• Journal of Industrial Technology
• /
• v.23 no.A
• /
• pp.139-147
• /
• 2003

The purpose of this study was to investigate and analyze the fatigue test data of pavement concrete. The static strength tests were carried out to check the compressive strength, flexural strength, and split tension strength at 56 days in order to minimize strength variation effect during test. The specimens were fabricated at twelves sections at a construction site of highway. The stress level and stress ratio of fatigue test were determined from static test results. The results are as follow: The flexural strength at 28 days mostly satisfied the criterion for design, but the compressive strength at 28 days were slightly below the criterion even though it satisfied at 56 days. The fatigue limit was 2 million cycles if the specimen was not failed to that cycles. The S-N curves were developed from the fatigue test results at each stress levels and each stress ratio. Then, the fatigue life of pavement concrete at a given stress level and fatigue strength of pavement concrete could be derived from these curves. Analysis using method No.2 was more acceptable because resulting of comparison and analysis using method No.2 was presented 2 sections were presented $R^2$ < 0.7, and other 2 sections were presented 0.7 < $R^2$ < 0.8, and the others 8 sections were $R^2{\geq}0.8$.

• Journal of Ocean Engineering and Technology
• /
• v.32 no.6
• /
• pp.458-465
• /
• 2018

Ice-going ships such as icebreakers, icebreaking tankers, and icebreaking LNG carriers are subjected to wave loads in open water and ice loads in ice-covered water. In terms of the ship's structural design, the local ice load is important. The fatigue failure due to repeated ice loads is also important. ISO 19906 specifies the assessment of the fatigue limit for a polar offshore structures. In addition, Lloyd's Register refers to fatigue damage based on ShipRight FDA ICE. In ShipRight FDA ICE, the fatigue damage indices due to wave and ice loads are simply presented as 0.5 for each load. It also states that the sum of the two fatigue damage indices should not exceed one. This study calculated and analyzed the fatigue damage index and fatigue life considering ARAON's voyage schedules and the assumed Antarctic voyage based on data measured during the Arctic voyage of ARAON in 2010.

• Journal of the Korean Society of Industry Convergence
• /
• v.24 no.4_2
• /
• pp.435-441
• /
• 2021

This research was performed to study the effect of the Ni + Mo addition on the fatigue properties in 12Cr steel. After heat treatment of 12Cr steel and 12Cr-Ni-Mo steel, tensile tests, impact tests, hardness tests, and rotary bending fatigue tests were performed, respectively. The fatigue fracture surface was observed and analyzed using SEM and EDS. The fatigue limit of 12Cr steel was 554 MPa, which was 49 MPa higher than 505 MPa of 12Cr-Ni-Mo steel. Striations, which are the shape of the typical fatigue fracture surface, were observed at the fracture surface near the starting point of fatigue fracture in the 12Cr steel and 12Cr-Ni-Mo steel. However, unlike the case of 12Cr steel, 12Cr-Ni-Mo steel also had a mixed fracture surface with the fatigue and the ductile fracture surface. When brittle non-metallic inclusions exist near the starting point of fatigue failure, the crack propagation was further promoted and the fatigue life was drastically reduced.

• Journal of Korean Society of Steel Construction
• /
• v.10 no.3 s.36
• /
• pp.423-435
• /
• 1998

In this paper fatigue strength reduction of butt weld with penetration defect, which can be seen frequently in the steel bridge, was assessed quantitatively. S-N curves were derived and investigated through the constant amplitude fatigue test of fully or partially penetrated welded specimen made of SWS490 steel. The fracture mechanical method was applied in order to calculate the remaining fatigue life of the partially penetrated butt welds. The fatigue limit of the fully penetrated butt welds was higher than that of category A in AASHTO's fatigue design curves, and the slope of S-N curves with 5.57 was stiffer than that of other result for welded part generally accepted as 3. The fatigue strength of the partially Penetrated butt weld was strongly influenced by the size of lack of penetration, D. It decreased drastically with increasing D from 3.9 to 14.7mm. Fracture behaviour of the partially penetrated butt weld is able to be explained obviously from the beach mark test that a semi-elliptical surface crack with small a/c ratio initiates at a internal weld root and propagates through the weld metal. To estimate the fatigue life of the partially penetrated butt weld with fracture mechanics, stress intensity factors K of 3-dimensional semi-elliptical crack were calculated by appling finite elements method and fracture mechanics parameters such as C and m were derived through the fatigue test of CT-specimen. As a result, the fatigue lives obtained by using the fracture mechanical method agreed well with the experimental results. The results were applied to Sung-Su bridge collapsed due to penetration defects in butt weld of vertical member.

• Transactions of the Korean Society of Mechanical Engineers A
• /
• v.39 no.7
• /
• pp.713-720
• /
• 2015

The sizes of the final blades of a low-pressure (LP) steam turbine have been getting larger for the development of high-capacity power plants. They are also larger than the other blades in the same system. As a result, fatigue damage is caused by a large centrifugal force and a low natural frequency of the blade. Recently, many failure cases have been reported due to repeated turbine startups and their prolonged use. In this study, the causes and mechanism of failure of a LP turbine blade were analyzed by using a finite element method to calculate the centrifugal force, the natural frequency of a stress-stiffening effect, and the harmonic response. It was observed that the expected fatigue damage position matched the real crack position at the airfoil's leading edge, and an equivalence fatigue limit approached a notch fatigue limit.

• Transactions of the Korean Society of Mechanical Engineers
• /
• v.12 no.3
• /
• pp.505-512
• /
• 1988

Since the propagation of a short fatigue crack is directly related to the large crack which causes the fracture of bulk specimen, the detailed study on the propagation of the short crack is essential to prevent the fatigue fracture. However, a number of recent studies have demonstrated that the short crack can grow at a low applied stress level which are predicted from the threshold condition of large crack. In present study, the threshold condition for the propagation of short fatigue crack is examined with respect to the microstructure and cyclic loading history. Specimens employed in this study were decarburized eutectoid steels which have various decarburized ferrite volume fraction. Rotating bending fatigue test was carried out on these specimens with the special emphasis on the '||'&'||'quot;critical non-propagating crack length.'||'&'||'quot; It is found that the reduction of the endurance limit of their particular microstructures can be due to the increase of the length of critical non-propagating crack, and the quantitative relationship between the threshold stress .DELTA. .sigma. $_{th}$ and the critical non-propagating crack length Lc can be written as .DELTA. .sigma. $_{th}$, Lc=C where m, C is constant. Further experiments were carried out on the effect of pearlitic structure and cyclic loading history on the length of critical non-propagating crack. It is shown that the length of critical non-propagating crack is closely related to both pearlite interlamellar spacing and cyclic loading history.ory. cyclic loading history.

• Transactions of the Korean Society of Mechanical Engineers A
• /
• v.25 no.4
• /
• pp.654-661
• /
• 2001

In this study, fatigue growth behavior of the transverse crack, which was the most dangerous damage among the various types of rail defects, was investigated using the notched keyhole specimen under constant amplitude loadings. Fatigue limit of smooth specimen in rail steel at R=0 was 110MPa, and the fatigue crack initiation life in the region of the low stress amplitude (ie. long life) occupied the major portion of the total fatigue life. The fatigue strength under variable amplitude loading was converted to the equivalent fatigue strength based upon. Miners rule, which was estimated approximately 9% lower than that under constant amplitude loading. Also, in the low ΔK(sub)rms region ($\leq$21MPa√m), fatigue crack growth rate (da/dN) under constant amplitude loading was higher than that under variable amplitude loading, whereas the tendency was reversed in the high ΔK(sub)rms region. It is believed that this behavior is due to the transition of fracture appearance.

• Transactions of the Korean Society of Mechanical Engineers A
• /
• v.24 no.3 s.174
• /
• pp.634-644
• /
• 2000

Fatigue crack growth and life is estimated by various fracture mechanical parameters but affected by load, material and environment. Fatigue character of component without surface notch cannot be e valuated by above-mentioned parameters due to microstructure of in-service material. Single fracture mechanical parameter or nondestructive parameter cannot predict fatigue damage in arbitrary boundary condition but multiple fracture mechanical parameters or nondestructive parameters can Fatigue crack growth modelling with three point representation scheme uses this merit but has limit on real-time monitoring. Therefore, this study shows fatigue damage model using backpropagatior. neural networks on the basis of X-ray half breadth ratio B/$B_o$ fractal dimension $D_f$ and fracture mechanical parameters can predict fatigue crack growth rate da/dN and cycle ratioN/$N_f$ at the same time within engineering estimated mean error(5%).

• Journal of Mechanical Science and Technology
• /
• v.18 no.1
• /
• pp.106-113
• /
• 2004

Recently, a new issue in designing spot welded structures such as automobile and train car bodies is to predict an economical fatigue design criterion. One of the most typical and traditional methods is to use a ΔP-N$\sub$f/ curve. However, since the fatigue data on the ΔP-N$\sub$f/ curve vary according to the welding conditions, materials, geometry of joint and fatigue loading conditions, it is necessary to perform the additional fatigue tests for determining a new fatigue design criterion of spot-welded lap joint having specific dimension and geometry. In this study, the stress distributions around spot welds of various spot welded lap joints such as in-plane bending type (IB type), tension shea. type (TS type) and cross tension type (CT type) were numerically analyzed. Using these results, the ΔP-N$\sub$f/ curves Previously obtained from the fatigue tests for each type were rearranged into the Δ$\sigma$-N$\sub$f/ relations with the maximum stresses at the nugget edge of the spot weld.

• Journal of the Korean Society of Manufacturing Technology Engineers
• /
• v.7 no.5
• /
• pp.66-71
• /
• 1998

It is required the superior materials for the parts of machines or structures, which could be endurable in severe load and environment. According to advancement of casting technology, nodular graphite cast iron is used as suitable for such condition. But nodular graphite cast iron is scattering of fatigue strength and low reliability. Therefore in this study, the effect of matrix structure and number of nodular graphite on the initiation of fatigue crack and fatigue strength. It was found that the material which has relatively high ferrite volume fraction was more easily cracked than other materials and fatigue limit was low. The material which has not found pinhole on the surface, the crack was initiated in graphite went through ferrite and propagated into through graphite, but separated graphite and ferrite grain boundary and combined with other cracks to fro large one.