Water entry of spheres with various contact angles

It is well known that the water entry of a sphere causes cavity formation above a critical impact velocity as a function of the solid–liquid contact angle; Duez et al. (Nat. Phys., vol. 3 (3), 2007, pp. 180–183). Using a rough sphere with a contact angle of $120^{\circ }$ , Aristoff & Bush (J. Fluid Mech., vol. 619, 2009, pp. 45–78) showed that there are four different cavity shapes dependent on the Bond and Weber numbers (i.e., quasistatic, shallow, deep and surface). We experimentally alter the Bond number, Weber number and contact angle of smooth spheres and find two key additions to the literature: (1) cavity shape also depends on the contact angle; (2) the absence of a splash crown at low Weber number results in cavity formation below the predicted critical velocity. In addition, we use alternate scales in defining the Bond, Weber and Froude numbers to predict the cavity shapes and scale pinch-off times for various impacting bodies (e.g., spheres, multidroplet streams and jets) on the same plots, merging the often separated studies of solid–liquid and liquid–liquid impact in the literature.

Numerical Simulation of Projectile Material Phase Transformation during Penetration

It was found from the experiment that the high temperature caused by target impact led to projectile austenite phase transformation. Based on this finding, a phase transformation model was established and verified by experiments, which proved the model had a relatively high calculation precision. Then, the model was used for numerical simulation. The result showed that the phase transformation zone was at the projectile head surface and the area increased as the impact velocity increased. The critical impact velocity which would cause phase transformation and affect projectile performance was around 700m/s.

Critical Scales Govern the Mechanical Fragmentation Mechanisms of Biomolecular Assemblies

Fragmentation mechanisms of peptide assemblies under shock deformation are studied using molecular dynamics simulations and are found to depend strongly on the relative magnitude of the shock front radius to the fibril length and the ratio of the impact energy to the fibril cohesive energy. The competition between size scaling of curvature and impact energy leads to a mechanism change at a critical impact velocity, developing a stark contrast in the size scaling of fragmentation at low and high strain rates. We show that the fragmentation mechanisms can be classified on the basis of the length and time scales of deformation and relaxation to provide new insight into experimental observations.

Effect of Random Defects on Dynamic Response of Honeycomb Structures

The dynamic crushing behavior of cellular metals is closely related to their microstructure. Two types of random defects by randomly thickening/removing cell walls are investigated in this paper. Their influences on the deformation modes and plateau stresses of honeycombs are studied by finite element simulation using ABAQUS/Explicit code. Three deformation modes, i.e. the Homogeneous Mode, the Transitional Mode and the Shock Mode, are used to distinguish the deformation patterns of honeycombs under different impact velocities. The critical impact velocity for mode transition between the Homogeneous and Transitional modes is quantitatively determined by evaluating a stress uniformity index, defined as the ratio between the plateau stresses on the support and impact surfaces. It is found that the critical impact velocity decreases with increasing thickening ratio but increases with increasing removing ratio. The plateau stress on the impact surface heavily depends on the impact velocity due to the inertia effect. The random defects lead to a weakening effect on the plateau stress. For the honeycombs with randomly removing cell walls, the weakening effect is especially obvious at a moderate impact velocity. For the honeycombs with randomly thickening cell walls, the weakening effect is particularly severe at a low impact velocity, but this effect almost disappears when the impact velocity is high enough.

Critical impact velocity for ice fragmentation

The fragmentation process is a main concern in many engineering applications such as preventing flameouts of aircraft engines. The authors of this article are interested in measuring the critical impact velocity for ice fragmentation. Precisely, a dropweight technique was applied to study the ice ball impacts on glass plates. The influence of ice ball temperature, diameter and impact angle is also investigated. The after-impact ice ball state was found to be classified into two cases: an altered state and a non-altered state. The critical impact velocity is defined as the minimum impact velocity for which the ice ball is altered after impact or the maximum impact velocity for which the ice ball is not altered after impact. The experimental results are analysed by a model, assuming that the alteration regime is observed as soon as the ice ball normal kinetic energy is higher than a critical value of its deformation energy. This model depends on one parameter which is determined in this study. This last result is the major achievement of this article as there is almost no measurement of this parameter in the literature.

Foreign Object Damage in Gas-Turbine Grade Silicon Nitrides by Silicon Nitride Ball Projectiles

Foreign object damage (FOD) behavior of two gas-turbine grade silicon nitrides (AS800 and SN282) was determined with a considerable sample size at ambient temperature using impact velocities ranging from 50 to 225 m/s by 1.59-mm diameter silicon nitride ball projectiles. The degree of impact damage as well as of post-impact strength degradation increased with increasing impact velocity, and was greater in SN282 than in AS800 silicon nitride. The critical impact velocity in which target specimens fractured catastrophically was remarkably low: about 200 and 130 m/s, respectively, for AS800 and SN282. The difference in the critical impact velocity and impact damage between the two target silicon nitrides was attributed to the fracture toughness of the target materials. The FOD by silicon nitride projectiles was significantly greater than that by steel ball projectiles. Prediction of impact force was made based on a yield model and compared with the conventional Hertzian contact-stress model.