Impact Momentum Research Articles

Predicting Leidenfrost Temperature and Effects of Impact Conditions on Droplet Size Distribution in Film-Boiling Regime

Abstract The interaction of a droplet with a solid wall is relevant in various engineering applications. The properties of the resulting secondary droplets are determined by the wall temperature, ambient pressure, impact momentum, and impact angle. This paper presents a comprehensive characterization of drop-wall interactions and the subsequent atomization as a function of the combined effects of such parameters. A drop-wall interaction model is derived for F-24 droplets using smoothed particle hydrodynamics (SPH). The model can predict different impact outcome regimes (deposition, rebound, contact-splash, and film-splash) for different ambient pressures, wall temperatures, and impact parameters. The model also addresses the effect of ambient pressure on the Leidenfrost behavior. Size distributions of secondary droplets are compared for vertical and nonvertical impacts of F-24 droplets on superheated surfaces in the film-boiling regime. The non-dimensional Sauter mean diameter (SMD) of the secondary droplets varies based on the position in the impact plane for all the nonvertical impacts but remains almost unchanged for vertical impacts. The leading arc for nonvertical impact consists of larger secondary droplets, and the size decreases toward the trailing arc. An empirical relation is proposed to represent this trend. This research sheds light on successive droplet impacts by studying the effects of impact frequency on SMD evolution. The results are compared to single droplet impact cases for different fuels and Weber numbers. The size of secondary droplets for successive impacts is observed to be nearly indistinguishable from that of single-droplet vertical impacts.

Elastoplastic plate shakes down under repeated impulsive loadings

When subjected to repeated dynamic impacts at identical load level, a metallic monolithic beam/plate may reach a stable state wherein measurable deformation ceases (i.e., shakedown in elastic state) after undergoing a sequence of elastoplastic deformations, which has been termed as “pseudo-shakedown” (P-S) (Jones, 1973, Shen and Jones, 1992). While the response of a single beam/plate under repeated low-velocity impacts has been thoroughly studied, its dynamic behavior under high-velocity impacts, such as explosive or alternating impulsive loads, is difficult to measure experimentally, due mainly to high costs and setup challenges. In the current study, the method of metallic foam projectile impact was employed to produce repeated impulsive loadings on a fully-clamped elastoplastic monolithic plate made of L907A (a Chinese standard shipbuilding steel). Its dynamic responses, including mid-point deflection versus time histories, final deflections, and deformation modes after each impact, were systematically measured. The phenomenon of dynamic shakedown was observed. To further explore this phenomenon, the method of finite elements (FE) was employed to simulate the repeated impulsive impact test, and its prediction accuracy was validated against experimental results. Unlike an elastoplastic (e.g., steel) monolithic plate subjected to repeated low-velocity impacts, which exhibits zero plastic energy dissipation in the P-S (pseudo-shakedown) state, the same plate under repeated high-velocity impacts shows a small level of plastic energy dissipation in the P-S state, mainly due to more extreme loading conditions. The initial impact momentum, yield strength, and tangent modulus of the material the plate is made of significantly affect both the stable deflection in the P-S state and the number of impacts needed to reach it, while the elastic modulus has limited influence. A modified dimensionless impulse loading number, accounting for the strain-hardening effect, is proposed. An approximately linear relationship between stable deflection in the P-S state and impulsive loading is found in dimensionless form.

Low velocity impact resistance of hybrid CFRP‐elastomer‐metal laminates: Influence of stacking sequence and impact conditions on damage mechanisms

AbstractFiber‐metal laminates (FMLs) are generally regarded as excellent lightweight materials for advanced structure design. To enhance the mechanical properties, the common FMLs can be optimized using carbon fibers. However, the combination of carbon fibers with aluminum induces interfacial challenges. Preventing galvanic corrosion with elastomeric interlayers is an effective solution. The lay‐up configuration greatly effects the impact damage resistance of hybrid CFRP‐elastomer‐metal laminates (HyCEMLs). In this work, micro‐CT scans and optical micrographic inspections on HyCEMLs are conducted after impact tests to ascertain the microstructural origins behind the mechanical performance changes. In addition, finite element models of different HyCEML configurations are built to complement the limited experimental data. The damage mechanisms of HyCEML with different configurations under various impact conditions are further compared. The numerical results suggest that impact energy is a more informative measure in terms of damage mode and size than impact velocity and momentum. Results also indicate that when the thickness for each sub‐laminate of HyCEML is maintained the same, hybrid laminates with aluminum stacked outside is beneficial for delaying the occurrence of matrix cracking and delaminations, and enhances HyCEML's resistance to global deformation. These findings will contribute to engineering hybrid laminates with desired impact performance for lightweight load‐bearing structures.Highlights The hybrid laminate with elastomeric interlayers is a forward‐looking solution in impact applications. Impact energy is a more informative measure in terms of assessing the damage mode and extent in HyCEMLs. The influence of stacking sequence on damage mechanisms of HyCEMLs is evaluated. Microstructural origins behind variations of hybrid laminates in the impact resistance are revealed.

Impact Momentum Transfer—Insights from Numerical Simulation of Impacts on Large Boulders of Asteroids

Asteroids pose potential hazards to Earth. The recent NASA Double Asteroid Redirection Test mission successfully demonstrated the change of an asteroid’s orbit by a kinetic impactor. This study focuses on impact-induced vertical momentum transfer efficiency (β − 1) considering various impact angles and subsurface boulder arrangements. Utilizing the iSALE-3D shock physics code, we simulate oblique impacts on different subsurface boulder configurations. Our results show that vertical ejecta momentum decreases with obliquity, with buried boulders inducing an anti-armoring effect. We define the direct impact-contacted boulder as the primary boulder and the surrounding boulders as secondary. The anti-armoring effect is most pronounced when the primary boulder is just below the surface, amplifying β – 1 by 50%. Impact angles between 60° and 75° exhibit a critical drop in ejecta momentum. An in-depth exploration of subsurface boulder arrangements reveals that secondary boulders have a minimal effect on vertical momentum transfer efficiency. Varying the size and separation of secondary boulders suggests that these subsurface features can either enhance or diminish the overall β − 1, providing insights into the dynamics of rubble-pile asteroids. In addition, impact melting is explored in our simulations, which suggests a minimal melt retention on Dimorphos’s surface. Volumes of retained melt differ by an order of magnitude for impacts on the homogeneous regolith and on targets with buried boulders. In summary, this study provides insights into the effect of subsurface boulders and impact angles on vertical momentum transfer efficiency, which is crucial for understanding asteroid deflection by a kinetic impactor.

Study on Dynamic Mechanical Properties of Sandwich Beam with Stepwise Gradient Polymethacrylimide (PMI) Foam Core under Low-Velocity Impact.

Different PMI foam materials of 52, 110, and 200 kg/m3 were used to design stepwise gradient cores to improve the impact resistance of the sandwich beam. The stepwise gradient core consists of three layers arranged in positive gradient, negative gradient, and sandwich-core (e.g., 200/52/200). These sandwich beams were subjected to the impact of a steel projectile under impact momentum of 10 to 20 kg·m/s, corresponding to impact energy in the range of 12.5 to 50 J. During the test, the impact force was recorded by an accelerometer, and the different failure modes were also obtained. Subsequently, the influence of the layer arrangement on the energy absorption and load transfer mechanism between the different layers was analyzed. The results showed that the top layer with a large density can improve the impact force, but the middle/bottom layer with a low density promoted specific energy absorption. Thus, based on these two points, the negative gradient core (200/110/52) had an excellent specific energy absorption because it can transfer and expand the area to bear the load layer by layer, which improved the energy absorption in each layer. Combined with the failure modes, the load transfer and deformation mechanisms between the layers were also discussed. The present work provided a valuable method to design an efficient lightweight sandwich structure in the protection field.

Numerical modelling of impact seismic sources using the stress glut theory

SUMMARY Meteorite impacts have proved to be a significant source of seismic signal on the Moon, and have now been recorded on Mars by InSight seismometers. Understanding how impacts produce seismic signal is key to the interpretation of this unique data, and to improve their identification in continuous seismic records. Here, we use the seismic Representation Theorem, and particularly the stress glut theory, to model the seismic motion resulting from impact cratering. The source is described by equivalent forces, some resulting from the impactor momentum transfer, and others from the stress glut, which represents the mechanical effect of plasticity and non linear processes in the source region. We condense these equivalent forces into a point-source with a time-varying single force and nine-component moment tensor. This analytical representation bridges the gap between the complex dynamics of crater formation, and the linear point-source representation classically used in seismology. Using the multiphysics modelling software HOSS, we develop a method to compute the stress glut of an impact, and the associated point-source from hypervelocity impact simulations. For a vertical and an oblique impact at 1000 m s−1, we show that the moment tensor presents a significant deviatoric component. Hence, the source is not an ideal isotropic explosion contrary to previous assumptions, and draws closer to a double couple for the oblique impact. The contribution of the point force to the seismic signal appears negligible. We verify this model by comparing two signals: (1) HOSS is coupled to SPECFEM3D to propagate the near-source signal elastically to remote seismic stations; (2) the point-source model derived from the stress-glut theory is used to generate displacements at the same distance. The comparison shows that the point-source model is accurately simulating the low-frequency impact seismic waveform, and its seismic moment is in trend with Lunar and Martian impact data. High-frequencies discrepancies exist, which are partly related to finite-source effects, but might be further explained by the difference in mathematical framework between classical seismology and HOSS’ numerical modelling.

Dynamic response of clamped metallic thin-walled cylindrical shells under lateral shock loading

A combined experimental and numerical study was carried out to explore the dynamic response of Q235 thin-walled cylindrical shells under lateral shock loading. The machined Q235 specimens were clamped on both sides and subjected to centered lateral simulated shock loading via foam projectile impact tests, with their dynamic deformation evolution, mid-point deflections, and final deformation modes experimentally measured. The mid-point deflections of the impact side are all positive (the direction of deformation is the same as the impact direction) while those of the rear side all exhibit negative values (the direction of deformation is opposite to the impact direction). To further explore this phenomenon, the method of finite element (FE) was employed to simulate the foam projectile impact, with good agreement against experimental measurements achieved. Using the validated numerical model, the effects of impact velocity, length-to-diameter ratio, and thickness-to-diameter ratio on the dynamic response of the thin-walled cylindrical shell were further analyzed. The direction of both side deflections and deformation modes are significantly affected by the impact level and the shell geometries. For the rear side, within the given range of impact momentum and geometric parameters, the numerically predicted deflection varies from −1.775 mm to 2.45 mm. Thereupon, the coupling of indentation and bulge deformation patterns are indicated, and their corresponding contribution changes are considered the main mechanism for determining the final deformation of the thin-walled cylindrical shell's rear side.

Shakedown of metallic sandwich beams when subjected to repeated impact loadings

When a monolithic metallic beam/plate subjected to repeated dynamic loadings with fixed impact level experiences elastoplastic deformation, its measurable deformations may cease to develop for further repetitions of the same dynamic load: this phenomenon was referred to as “pseudo-shakedown” in previous studies (Jones, 2014; Shen and Jones, 1992). Would pseudo-shakedown occur in a metallic sandwich structure under repeated shock loadings remains elusive. A combined experimental and numerical study was carried out to explore whether dynamic shakedown could occur in all-metallic ultralightweight corrugated core sandwich beams. For repeated impact tests, the sandwich beams were fabricated via the sequential process of cutting-stamping-vacuum brazing. When subjected to repeated impact loads (achieved via aluminum foam projectiles launched from a light gas gun), the dynamic responses of each sandwich specimen - including structural evolutions, beam deflections, and deformation/failure modes - were measured. Experimental results revealed that, depending upon the level of impact momentum applied to the sandwich beam, either dynamic shakedown or progressive failure could occur. The method of finite elements (FE) was subsequently employed to simulate the repeated shock tests, which was validated against experimental measurements, with good agreement achieved. To explore the physical mechanisms underlying the observed dynamic shakedown, the FE results of final plastic energy and effective plastic strain distribution in the sandwich beam were extracted and analyzed. When shakedown occurred, the measurable permanent deformation of the sandwich beam kept a relatively stable state, but its plastic energy remained positive, which is different from its monolithic beam/plate counterpart (i.e., plastic energy dropping to zero during dynamic shakedown). Such difference is mainly attributed to stress concentration occurring at the connecting joint points between the corrugated core and face sheets of sandwich and clamped endings, which produces plastic deformations that are difficult to reflect in overall large deformations of the beam. Consequently, the dynamic shakedown of metallic corrugated core sandwich beam captured in this study appears to be inexhaustive and hence may be termed as the “apparent pseudo-shakedown” (with danger lurking in the welding joints as well as clamped endings). Finally, based on a simplified bi-linear hardening constitutive law, the mechanisms underlying the dynamic shakedown of sandwich beam were further discussed from a purely base material point of view.

Mixed convection of a viscoplastic fluid with a variable yield stress in a lid-driven cavity

This study is a numerical investigation on heat and momentum transfer in viscoplastic fluids that exhibit a variable yield stress. Viscoplastic fluids are recognized for transitioning from solid to liquid under flow-induced shear-rate. However, these materials exhibit intricate rheological behaviors beyond this fundamental characteristic, often linked to thixotropy. Thixotropy delineates reversible, time-dependent alterations in a fluid's viscosity at a specific shear-rate. The temporal changes in viscosity stem from variations in the fluid's microstructure, responsive to the induced shear-rate. When subjected to shear, the fluid's microstructure breaks down into smaller units, countered by Brownian motion, resulting in a rearrangement of the microstructure due to attractive forces between microconstituents. These microstructural variations are thus reversible. Notably, these changes affect not only viscosity but also the yield stress of the fluid, categorizing it as a non-ideal yield-stress fluid with yield-stress variations linked to microstructure, termed isotropic hardening. This study aims to explore how variations in yield-stress fluid microstructure impact heat and momentum transfer. As a starting point, this study considers the lid-driven cavity flow with differentially heated walls in the presence of an external magnetic field. Addressing the yield-stress fluid microstructure variations involves utilizing the Houska–Papanastasiou model, a regularized model capturing thixotropy and isotropic hardening. The resulting governing equations are made dimensionless and numerically solved through the finite-element method. The findings indicate that a more pronounced breakdown of the fluid's microstructure correlates with a higher Nusselt number at the hot wall. Additionally, variations in fluid microstructure influence both the size and location of unyielded zones.

Liquid inertia versus bubble cloud buoyancy in circular plunging jet experiments

When a liquid jet plunges into a pool, it can generate a bubble-laden jet flow underneath the surface. This common and simple phenomenon is investigated experimentally for circular jets to illustrate and quantify the role played by the net gas/liquid void fraction on the maximum bubble penetration depth. It is first shown that an increase in either the impact diameter or the jet fall height to diameter ratio at constant impact momentum leads to a reduction in the bubble cloud size. By measuring systematically the local void fraction using optical probes in the biphasic jet, it is then demonstrated that this effect is a direct consequence of the increase in air content within the cloud. A simple momentum balance model, including only inertia and the buoyancy force, is shown to predict the bubble cloud depth without any fitting parameters. Finally, a Froude number based on the bubble terminal velocity, the cloud depth and also the net void fraction is introduced to propose a simple criterion for the threshold between the inertia-dominated and buoyancy-dominated regimes.

Energy-absorption analyses of grooved Al-sheet stacks using modified split Hopkinson pressure bar

This study suggests that stacks of thin aluminum (Al) sheets with fine rectangular or triangular grooves are effective materials for energy absorption. The energy-absorbing performance of these materials was evaluated using a modified split Hopkinson pressure bar (SHPB). Two important energy-absorbing parameters, impact momentum (I) and maximum impact acceleration (amax), were measured from stress-time (σ-t) curves. These parameters were found to vary with groove shape, groove cavity fraction, and specimen thickness. Both I and amax showed a continuous decrease as the specimen thickness increased from 6 to 18 mm or as the groove cavity fraction increased from 29–30% to 38–39%. Analyzing the σ-t curve shapes revealed that the triangular grooved specimens exhibited broad-peak shaped curves, resulting in a greater reduction in I compared to the broadened plateau shape observed in the rectangular grooved specimens. Taking into account both I and amax, the overall energy-absorbing performance of the triangular grooved specimens was better than that of the rectangular grooved specimens. Notably, in the triangular grooved specimens with a high cavity fraction, the triangular embossing intruded into the groove cavities, resembling a 'zipper' mechanism, further enhancing the effectiveness of energy absorption. This study presents a promising approach for developing various grooved Al sheet stacks that exhibit reduced amax and I by strategically exploring suitable groove shapes, cavity fractions, and stack thicknesses, especially in dynamically compressed artillery environments.

Effect of iso-propanol additive on the impact dynamics of a Leidenfrost water droplet

This paper experimentally investigated the Leidenfrost dynamics of an impacting droplet of water and 2–8 vol% iso-propanol-water solutions in the film boiling regime up to 450 °C. A single millimetric droplet was released from a height of several centimeters to yield different impact velocities. The aim of the present study was to clarify the influence of iso-propanol additive on the dynamic Leidenfrost point temperature, maximum spreading factor and nondimensional residence time, which remains to be verified. Visualization study with high-speed imaging indicated that the droplet impact pattern depends on both surface temperature and iso-propanol additive. It was shown that the dynamic Leidenfrost point temperature increases with impact velocity, and was significantly elevated with a small amount of iso-propanol additive by approximately 40 °C at the same Weber number. Especially, the lowest value of the mixture droplet with the smallest impact momentum (430 °C at 0.44 m/s and We = 6.2–8.3) still surpassed the highest value of the water droplet with the largest impact momentum (410 °C at 1.33 m/s and We = 55.0) within the present test range. Meanwhile, the iso-propanol additive markedly enhanced the droplet spreading ability, which was speculated to be caused by lower surface energy. Results also demonstrated that the nondimensional residence time was highly dependent on the impact velocity, but was not significantly influenced by the iso-propanol additive. Empirical correlations of the maximum spreading factor and nondimensional residence time were proposed with satisfactory predictive accuracy and good universality.

Performance of CFST members internally strengthened with I-shaped CFRP under impact load

This study presents experimental and numerical research on the dynamic behaviors and resistances of square concrete-filled steel tubular members internally strengthened by I-shaped CFRP profile (SCFST-CFRP) with fixed boundary conditions subjected to lateral impact loads. Dynamic impact tests were conducted on five SCFST-CFRP specimens and a traditional concrete-filled steel tubular (SCFST) specimen, examining their dynamic responses with respect to deformation modes, impact force, displacement, and strain distribution at different levels of impact energies and impact momentums. The test results indicated that the deformation of the specimens was concentrated in the impact region, approximately 0.38 L above the base, due to the transient localized characteristics of the lateral impact load. In terms of displacement and impact force, the SCFST-CFRP member exhibited better impact resistance compared to the SCFST member. Finite element models of the SCFST-CFRP members were created using LS-DYNA and validated against the impact tests. These validated models were then utilized to explore the damage patterns, dynamic bending moment, and load transfer mechanism of the SCFST-CFRP member under lateral impact loading. Additionally, a parametric study was conducted to evaluate the effects of load and structural parameters, including CFRP profile, impact energy, and impact momentum, on the impact resistance and dynamic flexural capacity of the SCFST-CFRP member. The results demonstrate that the enhancement in impact resistance caused by the inner CFRP profile was more significant under higher impact energy. Based on the parametric analysis results, empirical equations were proposed to predict the dynamic flexural capacity of lateral impact-loaded SCFST-CFRP members.

Analyses of impact energy-absorbing performance of open- and closed-cell Al foams using modified split Hopkinson pressure bar

Al foams have been developed to meet increasing demands for effective reduction of impacts in military, automotive, civil-engineering, and aerospace applications. However, evaluating their energy-absorbing performance is very difficult, as most of the impacted energy is quickly dissipated as interior pores are rapidly closed. In this study, a split Hopkinson pressure bar (SHPB) was modified by utilizing the incident wave alone, using a deceleration-measuring module placed between the striker and incident bars to reliably evaluate the energy-absorbing performance of open- and closed-cell Al foams with different densities and porosities. The impact momentum (Ibar), which was regarded as the overall energy of the incident wave produced by the impact of the striker bar, and the maximum impact acceleration (amax), which indicated the largest change of impact velocity, were used as main evaluation parameters. The ratio of the reduced amount of amax in comparison with the amax of the none-specimen case, i.e., amax reduction ratio, was lowest in the open-cell low-density specimens and increased as the specimen thickness or density increased, i.e., in the order of the open-cell high-density, closed-cell low-density, and closed-cell high-density specimens. Thus, the thicker higher-density foam specimens worked more effectively for the better energy-absorbing performance. The present modified SHPB is outstanding for the energy-buffering concepts and mechanism-focused interpretations of the amax and Ibar data, since the quantitative energy-absorption analyses have hardly been conducted because of the energetic and mechanical complexities. It also provides an excellent idea to improve or develop various Al foams having high amax reduction ratios.

Multi-scale finite element analysis on the soft impact behavior of twill weave laminate

This work proposed an impact momentum flexibility (IMF) to precisely and concisely characterize the soft impact behavior of twill weave laminate based on multi-scale finite element analysis. A multi-scale finite element model taking into consideration the microstructure and properties of constituents is established for twill weave laminate. The equivalent properties of fiber tows and twill weave laminate are predicted by micro-scale model and meso-scale model, respectively. Then the soft impact behavior of the laminate is investigated by macro-scale model. The multi-scale model is validated on different scales by comparing the experimental and simulated results of quasi-static compression and soft impact. On this basis, the deformation and failure of the laminate under soft impact are analyzed. The soft impact response is categorized according to the damage morphology and impact velocity. Moreover, the IMF is proposed to depict the ability to resist soft impact deformation of the laminate, and can be further adopted to rank the soft impact property of different materials.

Dynamic Impact Properties of Carbon-Fiber-Reinforced Phenolic Composites Containing Microfillers.

The addition of nano- and microfillers to carbon-fiber-reinforced polymers (CFRPs) to improve their static mechanical properties is attracting growing research interest because their introduction does not increase the weight of parts made from CFRPs. However, the current understanding of the high strain rate deformation behaviour of CFRPs containing nanofillers/microfillers is limited. The present study investigated the dynamic impact properties of carbon-fiber-reinforced phenolic composites (CFRPCs) modified with microfillers. The CFRPCs were fabricated using 2D woven carbon fibers, two phenolic resole resins (HRJ-15881 and SP-6877), and two microfillers (colloidal silica and silicon carbide (SiC)). The amount of microfillers incorporated into the CFRPCs varied from 0.0 wt.% to 2.0 wt.%. A split-Hopkinson pressure bar (SHPB), operated at momentums of 15 kg m/s and 28 kg m/s, was used to determine the impact properties of the composites. The evolution of damage in the impacted specimens was studied using optical stereomicroscope and scanning electron microscope. It was found that, at an impact momentum of 15 kg m/s, the impact properties of HRJ-15881-based CFRPCs increased with SiC addition up to 1.5 wt.%, while those of SP-6877-based composites increased only up to 0.5 wt.%. At 28 kg m/s, the impact properties of the composites increased up to 0.5 wt.% SiC addition for both SP-6877 and HRJ-15881 based composites. However, the addition of colloidal silica did not improve the dynamic impact properties of composites based on both phenolic resins at both impact momentums. The improvement in the impact properties of composites made with SiC microfiller can be attributed to improvement in crystallinity offered by the α-SiC type microfiller used in this study. No fracture was observed in specimens impacted at an impact momentum of 15 kg m/s. However, at 28 kg m/s, edge chip-off and cracks extending through the surface were observed at lower microfiller addition (≤1 wt.%), which became more pronounced at higher microfiller loading (≥1.5 wt.%).

Alcohol-induced elevation in the dynamic Leidenfrost point temperature for water droplet impact

In the present study, an experimental investigation is conducted to explore the alcohol additive effect on water droplet impact on a smooth red copper surface and to construct impact regime maps. The observations show that the alcohol additives can effectively facilitate droplet atomization and breakup, and increase the dynamic Leidenfrost point temperature. An interesting phenomenon is noted that the dynamic Leidenfrost point temperatures of alcohol-added water droplets at lower impact velocities are obviously elevated compared with pure water at higher impact velocities. In other words, the addition of a small amount of alcohol additive can effectively compensate for the decrease in the dynamic Leidenfrost point temperature caused by smaller impact momentum. A new triggering mechanism of the dynamic Leidenfrost effect is proposed with the surface roughness effect taken into consideration. An approximation model based on the pressure balance criterion is then developed to discuss the underlying physics of the alcohol-induced elevation in the dynamic Leidenfrost point temperature. Combining the scaling analysis of vapor layer thickness, a scaling relation of the dynamic Leidenfrost point temperature is derived, and the predictions agree well with present and previous data. It is concluded that the present findings provide an effective method to expedite the transformation from film boiling to transition boiling for heat transfer improvement with an increased dynamic Leidenfrost point temperature, simply by adopting alcohol additives to regulate the droplet thermophysical properties.

Behaviors of steel–concrete-steel sandwich panel with aluminum foam-filled energy absorbing supports under low-velocity impact

A new steel–concrete-steel sandwich panel with energy absorbing supports (SSP-EAS) was firstly proposed, and its dynamic responses under impact loading were investigated through conducting impact tests and numerical simulations. Three types of impact response modes of SSP-EASs were categorized according to the different crushing magnitudes of energy absorbing supports (EASs). In addition, the steel–concrete-steel sandwich panel (SSP) exhibited the combination of the global flexural deformation, local indentation and local bulging during the impact. The EAS could effectively absorb impact energy through the plastic bending of steel plates and the crushing of aluminum foam blocks, except for the response mode I with insignificant crushing of EASs. Moreover, the effects of crushing force of EASs and initial impact momentum on the dynamic responses of SSP-EAS were numerically analyzed. The impact resistance of the SSP-EAS could be improved through properly designing the EAS and assuring its maximum displacement slightly smaller than the compaction displacement during impact. In addition, the crushing force of EAS should not exceed the ultimate resistance of the SSP to assure the EAS being triggered to crush. Moreover, the SSP-EAS exhibited enhanced impact performances under higher initial impact momentum.

Momentum transfer on impact damping by liquid crystalline elastomers

The effect of elastomeric damping pads, softening the collision of hard objects, is investigated comparing the reference silicone elastomer and the polydomain nematic liquid crystalline elastomer, which has a far superior internal dissipation mechanism. We specifically focus not just on the energy dissipation, but also on the momentum conservation and transfer during the collision, because the latter determines the force exerted on the target and/or the impactor—and it is the force that does the damage during the short time of an impact, while the energy might be dissipated on a much longer time scale. To better assess the momentum transfer, we compare the collision with a very heavy object and the collision with a comparable mass, when some of the impact momentum is retained in the target receding away from the collision. We also propose a method to estimate the optimal thickness of an elastomer damping pad for minimising the energy in impactor rebound. It has been found that thicker pads introduce a large elastic rebound and the optimal thickness is therefore the thinnest possible pad that does not suffer from mechanical failure. We find good agreement between our estimate of the minimal thickness of the elastomer before the puncture through occurs and the experimental observations.

Finite element analysis of steel-concrete-steel sandwich beams with novel interlocked angle connectors subjected to impact loading

The steel–concrete–steel (SCS) sandwich structures are increasingly adopted to improve the impact resistance of the buildings or infrastructures owing to their good structural performances. Recently, the SCS sandwich beams with interlocked angle connectors (SCSSB-IACs) have been developed, and their performances under impact loading have been experimentally studied. This paper presents detailed numerical analyses of the SCSSB-IACs subjected to impact loading and reveals its response mechanisms. The Finite Element (FE) models of the SCSSB-IACs were established using LS-DYNA, and the FE predictions showed a good agreement with the impact test data. The impact process as well as distribution and evolution of bending moment and shear force, stress and strain were discussed. Furthermore, the internal energy histories of the SCSSB-IACs were revealed by FE analyses. Numerical results indicated that the majority of impact energy was absorbed by the concrete and bottom faceplate, and the deformation of the SCSSB-IACs was dominated by a flexural manner. Finally, parametric studies were performed to reveal the effects of impact momentum, energy and location on the response of the SCSSB-IACs.