• 한국자기학회지
• /
• 제5권3호
• /
• pp.222-232
• /
• 1995

자기저항이란 외부 자기장에 의해 재료의 전기저항이 변화되는 현상을 일컫는다. Au와 같은 비자성도체 및 반도체 재료의 경우 외부에서 자기장이 가해지면 전도 전자가 Lorentz 힘을 받아 궤적이 변하므로 저항이 변화한다. 이러한 저항 변화 를 정상 자기저항(Ordinary Magnetoresistance, OMR)이라 하며 일반적으로 상당히 작은 저항의 변화를 나타낸다. 강자성도체 재료에서는 정상 자기저항 효과 외에도 부가적인 효과가 생긴다. 이는 스핀-궤도 결합에 기인한 효과로써 자기 저항은 강자성체의 자화용이축, 외부자계와 잔류간의 각도에 의존하며 이방성 자기저항(Anisotropic Magnetoresistance, AMR)이라 한다. AMR 비(%)는 일반적 으로 다음과 같이 정의된다. 즉 ${\Delta}{\rho}_{AMR}/{\rho}_{ave}=(\rho_{\|}-\rho_{T})/{\rho}_{ave}$로 여기서 $\rho_{\|}$는 자기장의 방향이 전류의 방향과 같을 때의 비저항 이고 $\rho_{T}$는 서로 수직일 때이며 ${\rho}_{ave}=(\rho_{\|}-\rho_{T})/3$이다. 기존의 MR 센서나 자기재생헤드(magnetic read head)에 사용되는 퍼머로이계 합금의 AMR 비는 상온에서 약 2% 정도의 저항변화를 보인다.

• International Journal of Aerospace System Engineering
• /
• 제2권2호
• /
• pp.24-28
• /
• 2015

Composite materials are highly sensitive to the presence of manufacturing and service-related defects that can reach a critical size during service condition and thereby may affect the safety of the structure. When the structure undergoes some kind of damage, its stiffness reduces, in turn the dynamic responses change. In order to avoid safety issues early detection of damage is necessary. The knowledge of the vibration behavior of a structure is necessary and can be used to determine the existence as well as the location and the extent of damage.

• 한국세라믹학회지
• /
• 제36권8호
• /
• pp.843-847
• /
• 1999

The dc resistivity dc magnetization and thermopower of layered perovskite La1.6Ca1.4Mn2O7.07 have been studied. The ceramic sample of La1.6Ca1.4Mn2O7.07 undergoes the metal-insulator transition at 120K while a first-order phase transition from a ferromagnetic phase to a paramagnetic phae is observed at 260 K=TC This behavior is quite different from that of the well-known double exchange ferromagnets such as La1-xCaxMnO3 This phenomenon could be understood by considering the effects of the anisotropic double exchange interaction caused by two dimensional Mn-O-Mn networks in this materials. The dc magnetization between 120K and 250K is nearly constant and decreases rapidly with increasing temperature above 250K The measurements of dc resistivity and thermopower indicate that Zener polaron hopping conduction takes place above 260 K.

• 연구논문집
• /
• 통권24호
• /
• pp.81-96
• /
• 1994

To investigate failure mechanisms observed in carbon-phenolic thermal insulators, differential equations which govern the decomposition process in a deformable anisotropic porous solid are derived for three-dimensional axisymmetric constructions. The governing equations not only couple the material deformation with pore pressure, but also couple pressure and temperature, which means that heat convected by the pyrolysis gases is properly accounted for. Then the Bubnov-Galerkin finite element method is applied to these equations to transform them into a semidescrete finite element system. A thermal insulation liner in the cowl region under typical operating conditions is analyzed to find a mechanism for plylift. The results from the structural analysis show across-ply failure in the cowl zone. The mechanism for plylift is hypothesized as a sequential procedure : 1) the across-ply failure which is the precursor to plylift and 2) the local fiber buckling caused by generation of excessive in-plane compressive stress. To prevent plylift, the across-ply stress can be reduced by using appropriate material ply angles in cowl zone design.

• 소성∙가공
• /
• 제21권4호
• /
• pp.252-257
• /
• 2012

Crystallographic texture evolution during forming processes has a significant effect on the anisotropic flow behavior of crystalline material. In this study, a crystal plasticity finite element method (CPFEM), which incorporates the crystal plasticity constitutive law into a three-dimensional finite element method, was used to investigate texture evolution of a face-centered-cubic material - an aluminum alloy. A rate-dependent polycrystalline theory was fully implemented within an in-house program, CAMPform3D. Each integration point in the element was considered to be a polycrystalline aggregate consisting of a large number of grains, and the deformation of each grain in the aggregate was assumed to be the same as the macroscopic deformation of the aggregate. The texture evolution during three different deformation modes - uniaxial tension, uniaxial compression, and plane strain compression - was investigated in terms of pole figures and compared to experimental data available in the literature.

• 소성∙가공
• /
• 제13권7호
• /
• pp.629-634
• /
• 2004

Based on plastic instability, analytical prediction of bursting failure on tube hydroforming processes under combined infernal pressure and independent axial feeding is carried out. Bursting is irrecoverable phenomenon due to local instability under excessive tensile stresses. In order to predict the bursting failure, three different classical necking criteria such as diffuse necking criterion for sheet and tube, local necking criterion for sheet are introduced. The incremental theory of plasticity for anisotropic material is adopted and then the hydroforming limit and bursting failure diagram with respect to axial feeding and hydraulic pressure are presented. In addition, the influences of the material properties such as anisotropy Parameter, strain hardening exponent and strength coefficient on bursting Pressure are investigated. As results of the above approach, the hydroforming limit in view of bursting failure is verified with experimental results.

• 한국기계가공학회지
• /
• 제20권4호
• /
• pp.80-86
• /
• 2021

Plastics are widely used in mechanical and other fields due to their light weight, design flexibility, and molding processability. In processing plastics, defective products are mixed and reprocessed to improve production efficiency and reduce costs. In this study, an experiment was conducted to confirm the coefficient of thermal expansion of HDPE during this reprocessing. The coefficient of thermal expansion was measured at different measurement directions and heating rates. As a result, we observed that the coefficient of thermal expansion in the direction perpendicular to the injection direction is greater than that in the horizontal direction.

• 한국기계가공학회지
• /
• 제17권5호
• /
• pp.143-148
• /
• 2018

3D printing has been expanding beyond the bio/nano field to the automobile and aviation industries. 3D-printing technology has to overcome real problems to have economic value compared to its unlimited usability. Typically, the difference in mechanical strength along the lamination direction requires sufficient research to ensure reliability. In this paper, we study the anisotropic properties of ABS based on the stacking method of FDM 3D printing. Specifically, the mechanical properties of ABS material are determined through a tensile test and 3-point bending test, and the in-plane orthotropic properties are ascertained.

• 소성∙가공
• /
• 제30권2호
• /
• pp.69-73
• /
• 2021

This paper addresses the product shape modification for spring-back reduction in the sheet metal forming process of the cabin sunroof which is applied to the construction equipment. Initially, the anisotropic material properties are measured in order to calculate the degree of spring-back by the numerical simulation of the sheet metal forming process. To reduce the spring-back of the stamped part, several design modifications are suggested according to the geometrical bead arrangement on the planar region. The degrees of spring-back are confirmed for various product designs with different use of the geometrical bead. Finally, the spring-back improvement was validated by manufacturing the tryout product with the modified die set for the optimized product shape.

• 한국소성가공학회:학술대회논문집
• /
• 한국소성가공학회 1999년도 춘계학술대회논문집
• /
• pp.62-65
• /
• 1999

To reduce the cost of finite element analyses for sheet forming a 3D hybrid membrance/sheel method has been developed to study the springback of anisotropic sheet metals. in the hybrid method the bending strains and stresses were analytically calculated as post-processing using incremental shapes of the sheet obtained previously from the membrane finite element analysis. To calculate springback a shell finite element model was used to unload the final shape of the sheet obtained from the membran code and the stresses and strains that were calculated analytically. For verification the hybrid method was applied to predict the springback of a 2036-T4 aluminum square blank formed into a cylindrical cup. the springback predictions obtained with the hybrid method was in good agreement with results obtained using a full shell model to simulateboth loading an unloading and the experimentally measured data. The CPU time saving with the hybrid method over the full shell model was 75% for the punch stretching problem.