• Proceedings of the Korean Society of Machine Tool Engineers Conference
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• 2004.04a
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• pp.58-63
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• 2004

This study is to investigate a fracture characteristics of carbon fiber reinforced plastics (CFRP) under the tensile loading as a function of acoustic emission (AE) according to the frequency analysis (transient mode) and AE source location (location mode). It was found that the fracture mechanism of AE frequency analysis was a useful tool for the estimation of different type of fracture in CFRP, i.e., matrix(epoxy resin) cracking, delamitation and fiber breakage same as AE amplitude distribution.

• Journal of Korea Foundry Society
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• v.15 no.3
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• pp.291-297
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• 1995

The mechanical properties and fracture toughness of permanent mold cast austempered gray cast iron(AGI) were compared to those of sand cast AGI. The iron was melted to eutectic composition in order to get better castability especially in permanent mold casting. Specimens prepared for tensile, impact and fracture toughness test were austenitized at $900^{\circ}C$ and austempered at $270^{\circ}C$ and $370^{\circ}C$ for 1 hour. The strength, impact and fracture toughness of permanent mold cast AGI were found to be superior to those of sand cast AGI. The maximum value of 836 MPA in tensile strength, was obtained at the austempering temperature of $270^{\circ}C$. But ductility of AGI was not improved by permanent mold casting.

• Transactions of Materials Processing
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• v.11 no.1
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• pp.61-68
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• 2002

The thermal shock and thermal fatigue test has been carried out to analyze the thermal characteristics of tool steels for hot forging and the effects of mechanical properties on this study have been investigated. The resistance to thermal shock is first of all a matter of good toughness and ductility. Therefore, a proper hot-work tool steel should be characterized by high fracture strength and high temperature toughness. Based on these results, some critical temperature($T_{fracture}$) at which fracture occur can be measured to characterize the thermal resistance of the materials. During thermal fatigue tests, the thermal fatigue cracks occur because of the repetitive heating and cooling of the die surface and the thermal fatigue damage was evaluated by analyzing different number of cycles to failure. The results showed that the resistance to thermal shock and thermal fatigue were found to be favoured by high hot tensile strength and high hot hardness, and thermal resistance of SKD61 was superior to that of ESC, SKT4 and this was caused by higher mechanical properties of SKD61.

• Proceedings of the Korean Society of Precision Engineering Conference
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• 1996.11a
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• pp.678-682
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• 1996

Acoustic emission(AE) is monitored during end milling with a sensor attached to the cutting fluid discharge nozzle. Cutting forces are also measured and compared with the AE signals to examine the reliability of the AE signals. It is demonstrated that the AE signals provide reliable informations about the cutting processes and tool states. Moreover, edge chipping and fracture of tools can be successfully detected using both the AE signals and cutting forces.

• Journal of Korean Neurosurgical Society
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• v.58 no.4
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• pp.346-349
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• 2015

Objective : To investigate the value of lumbar bone mineral density (BMD) in fracture risk assessment (FRAX) tool. Methods : One hundred and ten patients aged over 60 years were enrolled and divided into 2 groups as non-osteoporotic vertebral fracture (OVF) and OVF groups. The 10-year-risk of major osteoporotic vertebral fracture of each group was calculated by FRAX tool with femoral and lumbar spine BMDs to compare the usefulness of lumbar spine BMD in prediction of OVF. The blood level of osteocalcin and C-terminal telopeptide (CTX) as markers of activities of osteoblast and osteoclast, respectively were analyzed using the institutional database. Results : In the OVF group, the ratio of patients with previous fracture history or use of glucocorticoid was higher than those in non-OVF group (p=0.000 and 0.030, respectively). The levels of T-score of femur neck and lumbar spine in OVF group were significantly lower than those in non-OVF group (p=0.001 and 0.000, respectively). The risk of OVF in FRAX using femur BMD in non-OVF and OVF groups was $6.7{\pm}6.13$ and $11.4{\pm}10.06$, respectively (p=0.007). The risk of using lumbar BMD in the 2 groups was $6.9{\pm}8.91$ and $15.1{\pm}15.08$, respectively (p=0.002). The areas under the receiver operator characteristic curve in the FRAX risk with lumbar and femur neck BMD were 0.726 and 0.684, respectively. The comparison of osteocalcin and CTX was not significant (p=0.162 and 0.369, respectively). Conclusion : In our study, the 10-year risk of major osteoporotic fracture in the OVF group of our study was lower than the recommended threshold of intervention for osteoporosis. Hence, a lower threshold for the treatment of osteoporosis may be set for the Korean population to prevent OVF. In the prediction of symptomatic OVF, FRAX tool using lumbar spine BMD may be more useful than that using femur neck BMD.

• Journal of Welding and Joining
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• v.34 no.3
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• pp.23-30
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• 2016

This paper discusses the optimization of friction stir spot welding (FSSW) process parameters for joining Aluminum alloy (AA6061-T6) with Magnesium alloy (AZ31B) sheets. Prior to optimization an empirical relationship was developed to predict the Tensile Shear Fracture Load (TSFL) incorporating the four most important FSSW parameters, i.e., tool rotational speed, plunge rate, dwell time and tool diameter ratio, using response surface methodology (RSM). The experiments were conducted based on four factor, five levels central composite rotatable design (CCD) matrix. The maximum TSFL obtained was 3.61kN, with the tool rotation of 1000 rpm, plunge rate of 16 mm/min, dwell time of 5 sec and tool diameter ratio of 2.5.

• Proceedings of the Korean Society of Machine Tool Engineers Conference
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• 2004.10a
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• pp.334-339
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• 2004

Ultraprecision machining and MEMS technology have been taken more and more important position in machining of microparts. Micro endmilling is one of the prominent technology that has wide spectrum of application field ranging from macro parts to micro products. Also, the method of micro-grooving using micro endmilling is used widely owing to many merit, but has problems of precision and quality of products due to tool wear and tool fracture. This investigation deals with state monitoring using acoustic emission(AE) signal in the micro-grooving. Characteristic evaluation of AE raw signal, AE hit and frequency analysis for state monitoring is also presented in the paper.

• Journal of Welding and Joining
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• v.29 no.2
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• pp.80-87
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• 2011

In this study, the friction stir spot welding (FSSW) of Mg alloy sheets has been tried using an apparatus devised with a CNC milling machine to give the precise control of joining condition including tool speed. The probe tool used is made of hard metal and composed of cylindrical shoulder and pin parts. The variation of morphologies formed after the friction stir spot welding depending on the plunge speed of the tool were investigated at each rpm of tool. The history of the temperature distribution and the vertical load induced during the spot welding with friction time were measured by using an Infrared Thermal Imager (THERMA CAMTM SC2000) and a loadcell located below the specimen fixture, respectively. Tensile-shear tests were also performed to evaluate the fracture load of welded specimens. In order to characterize the friction stir spot welding of Mg alloy sheets, the variation of the fracture load was discussed on micrographic observations, temperature distribution during the FSSW according to the plunge speeds of tool.

• Proceedings of the Korean Society of Machine Tool Engineers Conference
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• 1998.10a
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• pp.254-260
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• 1998

Application of bonded dissimilar materials in various industries are increasing. When these materials are used in structures, it needs to investigate strength evolution applying fracture mechanics. Al/Epoxy bonded dissimilar materials with an interface crack and an interface crack emanating from an edge semicircular hole were prepared for the static tests so that experiment of fracture toughness were carried out. Stress intensity factors of interface cracks in bonded dissimilar materials were computed with boundary element method(BEM) and the fracture criterion of mixed mode crack were analyzed. From the results, the fracture criterion and the method of strength evolution by the fracture toughness in Al/Epoxy bonded dissimilar materials were proposed.

• Proceedings of the Korean Society of Machine Tool Engineers Conference
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• 1998.03a
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• pp.233-238
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• 1998

Recently, the uses of ceramic/metal bonded joints for structural materials have increased in various engineering fields such as automobiles, heat engines, and so on. A method of strength evaluation focussing on fracture criterion of mixed was investigated in Si3N4/metal bonded joint. Also, Fracture toughness tests of Si3N4/metal bonded joints with an interface crack were carried out and the stress intensity factors of these joints were analyzed by boundary element method. From the results, the fracture criterion and method of strength evaluation by the fracture toughness were proposed in Si3N4/metal bonded joints