Low Impact Velocities Research Articles

Numerical study on the dynamic behavior of circular honeycomb structure with concentrated filling inclusions defects

Circular honeycombs are extensively used to absorb impact energy considering its good energy absorption properties. Rather than focusing on the existing perfect honeycomb, effects of concentrated filling inclusions on the in-plane dynamic behavior of circular honeycomb structure are studied in detail by using finite element simulations. The crushing model of honeycomb under in-plane impact is established which then is used to investigate the influences of packing ratio, defect distribution area and impact velocity on the deformation mode and the plateau stress. Investigation results show that the deformation model of imperfect honeycomb can still be classified into three types: Quasi-static mode, transition mode and dynamic mode; However, the value of packing ratio and defect location may seriously influence localized deformed band of transition mode; The plateau stress of the honeycomb relies on the defect location except for the packing ratio; The filling inclusions defect concentrated in sub-region 2 is more beneficial to improve the energy-absorption capacity; Especially under middle or low impact velocities, it displays higher sensitivity; The crushing plateau stress of honeycomb with α = 0.36 and defect concentrated in sub-region 2 is even improved about 56.9 % under impact velocity 100 m/s. These results can provide valuable suggestions in the study and design of other honeycomb structures.

Deposition behavior, microstructure and mechanical properties of an in-situ micro-forging assisted cold spray enabled additively manufactured Inconel 718 alloy

Cold spray is capable to additively manufacture oxide-free metallic parts in open air due to its low processing temperature. However, for metals with relatively high hardness such as Inconel 718 superalloy (IN718), it is still a big challenge to get dense deposits by using low-priced nitrogen gas. In this work, In-situ micro-forging (MF) was introduced to cold spray by mechanically mixing big-sized 410 stainless steel (410SS) particles into the IN718 powder so that the deposited IN718 layer can be hammered and plastically deformed by the MF particles during spraying and results in fully dense deposits. As 50 vol% MF particles were mixed into the IN718 powder, the porosity was decreased from 5.6% to 0.26%. Due to the low impact velocity, 410SS particles were not embedded into the IN718 deposits and the possible contamination was avoided. An oxide scale removal induced increment in deposition efficiency was detected for the first time. Due to the lower porosity and enhanced inter-particle bonding, a great improvement in ultimate strength from 96 to 464 MPa was achieved by the in-situ MF effect. After heat treated at 1200 °C for 6 h, the sample fractured at a high strength of 1089 MPa and revealed a ductile fracture manner.

Atomic-level deformation of CuxZr100-x metallic glasses under shock loading

Plastic deformation mechanisms in CuxZr100-x bulk metallic glasses (MGs) subjected to shock are investigated using molecular dynamics simulations. MGs with Cu compositions between 30 and 70 at. % subjected to shock waves generated via piston velocities that range from 0.125 to 2.0 km/s are considered. In agreement with prior studies, plastic deformation is initiated via formation of localized regions of high von Mises shear strain, known as shear transformation zones (STZs). At low impact velocities, but above the Hugoniot elastic limit, STZ nucleation is dispersed behind the shock front. As impact velocity is increased, STZ nucleation becomes more homogeneous, eventually leading to shock-induced melting, which is identified in this work via high atomic diffusivity. The shear stress necessary to initiate plastic deformation within the shock front is independent of composition at shock intensities near the elastic limit but increases with increasing Cu content at high shock intensities. By contrast, both the flow stress in the plastically deformed MG and the critical shock pressure associated with melting behind the shock front are found to increase with increasing Cu content over the entire range of impact velocities. The evolution of the short-range order in the MG samples during shock wave propagation is analyzed using a polydisperse Voronoi tessellation method. Cu-centered polyhedra with full icosahedral symmetry are found to be most resistant to change under shock loading independent of the MG composition. A saturation is observed in the involvement of select Cu-centered polyhedra in the plastic deformation processes at a piston velocity around 0.75 km/s.

Specific Energy Absorption Evaluation on GFRP Laminate Plate by Optical, Thermographic and Tomographic Analysis

Over the years, a large number of studies have been carried out on glass fiber composite materials in order to analyze their impact behavior. This is very important for the automotive applications, for example for bumper or crash-box. This paper presents the analysis performed on GFRP (Glass Fiber Reinforced Polymer) laminated plate specimens (epoxy matrix reinforced with E-glass, twill woven, 200 g/m2) subjected to drop impact tests. The tests have been conducted at two different impact speeds in Low Impact Velocity LVI maintaining the same impact energy to estimate the strain-rate effect. All the tests were performed referring to ASTM 7136 at standard ambient temperature. Then the SEA (Specific Energy Absorption) of the specimens was calculated, using three different non-destructive measurement techniques (Optical Analysis, IRThermography and X-ray Computed) to analyze the surfaces of rupture and the specimen interlaminar damages. The chosen geometry of the specimens allowed to reduce the costs and to use a simplest test bench giving additional information to the material behavior.

Modeling ignition prediction of HMX-based polymer bonded explosives under low velocity impact

Visco-SCRAM and the hot spot models are applied to predict the ignition of HMX-based polymer bonded explosives (PBXs) under low velocity impact. The model is implemented in the commercial software DYNA3D. The confined Steven test simulation is done to verify the model and to understand the mechanical and thermal response of PBXs. The ignition time as a function of the impact velocity is analyzed. Higher impact velocity results in shorter ignition time. Moreover, the empirical formula is demonstrated to predict the ignition time. Five different dimensions of the specimen ϕ70 mm × 13 mm, ϕ98 mm × 13 mm, ϕ140 mm × 13 mm, ϕ98 mm × 26 mm and ϕ98 mm × 39 mm are applied to obtain the specimen size effect on the ignition. The temperature rise distribution inside the specimen based on the hot spot model and the impact velocity threshold value are successfully predicted. The impact velocity threshold value is in agreement with the experimental data. The velocities 44 m/s, 45 m/s, 45 m/s, 66 m/s and 75 m/s are for the five specimens mentioned above respectively. In addition, the ignition due to different shape projectiles is analyzed. The simulation result shows the impact velocity threshold value is 45 m/s, 97 m/s and 21 m/s for the oval projectile, the flat projectile and the pin projectile respectively. The differences of the temperature rise among the three projectiles are analyzed in detail. In these cases above, the ignition prediction matches experimental results well and the details of mechanical and thermal response of PBXs, such as the deformation of specimens and the temperature rise histories at different positions, are further discussed. Frictional work is considered as the main ignition mechanism of HMX-based PBXs under low impact velocity.

The nature of the L chondrite parent body's disruption as deduced from high‐pressure phases in the Sixiangkou L6 chondrite

AbstractThe disruption of the L chondrite parent body (LCPB) at ~470 Ma is currently the best‐documented catastrophic celestial impact event, based on the large number of L chondritic materials associated with this event. Uranium‐lead (U‐Pb) dating of apatite and its high‐pressure decomposition product, tuite, in the Sixiangkou L6 chondrite provides a temporal link to this event. The U‐Pb system of phosphates adjacent to shock melt veins was altered to varying degrees and the discordance of the U‐Pb system correlates closely with the extent of apatite decomposition. This suggests that the U‐Pb system of apatite could be substantially disturbed by high‐temperature pulse during shock compression from natural impacts, at least on the scale of mineral grains. Although many L chondrites can be temporally related to the catastrophic LCPB impact event, the shock conditions experienced by each individual meteorite vary. This could be due to the different geologic settings of these meteorites on their parent body. The shock pressure and duration derived from most meteorites may only reflect local shock features rather than the impact conditions, although they could provide lower limits to the impact conditions. The Sixiangkou shock duration (~4 s), estimated from high‐pressure transformation kinetics, provides a lower limit to the high‐pressure pulse of the LCPB disruption impact. Combined with available literature data of L chondrites associated with this impact event, our results suggest that the LCPB suffered a catastrophic collision with a large projectile (with a diameter of at least 18–22 km) at a low impact velocity (5–6 km s−1). This is consistent with astronomical estimates based on the dynamical evolution of L chondritic asteroids.

Air Bubble Entrainment, Breakup, and Interplay in Vertical Plunging Jets

The entrainment, breakup, and interplay of air bubbles were observed in a vertical, two-dimensional supported jet at low impact velocities. Ultra-high-speed movies were analyzed both qualitatively and quantitatively. The onset velocity of bubble entrainment was between 0.9 and 1.1 m/s. Most bubbles were entrained as detached bubbles from elongated air cavities at the impingement point. Explosion, stretching, and dejection mechanisms were observed for individual bubble breakup, and the bubble interaction behaviors encompassed bubble rebound, “kiss-and-go,” coalescence and breakup induced by approaching bubble(s). The effects of jet impact velocity on the bubble behaviors were investigated for impact velocities from 1.0 to 1.36 m/s, in the presence of a shear flow environment.

Influence of Molecular Effects on the Emission of Sound in a Low-Velocity Impact of a Drop on Water Surface

The dynamics of an impact pressure pulse and the evolution of the coalescence bridge between a drop and the surface of water are investigated in experiments on the impact of the drop on water surface in the range of low impact velocities U. Experimental sequences of radii r i of the bridge, which are approximated by a function of the form t1/2, are extrapolated to the instant of contact and are compared with radii r k of the outer contour of the cross section formed by the bottom part of the drop with the surface. The impact pressure pulse exhibits the critical dependence on ratio $${\varepsilon _{\text{i}}}(U) = {\dot r_i}/{\dot r_k}$$ of the velocities of spreading. The value of e = 1 determines the velocity threshold below (above) which the pressure is hydrodynamic (hydroacoustic) by nature.

초소수성 실린더 표면에 충돌하는 액적의 움직임에 관한 수치해석 연구

Droplet impact is widely used in various engineering applications. Various conditions could affect the droplet evolution during impacting process. We have focused on geometrical effect from curved surface. Due to curvature of the wall, dynamics of the droplet would be quite different compared to the flat surface. We also paid great attentions to the contact line behavior. To augment the curvature effect, cylindrical surface with superhydrophobic characteristic was considered in this study. Very high advancing (165o) and receding angle (160o) associated with superhydrophobic surface makes distinctive difference in its behavior between axial and circumferential direction. We have numerical studied the impact process of the 3D droplet on cylindrical surface using Level Contour Reconstruction(LCRM) method. We found that impact behavior is pretty much same as flat surface in the axial direction. In circumferential direction, contact diameter has certain limit for higher impact velocity. Contact time of the droplet is also quite different compared to the flat surface in circumferential direction. At lower impact velocity, contact time of the droplet decrease rapidly with increased impact velocity but has negligible change at higher values.

How to quantitatively evaluate safety of driver behavior upon accident? A biomechanical methodology.

How to evaluate driver spontaneous reactions in various collision patterns in a quantitative way is one of the most important topics in vehicle safety. Firstly, this paper constructs representative numerical crash scenarios described by impact velocity, impact angle and contact position based on finite element (FE) computation platform. Secondly, a driver cabin model is extracted and described in the well validated multi-rigid body (MB) model to compute the value of weighted injury criterion to quantitatively assess drivers’ overall injury under certain circumstances. Furthermore, based on the coupling of FE and MB, parametric studies on various crash scenarios are conducted. It is revealed that the WIC (Weighted Injury Criteria) value variation law under high impact velocities is quite distinct comparing with the one in low impact velocities. In addition, the coupling effect can be elucidated by the fact that the difference of WIC value among three impact velocities under smaller impact angles tends to be distinctly higher than that under larger impact angles. Meanwhile, high impact velocity also increases the sensitivity of WIC under different collision positions and impact angles. Results may provide a new methodology to quantitatively evaluate driving behaviors and serve as a significant guiding step towards collision avoidance for autonomous driving vehicles.

On the influence of the property gradient on the impact behavior of graded multilayer sandwich with corrugated cores

Graded lightweight structure is a new trend to improve energy absorption capacity of such structural materials. This study aims at the influence of property gradients on the overall behavior of graded multilayer sandwiches with corrugated cores under impact loading. The design of the property gradient of graded multilayer sandwiches as well as the manufacturing of different corrugated cores is presented at first. The graded multilayer sandwich is tested under various experimental configurations at rather low (9 m/s) and high (38 m/s) impact velocities. It turned out that no influence of gradient is found for low impact velocity because a quasi-static equilibrium state is reached. However, at high impact velocity, the test revealed a significant difference between different property gradient profiles. Numerical models are also built to simulate those tests. It allows for the further numerical analysis on a larger range of gradient profiles and higher impact velocities. A general trend for the design of the graded multilayer sandwiches with corrugated cores to improve energy absorption efficiency is proposed, which consists of placing the weakest layer near the protected structure and the hardest layer near the impacted end of the graded sandwich.

Micro-XCT analysis of damage mechanisms in 3D circular braided composite tubes under transverse impact

This paper reports on the damage mechanisms of 3D circular braided composite tubes under transverse impact, applied by a Split Hopkinson Pressure Bar (SHPB) system. X-ray computed tomography (XCT) was employed to obtain 3D images of the 3D braided composite tubes after the impact test. Effects of impact velocity, braiding angle and the number of braiding layers on the damage mechanisms were researched, using the XCT data. For low impact velocity, it was found that cracks nucleate in weak areas and propagate along the path of least resistance between yarns, which give rise to matrix cracking and debonding between yarns. For high impact velocities, cracks develop along a relatively straight path with fiber breakage under high shear and tensile stresses. The orientation of yarns in the tubes showed a great effect on crack propagation.

A novel tailor-made technique for enhancing the crashworthiness by multi-stage tubular square tubes

State-of-the-art studies of the energy absorption systems reveal various techniques for enhancing the crashworthiness of a structure, and each technique has its benefits and drawbacks. For crashworthy designers, it is a challenge to design an efficient energy absorbing system which fulfills its requirements in terms of crashworthiness. Therefore, this paper proposes tailor-made technique for crashworthiness design by performing a combination of two or more energy absorption techniques to fulfill the crashworthy designer requirements. The tailor-made technique is adapted to have the ability of implementation by different structural configurations; thus, tubular square tubes configuration has been utilized as an example. Tailor-made technique implementation is validated experimentally and numerically under quasi-static and axial impact loading, afterwards, its merits are discussed through conducting a comparative study. Results show that the novel tailor-made technique provides the designers the flexibility in designing of the protected structure. Furthermore, tailor-made technique improves the crashworthiness of the energy absorbing system in terms of crushing performance and energy absorption capacity. Moreover, it leads to energy absorbing systems applicable to a wide range of impact velocities through generating low reaction force under low impact velocity and logical crushing force under high impact velocity.

Three-dimensional woven carbon fibre polymer composite beams and plates under ballistic impact

Clamped rectangular orthogonal 3D woven carbon composite beams under ballistic impact at a velocity range 60ms-1⩽v0⩽190ms-1 were investigated in order to understand the damage mechanisms within the material and the role of through-the-thickness (TTT) reinforcement. Experimental tests revealed three distinct categories of beam response: (i) low velocity impacts (v0<110ms-1) which featured projectile rebound, with dominant matrix cracking and localised fibre fracture, (ii) medium velocity impact (110ms-1⩽v0<148ms-1) which exhibited a stretch-deformation dominated failure mechanism, and (iii) higher velocity impacts (v0⩾148ms-1) which resulted in projectile penetration, combined with longitudinal fibre fracture at the centre of the sample. Finite element (FE) simulations were conducted to understand the experimental outcomes, which showed sufficient fidelity and captured the three distinct beam response regimes. The presence of the TTT-reinforcement can suppress the inter-laminar matrix crack propagation and increase the material ballistic impact resistance for low velocity impact and high velocity impact. However, for medium velocity impact, the in-plane fibre fracture surface was found to be at the locations of TTT-reinforcement. This may suggest that the TTT-reinforcement creates weak points for the stretch-deformation dominated failure mechanism. The verified FE simulations were conducted to predict the multi-hit ballistic impact limit surfaces for the clamped circular 3D woven composite plates, and for the equivalent laminate composite without the presence of the TTT reinforcement. The numerical results suggested the presence of TTT reinforcement could improve the multi-hit ballistic resistance of the composite plates for multi-hit scenarios where the initial impact is 50%–95% of the ballistic limit of the plates.

Hydroelastic analysis of a rectangular plate subjected to slamming loads

A hydroelastic analysis of a rectangular plate subjected to slamming loads is presented. An analytical model based on Wagner theory is used for calculations of transient slamming load on the ship plate. A thin isotropic plate theory is considered for determining the vibration of a rectangular plate excited by an external slamming force. The forced vibration of the plate is calculated by the modal expansion method. Analytical results of the transient response of a rectangular plate induced by slamming loads are compared with numerical calculations from finite element method. The theoretical slamming pressure based on Wagner model is applied on the finite element model of a plate. Good agreement is obtained between the analytical and numerical results for the structural deflection of a rectangular plate due to slamming pressure. The effects of plate dimension and wave profile on the structural vibration are discussed as well. The results show that a low impact velocity and a small wetted radial length of wave yield negligible effects of hydroelasticity.

Auto Rickshaw Impacts with Pedestrians: A Computational Analysis of Post-Collision Kinematics and Injury Mechanics

Motor vehicle-related pedestrian road traffic collisions are a major road safety challenge and a leading public health issue, since they are a primary cause of death and serious injury worldwide. In many developing countries, the auto-rickshaw–a three-wheeled vehicle with a canvas roof and side curtains – poses a significant risk to pedestrian safety due to the poor impact energy absorption of its structures and materials. This study presents a parametric and comparative analysis of auto-rickshaw-related pedestrian impacts and pedestrian-ground impacts by computational simulation, using a Finite Element model of an auto-rickshaw and LS-DYNA, 50th percentile adult male and six-year-old child Hybrid III Anthropometric Test Devices (dummies). The comparative study explored the kinematic responses and injury metrics associated with both adults and children impacted by an auto-rickshaw, as well as the most commonly impacted areas of an auto-rickshaw and the injury metrics for the adult pedestrian produced by primary and secondary impacts. The output data of the impact simulation was correlated against reported injury metrics, Head Injury Criterion, Neck Injury Criterion, Combined Thoracic Index, Injury Abbreviated Injury Scale and reported risk level. The results suggest that adult pedestrians are subjected to a relatively high risk of head, neck and chest injuries during primary impacts at 10, 20 and 35km/h, respectively, and some of the impact simulations suggest a risk of fatality. The 6YO-child pedestrians are at risk of serious head and neck injury at 10 and 15km/h, respectively. During secondary impacts, defined as impacts with a floor surface, head and neck injuries produced from ground contact are significant, including fatal injury, at 10km/h and greater, while insignificant chest injuries were observed at 40km/h. Vehicle impact response was investigated and Aluminium-6016-T4 and magnesium-AZ31B windscreen frame materials and a polycarbonate windscreen were found to produce the lowest injury risk of all the materials investigated whilst offering the greatest safety at the lowest cost. The present study provides valuable evidence for informing a series of recommendations and guidelines to make the auto-rickshaw safer during impacts with pedestrians. Overall, it has found that impact velocity, vehicle contact region, impact position and pedestrian size significantly influence the post-kinematic response of a pedestrian impacted by an auto-rickshaw and injury risk during primary and secondary impacts. Moreover, child pedestrians are subject to a relatively higher risk, compared to adults, during primary impacts. Secondary impacts were associated with a greater risk of head and neck injuries compared to primary impacts, even at low-impact velocities. Secondary impacts, however, produced much lower chest injury risk compared to primary impacts. Thus, even at relatively low impact velocities, the auto-rickshaw cannot be considered a ‘pedestrian friendly’ vehicle for use in urban areas. iii A b s t r a c t Future suggestions to reduce the injury risk level and increase the safety of the auto-rickshaw should be the implementation of strict safety regulations and, or, consideration of engineering solutions, such as retrofitting injury mitigation technologies to those auto-rickshaw contact regions which pose the greatest risk of producing pedestrian injury. In addition, modification of the frontal end geometry of the vehicle is recommended to ensure that injury risk is minimised during primary and secondary impacts.

Experimental Investigations on the Coefficient of Restitution for Sphere–Thin Plate Elastoplastic Impact

In multiparticle simulations of industrial granular systems such as hoppers, tumblers, and mixers, the particle energy dissipation is governed by an important input parameter called the coefficient of restitution (COR). Oftentimes, the wall thickness in these systems is on the order of a particles diameter or less. However, the COR value implemented in event-driven simulations is either constant or a monotonically decreasing function of the impact velocity. The present work experimentally investigates the effect of wall thickness on the COR through sphere–thin plate elastoplastic impacts and elucidates the underlying impact phenomena. Experiments were performed on 0.635 cm and 0.476 cm diameter (d) spheres of various materials impacting aluminum 6061 plates of different thicknesses (t) with the low impact velocities up to 3.1 m/s. Besides COR, indentation measurements and numerical simulations are performed to gain a detailed understanding of the contact process and energy dissipation mechanism. As the “t/d” ratio decreases, a considerable amount of energy is dissipated into flexural vibrations leading to a significantly lower COR value. Based on the results, it can be concluded that using a constant COR input value in particle simulations may not always be an appropriate choice, especially, in the case of thin plates. However, these new COR results validate that when the wall thickness is more than twice the sphere diameter (i.e., t/d &gt; 2), a constant COR value obtained for an impact with semi-infinite plate can be reasonably used.

Experimental characterization of particle-wall interaction relevant to entrained-flow gasification of biomass

The present study addresses particle–wall interaction phenomena relevant to entrained-flow gasifiers. Two types of biomass have been investigated, i.e. wood chips and corn stover. The dynamics of char and ash particles as they are impacted onto a flat surface in cold and hot conditions has been characterized by means of high speed imaging and tracking. Particle–wall collisions were described in terms of normal, tangential and global coefficients of restitution as well as deposition efficiency. The influence of carbon conversion and impact velocity on the dynamical pattern of rebound and deposition has been investigated. The results indicate that, even at ambient conditions, some plastic deformation occurs during the impact. Experiments performed under hot conditions demonstrate that a drop of the coefficients of restitution takes place at high temperature, especially at low impact velocity, for both the tested biomasses. This feature can be explained by considering the change of mechanical properties of particles as the temperature increases, and the effect of the adhesion energy during the impact. Results highlighted that ash of wood chips is not prone to form a slag layer, while ash of corn stover extensively contributes to formation of ash deposits and melts under hot conditions. Results from experiments with corn stover indicate that char particles deposit onto the molten ash surface. The dissipation of momentum associated with particle impact promotes the establishment of a dense-dispersed phase in the near-wall zone of entrained-flow slagging gasifiers.

Rebound mechanics of micrometre-scale, spherical particles in high-velocity impacts.

The impact mechanics of micrometre-scale metal particles with flat metal surfaces is investigated for high-velocity impacts ranging from 50 m s-1 to more than 1 km s-1, where impact causes predominantly plastic deformation. A material model that includes high strain rate and temperature effects on the yield stress, heat generation due to plasticity, material damage due to excessive plastic strain and heat transfer is used in the numerical analysis. The coefficient of restitution e is predicted by the classical work using elastic-plastic deformation analysis with quasi-static impact mechanics to be proportional to [Formula: see text] and [Formula: see text] for the low and moderate impact velocities that span the ranges of 0-10 and 10-100 m s-1, respectively. In the elastic-plastic and fully plastic deformation regimes the particle rebound is attributed to the elastic spring-back that initiates at the particle-substrate interface. At higher impact velocities (0.1-1 km s-1) e is shown to be proportional to approximately [Formula: see text]. In this deeply plastic deformation regime various deformation modes that depend on plastic flow of the material including the time lag between the rebound instances of the top and bottom points of particle and the lateral spreading of the particle are identified. In this deformation regime, the elastic spring-back initiates subsurface, in the substrate.

Submillimeter-Sized Bubble Entrapment and a High-Speed Jet Emission during Droplet Impact on Solid Surfaces.

When a droplet impacts a solid surface, the entrapment of a submillimeter-sized bubble and the emission of a high speed jet can be observed at low impact velocities. In this work, we show that bubble entrapment occurs only on sufficiently hydrophobic surfaces within a narrow range of impact velocities. The bubble is entrapped on hydrophobic surfaces, whereas it is trapped into the top of the droplet on superhydrophobic surfaces. The collapse of the air cavity formed during droplet impact, which is dominated by inertia and influenced by surface wettability, is the cause of the bubble entrapment. The velocity of liquid jets emitted after cavity collapse for drop impact with and without bubble entrapment scales with their sizes according to different power laws, which is explained by simple scaling analyses.