• Proceedings of the KWS Conference
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• 2002.10a
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• pp.145-150
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• 2002

It has been well known that ductile fracture of steels is accelerated by triaxial stresses. The characteristics of ductile crack initiation in steels are evaluated quantitatively using two-parameters criterion based on equivalent plastic strain and stress triaxiality. It has been demonstrated by authors using round-bar specimens with circumferential notch in single tension that the critical strain to initiate ductile crack from specimen center depends considerably on stress triaxiality, but surface cracking of notch root is in accordance with constant strain condition. In order to evaluate the stress/strain state in the specimens, especially under dynamic loading, a thermal, elastic-plastic, dynamic finite element (FE) analysis considering the temperature rise due to plastic deformation has been carried out. This study provides the fundamental clarification of the effect of strength mismatching, which can elevate plastic constraint due to heterogeneous plastic straining, loading mode and loading rate on critical condition to initiate ductile crack from notch root using equivalent plastic strain and stress triaxiality based on the two-parameter criterion obtained on homogeneous specimens under static tension. The critical condition to initiate ductile crack from notch root for strength mismatched bend specimens under both static and dynamic loading would be almost the same as that for homogeneous tensile specimens with circumferential sharp notch under static loading.

• Carbon letters
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• v.5 no.3
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• pp.108-112
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• 2004

Effect of silicon infiltration on the bend and tensile strength of 2D cross-ply carbon-carbon composites are studied. It is observed that bend strength higher than tensile strength in both types of composite is due to the different mode of fracture and loading direction. After silicon infiltrations bend and tensile strength suddenly decreases of carbon-carbon composites. This is due to the fact that, after silicon infiltration, silicon in the immediate vicinity of carbon forms the strong bond between carbon and silicon by formation silicon carbide and un-reacted silicon as free silicon. Therefore, these composites consist of three components carbon, silicon carbide and silicon. Due to mismatch between these three components secondary cracks developed and these cracks propagate from $90^{\circ}$ oriented plies to $0^{\circ}$ oriented plies by damaging the fibers (i.e., in-situ fiber damages). Hence, secondary cracks and in-situ fiber damages are responsible for degradation of mechanical properties of carbon-carbon composites after silicon infiltration which is revealed by microstructure investigation study by scanning electron microscope.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.36 no.2
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• pp.187-194
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• 2012

The strength of particle-reinforced metal matrix composites is, in general, known to be increased by the geometrically necessary dislocations punched around a particle that form during cooling after consolidation because of coefficient of thermal expansion (CTE) mismatch between the particle and the matrix. An additional strength increase may also be observed, since another type of geometrically necessary dislocation can be formed during extensive deformation as a result of the strain gradient plasticity due to the elastic-plastic mismatch between the particle and the matrix. In this paper, the magnitudes of these two types of dislocations are calculated based on the dislocation plasticity. The dislocations are then converted to the respective strengths and allocated hierarchically to the matrix around the particle in the axisymmetric finite-element unit cell model. The proposed method is shown to be very effective by performing finite-element strength analysis of $SiC_p$/Al2124-T4 composites that included ductile failure in the matrix and particlematrix decohesion. The predicted results for different particle sizes and volume fractions show that the length scale effect of the particle size obviously affects the strength and failure behavior of the particle-reinforced metal matrix composites.

• Journal of Welding and Joining
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• v.32 no.3
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• pp.74-80
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• 2014

Due to environmental regulations (RoHS, WEEE and ELV) of the European Union, electronics and automotive electronics have to eliminate toxic substance from their devices and system. Especially, reliability issue of lead-free solder joint is increasing in car electronics due to ELV (End-of-Life Vehicle) banning from 2016. We have prepared engine control unit (ECU) modules soldered with Sn-40Pb and Sn-3.0Ag-0.5Cu (SAC305) solders, respectively. Degradation characteristics of solder joint strength were compared with various conditions of automobile environment such as cabin and engine room. Thermal cycle test (TC, $-40^{\circ}C$ ~ ($85^{\circ}C$ and $125^{\circ}C$), 1500 cycles) were conducted with automotive company standard. To compare shear strength degradation rate with eutectic and Pb-free solder alloy, we measured shear strength of chip components and its size from cabin and engine ECU modules. Based on the TC test results, finally, we have known the difference of degradation level with solder alloys and use environmental conditions. Solder joints degradation rate of engine room ECU is superior to cabin ECU due to large CTE (coefficient of thermal expansion) mismatch in field condition. Degradation rate of engine room ECU is 50~60% larger than cabin room electronics.

• Journal of the Korean Ceramic Society
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• v.48 no.2
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• pp.152-159
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• 2011

The green and hard machining characteristics of dental ceramics are of great interest to dental industry. The green bodies of TZP/$Al_2O_3$ composites were prepared by the cold isostatic pressing, and machined on the CNC lathe using PCD (polycrystalline diamond) insert under various machining conditions. With increasing nose radius of PCD insert, surface roughness initially increased due to increased cutting resistance, but decreased by the onset of sliding fracture. The lowest surface roughness was obtained at spindle speed of 1,300 rpm and lowest feed rate. Hard bodies were prepared by pressureless sintering the machined green bodies at several temperatures. The grinding test for sintered hard body was conducted using electroplated diamond bur with different grit sizes. During grinding, grain pull out in the composite was occurred due to thermal expansion mismatch between the alumina and zirconia. The strength of the composite decreased with alumina contents, due to increased surface roughness and high monoclinic phase transformed during grinding process. The final polished samples represented high strength by the elimination of a phase transformation layer.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.38 no.5
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• pp.483-487
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• 2014

In this study, joining methods with SiC powder as the joining adhesives were studied in order to avoid the residual stresses coming from CTE (Coefficient of Thermal Expansion) mismatch between substrate and joining layer. The shear strength and microstructure of joined material between SiC substrates are investigated. The commercial Hexoloy-SA (Saint-Gobain Ceramics, USA) used in this work as substrate material. The fine ${\beta}$-SiC nano-powder which the average particle size is below 30 nm, $Al_2O_3$, $Y_2O_3$, and $SiO_2$ were used as joining adhesives. The specimens were joined with 20MPa and $1400-1900^{\circ}C$ by hot pressing in argon atmosphere. The shear test was performed to investigate the bonding strength. The cross-section of the joint was characterized by using an optical microscope and scanning electron microscopy (SEM).

• Computational Structural Engineering
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• v.8 no.1
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• pp.173-186
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• 1995

It is well known that structures constructed by bonding two or more materials and then subjected to temperature change experience thermal stress. This stress results from thermal expansion mismatch of materials. The present paper derives formulas for the stresses in a bimaterial axisymmetric disk which is subjected to a uniform temperature change. First, an approximate solution following strength-of-materials principles is developed. However, the strength-of-materials solution has difficulty in predicting both the peak value of interfacial stresses and its associated distribution. Next, a solution consistent with the theory of elasticity is developed by way of an eigenfunction expansion approach. The eigenfunction analysis is compared with finite element stress analysis results for a specific numerical example. Finite element analysis results show that the interfacial stresses are adequately predicted by eigenfunction solution. Therefore, the method developed in this paper will be useful in determination of the interfacial stress state.

• KSII Transactions on Internet and Information Systems (TIIS)
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• v.11 no.5
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• pp.2701-2722
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• 2017

Key generation is essential for protecting wireless networks. Based on wireless channel reciprocity, transceivers can generate shared secret keys by measuring their communicating channels. However, due to non-simultaneous measurements, asymmetric noises and other interferences, channel measurements collected by different transceivers are highly correlated but not identical and thus might have some discrepancies. Further, these discrepancies might lead to mismatches of bit sequences after quantization. The referred mismatches significantly affect the efficiency of key generation. In this paper, an efficient key generation scheme leveraging wireless channel reciprocity is proposed. To reduce the bit mismatch rate and enhance the efficiency of key generation, the involved transceivers separately apply discrete cosine transform (DCT) and inverse discrete cosine transform (IDCT) to pre-process their measurements. Then, the outputs of IDCT are quantified and encoded to establish the bit sequence. With the implementations of information reconciliation and privacy amplification, the shared secret key can be generated. Several experiments in real environments are conducted to evaluate the proposed scheme. During each experiment, the shared key is established from the received signal strength (RSS) of heterogeneous devices. The results of experiments demonstrate that the proposed scheme can efficiently generate shared secret keys between transceivers.

• Progress in Superconductivity and Cryogenics
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• v.16 no.4
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• pp.49-52
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• 2014

REBCO coated conductor (CC) tapes with superior mechanical and electromechanical properties are preferable in applications such as superconducting coils and magnets. The CC tapes should withstand factors that can affect their performance during fabrication and operation of its applications. In coil applications, CC tapes experience different mechanical constraints such as tensile or compressive stresses. Recently, the critical current ($I_c$) degradation of CC tapes used in coil applications due to delamination were already reported. Thermal cycling, coefficient of thermal expansion mismatch among constituent layers, screening current, etc. can induce excessive transverse tensile stresses that might lead to the degradation of $I_c$ in the CC tapes. Also, CC tapes might be subjected to very high magnetic fields that induce strong Lorentz force which possibly affects its performance in coil applications. Hence, investigation on the delamination mechanism of the CC tapes is very important in coiling, cooling, operation and design of prospect applications. In this study, the electromechanical properties of REBCO CC tapes fabricated by reactive co-evaporation by deposition and reaction (RCE-DR) under transversely applied loading were investigated. Delamination strength of the CC tape was determined using the anvil test. The $I_c$ degraded earlier under transverse tensile stress as compared to that under compressive one.

• Journal of the Korean Ceramic Society
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• v.36 no.1
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• pp.61-68
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• 1999

To analyze several defects and residual stress in sintering of layer structure materials, multiayer materials with TZP/SUS and ZT/SUS, and bilayer materials with porcelain/alumina and porcelain/Y-TZP were fabricated by sintering method. Multilayer materials prepared by pressureless sintering show the sintering defect such as warping, splitting, cracking originated from the difference of sintering shrinkage between each layer, which could be controlled by the adjustment of number and thickness in interlayer. In tape casting, a certain pressure given during sintering relaxed the sintering defects, specially warping. The residual stress in bilayer was examined with Vickers indentation method. A small tensile stress in porcelain/alumina and a large compressive stress in porcelain/Y-TZP were generated on the porcelain interface due to the thermal expansion mismatch, which affected the strength of bilayer materials. As a consequence, the sintering defects of multilayer materials and the residual stresses of bilayer materials were dominantly influenced on material design and starting material constants.