Showing papers on "Shock (mechanics) published in 2022"

Identification of constitutive equations at very high strain rates using shock wave produced by laser

Abstract: A method coupling experiments and simulations, is developed to characterize the yield stress and strain hardening of several metals loaded at 1 0 6 s − 1 and 25 ns, typically involved during Laser Shock Peening . It was applied to four materials: pure aluminum, 2024-T3 and 7175-T7351 aluminum alloys and Ti6Al4V-ELI titanium alloy . Thin foils have been irradiated with high-power laser to induce high-pressure shock wave. Plastic deformation is activated through the thickness up to the rear free-surface of the foils. These experiments have been simulated using three material constitutive equations: Elastic–Perfectly Plastic model considering static yield stress, Johnson–Cook model without strain hardening and Johnson–Cook model with strain hardening. The material parameters of Johnson–Cook law were identified by comparison of the experimental and calculated velocity profiles of the rear-free surface. Results are shown and discussed.

Dynamic implosion of submerged cylindrical shell under the combined hydrostatic and shock loading

Abstract: Thin-walled cylindrical shells are commonly used in designing underwater structures. When subjected to the combination of hydrostatic pressure and underwater explosive loading, these cylindrical shells are prone to implosion failure. In this work the dynamic stability of submerged cylindrical shells subjected to underwater explosion was investigated through both numerical simulations and theoretical modeling. In particular, the effects of initial hydrostatic pressure and fluid–structure interaction on the dynamic stability of metallic cylindrical shells are revealed. The transient responses of cylindrical shells have been simulated using Abaqus/Explicit with the water domain discreted by acoustic elements. The simulation results show that the existence of initial hydrostatic pressure reduces the structure stiffness, and the structural response to the pressure wave loading primarily consists of initial axisymmetric vibrations and subsequent mode 2vibrations. There exists one critical threshold value for the initial hydrostatic pressure, below which these vibrations are stable and above which these vibration will trigger the implosion of the cylindrical shells. These threshold values are obtained via both numerical simulations and theoretical modeling. The analytic model considering the added mass effect of surrounding water and the effect of initial stress of the shell due to hydrostatic pressure shows good accuracy comparing with the numerical and experimental results. The dependence of the dynamic buckling strength of the metallic cylindrical shells on the material choice for either constant static buckling strength or constant weight is then discussed.

Energy dissipation and effective properties of a nominally elastic composite material

Abstract: Composites such as architected metamaterials made from elastic materials can display energy absorbing or dissipative behavior associated with snap-through and other instabilities. These instabilities result in conversion of elastic strain energy of the elastic parent material of the composite to kinetic energy which is then dissipated over longer time scales via processes including air drag. Here we propose a new approach at modelling such composites by appealing to ideas from statistical mechanics, viz. we postulate that the state of such a composite comprising a large number of unit cells is given by maximization of entropy. We first develop the theoretical homogenization framework for such a composite and provide expressions of the effective properties. Solution of the governing equations requires sampling over all the states that the composite can attain, and this is efficiently done by employing the Nested Sampling algorithm to construct a sample with the required distribution. Using this numerical scheme , we present predictions of the volumetric compressive response of two model materials, i.e. two-dimensional (2D) architected materials with honeycomb-like microstructures and cell walls made from an elastic material. Unlike traditional homogenization schemes, the volumetric stress versus strain response now depends on an additional internal state variable that characterizes the vibrational modes of the microstructure, i.e., the predictions are akin to what is commonly known as an Equation-of-State (EoS). We finally demonstrate the utility of the method by using the predicted EoS to extract the shock response of the architected material and compute the dissipation within the shock.

On spherical shock wave focusing in air — A computational study

Abstract: A detailed numerical study on the phenomenon of Shock Wave focusing in air is carried out. The focusing phenomenon is achieved with the help of a shock tube and a converging section attached to it. The planar shock generated inside the shock tube is converted to a spherical shock with the help of the converging section and is focused to a point. High-temperature effects like temperature-dependent C p variation and chemical reactions corresponding to dissociated air are included in the simulation. The chemical reactions includes the dissociation, recombination and ionization of nine species of air including three ions ( N 2 , O 2 , N, O, NO, Ar, N O + , O + , and A r + ) are monitored throughout the simulation. The effect of driven section filling conditions such as initial pressure and temperature on focusing parameters is studied. The variation in the initial fill temperature is found to affect the flow properties much more as compared to the change in initial fill pressure while maintaining the same shock strength. The effect of incident shock strength on shock wave focusing is also investigated. It is observed that as the strength of the shock increases, the conditions like temperature and pressure at the focusing point increase and thereby increasing the reaction rate of all the reactions.

Energy analyses in Kolsky bar experiments

Abstract: The Kolsky bar technique is based on one-dimensional stress wave propagation theory where mass and momentum are conserved such that force and displacement of a specimen are measured to obtain the stress-strain response of a material. Energy analysis in Kolsky bar experiments has not been systematically investigated despite potential insights into how materials dissipate or absorb energy during impact events. In this chapter, energy analysis in methods are presented in both the time and frequency domains, using a NiTi shape memory alloy as an example. Energy dissipation ratio is a useful means of quantifying the energy absorption capability of a material in the time domain. Energy analysis in the frequency domain provides additional information about how energy is distributed over a range of frequencies, which can inform better shock mitigation design. Interpretation of energy analysis in both domains is discussed for PMDI foam and TufFoam, silicone foam and rubber, as examples. The analysis techniques are used to investigate dissipation through a threaded joint interface as an example, though the analysis method is general and could be extended to any interface.