• Journal of the Korean Society of Safety
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• 제27권5호
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• pp.42-48
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• 2012

Peel-tension fatigue experiments were conducted for investigating on fatigue strength of mechanical press joints of SPCC steel sheet used in the field of the automobile industry. In addition, finite element method analysis on the peel-tension specimen was conducted using HyperMesh and ABAQUS softwares. The cold rolled mild steel was used to join the T-shaped peel-tension specimen with a button diameter of 5.4 mm and a punch diameter of 8.3 mm. The fatigue limit load amplitude was found to be 112.4 N at the number of cycles 106, indicating that the ratio of fatigue limit load to static peel-tension strength was about 8%. This value suggests that the mechanical press joint is highly vulnerable to peel-tension load rather than to tensile-shear load, considering that the ratio of fatigue limit load to static tensile-shear strength was about 43%. Fatigue failure mode was found to be interface-failure mode.

• Journal of the Korean Society of Safety
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• 제27권3호
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• pp.15-19
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• 2012

The fatigue behavior of AM60 magnesium alloy produced by equal channel angular pressing(ECAP) process was investigated through fatigue lifetime and fatigue crack propagation rate tests. The grain structure of the material was refined from 19.2 ${\mu}m$ to 2.3 ${\mu}m$ after 6 passes of ECAP at 493 K. The yield strength(YS) and ultimate tensile strength (UTS) increase after two passes but decrease with further pressing, although the grain size becomes finer with increasing pass number. The softening effect due to texture anisotropy overwhelmed the strengthening effect due to grain refinement after 2 passes. A large enhancement in fatigue strength was achieved after two ECAP passes. The current finding suggests that two passed material is better than the multi-passed material in view of the static strength and fatigue performance.

• Journal of the Korean Society of Safety
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• 제20권4호
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• pp.1-8
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• 2005

Effects of Nb(Niobium) contents and solution annealing on the strength and fatigue life of 18%Ni maraging steel commonly using in aircraft, space field, nuclear energy, and vehicle etc. were investigated. Also the fatigue life stress intensity factor were compared experiment result and FEA(finite element analysis) result. The more Nb content, the higher or the lower fatigue life on base metal specimens or solution annealed specimens showing that the fatigue lift was almost the same. The maximum stresses of X, Y, and Z axis direction showed about $2.12{\times}10^2MPa,\;4.40{\times}10^2MPa\;and\;1.32{\times}10^2MPa$ respectively. The Y direction stress showed the highest because of the same direction as the loading direction. The fatigue lives showed about 7% lower FEA result than experiment result showing almost invariable error every analyzed cycle. Stress intensity factor of the FEA result was lower about $3.5{\sim}10%$ than that of the experiment result showing that the longer fatigue crack length the higher error. It considered that the cause for the difference was the modeled crack tip having always the same shape and condition regardless of the crack growth.

• Journal of the Korean Society of Safety
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• 제28권3호
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• pp.18-22
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• 2013

The Co-base super heat resisting alloy ECY768 is employed in gas turbine because of its high temperature strength and oxidation resistance. The prediction of fatigue life for superalloy is important for improving the efficiency. In this paper, low cycle fatigue tests are performed as variables of total strain range and temperature. The relations between strain energy density and number of cycle to failure are examined in order to predict the low cycle fatigue life of ECY768 super alloy. The lives predicted by strain energy methods are found to coincide with experimental data and results obtained from the Coffin-Manson method. The fatigue lives is evaluated using predicted by Coffin-Manson method and strain energy methods is compared with the measured fatigue lives at different temperatures. The microstructure observing was performed for how affect able to low-cycle fatigue life by increasing the temperature.

• Journal of the Korean Society of Safety
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• 제16권1호
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• pp.31-36
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• 2001

Surface crack growth characteristics and influence of the stress amplitude in rotary bending fatigue test were evaluated for annealed S35C steel, and than fractal dimensions of fatigue crack paths estimated using the box counting method. The following results that will be helpful to understand the fatigue crack growth mechanism were obtained. (1) Crack growth rate ds/dN and db/dN (s : half crack length at the surface crack, b : crack depth) depended on stress amplitude (${\Delta}{\sigma}/2$), stress intensity factor range (${\Delta}K_A, {\Delta}K_C$) and crack length. (2) At the effect area of 0.3 mm hole notch (s＜0.5 mm) crack growth rate did not depend on these factors. (3) The fractal dimensions (D) increased with stress amplitude (${\Delta}{\sigma}/2$) but decreased with cyclic number.

• Journal of the Korean Society of Safety
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• 제18권1호
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• pp.38-43
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• 2003

In this paper, the low cycle fatigue(LCF) life tests were carried out under waveforms of asymmetric triangle($4{\times}10^{-3}$ and $4{\times}10^{-10}$ strain rate) and hold-time(1min and 10min) in strain control. In triangular waveform, the fatigue lives of fast-slow waveforms were decreased to about 63-73% and them of slow-fast waveforms were down to about 23-24% compared to them of fast-fast waveforms. The shapes of fracture surfaces were transgranular in fast-fast and fast-slow waveforms and intergranular in slow-fast ones. The fatigue lives of slow-fast waveforms were remarkedly shorter than them of hold-time waveforms even though cycle times of hold time waveforms were longer than them of slow-fast ones. The damage mechanisms of frature surfaces were mixed frature with both transgranular and intergranular, but intergranular fratures were r-type cavity in hold-time waveforms and w-type cavity in slow-fast ones.

• Journal of the Korean Society of Safety
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• 제17권1호
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• pp.18-24
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• 2002

This study verified the relationship between fracture mechanics parameters(${\Delta}K,\;K_{max}$) and X-ray parameters ($(\sigma}_r,;B$) for G365 steel at elevated temperature up to $300{\circ}C$. The fatigue crack propagation test were carried out and X-ray diffraction technique according to crack length direction was applied to fatigue fractured surface. The residual stress on the fracture surface was found to increase in low ${\Delta}K$ region, reach to a maximum value at a certain value of $K_{max}$ or ${\Delta}K$ and then decrease. Residual stress was independent on stress ratio by arrangement of ${\Delta}K$ and half value breadth was independent by the arrangement of $K_{max}$. The equation of ${\sigma}_r-{\Delta}K$ was established by the experimental data. Therefore, fracture mechanics parameters could be estimated by the measurement of X-ray parameters.

• Proceedings of the Korean Institute of Industrial Safety Conference
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• 한국안전학회 2003년도 춘계 학술논문발표회 논문집
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• pp.508-513
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• 2003

The objective of this paper is to investigate the lining board's capacity by using the static loading test and fatigue test. Specimens that constitute the H & Channel type lining board are adopted. The test is to inspect the possibility of retrofit and efficiency, which is required to upgrade the structure's capacity and to examine the effects of the improvements of specimen by using structural analysis, stress analysis, static loading test and fatigue test, respectively. The accumulated test results of stress condition and deflection by bending will be used to analyze the relation between the cause of fatigue crack occurrence and the behavior of both structure system and the steel.

• Journal of the Korean Society of Safety
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• 제25권1호
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• pp.1-8
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• 2010

The tensile and fatigue experiments were conducted with tensile-shear specimens for investigating the strength of adhesive bonded and mechanical press joints of SPCC steel sheet used in the field of the automobile industry. The optimal punch press force was evaluated 50kN for combining epoxy adhesive bonding and mechanical press joining with a diameter of 8.3mm using SPCC sheet with a thickness of 0.8mm. The combining epoxy adhesive bonding and mechanical press joining exhibits the maximum tensile force of 750N. The fatigue strengths of the combination of adhesive bond and mechanical press joint and pure adhesive joint were evaluated 370N and 320N at 106cycles, respectively. These values correspond to 22% and 20% of their maximum tensile forces, respectively. However, the fatigue strength of the combination of adhesive bond and mechanical press joining was much lower than that of pure mechanical press joining.

• Journal of the Korean Society of Safety
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• 제33권3호
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• pp.8-14
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• 2018

Ni-base superalloy has excellent resistance to extreme environments such as high temperatures and high stresses and are used as materials for large gas turbines. In this paper, the specimens were taken from the blade that were used for a long time, and their life span was studied by microstructure analysis and avoidance of cursing. The microstructural analysis of the specimens was carried out using a OM and SEM to observe the coarsening, carbides on gamma prime. Low-cycle fatigue tests were performed on new material and airfoil of long time-used blade. The test was conducted under various deformation conditions and temperature conditions of $760^{\circ}C$ and $870^{\circ}C$. The low cycle fatigue test was carried out using the Coffin-Manson equation and the fatigue life was predicted. After the test, crack path and fracture surface were analyzed using SEM.