Increasing Impact Speed Research Articles

Bounce of hollow balls on flat surfaces

The bounce of a hollow spherical ball from a hard flat surface results in large deformations of the thin-walled shell; these deformations increase the internal stresses in the shell near a knuckle of bending and also increase the internal gas pressure inside the shell. This paper calculates the deformation and consequent contact forces that repel the ball from the surface, and compares the results with both a finite element analysis and experimental data. A table tennis ball is considered as an example of a thin-walled elastic shell. Since for an elastic ball there are no energy losses due to inelastic material behaviour, the analysis assumes that all energy losses are due to either friction in the contact area or impulsive forces arising from the instantaneous rate-of-change of momentum in the knuckle. In practice, the table tennis ball shows energy losses that increase with increasing impact speed; these losses substantially exceed the losses taken into account by the elastic shell analysis.

An Analysis of the Rettitution Behavior of Strings Considering Mechanical Properties in Diametral Direction.

The restitution behavior of fiber strings was analyzed with considering the tensile and compressive properties. An impact test device in which the strings were tensioned and struck with a hammer was used and the impact speed and restitution coefficients were measured. A special string which the contact part was covered by a steel pipe was also used to examine the effect of diametral compressive deformation of strings. In ordinary test, the coefficient becomes small with increase of impact speed, while that becomes slightly large in the covered string test. Accordingly, this phenomenon is caused mainly by the compressive deformation. A dynamic restitution model considering diametral visco-elasticity of materials was suggested with approximating it by Voigt model. The restitution behavior was simulated by numerical computation. The calculation results showed good agreement with the experimental ones.

On the Cavitation Occurring at the Bottom of a Suddenly Accelerated Circular Cylinder. Effects of Air Content in Water on Incipient Cavitation.

This study is intended to investigate the cavitation at the end surface of a cylinder when this is impulsively put into a longitudinal motion along its axis. The principle is that a longitudinal stress wave accompanying the longitudinal particle displacement is generated at one end of the cylindrical rod by an impact given at the other end ; the stress wave then propagates into the adjacent water as pressure fluctuations causing cavitation when the negative pressure reaches a certain value with a certain time duration. The results of the investigation can be summarized as follows (1) Cavitation is more susceptible with the air content in water. The pressures at the onset of cavitation increase with the air content. (2) Though the minimum pressures at the test surface decreases with increasing impact speed of the rod, the cavitation occurs at a pressure higher than the minimum and this incipient pressure is almost independent of the impact speed. (3) The incipient cavitation pressure can be negative when the air content in water is reduced from the saturated value.

Analysis of failure modes in impulsively loaded pre-notched steel plates

We analyze transient plane strain thermomechanical deformations of a pre-notched 4340 steel plate impacted by a 4340 steel projectile in the direction of two parallel notches, and study the influence of the impact speed and notch tip radius on the localization of the deformation. The plate configuration is identical to that in Kalthoff's experiments (1987, 1988). There is no failure or fracture criterion included in our work. However, the computed evolution of stress and plastic strain fields strongly suggest that with an increase of impact speed and decrease in notch tip radius, there is a failure mode transition from a tensile crack opening at approximately 70° to the notch ligament to an adiabatic shear band propagating at −5° to −15° to the notch ligament. This is in qualitative agreement with Kalthoff's findings.

Soybean Threshing Mechanics: II. Impact

An experimental device was built to study the use of mechanical impact to thresh soybeans from standing plants. The maximum threshing efficiency of the impact device was 92.5%. Threshing efficiency increased significantly with decreasing pod moisture content. The seed damage was within acceptable levels despite an increasing trend with increasing impact speeds and decreasing pod moisture content. The estimated energy absorbed per pod at 10% moisture content (w.b.) was 0.0127 J. The energy used by conventional combines was 20% higher than that required by the impact device, and approximately 30 times higher than the total energy absorbed by pods during shattering.

Primary velocity dependence of impact ejecta parameters

From the light emitted during impacts of secondary particles produced during hypervelocity primary impacts, the velocities and relative masses of these ejecta were determined as a function of the angle between the ejection direction and the target surface. The velocity of the ejecta increases with increasing impact velocity and decreasing ejection angle. The ratio of the maximum ejecta velocity to the primary impact velocity decreases with increasing impact speed. The main fraction of the secondary particles is ejected in rather small angular intervals of about 10° width in elevation. The ejection angle of the main fraction of the ejecta mass increases with increasing impact velocity.