• Korean Journal of Materials Research
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• v.17 no.6
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• pp.342-346
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• 2007

Microstructure and tensile strength of the vacuum brazed stainless steel were investigated in this study. For vacuum brazing of the stainless steel 303 and 304, the BNi-2, 3, 4 and 7 were used as filler metals. Among these filler metals, the BNi-2 showed excellent wettability at $1050^{\circ}C$. Indeed, the brazed stainless steel using the BNi-2 showed the highest tensile strength (483 MPa) among all brazed specimens. This is attributed to degree of interfacial reaction between the filler metal and stainless steel. Brazed stainless steel with BNi-2, 3 filler metals showed almost elastic deformation followed by plastic yielding and strain hardening up to a peak stress. On the other hand, it is likely that the fracture of the brazed specimens with BNi-4, 7 was occurred in elastic range without plastic yielding up to a peak stress.

• Journal of Ocean Engineering and Technology
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• v.3 no.2
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• pp.617-617
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• 1989

This paper deals with a fatigue life prediction of a surface crack based on the experimentally obtained relationship between surface crack length ratio $a/a_{f}$ and cycle ratio $N/N_{f}$ using micro computer. Firstly $a/a_{f}$-$N/N_{f}$ curves obtained from experimental tests, were assumed as three curves UC(the upper limit curve), LC(the lower limit curve) and MC(the middle curve), and these were utilized to predict the fatigue life. Comparing the calculated values which represent the characteristics of crack growth behaviors from the three assumed curves with the experimental ones, it has been found that in the stable crack growth region, they coincide reasonably well each other. And the differences between the fatigue lives obtained from the assumed curves and the experimental fatigue life did not exceed 20%. Using the characteristics of $a/a_{f}$-$N/N_{f}$ curves, it is possible to predict the da/dN-Kmax curves and the S-$N_{f}$ curves.

• Journal of Ocean Engineering and Technology
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• v.3 no.2
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• pp.108-117
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• 1989

This paper deals with a fatigue life prediction of a surface crack based on the experimentally obtained relationship between surface crack length ratio $a/a_{f}$ and cycle ratio $N/N_{f}$ using micro computer. Firstly $a/a_{f}$-$N/N_{f}$ curves obtained from experimental tests, were assumed as three curves UC(the upper limit curve), LC(the lower limit curve) and MC(the middle curve), and these were utilized to predict the fatigue life. Comparing the calculated values which represent the characteristics of crack growth behaviors from the three assumed curves with the experimental ones, it has been found that in the stable crack growth region, they coincide reasonably well each other. And the differences between the fatigue lives obtained from the assumed curves and the experimental fatigue life did not exceed 20%. Using the characteristics of $a/a_{f}$-$N/N_{f}$ curves, it is possible to predict the da/dN-Kmax curves and the S-$N_{f}$ curves.

• Journal of Ocean Engineering and Technology
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• v.27 no.1
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• pp.37-42
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• 2013

The presence of a crack can increase the local stress or strain, which can cause inelastic deformation and significantly reduce the life of a component or structure. Therefore, in this study, the fatigue crack growth (FCG) behaviors of friction stir welded Al 2024-T3 and Al 7075-T6 specimens were examined, with fatigue cracks growing parallel to the dynamically recrystallized zone at variable ${\Delta}K$ values and an R ratio of 0.3. In addition, the FCG values of the base metal Al 2024-T3 and Al 7075-T6 were tested under the same conditions and parameters as comparative groups. The results showed that compared with the base metal Al 2024 specimen, which had the best fatigue property, the welded specimen had only 88% of the fatigue cycles.

• Proceedings of the Korean Society For Composite Materials Conference
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• 2002.10a
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• pp.265-268
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• 2002

In this work, the blend system of epoxy and PMR-15 polyimide is investigated in terms of the cure behaviors and thermal stabilities. The cure behaviors are studied in DSC measurements and thermal stabilities are also carried out by TGA analysis. DDM (4, 4'-diamino diphenyl methane) is used as curing agent for EP and the content of PMR-15 is varied within 0, 5, 10, 35, and 20 phr to neat EP. As a result, the cure activation energy ($E_a$) is increased at 10 phr of PMR-15, compared with that of neat EP. From the TGA results of EP/PMR-15 blend system, the thermal stabilities based in the initial decomposed temperature (IDT) and integral procedural decomposition temperature (IPDT) are increased with increasing the PMR-15 content. The fracture toughness, measured in the context of critical stress intensity factor ($K_{IC}$) and critical strain energy release rate ($G_{IC}$), shows a similar behavior with $E_a$. This result is probably due to the crosslinking developed by the interactions between intermolecules in the polymer chains.

• Transactions of the Korean Society of Mechanical Engineers
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• v.14 no.2
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• pp.298-309
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• 1990

In this paper, a new method is suggested to analyze impulsive stresses at loading poing of concentrated impact load under certain impact conditions determined by impact velocity, stiffness of plate and mass of impact body, etc. The impulsive stresses are analyzed by using the three dimensional dynamic theory of elasticity so as to analytically clarify the generation phenomenon of cone crack at the impact fracture of fragile materials (to be discussed if the second paper). The Lagrange's plate theory and Hertz's law of contact theory are used for the analysis of impact load, and the approximate equation of impact load is suggested to analyze the impulsive stresses at the impact point to decide the ranage of impact load factor. When impact load factors are over and under 0.263, approximate equations are suggested to be F(t)=Aexp(-Bt)sinCt and F(t)=Aexp(-bt) {1-exp(Ct)} respectively. Also, the inverse Laplace transformation is done by using the F.F.T.(fast fourier transform) algorithm. And in order to clarity the validity of stress analysis method, experiments on strain fluctuation at impact point are performed on a supported square glass plate. Finally, these analytical results are shown to be in close agreement with experimental results.

• Journal of Advanced Marine Engineering and Technology
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• v.10 no.2
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• pp.136-145
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• 1986

The perfect and accurate methods to control the wear are not made clear so far. For this phenomenon only mating surface has been studied. In order to control the wear the essence of it has to be made clear. It is reported that adhesive wear might occure as a result of plastic deformation, the fracture and removal or transfer asperities on close contacting surfaces. On this view point the plastic flow was attempted to compare with fluid or electromagnetic flow. The partial differential equations of equilibrium for the plane strain deformation will make use of the method of characteristics. The characteristic curves or characteristics of the hyperbolic equation coincide with the slip lines by R. Hill's papers. By Hencky's stress equation, it is evident that if P and .phi. are prescribed for a boundary condition then it may be possible to proceed along constant .alpha. and .betha. lines to determine the value of the hydrostatic pressure everywhere in the slip line field net work. A wedge formation mechanism has been considered for an explanation of this matters. The analysis shows that there is a critical value, which depends on the hardness ratio and the shear stress on the interface, for the top angle of asperity is less than this critical value, the asperity can yield plastically despite of being harder than the mating surface.

• Journal of the Korean Society for Precision Engineering
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• v.30 no.8
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• pp.809-814
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• 2013

The relationship between the small-punch creep test and the conventional creep test was investigated experimentally using a method similar to that of the Monkman-Grant relationship. Uniaxial and small-punch creep rupture tests were carried out on 9Cr-2W ferritic steel (Commercial Grade 92 steel: X10CrWMoVNb 9-2) at elevated temperatures. From the relation derived in the same manner as the Monkman-Grant relation, a correlation between the displacement rate in response to the small-punch creep test and the strain rate in the uniaxial creep test was found, and the creep life was calculated using this relation. Furthermore, the failure modes of the small punch creep test specimens were investigated to show that the fracture was caused by creep.

• Transactions of Materials Processing
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• v.13 no.1
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• pp.72-77
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• 2004

An Fe-25Cr steel was oxidized in Ar atmosphere at 973K with and without applying external stress of 30∼35 MPa. A 0.1$\mu\textrm{m}$ thick $Cr_2O_3$ scales formed during pre-treatment in Ar atmosphere. Initiation of cracking on the oxide scales took place at grain boundaries during the end of second creep stage, in which cracks were found nearly perpendicular to the tensile directions. On the contrary, a scale developed in $N_2$-0.1%$SO_2$ displaced a poor adherence on the metal substrate. In this sample, a fast grown of scales was observed during creep deformation, and the strength of materials was much lower than in Ar. The creep strain rate of $1.5{\times}10^{-7}/s$ and $5.8{\times}10^{-7}/s$ was determined in Ar and in $N_2$-0.1%$SO_2$ under 30MPa, respectively.

• Journal of the Korean Society of Safety
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• v.12 no.4
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• pp.57-62
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• 1997

The combined structure of hybrid composite made through the bonding process of materials of different properties greatly defines its mechanical characteristics, as the results of the experiments on materials of different properties show much dissimilarity. When carbon/epoxy materials are applied to hybrid composite, the carbon materials helps to improve the mechanical properties of the hybrid composite, and the epoxy reduces its fracture strain and impact resistance. Carbon fiber which is now in general commercialization is classified as high modulus or high strength system, and its manufacturing methods are various. The study of the materials having combined structure is focused on the numerical analysis of the layers of bonding surface in materials with difference modulus. The hybrid composite made through the multilayered bonding of reinforced aluminium sheets with aramid fiber now faces the marketing phase, and especially its excellent fatigue resistance and mechanical properties promote active researches on the similar products of hybrid composite. This study aims to investigate the effects of CFRP volume ratio and fiber's orientation over the properties of mechanical strength and fatigue life of the hybrid composite, AI7075/CFRP. To carry out this study, static tensile and fatigue tests were given to some of the panels which, made through the co-cure processing in an autoclave, have different CFRP volume ratio and carbon fiber orientations.