• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.41 no.1
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• pp.17-24
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• 2013

In this paper, a micro-mechanical failure prediction program is developed based on SIFT (Strain Invariant Failure Theory) by using the multi-scale modeling method for fiber-reinforced composite materials. And the failure analysis are performed for open-hole composite laminate specimen in order to verify the developed program. First of all, the critical strain invariants are obtained through the tensile tests for three types of specimens. Also, the matrices of strain amplification factors are determined through the finite element analysis for micro-mechanical model, RVE (Representative Volume Element). Finally, the microscopic failure analysis is performed for the open-hole composite laminate specimen model by applying a failure load obtained from tensile test, and the predicted failure indices are evaluated for verification of the developed program.

• Journal of the Korean Society for Nondestructive Testing
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• v.24 no.2
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• pp.158-162
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• 2004

Reliability of welded structures in power plant facilities is very important, and their reliability evaluation requires exact materials properties. But, the conventional PQR (Procedure Qualification Record) can hardly reflect the real material properties in the field because the test is only done on specimens with simulated welding. Therefore, a continuous indentation technique is proposed in this study for simple and non-destructive testing of in-field structures. This test measures the indentation load-depth curve during indentation and analyzes the mechanical properties such as the yield strength, tensile strength and work hardening index. This technique has been applied to evaluate the tensile properties of the weldment in the main steam pipe and hot reheater pipe in power plants under construction and in operation.

• Journal of the Korean Institute of Gas
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• v.15 no.3
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• pp.67-73
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• 2011

One of the important topics to prepare the up-coming era of so-called ‘hydrogen economy’ is hydrogen transmission. Pipeline is conceivably the most economic way to consistently and safely transport a large amount of hydrogen over a long distance, which may be strongly requested in hydrogen economy era. As a good starting point for the purpose, one might wonder whether conventional API pipeline steels as designed for natural gas transmission can be used as the hydrogen pipeline materials or not. To answer the question, here we performed a series of micro-/nano-indentations together with tensile tests on the hydrogen-charged API X65, X70 and X100 steels having different strength level. In this paper, from the results of tensile tests, the hydrogen effects on the mechanical behavior in the API steels are systematically evaluated.

• Composites Research
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• v.12 no.2
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• pp.53-61
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• 1999

A failure criterion must be considered in each failure mode and loading condition to provide easy determining strength parameters, flexibility and rational simplicity. In this study, new failure criterion was developed by introducing equivalent strength under biaxial loading of tension and torsion. The experimental results showed that the equivalent biaxial strength has a power law relation with respect to a parameter, cos($tan^{-1}R_b$). Failure strength under biaxial loadings could be predicted as a function of tensile strength, torsional strength and biaxial ratio. The scattering of experimental data could be predicted using a Weibull distribution function and the concept of equivalent biaxial strength. Also, in this study, a fatigue theory was developed based on a plane stress model which enabled the S-N curve for combined stress states to be predicted from the S-N data for uniaxial loading. The prediction models can be predicted a biaxial strength and fatigue life of general laminated composite naterials under multi-axial loadings.

• Journal of the Korean Geotechnical Society
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• v.36 no.10
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• pp.21-31
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• 2020

In this study, Hoek-Brown constants (m_i) were calculated through nonlinear regression analyses using the results of the triaxial compression tests for the basaltic intact rocks in Jeju Island. The relationships of the m_i with the uniaxial compressive strength (UCS), Brazilian tensile strength (BTS) and UCS/BTS of the Jeju basalts were investigated, respectively. In addition, a method that can be used in determining Hoek-Brown failure envelopes including the tensile and compressive failures of the Jeju basalts has been proposed. As results, the m_i values had no clear correlations with the UCS, BTS and UCS/BTS of the Jeju basalts, but there were two strong correlations between UCS and m_i/UCS, and between BTS and m_i/BTS of the Jeju basalts. In addition, it was found that the tensile strengths calculated by the Hoek-Brown failure criterion underestimate the tensile strengths of the Jeju basalts through the relationship between the m_i and UCS/BTS of the Jeju basalts. The method presented in this study is considered to be useful in determining the Hoek-Brown failure envelope for the tensile and compressive failures of the Jeju basalts.

• Applied Chemistry for Engineering
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• v.7 no.2
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• pp.315-320
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• 1996

In this paper, we investigated the retention amounts(ash contents) according to the addition amounts of retention aid and calcium carbonate and compared the mechanical and optical properties of sheets under the same ash content. As the addition amounts of retention aid increase, the retention of calcium carbonate, that is, ash contents of sheets increase in all retention system. In this case, the sheets included ash content as already expected is produced by adjusting the addition amounts of retention aid and calcium carbonate. Tensile index, burst index, tear index, internal bonding strength of sheets straightly decrease as the ash content of sheets increases. Especially, in the same ash content, all sorts of strength are high in compozil system, low in dual polymer system. Opacity increases along with according to the increase of ash content, and is high in hydrocol system, the worst in dual polymer system. In equal opacity, the strength of paper decreases compozil, hydrocol, and dual polymer system in order. But to judge she superiority or inferiority of retention aids, it should consider the various factors such as the optimum production and process conditions besides the retention amounts of filler and the sheet strength.

• Polymer(Korea)
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• v.37 no.1
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• pp.5-14
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• 2013

In this paper the effect of UV (ultraviolet) exposure on HDPE (high density polyethylene)-smooth and f-PP (flexible polypropylene) geomembranes is evaluated under UVB-313 (ultraviolet wavelength 290-315 nm) exposure. Tensile property, melt flow index (MFI), oxidation induction time (OIT), both standard-OIT and high pressure-OIT and Fourier transform infrared spectroscopy/attenuated total reflectance (FTIR/ATR) results are discussed. Although tensile properties of the exposed geomembrane samples remained unchanged, the depletion of antioxidants was found higher for f-PP than for HDPE geomembrane. Arrhenius model by extrapolation was used on the data to predict the antioxidant lifetime to a typical site temperature of $20^{\circ}C$. There was no significant difference between the MFI value of the virgin and UV exposed HDPE geomembrane samples but a decrease in MFI was found in f-PP geomembrane that signifies that crosslinking has occurred. From FTIR spectra, the small peak (near $1750\;cm^{-1}$) observed in the spectrum of UV exposed sample corresponds to a carbonyl (C=O) linkage, which suggests that oxidation has occurred in the polymer structure, and another new band for f-PP between 3100 and $3500\;cm^{-1}$ is attributed to a hydroxyl bond and/or hydroperoxide bond.

• Applied Chemistry for Engineering
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• v.25 no.1
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• pp.90-95
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• 2014

Nylon66 extrudates as a function of the extrusion number were prepared by a twin screw extruder. Chemical structures, thermal properties, melt index, crystal structures, mechanical properties such as the tensile strength, elongation at break and impact strength, and rheological property were measured by FT-IR, $^1H$-NMR, melt indexer, DSC, TGA, XRD, universal tensile tester, Izod impact tester, and rheometer. FT-IR and $^1H$-NMR characterizations indicated that the number of extrusions did not affect the chemical structure. The decrease in the molecular weight was checked by the melt index of extrudates. There were no effects of the thermal history on the melting and degradation temperature. The tensile and impact strength and modulus were found to be similar, regardless of the number of extrusions, but the elongation decreased significantly. The complex viscosity of extrudates at low frequencies decreased with the extrusion number. No structural changes after extrusion were confirmed from the fact that there was no change in the slope and shape of G'-G" plot.

• Composites Research
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• v.20 no.1
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• pp.23-31
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• 2007

In order to improve the mechanical properties of jute fiber/polypropylene(PP) composites, the property change with the addition of a coupling agent, maleic anhydride polypropylene(MAPP) was examined experimentally. The maleated coupler acts as an intermediate to chemically connect the polar nature of the fiber and non-polar nature of the polyolefin polymer resin. Furthermore, the decrease in viscosity of the resin which results from the melting point reduction by the MAPP, leads to an increase of contact area with the fiber interface. We discussed the improvement of the PP composite blend of the maleated coupler with the 80mm jute long fiber mat in conjunction with the change of physical parameters in the thermoplastic resin. We confirmed the extent of contribution to the mechanical physical enhancement by using the following parameters: melting flow index(MI) and viscosity, contact angle, thickness of the composite, interfacial shear strength and morphology observation etc. Especially it was observed that the MI and viscosity, MAPP mixture had a very strong relationship with the tensile and flexural strength and modulus, and interfacial shear strength(IFSS).

• Transactions of the Korean Society of Mechanical Engineers A
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• v.36 no.7
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• pp.713-721
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• 2012

The finite element simulations of fatigue crack growth are carried out. Using only the mechanical properties usually obtained from the tensile test as input data, we attempted to predict the fatigue crack growth behavior. The critical crack opening displacement is determined by monitoring the change in displacements at the node close to the crack tip. Crack growth is simulated by debonding the crack tip node. The exponent in the Paris law was determined and compared to the published exponent. Plotting with respect to the effective stress intensity factor range yielded more consistent results.