Effects of sand filling on the dynamic response of corrugated core sandwich beams under foam projectile impact
Introduction
Whereas extreme dynamic impact and blast loads greatly threaten the security of structural systems and their occupants, conventional commercial and private buildings are often designed not to withstand such impact and blast loadings, and it is inconvenient and expensive to strengthen existing building structures with permanent protective constructions. One of the solutions is using maneuverable prefabricated structures to shield buildings and other structures against extreme loads [[1], [2], [3], [4], [5]]. Such protective structures should not only be able to resist a variety of impact/blast threats, but also are easy to transport and install, preferably with modular designs. When extreme events happen, the maneuverable, quickly installed temporary protective structures can act as barriers to shield schools, hospitals, embassies and other vital facilities.
Sandwich panels with stiff, strong face sheets and high porosity cellular cores are ultralightweight and can dissipate a considerable amount of impact energy via large plastic deformation of the core and face sheets, thus attractive for constructing maneuverable temporary structures. A variety of core topologies including foams [[6], [7], [8], [9]], honeycombs [[10], [11], [12], [13]], folded plates (corrugations) [[14], [15], [16], [17], [18]] and pyramidal trusses [[19], [20], [21]] have been selected as the core of sandwich panels to resist impact/blast loading. In particular, corrugated core sandwich structures stand out due to outstanding anti-impact performance [22] and relatively simple and efficient techniques needed for fabrication, e.g., extrusion [23] and laser welding [16]. The dynamic response of a sandwich panel with corrugated core subjected to foam projectile impact [14] and air blast loading [15,16] exhibited a higher shock resistance than its monolithic counterpart having equal mass.
Recently, to further improve the impact/blast resistance of sandwich structures with lattice truss cores, the concept of hybrid core design has been proposed wherein the interstices of the lattice truss core (e.g., corrugated plates) are filled with foam or other materials. Typically, with PVC foam selected as the filling material, corrugated sandwich panels exhibited improved performance under shock tube impulse and air blast loading [24,25], whereas superior penetration resistance was achieved by inserting ceramic prisms into a corrugated core sandwich panel [26]. Further, shear thickening fluid was used as the filler to improve the dynamic energy absorption capacity of sandwich structures [27]. The main aim of this study is to investigate the feasibility of using sand as a filling material for sandwich constructions with enhanced blast resistance, particularly for large-scale applications requiring quick assembling and low cost.
Sand as a readily available, low-cost granular material has been shown effective in mitigating blast shock wave [[28], [29], [30], [31]]. The shock wave energy is dissipated during wave-sand interaction, as: (I) particle collision, friction and crushing through sand compaction will consume much energy; (II) high pressure gas filtration by sand attenuates the transmitted air shock wave; (III) if sand is not confined, the wave energy is partly transferred to the motion of sand particles. Sand barriers, e.g., sandbags [30] and sand-filled anti-blast walls [32,33], have therefore been widely used to shield against blast/shock effect. In recent years, sand-filled sandwich constructions as temporary protective structures have been envisioned. The filled sand can be emptied from the sandwich core with ease so that the system retains its ultralightweight for quick transportation and fast assembly. For instance, extruded aluminum corrugated sandwich panels filled with sand (and small rocks) have been proposed as mobile shelters with modular design for ISO containers [2,3]. Sand-filled fiber-reinforced polymer (FRP) composite sandwich panels were also investigated experimentally under blast and fragmentation loading, and the effects of panel thickness and inner core configuration were quantified [1]. At present, however, the dynamic response and sand-structure interaction of sand-filled metallic sandwich structures under shock loading are seldom studied, and how sand properties (e.g., density, grain size, surface texture and mineralogy [34]) affect the shock resistance remains elusive.
A combined experimental and numerical approach was used to characterize the dynamic response of sand-filled metallic corrugated core sandwich beams under shock loading, with the shock loading simulated using closed-cell aluminum foam projectiles [35]. A coupled discrete particle/finite element simulation scheme was proposed for numerical calculations, which was validated against experimental measurements. The validated numerical model was employed to systematically evaluate the effects of sand properties on shock resistance.
Section snippets
Specimen configuration and fabrication
With reference to Fig. 1, corrugated core sandwich beams used in the current study were comprised of two thin stainless steel faceplates, a stainless steel core with trapezoidal corrugation, and two low carbon steel block fixtures. Relevant geometric parameters included: sandwich beam span L and width W, face sheet thickness , horizontal core segment width , core height , core web thickness , inclination angle , steel block length , and bolt hole diameter . In the absence of filled
Experimental results
Table 2 summarized all the experiments performed on both empty corrugated sandwich beams (EC) and sand-filled corrugated sandwich beams (SC), including the mass of empty beam , the mass of filling sand , the permanent front face deflection , the back face deflection , and the core compressive strain . For each type of beam, 6 levels of initial momentum per unit area were applied by varying the initial velocity of the foam projectile, as detailed in Table 2.
Numerical models
Three-dimensional (3D) numerical simulations were performed with commercially available software LS-DYNA v971_R8.1.0. A coupled discrete particle/finite element (FE) simulation scheme was used to simulate interaction between the filled sand, corrugation members, face sheets and sealing tapes. Sand was modeled as discrete spherical particles, while the sandwich beam, foam projectile and sealing tapes were modeled with the method of finite elements. Fig. 11 displayed the FE model for empty
Model validation
As energy conservation of the whole system is the premise of obtaining correct numerical results, it was firstly investigated. Fig. 18 presented the predicted energy-time curves of empty specimen EC-3 and sand-filled specimen SC-3. It is seen that the total energy was equal to the sum of kinetic energy, internal energy, sliding energy and hourglass energy. Good energy balance was achieved since the sliding energy and hourglass energy were small, much less than 10% of the internal energy. For
Concluding remarks
Dynamic responses of fully clamped, sand-filled corrugated core sandwich beams under simulated shock loading with aluminum foam projectile were experimentally characterized. A numerical model was established using coupled discrete particle/finite element method and validated against experimental measurements. The validated numerical model was then utilized to evaluate how the key properties of filled sand affect beam shock resistance. Main findings were summarized as follows.
- (1)
Sand filling
CRediT authorship contribution statement
Run-Pei Yu: Conceptualization, Methodology, Software, Writing - original draft, Writing - review & editing. Xin Wang: Investigation, Data curation. Qian-Cheng Zhang: Formal analysis, Writing - review & editing. Lang Li: Software, Visualization. Si-Yuan He: Resources, Investigation. Bin Han: Investigation. Chang-Ye Ni: Resources. Zhen-Yu Zhao: Validation. Tian Jian Lu: Funding acquisition, Conceptualization, Supervision.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This work was supported by the National Key Research and Development Program of China (2017YFB1102801), the Open Project for Key Laboratory of Intense Dynamic Loading and Effect (KLIDLE1801), the National Natural Science Foundation of China (11972185), the Aviation Science Foundation Project (20170970002), the Open Fund of the State Key Laboratory of Mechanics and Control of Mechanical Structures (Nanjing University of Aeronautics and astronautics, MCMS-E0219K02) and the Open Project of State
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