Advances in Fracture and Strength

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作者: Choong Myeong Kim, Jung Kyu Kim, Chul Su Kim
摘要: The optimum welding condition for the input power was experimentally determined using the ERW simulator. The optimum condition derived from the nondestructive defect inspection and impact energy was the heat input power of 250kW with normalizing treatment at 900oC. In order to evaluate the fatigue life of ERW pipes, fatigue crack growth test for base metal and weld joint with the optimum condition were performed. As stress intensity factor range ( DKS) increased, the fatigue crack propagation rate (da/dN) of the weld joint became slower than that of the base metal. The fatigue life of ERW pipe was statistically estimated using Monte-Carlo simulation with the standard deviation of material constants (C and m) of the Paris law in the specimen. The fatigue life at failure probability PF = 50% is 2.3×105 cycle. Since the fatigue test of pipe in field has a lot of difficulties due to cost, equipment and time, the life derived from the simulation was identified using the common durability simulation software.
作者: Kyung Su Kim, Byung Ok Kim, Young Kwan Kim, Chang Hwan Lee, Sung Won Lee
摘要: Recently, most of fatigue cracks in ship structures are reported within a few years after delivery. This type of fatigue characteristics cannot be explained adequately by the S-N curve based on high cycle fatigue. Calculation results under critical loading conditions reveal that stress magnitude higher than three times the yield stress occurs at some critical locations. It shows the fatigue cracks are related to low cycle fatigue. But the existing recommended design procedures in maritime industry do not properly cover low cycle fatigue problems. This work represents the first step in an effort to develop a design code that addresses low cycle fatigue problems. Low cycle fatigue test for uniform round specimen made of base/weld metal and for cruciform welded joint are carried out under constant amplitude alternating load, controlled by strain. Strain-cycle curves for the base metal and weld joints show good agreement with published data as well as some code recommended design curves.
作者: Chang Su Woo, Wan Doo Kim, Wan Soo Kim, Jae Do Kwon
摘要: Fatigue lifetime prediction methodology of the vulcanized natural rubber was proposed by incorporating the finite element analysis and fatigue damage parameter determined from fatigue test. Finite element analysis of 3D dumbbell specimen of natural rubber was performed based on a hyper-elastic material model determined from the tension, compression and shear tests. Stroke controlled fatigue tests were conducted using fatigue specimens at different levels of mean strain. The Green-Lagrange strain at the critical location determined from the FEM was used for evaluating the fatigue damaged parameter of the natural rubber. It was shown that the maximum Green-Lagrange strain was proper damage parameter, taking the mean strain effects into account. Fatigue lives of the natural rubber are predicted by using the fatigue damage parameters at the critical location. Predicted fatigue lives of the natural rubber agreed fairly well the experimental fatigue lives a factor of two.
作者: Tae Wan Kim, Sang Don Lee, Yong Joo Cho
摘要: In this study, the simulation of contact fatigue based on stress analysis is conducted under Elastohydrodynamic Lubrication (EHL) state. To predict a crack initiation life accurately, it is necessary to calculate contact stress and subsurface stresses accurately. Contact stresses are obtained by contact analysis of a semi-infinite solid based on the use of influence functions and the subsurface stress is obtained using rectangular patch solutions. The numerical algorithm using newton-rapson method was constructed to calculate the EHL pressure. Based on these stress values, three multiaxial high-cycle fatigue criteria are used. As a result, the effects of EHL on contact fatigue life are calculated.
作者: Byeong Wook Noh, Young Woo Choi, Jung Il Song, Sung In Bae
摘要: The Combine is necessary equipment in the agricultural industry. The components of the Combine are worked for a long time under inferior environmental condition. Especially reaping knife is worked in high frequency domain, and submit fatigue load while use. So, fatigue test was performed to obtain the S-N curve of real component, and load history is measured through field test. The local stresses due to these loads have been calculated by FEM. These results have been used as the input values for the multi-axial fatigue analysis of real components. For the assessment of multi-axial fatigue damage, the critical plane methods have been employed. The used parameters of critical plane methods are Morrow’s, Smith-Waston-Topper’s, and Brown-Miller’s, those were modified for the high cycle fatigue region within elastic behavior. In addition, design improvement is performed about shape of reaping knife to increase the endurance limit, and durability test for improved knife is performed. It is found that the fatigue life of improved reaping knife is increased, and durability test result show that life of reaping knife is increased enough.
作者: Dae Hyun Ryu, Seung Hoon Nahm
摘要: A crack detection system was developed to observe growth behavior of fatigue crack using the digital image processing techniques. The crack propagation behavior was observed successfully by combining block matching method and inclination threshold value method. The data obtained with this new system agreed with the existent data and the standard deviation was 0.03%. If the proposed method is utilized to develop a crack monitoring system to observe the crack growth behavior automatically, the time and effort needed for a fatigue test could be dramatically reduced. And also it will be easier to estimate and to manage efficiently the safety and residual life of industrial facilities.
作者: Satoshi Hanawa, Masahiro Ishihara, Yoshinobu Motohashi
摘要: In the structural design of ceramics components especially for graphite materials, it is important to apply the realistic fracture model in the design method so as to reduce the large safety margin. In this study, we proposed the multiaxial strength model by expanding the microstructure based brittle fracture model applicable to both uniaxial tensile and compressive stress conditions. The advantage of the model is a treatment of the microstructural information such as grain size, pore size and pore size distribution. The proposed model was applied to biaxial strength prediction of near isotropic nuclear graphite using grain/pore related microstructure parameters. Predicted results were compared with biaxial strength data, and it was found that the proposed fracture model showed fairly good strength prediction.
作者: Jong Choon Kim, Do Jun Shim, Kang Ok Yoon, Jae Boong Choi, Yoon Suk Chang, Young Jin Kim, Song Chun Choi, Woo Sik Kim
摘要: Pipelines have the highest capacity and are the safest and the least environmentally disruptive means for transmitting gas or oil. Recently, failures due to corrosion defects have become a major concern in maintaining pipeline integrity. A number of solutions have been developed for the assessment of remaining strength of corroded pipelines. In this paper, a Fitness-For-Purpose (FFP)type limit load solution for corroded city gas pipelines is proposed. For this purpose, a series of burst tests with various types of machined defects were performed. Finite element simulations were carried out to derive an appropriate failure criterion. Based on such solution along with existing solutions, an integrity evaluation program for corroded city gas pipeline, COPAP-CITY, has been developed.
作者: Yong Hak Huh, Dong Iel Kim, Chang Doo Kee
摘要: To determine the micro-mechanical properties for micro/nano materials, it may be essential to measure the strain/deformation during micro-mechanical testing. Therefore, in this study, continuous measurement of in-plane tensile strain in micro-sized specimens of thin film materials was introduced using the micro-ESPI technique. TiN and Au thin films 1 and 0.47µm thick, respectively, were deposited on the silicon wafer and fabricated into the micro-sized tensile specimens by the electromachining process. The micro-tensile loading system and micro-ESPI system were developed to measure the tensile strain during micro-tensile loading. Micro-tensile stress-strain curves for these materials were determined using the algorithm for continuous strain measurement. Furthermore, the new algorithm for enhancing the sensitivity to measurement of in-plane tensile strain was suggested. Using the algorithm, micro-tensile strain data between interfringes were calculated. It is shown that the algorithm for enhancement of the sensitivity suggested in this study makes the sensitivity to the in-plane tensile strain increase.