Initial Impact Force Research Articles

Simple hydroelastic impact models for water-borne debris

Debris driven by tsunamis pose a significant threat to structures, and yet most building codes that include debris impact are based on rigid-body dynamics. However, the debris will most likely not be rigid compared to the structural components, such as walls and columns, that they impact. Impact by flexible, water-borne wood poles and shipping containers is considered in this paper. A relatively simple one-dimensional model for acoustic wave propagation, for which an analytical solution is obtained, is shown to provide a good estimate for the initial impact force and duration. This model is validated with small-scale, in-air tests. The simple model is also validated with two-dimensional fluid–structure interaction using a finite element code. The acoustic model works well for initial impact as long as the debris rebounds from the impact target. When the water prevents separation, then the acoustic model significantly overestimates the asymptotic force. The effect of the gravitational waves resulting from impact is to retard re-impacts, as compared to the acoustic model.

On the investigation of a reliable actuation control method for ohmic RF MEMS switches

Efficient control of RF MEMS switches is a very important issue as it is correlated to main failure mechanisms/modes such as the impact force and bouncing phenomena, which degrade their dynamic performance and longevity. This paper presents the control of specific ohmic RF MEMS switches under three different actuation modes, a tailored pulse optimization method based on Taguchi's technique (voltage mode actuation control), resistive damping (charge mode actuation control) and finally the Hybrid actuation mode, which is a combination of the tailored pulse, the resistive damping and Taguchi's optimization technique. Coventorware simulations indicate that under optimized Tailored pulse and Hybrid actuation modes, the impact velocity is reduced by around 90%, the initial impact force by around 75% and the maximum bouncing displacement during the release phase by around 95%, while the switching speed is increased by around 20% compared with the step pulse control mode. The resistive damping control mode is inappropriate for this type of switch and only partial improvement during the pull-down phase has been achieved. Finally, a comparison between Hybrid and optimized tailored modes shows that Hybrid actuation mode excels with better switching characteristics and most importantly offers immunity to manufacturing and operation tolerances.

Impact jetting by a solid sphere

We use a novel ultra-high-speed video camera to study the initial stage of the impact of a solid sphere onto a liquid surface, finding a high-speed horizontal jet which emerges immediately following the intial contact. For R e > 2 × 10 4 the jet emerges when the horizontal contact between the sphere and the liquid is only 12% of its diameter. For the largest Reynolds numbers this jet can travel at more than 30 times the impact velocity of the sphere. This jetting occurs sooner and at much higher normalized velocities than has been observed previously. The breakup of the jet into a spray of droplets sometimes occurs through formation of pockets in the liquid sheet. Early in the impact, the energy transferred to the jet and the subsequent spray sheet is estimated to be much larger than the energy associated with the added mass inside the liquid pool. The jetting will therefore greatly increase the initial impact force on the sphere

Multi‐layered energy absorber frames for tall buildings under high‐speed impact

AbstractSummaryBased on the advanced fractional derivative damping theory for vibration of large systems and the flexible surface‐to‐surface advanced contact theory, a new design strategy has been presented to deal with the high‐speed impact of flying objects into tall buildings. Using a multi‐layered frame system with external energy absorber frame and a new individual grid‐stiffened flooring system (IGSF), a new system has been design to resist the initial impact force and the progressive collapse phenomena. The system can also guide the heat waves caused by the explosion using a special frame system. These characteristics of the new design method have been shown graphically using a full documented numerical example, which highlights the efficiency of the new multi‐layered frame system under high‐speed impact. The advanced flexible surface‐to‐surface contact theory is used to model the actual initial impact force and the advanced fractional derivative damping theory is used to obtained the energy absorption characteristics of large‐scale dampers used in the external frame system. The purpose of this paper is to present a new design method, which improves the resulting dynamic responses of a tall building induced by a high‐speed impact of a flying object. Copyright © 2003 John Wiley & Sons, Ltd.

Influence of Heel Flare and Midsole Construction on Pronation Supination and Impact Forces for Heel-Toe Running

The purpose of this study was to determine the influence of lateral heel flare on pronation, external impact forces, and takeoff supination for different midsole constructions. Data were collected using force platforms and high-speed film cameras. Fourteen male subjects participated in the study, running heel-toe at a speed of 4 m/s. The analysis of kinetic and kinematic variables showed that changes in lateral heel flare of 16°, 0°, and a rounded heel can be used to influence initial pronation during heel-toe running. It could be shown that changes in lateral heel flare do not have a relevant influence on changes in total and/or maximal pronation. Changes in lateral heel flare do have an effect on vertical impact force peaks if the midsole is relatively hard but not if the midsole is relatively soft. Based on the present study, a running shoe with a relatively hard midsole material and a neutral flare would have low initial pronation values and low vertical impact force peaks.

Analysis of the impact force by a transient liquid slug flowing out of a horizontal pipe

An abrupt increase in the gas flow rate in a horizontal gas-liquid two-phase stratified or wavy flow induces a transient slug flow. When this transient liquid slug flows out of a pipe, it applies a large impact force by its impingement on a structure located outside the pipe. The shape of this impact force-time curve is characterized by such quantities as initial impact force, maximum force, acting period of force and total momentum. The influences of each volumetric flux of two phases and of the pipe diameter on the impact force-time curves and the dynamic behavior of the transient liquid slug are experimentally studied and theoretically analyzed on the basis of Dukler-Hubbard's model by applying equations of the integral balance of mass and momentum to the transient liquid slug. The calculated results agree well with the experimental results of the dynamic behavior of the transient liquid slug and of the impact force.

Impact force by transient liquid slug flowing out of horizontal pipes (1st report, Experimental study)

An impact force of a transient liquid slug was measured in air-water horizontal two-phase Flow system. The time response of the impact force was characterized by the initial impact force, maximum force, acting period of force and mean effective force which were discussed with reference to the volumetric liquid and gas fluxes and also to the Flow characteristics of the transient slug Flow in horizontal pipes.

An experimental investigation of the initial force of impact on a sphere striking a liquid surface

Detailed experimental results are presented for the initial impact force on a sphere striking a horizontal liquid surface vertically at speeds in the range 1-3 m s−1. Results are discussed in terms of an impact drag coefficient. Liquids having viscosities in the range 10−3−102 Pa s have been studied. For low viscosities the results have been compared with the theoretical calculations of Shiffman & Spencer. Good agreement has been found in most respects; in particular the impact force varies as the square root of the depth for depths less than a tenth of the radius. The impact drag coefficient has also been studied through the transition from inertia to viscosity-dominated conditions. The variation of the impact drag coefficient is presented as a function of Reynolds number, and its variation in the range 5 × 10−2 < Re < 5 × 103 is shown to resemble that of a fully immersed sphere moving steadily in a homogeneous fluid.

Techniques in the study of impact and sliding wear of Zircaloy-4

A wear test apparatus has been designed to conduct detailed measurements of the contact forces and motion during impact and sliding (and combinations of the two). After carrying out pure impact and sliding tests on Zircaloy-4 couples in air at room temperature, it appears that the test apparatus has acceptable dynamic characteristics for wear studies. In pure impact, the wear rate of Zircaloy-4 was primarily influenced by the initial impact force which was measured dynamically. Secondary vibrations had a minimal effect on the wear rate. The wear rate in pure sliding was dependent upon the characteristics of the apparatus, such as its compliance, and the test conditions. Consequently the sliding amplitude should be monitored continuously throughout the test. In general, sliding wear was more severe than impact wear and good correlation between the average sliding amplitude and weight loss was observed. An indication of the relative motion between the wear specimens was given by the magnitude and form of the coefficient of friction which tended to increase with weight loss.