specimen, from which the compacted strain εd can be determined. The total absorbed energy E can be calculated using Eq. (3)8:

$$E = \int_{0}^{{l_{d} }} {P\left( l \right)dl}$$
(3)

where P(l) is the axial load corresponding to the compression displacement l, and ld is the compression displacement corresponding to the compacted strain εd.

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Compaction strain diagram of 300 kg/m3 EFCC test block.

The specific energy absorption (ESA) is defined as the ratio of the total absorbed energy E to the mass m of the material, expressed as8 :

$$E_{SA} = \frac{{\int_{0}^{{l_{d} }} {P\left( l \right)dl} }}{m}$$
(4)

Based on the stress–strain curves in Fig. 6 and Eqs. (3) and (4), the total absorbed energy E and specific energy absorption ESA for EFFC specimens of different densities were obtained, as shown in Fig. 8. It can be observed from the figure that as the specimen density increases, the total absorbed energy E increases, while the ESA first increases and then decreases. Equation (3) indicates that the total absorbed energy E of the specimens is determined by both the plateau stress of the material’s stress–strain curve and the compacted strain. With increasing density, although the compacted strain of the specimens decreases, the plateau stress increases significantly. Therefore, the total absorbed energy of high-density foam concrete specimens is higher than that of low-density foam concrete specimens. Additionally, changes in density also imply changes in mass; although high-density foam concrete has higher energy absorption, its larger mass results in a lower ESA. Based on this analysis, the 500 kg/m3 foam concrete exhibits superior energy absorption capabilities compared to specimens of other densities. Therefore, this density of EFFC was selected to fill the thin-walled metallic square tubes for further research.

Fig. 8
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Comparison of total energy absorption and specific energy absorption for test specimens at various densities.

Compressive experimenting of EFFC-filled thin-walled metallic square tube rigid-flexible coupled body

Fabrication of thin-walled metal square tubes

Thin-walled metallic tubes are widely utilized as energy-absorbing components in engineering applications. Currently, various configurations of thin-walled metallic energy absorbers have been implemented in mining rigid-flexible coupled energy dissipation devices. Considering the operating conditions of test specimens and experimental manufacturing costs, standard square tubes were selected with dimensions of 50 mm × 50 mm, fabricated from 45# medium carbon steel (equivalent to AISI 1045) with a wall thickness of 3 mm. The selected tubes comply with the Chinese national standard GB/T6728-2002. Axial compression testing of thin-walled metallic square tubes has revealed unstable stacking deformation patterns, resulting in inconsistencies in the buckling prestress. Research on the energy absorption and deformation mechanisms of metallic tubes under axial compression42 has demonstrated that strategically introducing defects on the tube surface can direct the specimens to deform in predetermined directions. This controlled deformation yields stable and predictable deformation patterns, leading to a more consistent and smooth platform buckling prestress. Based on these findings, the present study incorporates induced defects on the tested thin-walled metallic square tubes, as illustrated in Fig. 9. The induced defect was in the form of a rectangular groove with a width of 2 mm and a depth of 0.35 mm.

Fig. 9
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Square tube specimen with different induced defect types.

To accommodate the dimensions of the test specimens, five distinct defect-inducing configurations were developed: single-groove (A-1), double-groove (B-1, B-2), and triple-groove (C-1, C-2). The primary difference between the double-groove configurations B-1 and B-2 lies in the spatial orientation of the inducing grooves on the tube surface—they are positioned on non-parallel faces. Similarly, the triple-groove configurations (C-1, C-2) also feature inducing grooves on distinct faces. Taking C-2 as an example, the part drawing of the test square tube is shown in Fig. 10.

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Part drawing of C-2.

Simulation study on energy absorption characteristics of rigid-flexible coupled body

To investigate the energy absorption characteristics of the rigid-flexible coupled body, a numerical simulation of the C-2 specimen filled with foamed concrete (denoted as FC-2) was conducted using LS-DYNA software. The finite element model comprised a metallic square tube (C-2 type with pre-folded grooves), internally filled foamed concrete, and upper/lower loading plates. The square tube material was 45# steel (equivalent to AISI 1045) modeled with a Lagrangian mesh, while the foamed concrete was simulated using smoothed particle hydrodynamics (SPH). In the simulation, the lower plate was fully constrained, and the upper plate was displaced by 80 mm at a constant velocity over 30 ms. Both plates were tied to the square tube.

The simulation model and loading process are illustrated in Fig. 11, with the force–displacement and energy-displacement curves shown in Fig. 12. As observed in Fig. 11, initial stress concentrations occurred near the pre-folded grooves of the metallic tube and at the upper plate-foamed concrete interface. Progressive compression induced buckling in the metallic tube, leading to lateral compression of the foamed concrete. With increased compression, the tube walls exhibited outward convex or inward concave plastic deformation. Concurrently, axial compression of the foamed concrete caused fragmentation. The fragmented particles filled voids formed by the tube’s outward buckling, thereby resisting further buckling progression. This mutual coupling between the metallic tube and foamed concrete significantly influenced the deformation mechanics.

Fig. 11
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Numerical simulation of the FC-2 compression process.

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Simulation curve of FC-2.

The force–displacement curve in Fig. 12 reveals a rapid load drop post-elastic stage, followed by a gradual decline until mid-compression. Beyond 50% compression, lateral deformation of the tube stabilized, and the foamed concrete densification drove a sustained load increase. To further elucidate the energy absorption mechanisms of the metallic tube and EFFC during compression, experimental validation is required to complement the numerical findings.

Compression experiment on rigid-flexible coupled specimens

To study the mechanical properties of EFFC-filled thin-walled metallic square tubes, axial compression experiments were conducted on thin-walled metallic square tubes filled with 500 kg/m3 EFFC and unfilled thin-walled metallic square tubes. The loading was displacement-controlled with a loading rate of 20 mm/s, and the load–displacement curves were recorded. The experimental process is shown in Fig. 13.

Fig. 13
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Compression of the FC-2 rigid-flexible coupled experiment specimen.

The load–displacement curves for different defect-induced specimens are shown in Fig. 14. Curves A, B, and C in Fig. 14 correspond to the loading curves of unfilled metal tubes with single-groove, double-groove, and triple-groove configurations, respectively, while curves FA, FB, and FC represent the loading curves of EFFC-filled rigid-flexible coupled specimens. The experiment results show that the five types of defect-induced thin-walled metallic square tubes exhibited the same deformation mode. Taking the compression process of EFFC-filled thin-walled metallic square tubes with single-groove induced defect as an example, the deformation pattern is analyzed.

  • Initial Stage of Compression: In the initial stage of compression, the specimen with rigid-flexible coupled is in the elastic deformation stage, with stress concentrated at the induced defect of the square tube. When the yield load is reached, the square tube indents inward, resulting in the first buckling deformation.

  • Intermediate Stage of Compression: As the compression displacement increases, the lower part of the square tube bulges outward and stacks, leading to the second buckling deformation of the specimen.

  • Advanced Stage of Compression: As the axial displacement continues to increase, the upper part of the specimen bulges and stacks, causing the third buckling deformation.

  • Final Stage of Compression: In the final stage of loading, the specimen gradually enters the densification stage.

Fig. 14
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Comparative analysis of rigid-flexible coupled experiment specimens with various induced defects.

Comparative analysis of simulation and experimental results demonstrates strong agreement in deformation patterns and mechanical responses. Numerical simulations (Figs. 11 and 12) indicate that the compression process of the FC-2 specimen involves an initial sudden load drop after the elastic stage, load fluctuations induced by progressive buckling, and a subsequent load rise driven by the densification of the foamed concrete. Experimental data (Fig. 14) further validate this pattern: all filled specimens with various defect types (FA, FB, FC) exhibit similar three-stage load curve characteristics, with significantly higher load-bearing capacities than their unfilled counterparts (A, B, C). Notably, the simulations successfully replicate the core deformation mechanism observed experimentally—sequential buckling of the metallic tube at the pre-folded grooves (Fig. 11), while the fragmentation and filling action of the foamed concrete effectively restrain the propagation of tube wall instability (Figs. 13 and 14). This rigid-flexible coupling effect manifests in both simulations and experiments as a gentle load decline phase followed by a late-stage strengthening trend in the load–displacement curves, demonstrating that the numerical model effectively predicts the energy absorption characteristics of the filled structure.

Analysis of energy absorption characteristics in EFFC-filled thin-walled metallic square tube rigid-flexible coupled body

Energy absorption characteristics of rigid-flexible coupled specimens

Figure 14 shows the load–displacement comparison for the five types of defect-induced rigid-flexible coupled specimens and thin-walled metallic square tube specimens. The specimens labeled with series F represent the EFFC-filled thin-walled metallic square tube rigid-flexible coupled body, and ΔFy denotes the load drop. It can be observed that the load fluctuation trend of the rigid-flexible coupled specimens is similar to that of the thin-walled metallic square tube specimens, with three load drops, each corresponding to a buckling deformation until the final densification stage. Compared to the thin-walled metallic square tube specimens, the rigid-flexible coupled specimens exhibit higher plateau stress, indicating a higher total energy absorption. The compaction displacements ld0 and ld of the rigid-flexible coupled specimens and the thin-walled metallic square tube specimens were determined using Eq. (3). Due to the internal filling of EFFC, the rigid-flexible coupled specimens enter the densification stage earlier than the thin-walled metallic square tube specimens (ld0 < ld).

The energy absorption of rigid-flexible coupled specimens with different induced defects was compared with the algebraic sum of the energy absorption of thin-walled metallic square tubes and EFFC, as shown in Fig. 15. The results demonstrate that, due to the coupled effect between the enriched fiber foam concrete and the metallic square tubes, the total energy absorbed by the rigid-flexible coupled specimens is higher than the algebraic sum of the energy absorbed by the thin-walled metallic square tube specimens and EFFC. The lowest energy absorption enhancement ratio is 27.81%, and the highest is 94.5%.

Fig. 15
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Comparative analysis of energy absorption in rigid-flexible coupled experiment specimens with various induced defects.

Load analysis of rigid-flexible coupled body

Studies on the load reduction of energy absorption components have shown that the smaller the load reduction of mining energy absorption components, the lower the impact tendency, and the less likely the components are to fail43. Therefore, load reduction is an important indicator for evaluating the structural stability of energy absorption components. Figure 16 shows a comparison of the load reduction of rigid-flexible coupled specimens with induced defects and thin-walled metallic square tube specimens. The results indicate that the load reduction of the rigid-flexible coupled specimens is significantly lower than that of the thin-walled metallic square tube specimens. The maximum load reduction for the thin-walled metallic square tube specimens is 95.9%, while the maximum load reduction for the rigid-flexible coupled specimens is 66.7%. Filling with EFFC can significantly reduce the load reduction of the unfilled metallic square tube specimens, thereby decreasing their tendency for impact. The reason for this result is that when the thin-walled metallic square tube undergoes buckling deformation, the load shows a downward trend. In contrast, the EFFC inside the rigid-flexible coupled specimens provides internal support during buckling, thereby reducing the magnitude of the load drop. This phenomenon can be attributed to the buckling-induced load reduction of the thin-walled metallic square tube. The EFFC within the rigid-flexible coupled body exerts a stabilizing effect during the buckling process, mitigating the magnitude of load reduction by providing internal structural support.

Fig. 16
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Comparison of load reduction in experiment specimens with different induction defects.

Comparative analysis of E SA for rigid-flexible coupled body specimens

Research on energy absorption components of mine anti-impact support equipment has demonstrated that, due to variations in materials and design parameters, significant differences in the total energy absorption capabilities of these components exist44,45,46. Therefore, in the analysis of energy absorption characteristics of energy absorption components, in addition to considering the total energy absorbed, specific energy absorption (ESA) and load fluctuation coefficient Δ are introduced as indicators to evaluate and compare the energy absorption characteristics of different energy absorption components. The load fluctuation coefficient Δ44 characterizes the smoothness of energy absorption and buffering during the process of load application. It evaluates the stability of load in the anti-impact process. The closer it is to 1, the better the energy absorption effect. It is defined as follows:

$$\Delta = \frac{{F_{\max } }}{{F_{{{\text{mean}}}} }} = \frac{{F_{\max } }}{{\frac{1}{{l_{d} }}\int_{0}^{{l_{d} }} {P\left( l \right)dl} }}$$
(5)

In the formula: Fmax represents the peak yield load of the energy absorption component; Fmean denotes the average yield load. The comparison of specific energy absorption (ESA) and load fluctuation parameters between the EFFC-filled thin-walled metallic square tubes studied in this work and other typical energy absorption components is shown in Table 2.

Table 2 Comparative analysis of evaluation indices for energy absorbing components.
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The energy-absorbing structures listed in Table 1 comprise three foam-filled metallic tube configurations (Foam concrete filled aluminum tube-700, Aluminum foam-filled multi-cell-d1, EFFC-filled thin-walled metallic square tube) and three anti-impact energy-absorbing components for coal mines (Pre-folded square tube, Corrugated tube expanding, Circular tube expanding). Data comparison reveals that while the Aluminum foam-filled multi-cell achieves the highest specific energy absorption (SEA), its sparking hazards during impact compression—a critical safety concern in methane-rich mining environments—preclude practical deployment. Conversely, the rigid-flexible coupled EFFC-filled thin-walled metallic square tube exhibits superior SEA relative to other structures, demonstrating enhanced energy absorption capacity. Furthermore, its load fluctuation coefficient (Δ = 1.09) approaches the ideal value of 1, indicating exceptional load stability. Considering manufacturing feasibility, production costs, and operational constraints in underground mines, the EFFC-filled thin-walled metallic square tube represents a more suitable solution for coal mining enterprises.

Conclusion

Three key findings emerge from this study:

First, while increased density elevates the yield load and platform stress of Fiber-Enriched Foamed Concrete (EFFC), the accompanying mass gain reduces specific energy absorption (SEA). Experimental analysis identifies 500 kg/m3 as the optimal density for metal component filling, delivering peak SEA performance.

Second, the EFFC-filled thin-walled metallic square tube rigid-flexible coupled system demonstrates exceptional stability. Under quasi-static loading, it exhibits 27.81–94.5% higher energy absorption and superior platform stress compared to unfilled tubes. Critically, its maximum load reduction (66.7%) is significantly lower than that of bare tubes (95.9%), confirming enhanced structural resilience.

Third, evaluation using specific energy absorption (24.2 kJ/kg) and load fluctuation coefficient (1.09) reveals superior performance: The composite absorbs 18% more energy per unit mass than conventional pre-folded square tubes (21.1 kJ/kg), while its near-unity fluctuation coefficient (1.09 vs. 1.14–1.64 in benchmarks) indicates exceptional load stability.

Finally, EFFC proves to be an economically viable filler for metal energy absorbers, simultaneously increasing total energy absorption by up to 94.5% and reducing impact propensity through stabilized load response. This cost-effective solution offers valuable insights for selecting composite fillers in impact protection systems.