Propagation Of Impact-induced Waves Research Articles

Ballistic impact induced wave propagation and dislocation of three-dimensional graphene origami/copper nanocomposites

Graphene (Gr)-reinforced nanocomposites have great potential as impact protection materials due to their exceptional mechanical properties. However, the intrinsic brittleness of Gr originating from its two-dimensional (2D) geometry considerably hinders its application. In this work, a three-dimensional (3D) graphene origami (GOri) reinforced copper (Cu) nanocomposite is designed and its wave propagation characteristics under a high-velocity ballistic impact are studied using the molecular dynamics (MD) method. It is found that the intrinsic brittleness of Gr can be alleviated by transforming its original 2D shape into 3D origami shape and embedding GOri in Cu matrix can lead to effectively increased out-of-plane wave propagation speed thereby boost the dissipation of impact energy by promoting rapid dislocation propagation within the Cu matrix during the initial stage of impact.

Transient contact-impact behavior for passive walking of compliant bipedal robots

When bipedal robots walk, the compliance of local contact zone and whole structure usually causes high amplitude contact force and wave propagation excited by impact. The object of this paper is to analyze the transient contact-impact behavior during the walking of compliant bipedal robots (CBR) using a completely flexible body model (CFBM) and obtain its fine contact modes. The applicability of the traditional method based on completely rigid body model (CRBM) to the compliant bipedal robots is also discussed. In CFBM, the structural deformation field and inertial field are discretized by finite element theory, and the penalty function method is adopted to account for the unilateral contact constraint. The dynamic equations, strain and stress equations for the CFBM are derived. The contact forces, impact-induced wave propagation and total mechanical energy of the CBR are calculated. The results show that the relative contact motions between the foot and the slope experience two phases: (1) macroscopic oblique impact phase (duration time is 0.1∼0.2 s) and (2) rolling phase (duration time is 0.6∼0.7 s). Phase 1 consists of a series of fast normal contact-separation switches and tangential stick–slip switches. Phase 2 means there are continuous normal compression and tangential stick. Since the traditional method neglects the first phase, it is not comprehensive for investigating the CBR with impact-induced waves and vibration, especially for the case of lower coefficient of friction (COF) μ. In addition, a ‘dynamic self-locking’ phenomenon is found as μ>0.9, which will lead to the walking unstable.

Transient impact response analysis of an elastic–plastic beam

• A hybrid numerical–analytical model is presented for elastic–plastic beam impact. • The experiment of sphere-beam impact has been conducted. • The model has been verified by the experiments. • The impact-induced waves have been detected by the experiments and the model. • Effects of various impact parameters are investigated by the present model. A hybrid, numerical–analytical model is presented to investigate the transient response of a simply supported elastic–plastic beam subjected to impact of a sphere. The model couples a finite difference model based on the Rayleigh's beam theory with a theoretical contact model, named refined Stronge's model. The elastic–plastic transient impact responses of the beam and the wave propagation are solved by the finite difference model. The local elastic–plastic contact behavior is analyzed by the refined Stronge's model. The presented model is verified by the experiments. The comparisons of the numerical results with the experimental results show that the presented model is valid and can predict accurately the transient impact response and impact-induced wave propagation. The comparison of the calculating efficiency with the 3D finite element model shows that the presented model is highly efficient and suitable for parametric study. The effects of the various impact parameters, such as impactor mass, velocity, plasticity and impact location on the impact response, the energy loss and the coefficient of restitution are investigated. It has been found that the impact-induced wave propagation influences significantly the impact force response, the contact time, the energy loss and the coefficient of restitution. A jumping phenomenon of the energy loss and the Newton coefficient of restitution has been found, and it is resulted from the impact-induced wave propagation. The presented model appears to be suitable and convenient for the impact response analysis of elastic–plastic beam, especially for the study of impact-induced wave effects.

Propagation of solitary waves in 2D granular media: A numerical study

The impact-induced wave propagation in a model granular material composed of closely packed linearly elastic spherical particles interacting through Hertzian contact is investigated numerically using a specially adapted molecular dynamics framework. Of particular interest is the effect of the stiffness and density mismatch between main and interstitial beads on the anisotropic nature and speed of propagation of the primary compressive wave generated by a localized impact event in an extended square-packed granular medium. Two propagation regimes are observed in the numerical simulations: the first one described by a solitary wave pattern emanating from the point of impact, and the other characterized by a directional propagation of the impact energy in the principal directions of the pack. A simple model is proposed to describe the bounds between these two propagation regimes in the parametric space defined by the mass and stiffness ratios between main and interstitial particles. Maps of the normalized maximum compressive force and wave speed are presented to quantify the anisotropy of the wave propagation response.

Propagation of Impact-Induced Longitudinal Waves in Mechanical Systems With Variable Kinematic Structure

In previous publications by the authors of this paper it was shown that elastic media become dispersive as the result of the coupling between the finite rotation and the elastic deformation. Impact-induced harmonic waves no longer travel, in a rotating rod, with the same phase velocity and consequently the group velocity becomes dependent on the wave number. In this investigation, the propagation of impact-induced longitudinal waves in mechanical systems with variable kinematic structure is examined. The configuration of the mechanical system is identified using two different sets of modes. The first set describes the system configuration before the change in the system topology, while the second set describes the configuration of the system after the topology changes. In the analysis presented in this investigation, it is assumed that collision between the system components occurs first, followed by a change in the system topology. Both events are assumed to occur in a very short-lived interval of time such that the system configuration does not appreciably change. By using the first set of modes, the jump discontinuity in the system velocities is predicted using the algebraic generalized impulse momentum equations. The propagation of the impact-induced wave motion after the change in the system topology is described using the Fourier method. The series solution obtained is used to examine the effect of the topology change on the propagation of longitudinal elastic waves in constrained mechanical systems. It is shown that, while, for a nonrotating rod, mass capture or mass release has no effect on the phase and group velocities, in rotating rods the phase and group velocities depend on the change in the system topology. In particular the phase velocities of low harmonic longitudinal waves are more affected by the change in the system topology as compared to high frequency harmonic waves.

Spurious wave reflection at the contact interface for finite element impact analysis

In finite element elastic impact analysis, the propagation of impact-induced waves through finite element meshes normally causes spurious components in the numerical solutions. In this paper the possible spurious wave reflection at the impact interface is investigated, where the impact interface is modelled by using the penalty function method. The amplitudes of the reflected and transmitted waves are derived as functions of frequencies and wavenumbers, which are related by the dispersion relations of the finite element meshes. It is observed that significant spurious wave reflections may appear at the impact interface when a small penalty parameter is used. The current investigation is restricted to one-dimensional cases with constant strain or linear strain elements.