• 한국재료학회지
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• 제22권1호
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• pp.29-34
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• 2012

The hydrogen embrittlement of high strength steel for automobiles was evaluated by small punch (SP) test. The test specimens were fabricated to be 5 series, having various chemical compositions according to the processes of heat treatment and working. Hydrogen charging was electrochemically conducted for each specimen with varying of current density and charging time. It was shown that the SP energy and the maximum load decreased with increasing hydrogen charging time in every specimen. SEM investigation results for the hydrogen containing samples showed that the fracture behavior was a mixed fracture mode having 50% dimples and 50% cleavages. However, the fracture mode of specimens with charging hydrogen changed gradually to the brittle fracture mode, compared to the mode of other materials. All sizes and numbers of dimples decreased with increasing hydrogen charging time. These results indicate that hydrogen embrittlement is the major cause of fracture for high strength steels for automobiles; also, it is shown that the small punch test is a valuable test method for hydrogen embrittlement of high strength sheet steels for automobiles.

• 대한전기학회:학술대회논문집
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• 대한전기학회 2000년도 추계학술대회 논문집 학회본부 A
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• pp.265-268
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• 2000

In this study, a small punch creep (SP-Creep) test using miniaturized specimen has been described for the development of the new creep test method for high temperature structural components such as headers and tubes of boiler, turbine casing and rotor, and reactor vessel. The SP-Creep testing technique has been applied to 1Cr-0.5Mo steel used widely as boiler header material and the creep test temperature are varied at $550^{\circ}C{\sim}600^{\circ}C$. From the experimental results, e.g. SP-Creep curve behaviors, the creep rate in steady state and creep rupture life with test temperature and load, the load exponential value(n, m), the activation energy($Q_{spc}$), the Monkman-Grant relation and the creep life assessment equation etc., it can be summarized that the SP-Creep test may be a useful test method to evaluate the creep properties of the heat resisting material such as boiler header.

• 대한기계학회:학술대회논문집
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• 대한기계학회 2001년도 추계학술대회논문집A
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• pp.143-148
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• 2001

This research was undertaken to clarify effects of thermal aging on electrochemical and mechanical properties of superaustenitic stainless steel. The steel was artificially aged at $300{\sim}650^{\circ}C$ for $240{\sim}10,000hrs$. and investigated at $-196{\sim}650^{\circ}C$ using small punch(SP) test. Also, the change in electrochemical properties caused by effects of thermal aging was investigated using electrochemical anodic polarization test in a KOH electrolyte. Carbides and ${\eta}-phase(Fe_2Mo)$ precipitated in the grain bounderies seem to deteriorate the mechanical properties by decreasing cohesive strength in the grain bounderies and promote the current density observed in electrochemical polarization curves. The electrochemical and mechanical properties of superaustenitic stainless steel was drastically decreased in the specimen aged at $650^{\circ}C$.

• 대한기계학회논문집A
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• 제20권10호
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• pp.3234-3242
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• 1996

This paper describes experimental results of modified small punch( MSP) test conducted to evaluate the fracure characteristics and mechanical properties of plasma sparayed zirconia ($ZrO_2$ stabilized with 8wt. % $Y_20_3$ : YSZ) NiCrAlY composite. The mixing ratios of YSZ/NiCrAlY were 0/100, 25/75, 50/50, 100/0 v.%. Test temperatures ranged from 293K to 1473K. This study is directed at development of thermal barrrier coating(TBC) system with superior heat resistance and mechanical properties. The microstructure and fracture process of the composite were examined by SEM and AE method. The mechanical properties of 100% YSZ were nearly independent of the temperatures tested in this study. In contrast, the NiCrAlY-containing composites showed a significant decrease of the mechanical properties above 1273K, showing a ductile- brittle transition behavior up to the temperature. Furthermore, it can seen that 25% YSZ/75% NiCrAlY composite gave the highest fracure strength and fracture energy among the mixing ratio tested over the temperature range.

• 대한기계학회논문집A
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• 제26권7호
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• pp.1332-1339
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• 2002

This research was undertaken to clarify effects of thermal aging on electrochemical and mechanical properties of superaustenitic stainless steel and to detect the material degradation nondestructively. The steel was artificially aged at $300{\sim}650^{\circ}C$ for $240{\sim}10,000h$ and the mechanical properties were investigated at $-196{\sim}650^{\circ}C$ using small punch(SP) test. Also, the change in electrochemical properties caused by effects of thermal aging on superaustenitic stainless steel was investigated using electrochemical anodic polarization test in a KOH electrolyte. Carbides and ${\eta}-phase(Fe_2Mo)$ precipitated in the grain boundaries seem to deteriorate the mechanical properties by decreasing cohesive strength in the grain boundaries and to promote the current density observed in electrochemical polarization curves, The electrochemical and mechanical properties of superaustenitic stainless steel decreased significantly in the specimen aged at $650^{\circ}C$ corresponding to the sensitization temperature for conventional austenitic stainless steels.

• 대한기계학회논문집
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• 제19권3호
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• pp.631-638
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• 1995

In this study, mechanical characteristics test of Functionally Gradient Materia (FGM) was performed by means of Modified Small Punch (MSP) Test with FGM; NiCrAlY-8YSZ and PSZ-Ni. To determine fracture mechanic factor, it was carried out MSP test that has possibility with small specimen (10*10*0.5 mm$^{t}$ ) and AE test to analyze micro fracture mechanism. As a result, fracture behavior became varied from brittle fracture to ductile as the content of Ni(or NiCrAlY) composition was increased and fracture energy was increased too. AE characteristics demonstrated that AE technique can detect the onset of fracture processes and AE energy was suddenly increased in the vicinity of maximum load. Since Young's modulus, fracture stress and fracture toughness was determined by MSP test, it can be known that the composition of NiCrAly 75%/8YSZ25% has the best mechanical property and furthermore this result is supported with fracture surface observation.

• Journal of Welding and Joining
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• 제12권4호
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• pp.63-75
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• 1994

The conventional SCC(stress corrosion cracking) test methods have much difficulty in evaluating the SCC behaviors of the localized zone like HAZ, bond line and weld metal because of the specimen size. Accordingly, the purpose of this paper is to develop the new SCC test method of the welded zone by evaluating the SCC susceptibility on parent metal and various microstructures of the welded zone by SP(small punch) test method using miniaturized small specimen and SSRT(slow strain rate test) method(SP-SSRT). Besides, this study is to verify the efficiency of the SP-SSRT results through AE(acoustic emission) test which is a useful technique to monitor the microfracture processes of the material. From the results of SCC susceptibility, SEM observation and AE test, it can be concluded that the SP-SSRT test using miniaturized small specimen(10mm*10mm*0.5mm) will be a good test method to evaluate the SCC susceptibility on the local zone such as the welded zone.

• 한국소성가공학회:학술대회논문집
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• 한국소성가공학회 2005년도 추계학술대회 논문집
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• pp.283-286
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• 2005

When subjected to severe shear deformation by ECAP, microstructure of Al2024 becomes nanocrystalline grained texture material. To measure the strength of that, small punch (SP) testing method was adopted as a substitute for the conventional uniaxial tensile testing because the size of material processed by ECAP were limited to $\varphi12mm$ in transverse direction. SP tests were performed with specimens in longitudinal and transverse directions of Al 2024 ECAP metal. For comparing the strength values with those assessed by SP tests, uniaxial tensile tests were also conducted with specimens in longitudinal direction. Failure surfaces of the tested SP specimens showed that failure mode was shear deformation and Al 2024 ECAP metal has an anisotropy in strength. Thus, conventional equations proposed for assessing the strength characteristics were improper to assess those of Al2024 ECAP metal. In this paper a way of assessing the strength of Al 2024 ECAP metal was proposed and was proven to be effective.

• 한국표면공학회지
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• 제46권1호
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• pp.42-47
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• 2013

The strain-induced phase transformation from austenite to martensite is responsible for the high strength and ductility of TRIP steels. However high strength steels are susceptible to hydrogen embrittlement. This study aimed to evaluate the effects of hydrogen on the behavior of hydrogen delayed fracture in TRIP steel with hydrogen charging conditions. The electrochemical hydrogen charging was conducted at each specimen with varying current density and charging time. The relationship between hydrogen concentration and mechanical properties of TRIP steel was established by SP test and SEM fractography. The maximum loads and displacements of the TRIP steel in SP test decreased with increasing hydrogen charging time. The results of SEM fractography investigation revealed typical brittle mode of failure. Thus it was concluded that hydrogen delayed fracture in TRIP steel result from the diffusion of hydrogen through the ${\alpha}$' phase.

• 대한기계학회논문집A
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• 제28권5호
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• pp.554-562
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• 2004

An empirical relationship between parameters from SP curves and tensile properties has been systematically investigated by experimental tests and FEM simulations. A series of SP and tensile tests were performed. SP tests were also simulated by FE analysis with various tensile properties. It was found that the yield loads(Py) and the maximum loads( $P_{MAX}$) in SP curves were linearly related with the yield strength($\sigma$$_{o}$) and the tensile strength($\sigma$$_{UTS}$), respectively. The yield loads defined from the intersection point of two lines tangent to the elastic bending region and plastic bending region showed better relation to the yield strength than those from offset line. The maximum loads in SP curves showing plastic instability region was linearly related with the tensile strengths. The slope of SP curves in simulation results had a close correlation with the hardening coefficient and hardening strength as well.l.l.l.