Zhijun Zheng

A Flexible and Highly Pressure-Sensitive Graphene- Polyurethane Sponge Based on Fractured Microstructure Design

A fractured microstructure design: A new type of piezoresistive sensor with ultra-high-pressure sensitivity (0.26 kPa(-1) ) in low pressure range (<2 kPa) and minimum detectable pressure of 9 Pa has been fabricated using a fractured microstructure design in a graphene-nanosheet-wrapped polyurethane (PU) sponge. This low-cost and easily scalable graphene-wrapped PU sponge pressure sensor has potential application in high-spatial-resolution, artificial skin without complex nanostructure design.

Low-velocity perforation behavior of composite sandwich panels with aluminum foam core:

Perforation response and failure of sandwich panels with composite face sheets and aluminum foam core are investigated experimentally in this paper. Quasi-static perforation and low-velocity impact tests are carried out by using a material test system and a drop weight machine, respectively. The load-displacement response, energy absorption and energy-absorbing effectiveness of sandwich panels are obtained and compared for quasi-static and impact tests. Effects of some key parameters on the overall energy absorption behavior of the panels are explored, such as impact energy, face sheets and core thickness, core density and indenter nose shape.

Crashworthiness design of density-graded cellular metals

Crashworthiness of cellular metals with a linear density gradient was analyzed by using cell-based finite element models and shock models. Mechanisms of energy absorption and deformation of graded cellular metals were explored by shock wave propagation analysis. Results show that a positive density gradient is a good choice for protecting the impacting object because it can meet the crashworthiness requirements of high energy absorption, stable impact resistance and low peak stress.

Localized indentation of sandwich beam with metallic foam core

The localized behavior of sandwich structures with foam core subjected to indentation loading is investigated in this article. Based on the principle of virtual velocities, concisely explicit solutions for the indentation forces and shape functions of deformation zones of sandwich beams are derived. Both flat and cylindrical indenters are considered. The indentation force varies linearly with the square root of indenter displacement and thesurface deformation profiles are proportional to the square of the distance from the contact center along the beam direction. Finite element models have been established using the ABAQUS/Explicit code to verify the validity and applicability of the analytical solutions. The theoretical predictions of the profiles of deformation zones and the denting loads of indenters are in good agreement with those by numerical simulation.

Impact Resistance and Energy Absorption of Functionally Graded Cellular Structures

The dynamic response of functionally graded cellular structures subjected to impact of a finite mass was investigated in this paper. Compared to a cellular structure with a uniform cell size, the one with gradually changing cell sizes may improve many properties. Based on the two-dimensional random Voronoi technique, a two-dimensional topological configuration of cellular structures with a linear density-gradient in one direction was constructed by changing the cell sizes. The finite element method using ABAQUS/Explicit code was employed to investigate the energy absorption and the influence of gradient on stress wave propagation. Results show that functionally graded cellular structures studied are superior in energy absorption to the equivalent uniform cellular structures under low initial kinetic energy impacts, and the performance of such structures can be significantly improved when the density difference is enlarged. The stress levels at the impact and support ends may be reduced by introducing a gradual change of density in cellular structures when the initial impact velocity is low.

Blast Alleviation of Cellular Sacrificial Cladding: A Nonlinear Plastic Shock Model

The anti-blast behavior of cellular sacrificial cladding is investigated based on a continuum-based nonlinear plastic shock model. A rate-independent, rigid–plastic hardening (R-PH) model with two material parameters, namely the initial crushing stress and the strain hardening parameter, is employed to idealize the cellular material. The governing equation of the motion of cover plate is obtained and solved numerically with a fourth-order Runge–Kutta scheme. A comparison of the crushing percentage contours of sacrificial cladding based on the R-PH model and the rigid–perfectly plastic–locking (R-PP-L) model is carried out. Results transpire that the R-PP-L model is not accurate enough to evaluate the energy absorption. Dimensional analysis is employed to study the critical length of cellular sacrificial cladding and an empirical expression is determined by the controlling valuable method. An asymptotic solution is also obtained by applying the regular perturbation theory. Finally, the design criteria of cellular sacrificial cladding based on the R-PH shock model is verified by a cell-based finite element model.

Indentation of composite sandwich panels with aluminum foam core: An experimental parametric study

Quasi-static indentation tests were carried out to investigate the mechanical responses of sandwich panels with composite face sheets and aluminum foam core. The energy absorption, specific energy absorption and energy-absorbing effectiveness factor of rigid-supported composite sandwich panels were evaluated and compared. The deformation of upper face sheets, foam cores and lower face sheets was captured. The effects of several key parameters, including the face sheet thickness, the core thickness and relative density, and the indenter nose shape, on the energy absorption behavior of the rigid-supported sandwich panels were explored. The dependency of the load–displacement response of sandwich panels on boundary conditions was also discussed. It was found that the rigid-supported sandwich panels absorb the greatest energy and own the highest energy absorption efficiency, while fully fixed panels absorb the least energy and own the lowest energy absorption efficiency.

Influences of Inertia and Material Property on the Dynamic Behavior of Cellular Metals

Cellular metals are widely used in light-weight structures and energy absorption devices. Although many experimental studies on the dynamic behavior and rate sensitivity of cellular metals have been reported in the literature, there are some conflicting conclusions on the rate effect of metallic foams. In this paper, some numerical tests are presented to explore the effects of inertia, strain hardening and strain-rate hardening of the cell wall material on the behavior of Voronoi honeycomb samples under dynamic compression. Three deformation modes are found and corresponding nominal stress-strain curves and the plateau stress of the “specimens” are obtained. The results reveal that inertia plays an important role in Shock Mode and Transitional Mode but it does not affect the compressive stress-strain curve of the honeycomb. The strain-rate sensitivity of the honeycombs is less significant than that of the cell-wall material and becomes negligible under high impact velocities. The strain-hardening effect of the cell-wall material is of less importance.

MECHANISMS UNDERLYING TWO KINDS OF SURFACE EFFECTS ON ELASTIC CONSTANTS

Recently, people are confused with two opposite variations of elastic modulus with decreasing size of nano scale sample: elastic modulus either decreases or increases with decreasing sample size. In this paper, based on intermolecular potentials and a one dimensional model, we provide a unified understanding of the two opposite size effects. Firstly, we analyzed the microstructural variation near the surface of an fcc nanofilm based on the Lennard-Jones potential. It is found that the atomic lattice near the surface becomes looser in comparison with the bulk, indicating that atoms in the bulk are located at the balance of repulsive forces, and the elastic moduli decrease with the decreasing thickness of the film accordingly. In addition, the decrease in moduli should be attributed to both the looser surface layer and smaller coordination number of surface atoms. Furthermore, it is found that both looser and tighter lattice near the surface can appear for a general pair potential and the governing mechanism should be attributed to the surplus of the nearest force to all other long range interactions in the pair potential. Surprisingly, the surplus can be simply expressed by a sum of the long range interactions and the sum being positive or negative determines the looser or tighter lattice near surface respectively. To justify this concept, we examined ZnO in terms of Buckingham potential with long range Coulomb interactions. It is found that compared to its bulk lattice, the ZnO lattice near the surface becomes tighter, indicating the atoms in the bulk are located at the balance of attractive forces, owing to the long range Coulomb interaction. Correspondingly, the elastic modulus of one-dimensional ZnO chain increases with decreasing size. Finally, a kind of many-body potential for Cu was examined. In this case, the surface layer becomes tighter than the bulk and the modulus increases with deceasing size, owing to the long range repulsive pair interaction, as well as the cohesive many-body interaction caused by the electron redistribution.

STRESS DISTRIBUTION IN GRADED CELLULAR MATERIALS UNDER DYNAMIC COMPRESSION

DYNAMIC COMPRESSION BEHAVIORS OF DENSITY-HOMOGENEOUS AND DEN-SITY-GRADED IRREGULAR HONEYCOMBS ARE INVESTIGATED USING CELL-BASED FINITE ELEMENT MODELS UNDER A CONSTANT-VELOCITY IMPACT SCENARIO. A METHOD BASED ON THE CROSS-SECTIONAL ENGINEERING STRESS IS DEVELOPED TO OBTAIN THE ONE-DIMENSIONAL STRESS DISTRIBUTION ALONG THE LOADING DIRECTION IN A CELLULAR SPECIMEN. THE CROSS-SECTIONAL ENGINEERING STRESS IS CONTRIBUTED BY TWO PARTS: THE NODE-TRANSITIVE STRESS AND THE CONTACT-INDUCED STRESS, WHICH ARE CAUSED BY THE NODAL FORCE AND THE CONTACT OF CELL WALLS, RESPECTIVELY. IT IS FOUND THAT THE CONTACT-INDUCED STRESS IS DOMINANT FOR THE SIGNIFICANTLY ENHANCED STRESS BEHIND THE SHOCK FRONT. THE STRESS ENHANCEMENT AND THE COMPAC-TION WAVE PROPAGATION CAN BE OBSERVED THROUGH THE STRESS DISTRIBU-TIONS IN HONEYCOMBS UNDER HIGH-VELOCITY COMPRESSION. THE SINGLE AND DOUBLE COMPACTION WAVE MODES ARE OBSERVED DIRECTLY FROM THE STRESS DISTRIBUTIONS. THEORETICAL ANALYSIS OF THE COMPACTION WAVE PROPAGATION IN THE DENSITY-GRADED HONEYCOMBS BASED ON THE R-PH (RIGID–PLASTIC HARDENING) IDEALIZATION IS CARRIED OUT AND VERIFIED BY THE NUMERICAL SIMULATIONS. IT IS FOUND THAT STRESS DISTRIBUTION IN CELLULAR MATERIALS AND THE COMPACTION WAVE PROPAGATION CHARACTER-ISTICS UNDER DYNAMIC COMPRESSION CAN BE APPROXIMATELY PREDICTED BY THE R-PH SHOCK MODEL.