Zhi Hua Wang

Effects of cell size on compressive properties of aluminum foam

The effects of cell size on the quasi-static and dynamic compressive properties of open cell aluminum foams produced by infiltrating process were studied experimentally. The quasi-static and dynamic compressive tests were carried out on MTS 810 system and SHPB(split Hopkinson pressure bar) respectively. It is found that the elastic moduli and compressive strengths of the studied aluminum foam are not only dependent on the relative density but also dependent on the cell size of the foam under both quasi-static loading and dynamic loading. The foams studied show a significant strain rate sensitivity, the flow strength can be improved as much as 112%, and the cell size also has a sound influence on the strain rate sensitivity of the foams. The foams of middle cell size exhibit the highest elastic modulus, the highest flow strength and the most significant strain rate sensitivity.

Tribological Properties of AlCrCuFeNi2 High-Entropy Alloy in Different Conditions

In order to understand the environmental effect on the mechanical behavior of high-entropy alloys, the tribological properties of AlCrCuFeNi2 are studied systematically in dry, simulated rainwater, and deionized water conditions against the Si3N4 ceramic ball at a series of different normal loads. The present study shows that both the friction and wear rate in simulated rainwater are the lowest. The simulated rainwater plays a significant role in the tribological behavior with the effect of forming passive film, lubricating, cooling, cleaning, and corrosion. The wear mechanism in simulated rainwater is mainly adhesive wear accompanied by abrasive wear as well as corrosive wear. In contrast, those in dry condition and deionized water are abrasive wear, adhesive wear, and surface plastic deformation. Oxidation contributes to the wear behavior in dry condition but is prevented in liquid condition. In addition, the phase diagram of AlxCrCuFeNi2 is predicted using CALPHAD modeling, which is in good agreement with the literature report and the present study.

Plastic Deformation of Al0.3CoCrFeNi and AlCoCrFeNi High-Entropy Alloys Under Nanoindentation

The mechanical properties of Al0.3CoCrFeNi and AlCoCrFeNi high-entropy alloys (HEAs) were investigated by instrumented nanoindentation over a broad range of loading rates. It was found that the loading portion of the two HEAs exhibited apparent discontinuities at low loading rates. However, the discontinuity became less pronounced with increasing the loading rate. The experimental results that the hardness, elastic modulus, and yield strength of AlCoCrFeNi HEAs are larger than those of Al0.3CoCrFeNi HEAs can be elucidated in terms of thermodynamic and topological parameters of the constituent elements and solid solution strengthening, respectively. In situ scanning images displayed a significant pile-up around the indents, demonstrating that a highly localized plastic deformation occurred under nanoindentation. Furthermore, the resistance for creep behavior increases as the Al concentration is increased due to the enlarged lattice distortion related to a solution strengthening effect.

Effects of heat treatment on dynamic compressive properties and energy absorption characteristics of open-cell aluminum alloy foams

Abstract The effects of heat treatment on the dynamic compressive properties and energy absorption characteristics of open cell aluminum alloy foams (Al-Mg-Si alloy foam and Al-Cu-Mg alloy foam) produced by infiltrating process were studied. Two kinds of heat treatment were exploited: age-hardening and solution heat treating plus age-hardening (T6). The split Hopkinson pressure bar (SHPB) was used for high strain rate compression test. The results show that both age-hardened and T6-strengthened foams exhibit improved compression strength and shortened plateau region compared with that of foams in as-fabricated state under high strain rate compression, and the energy absorption capacity is also influenced significantly by heat treatment. It is worthy to note that omitting the solution treating can also improve the strength and energy absorbed much.

A numerical simulation of metallic cylindrical sandwich shells subjected to air blast loading

The dynamic response of cylindrical sandwich shells with aluminum foam cores subjected to air blast loading was investigated numerically in this paper. According to KNR theory, the nonlinear compressibility of the air and finite shock conditions were taken into account in the finite element model. Numerical simulation results show that the compression strain, which plays a key role on energy absorption, increases approximately linearly with normalized impulse, and reduces with increasing relative density or the ratio of face-sheet thickness and core thickness. An increase of the impulse will delay the equalization of top and bottom face-sheet velocities of sandwich shell, but there is a maximum value in the studied bound. A limited study of weight optimization was carried out for sandwich shells with respect to the respective geometric parameters, including face-sheet thickness, core thickness and core relative density. These numerical results are of worth to theoretical prediction and engineering application of cellular metal sandwich structures.

Elasto-plastic constitutive model of aluminum alloy foam subjected to impact loading

A multi-parameter nonlinear elasto-plastic constitutive model which can fully capture the three typical features of stress-strain response, linearity, plasticity-like stress plateau and densification phases was developed. The functional expression of each parameter was determined using uniaxial compression tests for aluminum alloy foams. The parameters of the model can be systematically varied to describe the effect of relative density which may be responsible for the changes in yield stress and hardening-like or softening-like behavior at various strain rates. A comparison between model predictions and experimental results of the aluminum alloy foams was provided to validate the model. It was proved to be useful in the selection of the optimal-density and energy absorption foam for a specific application at impact events.

Twinning-induced plasticity (TWIP) and work hardening in Ti-based metallic glass matrix composites

The present study demonstrates that Ti-based metallic glass matrix composites (MGMCs) with a normal composition of Ti43Zr32Ni6Ta5Be14 containing ductile dendrites dispersed in the glass matrix has been developed, and deformation mechanisms about the tensile property have been investigated by focusing on twinning-induced plasticity (TWIP) effect. The Ti-based MGMC has excellent tensile properties and pronounced tensile work-hardening capacity, with a yield strength of 1100u2009MPa and homogeneous elongation of 4%. The distinguished strain hardening is ascribed to the formation of deformation twinning within the dendrites. Twinning generated in the dendrites works as an obstacle for the rapid propagation of shear bands, and then, the localized necking is avoided, which ensures the ductility of such kinds of composites. Besides, a finite-element model (FEM) has been established to explain the TWIP effect which brings out a work-hardening behavior in the present MGMC instead of a localized strain concentration. According to the plasticity theory of traditional crystal materials and some new alloys, TWIP effect is mainly controlled by stacking fault energy (SFE), which has been analyzed intensively in the present MGMC.

Studies on Wavelet Packet-Based Crack Detection for a Beam under the Moving Load

The objective of this study is to show the potential of the crack detection method based on Wavelet Packet Transform (WPT), which is depending on the response at a single point on a beam subject to moving load. In this paper, an ANSYS model of a cracked beam is established. The moving load is transient analyzed by shifting the point of the concentrated force. The response at mid-span of the beam is calculated and wavelet packet transformed. The crack on the beam can be found by the abnormal signal in WPT branches. The size is also estimated by a defined damage index (Dindex) which relates to the energy of the abnormal signal. Finally, the effects of both crack location and wavelet selection on Dindex are discussed in detail.

Studies on the Dynamic Compressive Properties of Open-Celled Aluminum Alloy Foams

The compressive deformation behavior of open-cell aluminum foams with different densities and morphologies was assessed under quasi-static and dynamic loading conditions. High strain rate experiments were conducted using a split Hopkinson pressure bar technique at strain rates ranging from 500 to 1 2000 − s . The experimental results shown that the compressive stress-strain curves of aluminum foams also have the “ three regions” character appeared in general foam materials, namely elastic region, collapse region and densification regions. It is found that density is the primary variable characterizing the modulus and yield strength of foams and the cell appears to have a negligible effect on the strength of foams. It also is found that yield strength and energy absorption is almost insensitive to strain rate and deformation is spatially uniform for the open-celled aluminum foams, over a wide range of strain rates.

Experimental Study on the Dynamic Splitting Tensile Behaviour of Concrete

Many structures and buildings such as nuclear power station and chemical plant are often subjected to impact and explosive loadings. The understanding of material response to highamplitude, short-duration, impulse loads is very important, dynamic behavior of concrete under high strain rate has been paid much attention to. In the present paper, experimental study on the dynamic tension behavior of concrete is carried out. Based on the former theoretical introduction, dynamic splitting tensile tests at different strain rates are conducted on 74mm diameter concrete specimens in a Split Hopkinson Pressure Bar to study the effect of strain rate on the dynamic tension behavior of concrete. The mechanism and speed of crack propagation of concrete cylinder planar surface in dynamic splitting tensile test are discussed briefly.