A study on the unloading modulus of Al-alloy foam

Tunvir, Kazi,Jeong, Gil-Do,Kim, Amkee,Cheon, Seong-Sick,Ahn, Byung-Wook 한밭대학교 생산기반기술연구소 2004 생산기반기술연구소 논문집 Vol.5 No.1

Behavior of unloading modulus of closed cell Al-alloy foam has been investigated through experiment and finite element analysis. Al-Si-Ca alloy foam having an isotropic material property Produced by melt based process was used in the evaluation of the behavior of unloading modulus of closed cell aluminum alloy foam. In the evaluation process a finite element analysis was accomplished for which a multiple cell finite element model having cruciform and hemispherical type cells originally proposed by Santosa et at. was used. The Santosa model was modified by introducing different wall thicknesses for cruciform and hemisphere cells on the basis of microstructural examination of the real foam cell wall structure. The elastic modulus of the form specimen has been found to increase with the monotonic compression. In case of finite clement analysis the unloading behavior of the monotonic compression also showed the same trend as the experimental result but with the discrepancy between finite clement analysis and experiment in low deformation range. This discrepancy may be associated with the different cell morphologics which caused the different deformation patterns of the cells. The influence of the loading condition on the unloading modulus was also studied by performing a compression-compression fatigue test measuring the unloading modulus by measuring the cyclic stress-strain during each fatigue cycle. In this case, the unloading elastic modulus showed a decreasing trend with increase of strain unlike in case of the monotonic compression.

The effect of two neighboring defects on the mechanical properties of carbon nanotubes

Tunvir, Kazi,Kim, Amkee,Nahm, Seung Hoon IOP Pub 2008 Nanotechnology Vol.19 No.6

<P>The tensile behavior of single-walled nanotubes (SWNTs) having two defects (vacancy or Stone–Wales) positioned next to each other was simulated in this study to investigate the influence of the spatial arrangement of defects on the mechanical properties. The simulations were performed using classical molecular dynamics (MD) at the atomic scale. Two neighboring vacancy defects reduced the failure strength as much as 46% and the failure strain as much as 80% in comparison with those of pristine SWNTs, while two neighboring Stone–Wales defects reduced them as much as 34% and 70% respectively. SWNTs having two defects in the loading (axial) direction showed higher failure strength than SWNTs with defects perpendicular to the loading direction. For both types of defect, the closer the defects, the weaker the SWNTs. As result, the defect arrangement in the SWNT structure must be one of the key factors in determining its mechanical properties, as well as the population of defects.</P>

Analytical Solution for Crushing Behavior of Closed Cell Al-Alloy Foam

Tunvir, K.,Kim, A.,Cheon, S. S. unknown 2007 MECHANICS OF ADVANCED MATERIALS AND STRUCTURES Vol.14 No.5

<P> Crushing behavior of closed cell aluminum (Al) alloy foam was studied. Al-alloy foam was represented by an array of closely packed large cubic cells truncated at the corners by small spherical cells. The membrane stretching and bending energy associated with the crushing of a cell wall in conjunction with minimum energy principle in plasticity was considered in the analytical formulation. The analytical solutions of cubic-spherical lattice for crushing resistance were found to be in good agreement with the results of FE analysis. Finally, those were compared with available crushing resistances of commercial Al-alloy foams such as Al-Si-Cu-Mg, CYMAT, ALCAN and IFAM foams.</P>

Mechanical Properties of Carbon Nanotubes with Randomly Distributed Vacancy Defects

Kazi Tunvir,Seung Hoon Nahm,Amkee Kim,이학주 한국물리학회 2007 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.51 No.III

Carbon nanotubes (CNTs) offer very high mechanical properties with huge scatter. The scatter in the properties is believed to occur mainly due to the defects originated inherently during production. CNTs under a tensile load having randomly distributed vacancy defects are simulated to investigate the effect of the spatial distribution of defects on the mechanical properties. A simple random unit generation method was used to allocate the defects randomly in the single-walled nanotube's structure. The simulation was carried out a using classical molecular dynamics (MD) simulation technique in atomic scale. Defect density of 1 \% reduced the failure strength, the failure strain and Young's modulus of CNTs by as much as 42 \%, 65 \% and 2 \%, respectively, while a defect density of 8 \% lowered the properties by as much as 52 \%, 71 \% and 14 \%, respectively. The scatter in the properties due to the random distribution of the defects was found to increase with increasing number of defects in the SWNTs. For example, for a defect density of 8 \%, the standard deviations of the data for 20 sample simulations having different distributions normalized to the mean values calculated for the failure strength, the failure strain and Young's modulus were about 11 \%, 20 \% and 8 \%, respectively. The defect arrangement in the SWNT's structure is one of the key factors in determining its mechanical properties and the population of defects.

A study on the unloading modulus of Al-alloy foam

Kazi Tunvir,Gil-Do Jeong,Amkee Kim,Seong-Sick Cheon,Byung-Wook Ahn 한밭대학교 생산융합기술연구소 2005 생산융합기술연구소 논문집 Vol.5 No.1

Time–temperature superposition for foaming kinetics of Al-alloy foams

Kim, Amkee,Tunvir, Kazi,Nahm, Seung-Hoon,Cho, Seong-Seock Elsevier 2008 Journal of materials processing technology Vol.202 No.1

<P><B>Abstract</B></P><P>Applicability of time–temperature superposition principle to the foaming kinetics of aluminum (Al)-alloy foams produced by powder metallurgical method was investigated. Foaming kinetics above melting temperatures of Al–Si–Cu–Mg foams was studied. The expansion data at various furnace temperatures were collected. Well-known superposition parameters such as Larson-Miller, Orr-Sherby-Dorn, Goldhoff-Sherby and Manson-Succop were established based on the linear iso-expansion lines in plots of log(heating time) versus furnace temperature and log(heating time) versus inverse furnace temperature. In order to study the expansion kinetics of the Al-alloy foams, the expansions were measured in terms of pore fraction using an image analyzer. Finally, the linear relationship between the porosity and the superposition parameters was established.</P>

Study of Al-Alloy Foam Compressive Behavior Based on Instrumented Sharp Indentation Technology

Kim Am-Kee,Tunvir Kazi The Korean Society of Mechanical Engineers 2006 JOURNAL OF MECHANICAL SCIENCE AND TECHNOLOGY Vol.20 No.6

The stress-strain relation of aluminum (Al) alloy foam cell wall was evaluated by the instrumented sharp indentation method. The indentation in a few micron ranges was performed on the cell wall of Al-alloy foam having a composition or Al-3wt.%Si-2wt.%Cu-2wt.%Mg as well as its precursor (material prior to foaming). To extract the stress-stram relation in terms of yield stress ${\sigma}_y$, strain hardening exponent n and elastic modulus E, the closed-form dimensionless relationships between load-indentation depth curve and elasto-plastic property were used. The tensile properties of precursor material of Al-alloy foam were also measured independently by uni-axial tensile test. In order to verify the validity of the extracted stress-strain relation, it was compared with the results of tensile test and finite element (FE) analysis. A modified cubic-spherical lattice model was proposed to analyze the compressive behavior of the Al-alloy foam. The material parameters extracted by the instrumented nanoindentation method allowed the model to predict the compressive behavior of the Al-alloy foam accurately.

Study of Al-Alloy Foam Compressive Behavior Based on Instrumented Sharp Indentation Technology

Amkee Kim,Kazi Tunvir 대한기계학회 2006 JOURNAL OF MECHANICAL SCIENCE AND TECHNOLOGY Vol.20 No.6

The stress-strain relation of aluminum (Al) alloy foam cell wall was evaluated by the instrumented sharp indentation method. The indentation in a few micron ranges was performed on the cell wall of Al-alloy foam having a composition of Al-3wt.%Si-2wt.%Cu-2wt.%Mg as well as its precursor (material prior to foaming). To extract the stress-strain relation in terms of yield stress σy, strain hardening exponent n and elastic modulus E, the closed-form dimensionless relationships between load-indentation depth curve and elasto-plastic property were used. The tensile properties of precursor material of Al-alloy foam were also measured independently by uni-axial tensile test. In order to verify the validity of the extracted stress-strain relation, it was compared with the results of tensile test and finite element (FE) analysis. A modified cubic-spherical lattice model was proposed to analyze the compressive behavior of the Al-alloy foam. The material parameters extracted by the instrumented nanoindentation method allowed the model to predict the compressive behavior of the Al-alloy foam accurately.

Study on Compressive Behavior of Heterogeneous Al-alloy Foam by Cruciform-Hemisphere Model

Amkee Kim(김엄기),Kazi Tunvir(카지 턴비르),Sung-Jun Park(박성준),Gil-Do Jeong(정길도),Hasan(하산),Seong-Sik Cheon(전성식) 대한기계학회 2005 대한기계학회 춘추학술대회 Vol.2005 No.5

Compressive behavior of closed cell aluminum (Al) alloy (Al-Si-Ca and Al-Si-Cu-Mg) foams having an isotropic material property was investigated by finite element (FE) analysis of a multiple lattice finite element model composed of cruciform and hemispherical sections. Different wall thickness for the hemisphere and the cruciform sections were considered in the model on the basis of micro structural examination of real foam. The simulation for Al-Si-Ca foam being ductile showed a good agreement with the experiment in the entire deformation range. However Al-Si-Ca-Mg foam being brittle in nature showed less resistance to the large deformation compared to the simulation.