Zhenyu Xue

Preliminary assessment of sandwich plates subject to blast loads

Abstract The question motivating the present study is whether metal sandwich plates with sufficiently strong cores are able to sustain substantially larger blast loads than monolithic solid plates of the same material and total mass. Circular plates clamped at their edges are considered under blast loads large enough to produce substantial deflections. The material is elastic–perfectly plastic. Material strain-rate dependence and fracture are neglected. A dynamic finite element formulation for elastic–plastic solids is employed to analyze the plate response. Uniformly distributed blast impulses are considered. As a basis for comparison, complete results are obtained for solid plates for both zero-period and finite-period impulses. Similar computations are carried out for a set of sandwich plates having tetragonal truss cores. The potential for superior strength and energy absorbing capacity of the sandwich plates is demonstrated compared with solid plates having the same mass. The importance of both the strength and energy absorbing capacity of the core are highlighted for superior blast resistance. Proposals for further research are made.

Indentation of a soft metal film on a hard substrate: strain gradient hardening effects

A new type of nanoindentation experiment showing the effect of a strain gradient on the flow strength of a crystalline material is conducted and analyzed. We show that by indenting a soft metal film (Al) on a hard substrate (glass) with a sharp diamond indenter a strong gradient of plastic strain is created. The true hardness of the film is observed to increase with increasing depth of indentation when the indenter tip approaches the hard substrate, in sharp contrast to the falling hardness with increasing depth in bulk materials. We associate this rise in hardness with the strong gradient of plastic strain created between the indenter and the hard substrate. We use the mechanism-based strain gradient (MSG) plasticity theory to model the observed indentation behavior. The modeling shows that the MSG plasticity theory is capable of describing not only the decreasing hardness with increasing depth of indentation at shallow indentations, as observed in bulk materials, but also the rise in hardness that occurs when the indenter tip approaches the film/substrate interface.

The strain gradient effect in microelectromechanical systems (MEMS)

Metallic materials display strong size effect when the characteristic length of deformation is of the order of microns. The theory of mechanism-based strain gradient (MSG) plasticity established from the Taylor dislocation model has captured this size dependence of material behavior at the micron scale very well. The strain gradient effect in microelectromechanical systems (MEMS) is investigated in this paper via the MSG plasticity theory since the typical size of MEMS is of the order of microns (comparable to the internal material length in MSG plasticity). Through an example of a digital micromirror device (DMD), it is shown that the strain gradient effect significantly increases the mechanical strain energy in the DMD, and reduces the rotation time of the micromirror. However, the strain gradient has no effect on the critical bias voltage governing the fast rotation of the micromirror.

Modeling of Metallic Sandwich Structures

All-metal sandwich construction holds promise for significant improvements in stiffness, strength and blast resistance for built-up plate structures. Analysis of the performance of sandwich plates under various loads, static and dynamic, requires modeling of face sheets and core with some fidelity. While it is possible to model full geometric details of the core for a few selected problems, this is unrealistic for larger complex structures under general loadings. A constitutive model can be proposed as an alternative means of modeling the sandwich core. The constitutive model falls within the framework of a compressible rate-independent, anisotropic elastic-plastic solid. In this paper, the model will be presented in details, along with numerical implementation in a finite element code, and benchmarks its performance against existing constitutive models.Copyright