Amkee Kim

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

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 stressσy, strain hardening exponentn and elastic modulusE, 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 cubicspherical 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.

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

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.

A multi-cell FE-model for compressive behaviour analysis of heterogeneous Al-alloy foam

Compressive mechanical behaviours of closed cell Al-alloy foams produced by melt based and powder metallurgical methods were investigated by a finite element model composed of multiple unit lattices. The unit lattice consists of spherical and cubic sections possessing a thickness ratio between them. A Gaussian distribution of the relative density among the lattices and the random allocation of lattices were implemented in the model to address the structural heterogeneity. The constitutive relation for the lattice material was determined by a nondestructive instrumented sharp indentation test on the cell wall. The simulated compressive stress–strain curves for the ductile Al–Si–Ca foam were found to be in good agreement with the experimental ones over the entire strain range while the resistance of brittle Al–Si–Cu–Mg foam to the deformation turned out to be lower than that predicted by the model after about 0.50 strain. The discrepancy between experimental and simulation results for Al–Si–Cu–Mg foam in high strain range was associated with the disintegration of cell walls broken by the large deformation.

In-situ Resistance Measurements during Tensile Test of Carbon Nanotube using Nano-manipulator

A variety of outstanding experimental results on mechanical properties of carbon nanotube (CNT) are fast appearing. Nevertheless, mechanical properties of CNT have been rarely understood because of measurement difficulties. The response of multi walled carbon nanotube (MWNT) to mechanical strain applied with a nano-manipulator was investigated inside scanning electron microscope (SEM). MWNTs were produced by arc-discharge method and the protruded sample for tensile test was selected among the MWNTs fragment which has rectilinearity and purity above 40%. The average diameter of MWNT was approximately 15 nm. In order to measure the resistance of a MWNT during tensile test, an individual MWNT was attached at the tungsten (W) tip using electron beam induced deposition. We confirmed that the contact resistance between a MWNT and the W tip decrease during the exposure of electron beam. And then the W tip were manipulated and controlled by nano-manipulator and personal computer. We observed that resistance of sample was significantly changed until the MWNT fracture

Evaluation of fracture toughness of degraded Cr-Mo-V steel using electrical resistivity

More effective nondestructive technology for the estimation on material properties has been sought. In this research, a new electrical resistivity method was attempted for the estimation of the fracture toughness of a degraded turbine rotor steel. 1Cr-1Mo-0.25V turbine rotor steel specimens with seven different periods of aging were prepared by an isothermal heat treatment at 630°C and the electrical resistivity was determined by a direct current four points potential method. The electrical resistivity at room temperature monotonously decreased with the extent of degradation of the material. It was also observed that the fracture toughness was correlated with the electrical resistivity. The microstructural changes of material during aging was examined by X-ray diffractometer and electron probe micro analyzer (EPMA). A larger amount Mn23C6 and (Cr2.5Fe4.3Mo0.1)C3 appeared in grain boundaries of the more heavily aged material.

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

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.

Foaming behaviour of Al–Si–Cu–Mg alloys

Abstract The powder metallurgical route was utilised to obtain the Al–5Si–4Cu–4Mg (alloy 544) and Al–3Si–2Cu–2Mg (alloy 322) foams. Various steps such as centrifugal atomisation, mixing alloy powder and foaming agent (1 wt-%TiH2), cold compaction of mixture, hot extrusion and foaming in a preheated furnace were performed. Foaming behaviour of the alloys was investigated by digital microscopy, image analysis, scanning electron microscopy (SEM) and energy dispersive X-ray (EDX) mapping in this study. It was found that alloy 544 takes a shorter period of time to initiate pore nucleation than alloy 322. Alloy 544 had a higher pore growth rate than alloy 322 at the same pre-set furnace temperature. In both alloys, crack-like pore nucleation occurred between aluminium alloy powders elongated in a direction parallel to the extrusion direction. Both alloys showed the same foaming sequence of crack-like pore nucleation, spherical pore growth, coalescence of neighbouring pores and collapse of pores adjacent to the free surface of specimen. The time required to start pore nucleation decreased with the increase of foaming temperature. The cell walls of both alloys consisted of α-Al phase and eutectic phase.

Study on Erosion of Carbon Fiber Reinforced Plastic Composite

The solid particle erosion behaviour of unidirectional carbon fiber reinforced plastic (CFRP) composites was investigated. The erosive wear of these composites was evaluated at different impingement angles (, , , ), different impact velocities (40, 55, 60, 70m/s) and at three different fiber orientations (, , ). The erodent was SiC sand with the size of irregula. shapes. The result showed ductile erosion behaviour with maximum erosion rate at impingement angle. The fiber orientations had a significant influence on erosion. The erosion rate was strongly dependent on impact velocity which followed power law . Based on impact velocity (V), impact angle () and fiber orientation angle (), a method was proposed to predict the erosion rate of unidirectional fiber reinforced composites.

Influence of Foaming Temperature and Time on the Hardness of Cellular Al-Si-Cu-Mg Alloys

ABSTRACT Micro Vickers hardness was measured on the cell wall of A1-Si-Cu-Mg alloy foams having different compositions and subjected to different foaming temperature and period of time during production. The powder metallurgical route was utilized to obtain aluminium alloy foam. A1-5%Si-4%Cu-4%Mg-l%TiH2 alloy precursor showed higher hardness value than A1-3%Si-2%Cu-2%Mg-l %TiH2 alloy because of higher alloying element. The hardness value was increased with the increase of foaming period of time at all foaming temperatures while foams having same composition but higher foaming temperature showed higher hardness value regardless of material. These might be due to the fine eutectic grain size effect. Local hardness of foams at α-A1 grain boundaries and junctions of multiple grains were found to be higher than that inside the α-A1 grains. Higher hardness value of eutectic phase and accumulation of eutectic phase near grain boundaries are responsible for this higher hardness value at α-A1 grain boundaries.

Time dependent process-zone growth in polycarbonate

The process zone evolution strongly influences the crack growth of polycarbonate. A methodology for determination of the kinetics of the process zone evolution was developed by decoupling these two process. Constant displacement (stress relaxation) conditions under which the crack length remained. A thermodynamic model for equilibrial process zone in polycarbonate is developed from the modification of Chudnovsky model. Based on the model the driving force and a new kinetic equation for evolution of the process zone are proposed after considerly of the irreversible thermodynamics and chemical reaction theories. In addition, the validity of model and new kinetic equation are examined experimentally.