Small Impact Velocities Research Articles

Atomistic simulations of shock waves in cubic silicon carbide

Molecular dynamics simulations were performed to investigate the mechanical properties of shock waves in cubic silicon carbide (3C–SiC, β–SiC). Shock wave was produced and distinctly demonstrated when a moving sample material impacted to the static sample material on one side. At rather small impact velocity, compressive shock wave traveled with a velocity equal to the longitudinal sound speed. A linear Hugoniot relationship for normalized particle velocity was verified from our atomistic simulation results, which is comparable with the recent shock experiments on SiC powders. Part of cubic lattice atoms transforms into amorphous state when the impact velocity exceeds the critical value of 4.91 km/s. The size of the amorphous region is well proportional to the particle velocity.

Numerical Simulation of Two Droplets Impinging Successively on a Hot Solid in the Film Boiling Regime

The deformation mechanics of two droplets impinging successively on a hot solid at small impact velocities was investigated by carrying out computer simulations. The conservation equations for mass, momentum, and energy for unsteady incompressible viscous fluids in an axisymmetric coordinate system were approximated and solved with a finite difference method, taking account of gravity, viscosity, and surface tension. It was assumed that a thin vapor layer forms between the water droplet and the solid surface immediately after impact. The numerical results were compared with those of experiments. It was found that the liquid deforms into a crown shape during the successive impacts. The fluid mechanics of the interactions of the droplets is discussed in detail.

Transient clusters in granular gases

The most striking phenomenon in the dynamics of granular gases is the formation ofclusters and other structures. We investigate a gas of dissipatively colliding particles with avelocity dependent coefficient of restitution where cluster formation occurs as a transientphenomenon. Although for small impact velocity the particles collide elastically,surprisingly the temperature converges to zero.

Finite Duration Impacts With External Forces

Abstract This work considers the effect of external forces during finite duration collisions using an incremental model of impact. The deformation of the “rigid” body is modeled through an elastic element and the time interval over which contact occurs is of finite duration. Moreover, the work done by the external forces is nonzero during the collision. This model allows for an equivalent coefficient of restitution e to be identified. In the presence of a constant external force the coefficient of restitution depends not only on the system parameters, but the initial relative velocity at the point of impact. For external forces which tend to bring the colliding bodies together, the colliding bodies remain in contact for sufficiently small impact velocities (e=0) while for larger incoming speeds, the coefficient of restitution is positive. This state dependent restitution arises from the coupling of external forces to the collision model, and is not seen in more familiar models of impact. Finally, based on the results of the experimental system and the incremental model, the standard algebraic model of restitution is modified to include these finite duration effects.

Impact Velocities of the Teeth after a Sudden Unloading at Various Initial Bite Forces, Degrees of Mouth Opening, and Distances of Travel

A potentially dangerous situation arises when an individual bites on hard and brittle food which suddenly breaks, since the impact velocity of the lower teeth onto the upper teeth after the food is broken can be high and may cause dental damage. The present experiments were designed to study the magnitude of the impact velocity after a sudden unloading at various initial bite forces, degrees of mouth opening, and distances of travel. Subjects were asked to perform a static biting task during which the resistance to the bite was suddenly removed. The upward mandible movement was arrested after a certain distance. The velocity of the lower teeth at impact was calculated just before the mandible came to a standstill in combinations of 4 different bite forces (100, 80, 60, and 40 N), 4 different initial degrees of mouth opening (33.5, 30.5, 27.5, and 24.5 mm), and 3 different distances of travel of the mandible (4.5, 3.0, and 1.5 mm). We found that the bite force rapidly declined after the unloading, resulting in a small impact velocity of the lower front teeth. This impact velocity largely depended on the magnitude of the initial bite force and the distance traveled; it was barely sensitive to variations in degree of initial mouth opening. The maximal velocity of the lower teeth was 0.43 m/s (at an initial bite force of 100 N). This maximum was reached after a distance of travel of about 4 mm in 12 ms. The data suggest that the rapid decline in bite force coupled with a limitation of impact velocity is due to the force-velocity properties of the active jaw muscles and is not caused by neural control.

Measurements of the 2P3/2 Vacancy State Alignment of Au Induced by Proton, Lithium and Boron Ion Impact

In this paper, the degree of alignment of 2P3/2 vacancy state in Au induced by proton, lithium and boron ion impact was measured using a Si(Li) detector. Also the projectile-energy dependence of the degree of alignment was investigated over the projectile energy range from 0.2 MeV/amu to 2.0MeV/amu. The de-alignment phenomenon was observed for small relative impact velocities.

Nonsteady penetration of longs rods into semi-infinite targets

This paper describes a new one-dimensional theory of nonsteady penetration of long rods into semi-infinite targets. The target is viewed as a “finite mass” that resides within the semi-infinite target space. Thus, an equation of motion for the target was constructed so that together with erosion and penetrator deceleration equations, expressions for penetration rates and depths were obtained. Forces acting on the target and penetrator are defined in terms of only ordinary strength levels usually associated with dynamic properties or work-hardened material states. Also, the concept of critical impact velocity was used to establish the onset of penetration in this formulation. This penetration equation corresponds in exact form to hydrodynamic theory in the limits of small strengths and/or high impact velocity. Results for penetration rates agree well with hydrocode calculations, and predicted penetrations agree with experimental data over an impact velocity range of 0–5,000 m/s.

Elastic impact of a bar on a half-space

When a semi-infinite bar impacts on a half-space, the pressure and particle velocity at the contact surface jump to a value which is given by the impedances of the two materials. The presence of the half-space is gradually felt by the bar as an increase of effective impedance. As a result, the contact pressure starts to increase while the velocity keeps decreasing. When the contact surface eventually comes to rest, the pressure reaches a final value. This transition between the initial jump and the final state is investigated. A simple model is derived for small impact velocities and linear elastic material behaviour based on the assumption of a linear pressure-penetration relationship. Impact experiments of an aluminium and a plexiglas bar on a plexiglas block with a velocity of 1 to 2 m/s show good agreement with results from the model.

On the rings of Saturn

Results from numerical simulations of jetstreams are used to discuss certain aspects of the dynamics of the rings of Saturn. The probable velocity distribution inside the ring system is strongly non-Maxwellian. For the rings to form and remain a minimal degree of inelasticity is required. The energy consumption decreases rapidly with decreasing thickness of the rings. As we expect the degree of inelasticity to decrease for very small impact velocities, a minimal thickness should be reached, somewhat lower than the observed value.

Response characteristics of human body to mechanical shock

Mechanical shock has become to be one of the important factors with human environment but little is known about the response characteristics of human body to mechanical shock. The present study was performed to investigate the physical characteristics of response to impact with human body. An instrument which was composed of a striker to produce applied shock and a meter to measure the acceleration was constructed for the present study.Operations revealed that the impact acceleration of the fore head was the largest among those measured and that an impact with small impact velocity was suitable to distinguish parts with different characteristics. The impact duration with gluteal region was the longest, which was 0.09 [sec] in case of the impact velocity of 0.077 [m/sec]. Mechanical impedance with the regions which was mostly composed of soft tissues was below 5 [kg·sm-1] and that with the regions to which bones related was over 5 [kg·sm-1]. Dynamic stiffness with the gluteal region was 0.5×103 [kg/m] and that with the fore head was 27.5×103 [kg/m] and these values reflect well the state of springs.