Gui Tong Yang

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.

Effect of Cell Size on the Dynamic Compressive Properties of Aluminum Alloy Foams

The static and dynamic compressive behaviors of open-cell aluminum alloy foams with virtually the same relative density of 0.4 were investigated. The foams have different cell sizes (0.5mm, 1.5mm, 2.5mm) but similar cell morphology and microstructure. The yield strength of these foams was characterized as a function of strain rate and cell morphology. The experimental results indicated that the mechanical responses of foams are sensitive to strain rate, and dependent of the cell size. The present results are compared in details with recent findings obtained from the aluminum foams.

Counterintuitive Dynamic Behavior in Elastic-Plastic Structures

The counterintuitive phenomena of elastic, perfectly plastic beam, circular plate and square plate are investigated numerically and experimentally. The unsteady areas and uncertainty of response are observed numerically. At the end, the law of thermodynamics and the theorem of Lyapunov instability are employed to state the formation mechanism of counterintuitive behavior.

Penetration of Cone-Nose Projectile to the Laminated Composite Aluminum Alloy Foam Target

A theoretical study is presented herein on the penetration of the laminated composite aluminum alloy foam target struck normally by conical-nosed projectiles. Two layers were arranged according to the density of the respective foam; configuration 1 consisted of 10mm/semi-infinite continuous foams and configuration 2 consisted of 20mm/ semi-infinite continuous foams. The dynamic cavity expansion theory is applied to formulate analytical model. The penetrating process can be divided into 6 stages. The resistance equations during every stage are derived. Penetrating depth of projectile are analyzed. The effect of initial velocity, mass density of foam material and the thickness of the upper layered foam on the penetration resistance are investigated. It is found that composite target have a higher penetrating resistance than the monolithic foam material target of equal mass. The analytical results show that configuration 1 outperformed configuration 2 in regards to their penetrating resistance. The thickness of the upper layered foam within 5-20mm has significant influence on penetrating depth. The energy absorption capacity of the composite target material is evaluated.

Effect of heat treatment on compressive properties of open cell aluminum foams

This paper presents a study of heat treatment on the quasi-static and dynamic compressive properties of the open cell aluminum alloy foams in as-fabricated (F), age-hardened (A) and T6-strengthened (T6) conditions. Although the strain rate and heat treatment of foams are different, all exhibit similar deformation behavior in the subsequent deformation. The yield stress of foams at different strain rates are improved by heat treatment, all exhibit some strain rate sensitivity. However, the densification strain of foams is not sensitive to heat treatment.

Investigation of the Dynamic Buckling in the Carbon Nanotube under Impact Torque

The dynamic buckling caused by propagation of a stress wave in single-wall carbon nanotube subjected to impact torque is investigated. The single-wall carbon nanotube is modeled by a cylindrical shell with semi-infinite length, and the dynamic buckling under impact torque is reduced to a bifurcation problem caused by the propagation of torsion stress wave. The bifurcation problem can be converted to solving a group of nonlinear algebraic equations. The numerical computation is carried out, and the effects of the different parameters on dynamic buckling are discussed. It is found that if critical buckling time of the carbon nanotube is different, the corresponding buckling model is different, too. Relation between the critical buckling stress and the critical buckling time is given. Molecular-dynamic simulations of torsional deformation of a single-wall carbon nanotube have been used to obtain the critical buckling strain, which is 0.064. In this work, the critical buckling strain obtained by the continuum model is 0.061, which is very close to the value 0.064. Single-wall carbon nanotubes have very much powerful anti-impact torque, and the critical buckling shearing stress can reach up to 132GPa.

Dynamic Compressive Strength of Aluminum Alloy Foams

The dynamic compressive behavior of open-cell aluminum alloy foams with different length of specimens was investigated using the split Hopkinson pressure bar technique. Plastic strength was measured for aluminum alloy foam specimens having the three cell sizes but similar cell microstructure. Longer specimens exhibited lower mean strength and broader scattering of the strength values than the shorter ones. It can be observed that mechanical response of aluminum alloy foams appear to be dependent of the cell size for both the shorter and longer specimens.

Numerical Study on Damage of Half-Infinite Bar

Drillstring in oil drilling is simplified as a half-infinite bar. Taking account of Peierls-Nabarro(P-N) force and viscous effect of solid, dynamic response is simulated under harmonic driving at the end of the bar. With the physical properties specified, periodic, quasi-periodic and chaotic motions would appear as a function of the amplitude of external driving. We find that all the particles of the bar have the same qualitative characters, while vibrating amplitude decreases along the bar. It appears that particles of the bar reach chaotic motion through quasi-periodic motion. Investigation of this half-infinite bar will provide references for design of drillstring to keep motion away from chaotic region.

A Test Method Used for the Study on Dynamic Behavior of Porous Materials

There is certain difficulty in determining the dynamic properties of metallic porous materials by using the SHPB technique due to special properties of these materials. In order to identify the initial dynamic collapse strength and “plateau” stress and investigate the effect of strain rate and the effect of velocity by tests, a direct impact device is designed based on the current SHPB experimental device in the present paper. The direct impact device is dynamically calibrated .The transfer function h(t) is given by using the deconvolution in inverse analysis with computer simulation. The data processing system is performed and an efficient method to investigate the dynamic behavior of metallic porous materials is afforded.

Experimental Study on Dynamic Mechanical Properties of Al2O3 Ceramics

Several modifications on classical Split Hopkinson Pressure Bar apparatus were implemented in order to improve the capability of systems for testing on high-strength materials. According to the theory of one-dimensional stress wave, the modification method on data handling was given. The dynamic mechanical properties of Al2O3 ceramics were investigated using the improved SHPB technique. It is shown that ceramics is a nonlinear elastic-brittle material. This material is sensitive to strain-rate at higher strain-rate. The dynamic modulus of elasticity and dynamic compressive strength of ceramics increase with increase in strain rate. A dynamic constitutive equation is obtained by fitting the test data. Introduction The desirable mechanical properties of Al2O3 ceramics, including high stiffness, high strength and low density, make it attractive for a wide range of armor applications[1]. Therefore, the study on the dynamic behavior of the ceramics under impact loading is significative. Some experimental studies have been made in recent years to research the dynamic mechanical properties of ceramics at high strain-rate by performing plate-impacting experiment [2,3]. Split Hopkinson Pressure Bar apparatus can be used to characterize the dynamic stress-strain relations under uniaxial compression. However, Al2O3 ceramics is a brittle material with high strength so that the breaking strain is tiny. It is very difficult to study the dynamic behavior of ceramics by classical SHPB technology. A study on the compressive strength of two-phase TiB2+Al2O3 ceramics was made by G.Kennedy[4]. The stress-time curves were obtained by using SHPB technology. The dynamic behaviors of the 95%,97% and 99% Al2O3 ceramics were investigated by L.Huang [5].However, only some preliminary results were obtained in the former research works. The stress-strain curves of Al2O3 ceramics were measured adopting the classical SHPB apparatus for attempt. Several problems in classical SHPB technology were found: (1) The strain rate in specimen is lower;(2) The stress level in specimen is lower; (3) The hardness of the specimen is greater than that of the SHPB bars. As a result, the bar ends will be damaged in the tests. In the present paper,In order to obtain more desirable results, several modifications on classical SHPB technology were implemented. by the use of an improved SHPB apparatus, the uniaxial compression tests for Al2O3 ceramics in the strain-rate range from 560~750 s-1 were carried out and the stress-strain curves of Al2O3 ceramics were obtained. From the experimental results, it is shown that ceramics is nonlinear elastic-brittle material. This material is sensitive to strain-rate at higher strain-rate. The dynamic modulus of elasticity and dynamic compressive strength of ceramics increase with increase in strain rate. A dynamic constitutive equation is obtained by fitting the test data. Key Engineering Materials Online: 2004-10-15 ISSN: 1662-9795, Vols. 274-276, pp 835-840 doi:10.4028/www.scientific.net/KEM.274-276.835