Advances in Fracture and Failure Prevention

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Authors: Do Won Seo, Yang Bae Jeon, Jae Kyoo Lim
Abstract: Spot welding is a process that sheet metals are joined in one or more spot by heating at the faying interface. In this process, the spatter is dispersed from melted area. It has been reported that spatter generation has adverse effects on weld quality. However, no systematic study has been carried out to find out its effect on weld quality in resistance spot welding processes. In this study, specially designed specimens are used to perform experimental investigation of spatter generation and its effect. Major finding of this study show trends in tensile-shear strength for various amounts of spatter generated during spot welding process. Thus, optimum welding conditions are proposed in view of spatter generation and tensile-shear strength.
Authors: J.B. Kim
Abstract: This study presents a three-dimensional dynamic stress analysis of the sabot/rod system during launch. A study was undertaken to investigate the influence of forward sabot diameter and rod length on the stress of projectile rod traveling in the gun tube. Three types of gun tube profile were simulated. The first profile is straight. The second profile of the tube droop is caused only by gravity and the third is combined droop profile which is caused by gravity with the manufacture status. Numerical simulation results showed that the maximum von Mises stress due to in-bore behavior of the sabot/rod system could be decreased by changing the forward sabot diameter in a tube drooped by gravity loads with the manufacture status. The results also showed that the stress of rod was affected by the factor of at·L/D· ( at ; combined acceleration, L ; length of rod, D ; diameter of rod), and it becomes minimum when the aspect ratio L/D becomes 14 in the model used in the study. in the study
Authors: Seok Hwan Ahn, Ki Woo Nam, Kotoji Ando
Abstract: Four kinds of brittle materials were used to evaluate the bending strength under three-point bending and the characteristics of the elastic wave signal by Vickers indentation. The bending test was carried out under room temperature and high temperature. A crack was made at the tension side of the specimen with a Vickers indenter to investigate fracture strength. Fracture wave detector was used to evaluate characteristics of waveform and frequency of the elastic wave signal.
Authors: Kenji Machida, Mamtimin Gheni
Abstract: The thickness dependency of the temperature image obtained by an infrared thermography was investigated using specimens with three kinds of materials and four kinds of the thickness of the specimen. Only the sum of the principal stresses which is the first invariant of stress tensor is measured, and it is impossible to measure individual stress components directly. Then, the infrared hybrid method was developed to separate individual stress components. Although the form of the contour line of low stress side differs greatly, the distribution form of high stress side was considerably alike. The stress intensity factor of material with low thermal conductivity can be estimated with high accuracy by the infrared hybrid method. On the crack problem, it was elucidated that the influence of thermal conduction is large and an inverse problem analysis is required.
Authors: Sung Gyu Jung, In Gyu Park, Chang Soon Lee, Myung Jo Jhung
Abstract: To prevent the potential failure of the reactor pressure vessel (RPV), it is requested to operate RPV according to the pressure-temperature (P-T) limit curve during the heat-up and cool-down process. The procedure to make the P-T limit curve was suggested in the ASME Code but it has been known to be too conservative for some cases. In this paper, the conservatism of the ASME Code Sec. XI, App. G was investigated by performing a series of sensitivity analyses. The effects of six parameters such as crack depth, crack orientation, clad thickness, fracture toughness, cooling rate, and neutron fluence were analyzed. The results of P-T limit curves are compared to one another.
Authors: Eisaku Umezaki, T. Suzuki
Abstract: The two-dimensional deformation of wood under shearing loads is measured using an electronic speckle pattern interferometry (ESPI) technique. The radical and tangential sections of Douglas firs are used as specimens in the longitudinal direction. Results reveal that large two-dimensional deformation of the wood can be measured using the ESPI technique, the deformation values significantly vary for every part of the specimens, and the ring directions of earlywood and latewood, which compose the annual rings, have an effect on the two-dimensional deformation of wood.
Authors: Y.H. Seo, Byoung Kee Kim, H.D. Son
Abstract: Wire cutting(EDM) or blanking is used to made workpieces from sheet metal. Wire EDM provides a relatively simple method for making holes of any desired cross section in material. But EDM requires a lot of working time and the high unit cost of production. In conventional blanking, for the production of precision devices or assemblies, it is always necessary that at least two, but generally more, secondary operations are required per piece part. Using the fine blanking process, a precise finished part with inner and outer forms clearly sheared over the whole material thickness are produced in one single operation. In this study an attempt is made to manufacture a sprocket with fine blanking process. The sprocket is parts for the tape feeder of surface mount system in electronic parts. First, a change of the existing design is made in a sprocket. The materials selected are three kinds of stainless steel, SUS304, SUS316 and SUS430. And the mechanical properties are investigated through the tensile test. After fine blanking, hardness and precision are examined with hardness test and 3-dimensional coordinate measuring for samples. The results of investigations of fine-blanking process with the help of FEM code, DEFORM 2D, are presented. For the simulation, SUS304 and SUS316 are used as materials. The damage model of Cockroft and Latham is used to calculate damage. Die-roll height, die-roll width, burnish zone and fracture zone from the fine blanking simulation are investigated in comparison with them of samples. And the applied force at each part of fine-blanking die is estimated with load-stroke diagram.
Authors: J.D. Kim, Yun Hae Kim, Jin Seok Oh
Abstract: The authors have observed directly and simultaneously the laser-induced plasma and keyhole behavior by high speed frame/streak camera using a special optical systems during the pulsed YAG laser welding of Al-Mg alloys in air and argon atmospheres. The dynamic behavior of Al-Mg alloys plasma was very unstable and this instability fluctuation period was about 440µs. After laser termination, abrupt collapse of keyhole within 600µs was observed and it was related to the formation of porosity. The authors also performed the spectroscopic analysis of laser-induced plasma, and clarified the structure and composition of evaporated particles during pulsed laser welding of Al-Mg alloys. In the air environment, the intensities of molecular spectrum of AlO and MgO were different each other depending on the power density of laser beam. Under the low power density irradiation condition, the MgO band spectrum was predominant in intensity, while the AlO spectrum became much stronger in higher power density. The same result was revealed in analyzing the composition of evaporated particles. These results were attributed to evaporation phenomena of metals with different boiling points and latent heats of vaporization.
Authors: Abdel-Hamid I. Mourad, N. Bekheet, A. El-Butch, L. Abdel-Latif, D. Nafee

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