Compaction Wave Research Articles

ABAQUS-based research on the parameters of highway subgrade vibratory compaction and vibration wave propagation laws

Vibratory rollers are generally used in the process of highway subgrade compaction. In the paper, the vibratory roller—subgrade finite element model was established to simulate the field construction by using ABAQUS. We used Hilbert–Huang Transform to analyze the compaction in the field test from the time–frequency domain. By changing the parameters of the vibratory roller and the filler, the comprehensive influence of the parameters such as the elastic modulus of the filler, rolling speed, excitation force, vibration frequency and thickness of the filler on the compaction quality of the subgrade was investigated. We studied the propagation pattern of vibration waves in three-dimensional space. The study shows that the signals of different frequency bands in the Hilbert spectrum represent the compaction degrees of fillers in different zones. The peak acceleration generally decreases with the increase of horizontal and vertical distance, but there is an increase at the boundary of the vibration field. There is an optimal combination of the excitation force, vibration frequency and thickness of the filler. The vibration waves propagate in the form of an ellipsoid in three dimensions, and the amplitude decreases with distance.

Mesoscale simulation of granular materials under weak shock compaction–pore size distribution effects

This research established a systematic method to generate various pore-size distributions (PSDs) and studied the effect of PSDs on the shock compaction response of granular materials using two-dimensional mesoscale simulations under identical porosity. Simulations utilized various PSDs for three particle shapes (circle, ellipse, and square). Contacting particle configurations using three PSDs, characterized by spatially uniform distributed pores to heterogeneous distributed pores, and non-contacting particle configurations under a single case of PSD were tested. The PSD of generated particle sets was characterized using coordination number, mean diameter, and bimodality coefficient as statistical metrics. Mesoscale simulations showed that regardless of the conditions of pore distributions, shock compaction of granular materials consistently demonstrates a precursor, shock compaction front, and end. However, the shock compaction velocity of contacting particles was dependent on the PSDs despite the constant initial porosity. The compaction velocity was faster in particle configurations with relatively uniform pore distributions than in heterogeneous pore distributions, which our study demonstrated can be attributed to particle rearrangement during compaction. Circular-shaped particles had high sensitivity in shock compaction response to the various PSDs. Furthermore, a contacting particle configuration tended to propagate the shock compaction wave relatively faster than particles that were in a non-contact configuration. This study established the relative importance of considering PSD as a metric over the coordination number in studies of the shock compaction response of granular materials. Further, insights are provided on the evolving shock substructure to characterize the shock compaction response of granular materials.

Dynamic characteristics of density-graded cellular materials for impact mitigation

Due to their exceptional impact resistance capabilities, density-graded cellular materials have immense potential in applications where crashworthiness requirements are of prime importance. Under impact loading, the deformation in these materials is characterized by compaction front propagation. Previous studies have utilized numerical techniques to solve the equations governing compaction shock propagation for cellular materials with density gradation. In this study, analytical solutions are formulated using compaction front position as the independent variable. The expressions for the velocity of the impinging rigid mass, energy absorption, incident stress, and transmitted stress are determined. The analytical solutions are shown to be in excellent agreement with the cell-based finite element solutions. The effect of density gradient on energy absorption and stresses is studied. The impact resistance factor is employed to assess different density-graded cellular materials for their effectiveness in impact mitigation. This study demonstrates that density-graded cellular materials can offer superior impact protection to objects at both the incident and transmitted ends. Lower density toward the incident end enhances impact resistance to objects located there, and likewise, lower density at the transmitted end offers more impact resistance to objects at that end.

Numerical simulation of shock wave propagation over a dense particle layer using the Baer–Nunziato model

The present study examines the possibility of numerical simulation of a strong shock wave propagating over the surface of a dense layer of particles poured onto an impermeable wall using the Baer–Nunziato two-phase flow model. The setting of the problem follows the full-scale experiment. The mathematical model is based on a two-dimensional system of Baer–Nunziato equations and takes into account intergranular stresses arising in the solid phase of particles. The computational algorithm is based on the Harten–Lax–van Leer–Contact method with a pressure relaxation procedure. The developed algorithm proved to be workable for two-phase problems with explicit interfacial boundaries and strong shock waves. These issues are typical of problems arising from the interaction of a shock wave with a bed or a layer of particles. A comparison with the simulations and full-scale experiments of other authors is carried out. A reasonable agreement with the experiment is obtained for the angles of the transmitted compaction wave and granular contact, including their dependency on the intensity of the propagating shock wave. The granular contact angle increases with the incident shock wave Mach number, while the transmitted compaction wave angle decreases. An explanation is given of the phenomenon of the decrease in thickness of the compacted region in the layer with the increase in intensity of the propagating shock wave. The main reason is that the maximal value of the particle volume fraction in the plug of compacted particles in the layer rises with the increase in shock wave intensity.

Multi-objective optimization of additive manufactured functionally graded lattice structures under impact

A multi-objective optimization procedure to effectively design gradient lattice structures under dynamic loading is presented. It aims both at maximized energy absorption characteristics and a lightweight design. The reduction of the peak crushing force and a maximized specific energy absorption are simultaneously optimized. Design variables like the relative density and density gradient of the lattice structure are investigated, considering different topologies of the lattice unit cells. To model and simulate the lattice structures efficiently, a simplified beam-based simulation finite element model is introduced and validated against experimental tests. They cover the range from dynamic equilibrium up to compaction wave propagation as impact deformation phenomena. Based on large numbers of simulations, an artificial neural network is used to predict the energy absorbing characteristics for the given design parameters and to find the optimal configuration of the lattice structures. The neural network is trained using a multi response adaptive sampling algorithm allowing parallel simulation with automatically generated finite element models during each iteration step. Subsequently, a multi-objective genetic algorithm is used to find optimal combinations of the design parameters of lattice structures under different impact velocities and cell topologies.

Investigation of shock transmission and amplification/mitigation in aluminium foams

The present work investigates factors which control the stress generated at the back face of end wall-mounted shock-loaded aluminum foam. When the applied shock pressure is higher than the plastic strength of the foam, the foam deforms plastically generating a compaction wave traveling below shock velocity. Wave reflection and momentum transfer at the end wall result in shock amplification. With increasing foam length, the stress at the end wall decreases, finally leading to shock mitigation. A relation between compaction wave characteristics, foam densification, and stress generated at the end wall is established.

Deployable linear and spiral array structures based on a Kresling-inspired mechanism with integrated scissor arms

Recent developments have shown that spatial structures devised from origami or low-dimensional rigid linkage mechanisms can be used to construct deployable arrays for antennas or satellites. Yet, some of these structures are limited to deployment in fixed planes or directions, or do not define straightforward processes for deployment. To surmount these limitations, this research introduces a reconfigurable single-degree-of-freedom spatial structure devised from a Kresling-inspired mechanism with integrated scissor arms. Analytical models are constructed to demonstrate compaction, deployment, and acoustic wave guiding capabilities of the proposed, modular structure. The influences of the geometric parameters on compaction, deployment, and scissor arm orientation are also explored, and reveal modular scissor arm behavior and large deployment-to-compaction area ratios. The acoustic wave guiding capabilities of the Kresling-inspired scissor structure are exemplified via a structure using spiral scissor arms, thereby proposing a novel concept for the construction of deployable wave guiding arrays. Experimental studies with model arrays complement the analytical findings of both the geometric reconfigurations and wave guiding functionality. Finally, out-of-plane configurations are depicted to demonstrate the three-dimensional shape change capabilities of the Kresling-inspired scissor structure. The results in this study encourage broader exploration of the interfaces between origami inspired structures and rigid linkage mechanisms.

Structure of Shock Wave in Nanoscale Porous Nickel at Pressures up to 7 GPa

The structure of shock waves in pressed porous samples of nickel nanoparticles was investigated in a series of uniaxial planar plate impact experiments in the pressure range of 1.6-7.1 GPa. The initial porosity of the samples was about 50%. Wave profiles were obtained using laser velocimetry techniques. The nanomaterial demonstrated a complex response to shock loading including the development of a two-wave structure associated with precursor and compaction waves. The effect on profiles and measurements of the observed precursor reverberations propagating between the front of a compaction wave and a monitored sample surface was described. The obtained wave profiles were used to estimate the thicknesses of precursor and compaction wave fronts.

Shock compression behavior of stainless steel 316L octet-truss lattice structures

Lattice structures offer desirable mechanical properties for applications of energy absorption and impact mitigation but limited research has been carried out on their shock compression behavior. In this work, the shock compression behavior of stainless steel 316L (SS316L) octet-truss lattice structures was investigated through experimental techniques and numerical simulations. Plate impact experiments with high-speed imaging were conducted at impact velocities of 270–390 m/s on lattice specimens with 5 × 5 × 10 unit cell geometries additively manufactured (AM) using direct metal laser sintering. High-speed imaging together with digital image correlation was used to extract full-field measurements and define a two-wave structure consisting of an elastic wave and planar compaction (shock) wave which propagated along the impact direction. A linear shock velocity versus particle velocity relation was found to approximate the measurements with a unit slope and a linear fit constant equal to the crushing speed. The shock velocity versus particle velocity relation, full-field measurements, and elastic limit together with the Eulerian form of the Rankine–Hugoniot jump conditions were used to find relations for the stress and internal energy behind the shock. Stress behind the shock increased with relative density and particle velocity, and specific internal energy converged to a single curve similar to that of bulk AM SS316L. Explicit finite element analysis using the Johnson–Cook constitutive model demonstrated similar shock behavior observed in experiments and a linear shock velocity versus particle velocity relation and corresponding Hugoniot calculations were found to be in agreement with experimental results. Numerical simulations confirmed negligible effects of exterior versus interior measurements and further validated the application of one-dimensional shock theory.

One-step mechanical exfoliation and deposition of layered materials (graphite, MoS2, and BN) by vacuum-kinetic spray process

The exfoliation of micron-layered materials to few-layered nanosheets is still a great challenge due to their widespread use in various applications. Herein, we provide a new approach for one-step mechanical exfoliation and deposition of graphite, MoS2, and boron nitride at room temperature. The essence of this technique is that the shock compaction wave produced by the micron powder impact with the hard substrate at a high velocity induces fragmentation and exfoliation of layered materials. The lattice expansion due to removal of stacked layers and reduction of crystalline domain size in nanosized thin films is identified using x-ray diffraction and a high-resolution transmission electron microscope. Raman spectra of the nanostructured thin film indicate the improvement of disorder-related modes due to the kinetic-induced layer separation and powder fragmentation. X-ray photoelectron spectra analysis indicates the transformation of bond configuration from sp2 hybridization of bulk hexagonal structure to sp3 in the nanostructured thin film.

The effects of electrically exploding gold bridgewires into inert and explosive powder beds

The particle velocity created in beds of both low-density inert sugar and explosive PETN as a function of distance from an exploding bridgewire was measured using optical velocimetry and a silvered PMMA window. As expected, more violent bridge-bursts (from a greater-stored-energy capacitive discharge unit) resulted in greater particle velocities and a better supported compaction wave in sugar. In all cases, ramp waves, not shocks, were observed in the inert sugar. Large window velocities were observed for very powerful bursts (up to 270 m/s), but bursts required for stochastic detonator operation conditions resulted in sugar/PMMA window velocities of only 8–10 m/s 0.85 mm from the bridge location. In contrast, after a distance of only 0.65 mm, a building shock wave was observed in PETN under both threshold and reliable firing conditions. Subsequently a hot-spot-driven shock-to-detonation (SDT) process was observed prior to full detonation. The measured buildup process accounts for approx 66% of the so-called excess transit time (ETT) between the observed and theoretical total function time for the particular exploding-bridge-wire (EBW) detonator studied. The remainder must occur in the powerful output pellet region. In contrast to a common understanding, the ETT is found to be a weak function of the discharge energy. Thus, the operation of the detonator after a bridge-burst energy-to-powder reaction transition process is found to be hot-spot-driven SDT in both the low- and high-density pellets.

The Symmpact: A Direct-Impact Hopkinson Bar Setup Suitable for Investigating Dynamic Equilibrium in Low-Impedance Materials

BackgroundMeasuring the dynamic behavior of low-impedance materials such as foams is challenging. Their low acoustic impedance means that sensitive force measurement is required. The porous structure of foams also gives rise to dynamic compaction waves, which can result in unusual behavior, in particular if the foam material is so thick, that dynamic force equilibrium is not reached.ObjectiveThis work investigates comparatively large polyurethane foam specimens with densities in the range of 80 – 240 kg/m3 to deliberately achieve a state away from force equilibrium during high-rate compaction. The aim is to understand how an increase in strain rate leads to a reduction in strength for such materials.MethodsA specialized direct-impact Hopkinson bar is employed. It uses polycarbonate bars to achieve the required long pulse duration of 2.6 ms to compress the large specimens into the densification regime. In contrast to existing setups, both striker and output bar are instrumented with strain gauges to monitor force equilibrium. The absence of an input bar allows monitoring force equilibrium more accurately. Special attention is paid to the calibration of strain gauges, taking non-linear effects, wave dispersion and attenuation into account. Digital Image Correlation is employed to analyze elastic and plastic compaction waves by means of Lagrange diagrams.ResultsDepending on density, the specimens show saturation of dynamic strength increase at high rates of strain approx 500 /s, or even negative strain rate sensitivity in case of the lowest density. The occurrence of apparent negative strain rate sensitivity is accompanied by a localized structural collapse front, moving at a low velocity of approx 10 m/s through the material. This apparent strain rate sensitivity is a structural effect which is related to the thickness of the specimen.ConclusionsThe primary aim of material characterization using Hopkinson bars is to achieve a state of force equilibrium. For this reason, very thin specimens are usually employed. However, data gathered in this way is not representative for thick foam layers. Here, an increase of strain rate can lead to a decrease of strength if homogeneous deformation is replaced by a dynamic compaction wave. This behavior can occur at strain rates encountered under conditions such as automotive crash.

The effects of air gaps and inert layers on exploding bridgewire detonator function

ABSTRACT Exploding bridgewire (EBW) detonators work by electrically exploding a thin conducting wire as a result of a high voltage discharge from a capacitor. The process creates a strong UV (ultraviolet) emission and a shock compaction wave into the surrounding porous explosive bed. To investigate the significance of each, the research presented here utilized air gaps, an optical window, or inert porous powders between the bridge and explosive powder bed. The thickness of the air gap was varied to attenuate the air shock but allow the transmission of UV light. The optical properties of the inert porous bed were varied to allow a similar shock compaction to occur, but with different intensities of UV light. Finally, a thin optical window was used that blocked the shock, but allowed transmission of the UV light. It is clearly shown that both UV light and shock compaction are important to the functioning of EBW detonators at practical energy levels although each is individually capable of resulting in detonation if sufficient pure energy of either form is applied.

Influence of Multi-Layer Core on the Blast Response of Composite Sandwich Cylinders

The influence of a multi-layer core on the blast response of composite sandwich cylinders under internal explosive loading was investigated. Experiments were conducted first to obtain the fundamental deformation and failure patterns of composite cylinders with uniform, double-layer, and triple-layer profile cores. They were compared with the finite element model for prediction with good agreement. The mechanisms of energy absorption and deformation of a composite sandwich were explored by parametric analysis. Experimental results indicated that compaction wave in a double-layer core was initiated from the inner face sheet and then propagated to the outer face sheet when the gradient was positive; however, the core densification started at the inner surfaces of both layers and propagated to the outer face sheet together. The maximum radial deflection decreased with increasing face sheet thickness or decreasing blast loading. The percentage of energy absorbed by the core increased with decreasing face sheet thickness or increasing blast loading. This study revealed a possibility to reduce the maximum deflection (same structural mass and same energy absorption) for the sandwich cylinders by using a proper core distribution.

Low-Gas Detonation in Low-Density Mechanically Activated Powder Mixtures

We have analyzed experimental data on supersonic self-sustained propagation of an energy-release wave in low-density mechanically activated mixtures. Various mechanisms that can be responsible for this process have been investigated, and the mechanism for detonation-like propagation of reaction in powder mixtures has been proposed. It is shown that under certain conditions, this process possesses all features of detonation and must be treated as a variety of detonation. It is demonstrated that this type of detonation basically differs from classical “ideal” detonation: instead of a shock wave, a compaction wave propagates in a powder mixture, in which powder compaction and not compression of particle material occurs due to mutual displacement of particles. In this case, a chemical reaction is initiated due to mutual friction of oxidizer and fuel particles in the powder compaction wave.

Dynamic Compressive Behaviors of Two-Layer Graded Aluminum Foams under Blast Loading.

Experimental and numerical analyses were carried out to reveal the behaviors of two-layer graded aluminum foam materials for their dynamic compaction under blast loading. Blast experiments were conducted to investigate the deformation and densification wave formation of two-layer graded foams with positive and negative gradients. The shape of the stress waveform changed during the propagation process, and the time of edge rising was extended. Finite element models of two-layer graded aluminum foam were developed using the periodic Voronoi technique. Numerical analysis was performed to simulate deformation, energy absorption, and transmitted impulse of the two-layer graded aluminum foams by the software ABAQUS/Explicit. The deformation patterns were presented to provide insights into the influences of the foam gradient on compaction wave mechanisms. Results showed that the densification wave occurred at the blast end and then gradually propagated to the distal end for the positive gradient; however, compaction waves simultaneously formed in both layers and propagated to the distal end in the same direction for the negative gradient. The energy absorption and impulse transfer were examined to capture the effect of the blast pressure and the material gradient. The greater the foam gradient, the more energy dissipated and the more impulse transmitted. The absorbed energy and transferred impulse are conflicting objectives for the blast resistance capability of aluminum foam materials with different gradient distributions. The results could help in understanding the performance and mechanisms of two-layer graded aluminum foam materials under blast loading and provide a guideline for effective design of energy-absorbing materials and structures.

Compaction Wave Propagation in Layered Cellular Materials Under Air-Blast

The propagation of compaction waves in layered cellular material subjected to air-blast is analyzed to examine the mechanism of compaction wave and reveal the phenomena that develop at the interface between the cellular layers. Similar to the previous studies of cellular materials under dynamic loading, the topology of cellular materials is neglected and homogeneous properties are assumed. The rigid-perfectly plastic-locking (R-PP-L) material idealization and the simple shock theory are employed to analyze the compaction situations. Analytical solutions for compaction wave propagation of double-layer cellular materials with two gradient-arrangements under air-blast loading have been worked out. The densification wave occurs at the blast end and then gradually propagates to the distal end for layers’ densities increase in the propagation direction (positive gradient). While compaction waves simultaneously form in both layers and propagate to the distal end in the same direction for the negative gradient. The finite element (FE) models using the Voronoi technique are carried out with practical aluminum foam to verify the predictions of the theoretical analysis. The potential of layered cellular materials to design efficient structural components under air-blast load is discussed, which would outperform their corresponding single counterpart with equal mass.

Малогазовая детонация в низкоплотных механоактивированных порошковых смесях

Experimental data on supersonic self-sustaining propagation of the energy release wave in low-density mechanically activated powder mixtures are analyzed. Various mechanisms that may be responsible for this process are analyzed, and a mechanism for the detonation-like propagation of the reaction in powder mixtures is proposed. It is shown that under certain conditions this process has all the signs of detonation and should be recognized as one of the types of detonation. It is shown that this type of detonation is fundamentally different from the classical "ideal" detonation, for example, in gases: instead of a shock wave, a compaction wave propagates through the powder mixture, in which there is basically no compression of the particle material, but powder compaction occurs due to the mutual rearrangement of particles. In this case, the initiation of a chemical reaction occurs due to the mutual friction of the oxidizer and fuel particles in the powder compaction wave.

Assessment of three Techniques in Surpassing Ledges in Curved Canals

Abstract The ledge formation is an iatrogenic accident that occurs in a curved canal due to the incorect application of the preparation techniques. The aim of this study was to find out which of the three well-known obturation techniques can surpass better an artificial created ledge. Matherials and methods: artificial ledge were created using Gates-Glidden drills no.1 and 2 at 15 mm and no.3 at 13 mm of the working lenght of fortyfour resin blocks. Thirty blocks that didn’t allowed a #10 file to surpass the ledge were accepted for study, divided into three equal groups and filled by lateral condensation, thermal compaction and continuous wave System B technique. Results: System B resulted in the highest obturated area beyond the starting point of the ledge (p<0,05), while lateral condensation showed the worst results. There were no differences between thermal compaction and lateral condensation in surpassing the deviation. Conclusion: within the limitation of this study, System B seems to be the the most appropriate techique when trying to surpass a ledge.

Experimental characterization of compaction wave propagation in cellular polymers

A great-deal of literature is available in the study of compaction wave propagation in cellular materials. However, experimental investigations concerning the spatial variation of the deformation features of compaction waves are limited. In this study, the formation and propagation of compaction wave in a low density polymeric foam under intermediate velocity projectile impact loading is investigated experimentally. The results are discussed in terms of the compaction wave characteristic parameters such as compaction wave velocity, axial strain, particle velocity, etc. In addition, spatial distribution of inertia stress during compaction wave propagation, and the critical velocity required to form shock are estimated. The stress jump across the compaction wave is also calculated using an inertia stress analysis and compared with the shock theory. It was observed that an elastic precursor propagates, at a velocity of 740 m/s, along the material upon impact, and it decays as it propagates along the specimen. Whereas, the compaction wave formed following the elastic precursor was propagated at a constant velocity much slower than the elastic precursor. Inertia stress calculations shows that the fast moving elastic precursor is reflected from the distal end and reduces the inertia component of the total stress. On the other hand, the compaction thickness is seen to be approximately constant during the entire duration of the compaction wave propagation after the achievement of the quasi-steady condition.