• 대한기계학회논문집A
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• 제39권8호
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• pp.781-785
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• 2015

엔지니어링 플라스틱(Engineering Plastic ; EP)에 대한 내구성 평가를 위해 현재 새롭게 국내에서 개발한 초음파 피로시험법을 이용하여 EP 중에서도 결정화 속도가 빠르고 결정화도가 높은 범용 POM(Polyoxymethylene) 소재에 대한 초음파 가속피로시험 거동을 평가하고자 하였다. 이에 본 연구에서 사용된 POM 소재의 밀도는 $1.37g/cm^3$, 동탄성계수는 3.49 GPa 로 측정되었으며, 초음파피로시험 20kHz, 응력비 R= -1 의 판상시편 두께 (4t, 7t, 10t)에 따른 피로수명평가 결과 전체 응력진폭 5.0~6.0MPa 부위에서 피로한도를 확인하였다. 피로시험 후 파단 면을 관찰한 결과 7t, 10t 두께 시편의 크랙 시작위치에서 미소 공동들이 서로 연결된 형태의 잔금(crazing) 균열현상으로 파단된 dimple 구조형상을 전자현미경을 통해 확인할 수 있었다.

• Journal of Welding and Joining
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• 제28권1호
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• pp.34-40
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• 2010

In Friction Stir Lap Welding(FSLW), the movement of material within the weld was more important than the microstructure, due to the interface present between the sheets. Thus, The soundness of free defect, Effective Sheet Thickness(EST) and width of joint were most important factor of mechanical properties. Specimens by lap joint types that were 'A-type' and 'R-type' were made in this study. A-type tensile specimen was loaded at advancing side and R-type tensile specimen was loaded at retreating side. Macro-, micro-structural observation and mechanical properties of FSLW A5052-H112 alloy ware investigated under varying rotating and welding speed. The results were as follows: Material hook formed decreasing after sharply increasing was appeared at the end interface of joint area in advanced side, and material hook formed decreasing after smoothly increasing was observed at that in retreated side. Tensile load had no relation with defects. As rotating speed was higher, tensile strength was increasing and EST was decreasing regardless of joint types. joint efficiency was over 70%. In a result of fractography, fracture in A-type was partially occurred by dimple in SZ, and fracture in R-type was generally occurred by dimple in HAZ.

• 드라이브 ㆍ 컨트롤
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• 제13권1호
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• pp.27-33
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• 2016

In hydraulic machinery, the hydraulic fluid acts primarily as working fluid and secondarily as a lubricant. Hence, the viscous friction force acting on the sliding components should be reduced to improve the mechanical efficiency. It is now well known that the surface texturing is a useful method for friction reduction. In this study, using a commercial computational fluid dynamics (CFD) code, FLUENT, the lubrication characteristics of a surface textured slider bearing under high boundary pressure difference is studied. The streamlines, velocity profiles, pressure distributions, load capacity, friction force and leakage flowrate are highly affected by the film thickness ratio and the textured region. Partial texturing at the inlet region of the inclined slider bearing can reduce both friction force and leakage flowrate than in the untextured case. The present results can be used to improve the lubrication characteristics of hydraulic machinery.

• 한국안전학회지
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• 제9권1호
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• pp.9-18
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• 1994

In this study, we conducted experimental tests to evaluate fracture behaviors of fresh-butt welded metal by Acoustic Emission technique. We selected similar welding and dissimilar welding process, the one welded for SM45C, SS41 and SUS304 of each material, the other for SM45C and SS41, SM45C and SUS304 and SS41 and SUS304. The fracturing processes of weld metal were estimatied through the fracture toughness test with compact tension specimens and fractography analysis. In ASTM test method E-399, type I curves for materials of this study were obtained by load-cod diagram of fracture toughness test. and 5% offset load( $P_{5}$) was estimated as the estimated crack initial load( $P_{Q}$), The estimated crack initial load( $P_{Q}$) of similar welding materials generally lower than base matal, and then SM45C appeared greatly in decreasing rate of PB, SS41 and SUS304 appeared in order. $P_{Q}$ of dissimilar welding materials were lower than the similar welding materials. $P_{Q}$ of welding of SM45C and SS41 appeared in small, SUS304 and SS41 appeared greatly in dissimilar welding materials. In fracture toughness test, AE counts increased before the inflection point of the slope, decreased after that. It was found that increasing of AE counts were due to the microcrack formation at the crack tip near the $P_{5}$ point through AE data. For welding materials in this study, both low and high AE amplitude appeared simulataneously. It was confirmed that the low AE amplotude was due to formation of micro void, micro crack or micro dimple, the high AE amplitude was caused by microvoid coalescence and quasi-cleavage fracture through analyses of fractograpy.apy.apy.apy.

• 한국주조공학회지
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• 제15권2호
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• pp.138-145
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• 1995

The mechanical properties of $Al/Al_2O_3$ composites have been investigated in relation with manufacturing factors such as applied pressure of casting and binder amount of preform. It was found that tensile strength increases with an increase of applied pressure, but decreases with binder amount. Increase of tensile strength is attributable to refinement of microstructure, improvement of intefacial bonding between $Al_2O_3$ short fiber and matrix, decrease of porosity in the matrix. Due to the high thermal stability of alumina short fiber, tensile strength of composites at $150^{\circ}C$ was superior to matrix alloy at room temperature. To evaluate the strength of composites, modified Kelly-Tyson's equation was introduced. Manufacturing factor M was obtained calculating from experimental data. M values were increased with applied pressure, but decreased with binder amount. The initiation of microcrack appeared to be at interface and reinforcement colony. Amount of micro-dimple was increased with applied pressure, and interfacial debonding phenomenon was remarkable with an increase of binder amount.

• 한국해양공학회지
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• 제1권1호
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• pp.127-137
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• 1987

The evaluation of the elastic-plastic fracture toughness $J_{1C}$ was performed on the center of weld metal(CWM), the heat affected zone (HAZ) and the base metal (BM) of API 5LB steel pipes welded by the high frequency electric resistance welding. The $J_{1C}$ was evaluated by the JSME R-Curve and JSME SZW methods using the smooth and side-grooved specimens. The results are as follows; (1) The $J_{1C}$ values by the SZW method are overestimated as compared with those by the R-curve method, because the micro-crack is formed as SZW increase with the deformation at SZ after initiation of the ductile crack. (2) The everage of $J_{1C}$ values by the the R-curve and the SZW methods in side-grooved specimens tended to decrease in comparison with smooth specimens 9.42% at CWM, 4.2% at HAZ, 23.2% at BM, respectively. (3) The boundary of the fatigue pre-crack, stretched zone, and dimple regions appeared more clearly in side-grooved specimens, for the slight change of SZW in the direction of the plate thickness, as compared with smooth specimens.

• 대한기계학회논문집
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• 제11권5호
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• pp.717-724
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• 1987

본 연구에서는 한국중공업에서 실제로 제작한 터어빈 로우터를 시험편으로 하 여 -150.deg. C에서 +150.deg. C에 걸쳐 인장시험, 충격시험 및 파괴인성시험을 실시하여 각 온 도에서 기계적 성질과 파괴인성치를 시험적으로 구하고 Begley-Logsdon의 방법에 따라 파괴인성치를 예측하여 예측방법의 유용성 여부를 조사하고 또한 파단면의 양상을 함 께 검토하였다.

• Journal of Welding and Joining
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• 제19권1호
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• pp.65-74
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• 2001

A study on the corrosion behavior in case of As-welded and PWHT temperature 55$0^{\circ}C$ of welding part of RE36 steel for marine structure was investigated with parameters such as micro-Vickers hardness, corrosion potential measurement of weld metal(WM), base metal(BM) and heat affected zone(HAZ), both Al anode generating current and Al anode weight loss quantity under sacrificial anode cathodic protection conditions. And also we carried out slow strain rate test(SSRT) in order to research both limiting cathodic polarization potential for hydrogen embrittlement and optimum cathodic protection potential as well as mechanical properties by post-weld heat treatment(PWHT) effect. Hardness of HAZ was the highest among three parts(WM, BM and HAZ) and the highest galvanic corrosion susceptibility was HAZ. And the optimum cathodic polarization potential showing the best mechanical properties by SSRT method was from -770mV to -875mV(SCE). In analysis of SEM fractography, applied cathodic potential from -770mV to -875mV(SCE) it appeared dimple pattern with ductile fracture while it showed transgranular pattern (Q. C : quasicleavage) under -900mV(SCE). However it is suggested that limiting cathodic polarization potential indicating hydrogen embrittlement was under -900mV(SCE).

• 한국주조공학회지
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• 제14권1호
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• pp.52-61
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• 1994

The effect of Ni addition, on the mechanical properties and fracture characteristics of Mo-Cu and Mo-Ni-Cu alloyed ductile iron austenitized at $900^{\circ}C$ and austempering temperatures of $250^{\circ}C$, $300^{\circ}C$ and $350^{\circ}C$. The tensile strength, yield strength and hardness are decreased and elongation and impact value are increased in both Mo-Cu and Mo-Ni-Cu alloyed austempered ductile iron, with increased austempering temperature. According to the austempering temperature are increased, the amount of retained austenite are increased. Maximum value of fracture toughness is obtained at $350^{\circ}C$ austempering temperature at this condition, the amount of retained austenite came to 40% in Mo-Ni-Cu alloyed ADI and 34% in Mo-Cu alloyed ADI. The fracture surface of ADI which had represented high toughness are showed a quasi-cleavage pattern and a dimple pattern with micro void. Comparing the fracture characteristics of Mo-Cu alloyed ADI with that of Mo-Ni-Cu alloyed ADI, the latter was superior to the former.

• Journal of Welding and Joining
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• 제2권2호
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• pp.54-61
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• 1984

Construction of welding structure is greatly dependent upon welding heat cycle. Fracture toughness is decreased remarkablely due to coarse grained HAZ and inequal residual stress of three dimensions to originate in welding. Post weld heat treatment(PWHT) is carried out to increase the fracture toughness of HAZ and to remove the residual stress. There occur some problem such as toughness decrement and stress relief cracking(SRC) in the coarse grained HAZ subject to the effect of tempering treatment. Therefore, in this paper, the effect of heat inputs affecting cooling rate and PWHT under the no stress on fracture toughness were evaluated by crack opening displacement (COD), SEM and micro-hardness test. Experimental results are as follows; 1. Fracture toughness of weld HAZ is dependent upon weld heat cycle and it is decreased with increment of heat input, but the degree of improvement of fracture toughness after PWHT was linearly increased with heat input. 2. Hardness of the parent metal is not changed, but the softening of coarse grained HAZ is remarkable due to PWHT. 3. Fracture surface of as-weld show the perfect brittle fracture with the cleavage fracture, but after PWHT they appear the ductile fracture surface with dimple.