• Journal of the Korean Society for Nondestructive Testing
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• v.15 no.4
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• pp.549-560
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• 1996

The purpose of this study is to investigate the usefulness of small punch(SP) test using miniaturized specimens as a method for fracture strength evaluation of CANDU pressure tube embrittled by hydrogen. According to the test results, the fracture strength evaluation as a function of hydrogen concentration at $-196^{\circ}C$ was much better than that at room temperature, as the difference of SP fracture energy(Esp) with hydrogen concentration was more significant at $-196^{\circ}C$ than at room temperature for the hydrogen concentration up to 300ppm-H. It was also observed that the peak of average AE energy, the cumulative average AE energy and the cumulative average AE energy per equivalent fracture, strain increased with the increase of hydrogen concentration. From the results of load-displacement behaviors, Esp behaviors, macro- and micro-SEM fractographs and AE test it has been concluded that the SP test method using miniaturized specimen($10mm{\times}10mm{\times}0.5mm$) will be a useful test method to evaluate the fracture strength for CANDU pressure tube embrittled by hydrogen.

• Journal of the Korea Concrete Institute
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• v.14 no.5
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• pp.689-697
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• 2002

The dynamic loads and load-point displacements of concrete three-point bend (TPB) specimens had been measured. The average crack velocities measured with strain gages were 0.16 ㎜/sec ∼ 66 m/sec. The fracture energy for crack extension was determined from the difference of the kinetic energy for the load-point velocity and the strain energy without permanent deformation from the measure external work. For all crack velocities, there were micro-cracking for 23 ㎜ crack extension, stable cracking for 61 ㎜ crack extension at the maximum strain energy, and then unstable cracking. The unstable crack extension was arrested at 80 ㎜ crack extension except the tests of 66 m/sec crack velocity. The tests less than 13 ㎜/sec crack velocity and faster than 1.9 m/sec showed static and dynamic fracture behaviors, respectively. In spite of much difference of the load and load-point displacement relations for the crack velocities, the crack velocities of dynamic tests did not affect on fracture energy rate during the stable crack extension due to the reciprocal action of kinetic force, crack extension and strain energy. During stable crack extension, the maximum fracture resistances of the dynamic tests was 147％ larger than that of the static tests.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.37 no.12
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• pp.1159-1165
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• 2013

Powder blasting, which is an efficient micromachining method for glass, silicon, and ceramics, has a critical disadvantage in that the surface finish is poor owing to the brittle fracture of materials. Low-pressure waterjet machining can be applied to smoothen the rough surface inside the blasted structure. In this study, the surface roughness and sectional dimension of micro-channels are observed during the repetitive application of a waterjet to blasted micro-channels. The asperities and subsurface cracks created by blasting are removed by waterjet machining. Along with the surface roughness, it is found that the sectional dimension increases and the edges of the finished micro-channel become slightly round. Finally, a microfluidic chip is machined by the blasting-waterjet process and a transparent microfluidic channel is obtained efficiently.

• Journal of Welding and Joining
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• v.31 no.5
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• pp.71-76
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• 2013

The effect of zirconate having - NH functional group on the T-peel and lap shear strength of $CaCO_3$ containing structural epoxy adhesive for car body assembly was investigated. Curing behavior of epoxy adhesive samples were investigated by differential scanning calorimeter (DSC) techniques. The addition of zirconate up to 7.5 phr did not affect the curing mechanism of epoxy adhesive. While the small amount of zirconate addition less than 1.1 phr increased the cross-linking density, the excess addition of zirconate resulted in the increase of uncross-linked impurity. From the increase of T-peel and lap shear strength and the change of fracture mode from the adhesive failure to the mixed one, it was considered that the small addition of zirconate was effective in improving the adhesion strength of epoxy adhesive to the adherend and inorganic filler surfaces. The formation of uncross-linked impurity with the excess addition of zirconate was considered to decrease the joint strength by decreasing the cohesive strength of the cured epoxy.

• Proceedings of the Materials Research Society of Korea Conference
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• 2012.05a
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• pp.95.1-95.1
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• 2012

Very High Temperature gas cooler Reactor (VHTR) has been considered as one of the most promising nuclear reactor because of many advantages including high inherent safety to avoid environmental pollution, high thermal efficiency and the role of secondary energy source. The TRISO coated fuel particles used in VHTR are composed of 4 layers as OPyC, SiC, IPyC and buffer PyC. The significance of CVD-SiC coatings used in tri-isotropic(TRISO) nuclear coated fuel particles is to maintain the strength of the whole particle. Various methods have been proposed to evaluate the mechanical properties of CVD-SiC film at room temperature. However, few works have been attempted to characterize properties of CVD-SiC film at high temperature. In this study, micro tensile system was newly developed for mechanical characterization of SiC thin film at elevated temperature. Two kinds of CVD-SiC films were prepared for micro tensile test. SiC-A had [111]-preferred orientation, while SiC-B had [220]-preferred orientation. The free silicon was co-deposited in SiC-B coating layer. The fracture strength of two different CVD-SiC films was characterized up to $1000^{\circ}C$.The strength of SiC-B film decreased with temperature. This result can be explained by free silicon, observed in SiC-B along the columnar boundaries by TEM. The presence of free silicon causes strength degradation. Also, larger Weibull-modulus was measured. The new method can be used for thin film material at high temperature.

• Journal of the Korean Society for Nondestructive Testing
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• v.28 no.4
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• pp.352-357
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• 2008

The detection of micro cracks in materials at the early stage of fracture is important in many structural safety assurance problems. The nonlinear ultrasonic technique (NUT) has been considered as a positive method for this, since it is more sensitive to micro crack than conventional linear ultrasonic methods. The basic principle is that the waveform is distorted by nonlinear stress-displacement relationship on the crack interface when the ultrasonic wave transmits through, and resultantly higher order harmonics are generated. This phenomenon is called the contact acoustic nonlinearity (CAN). The purpose of this paper is to prove the applicability of CAN experimentally by detection of micro fatigue crack artificailly initiated in Aluminum specimen. For this, we prepared fatigue specimens of Al6061 material with V-notch to initiate the crack, and the amplitude of second order harmonic was measured by scanning along the crack direction. From the results, we could see that the harmonic amplitude had good correlation with COD and it can be used to detect the crack depth in more accurately than the common 6 dB drop echo method.

• Restorative Dentistry and Endodontics
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• v.32 no.2
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• pp.130-137
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• 2007

This in vitro study examined the effect of surface defects on cutting blades on the extent of the cyclic fatigue fracture of HEROShaper Ni-Ti rotary files using fractographic analysis of the fractured surfaces. A total of 45 HEROShaper (MicroMega) Ni-Ti rotary flies with a #30/.04 taper were divided into three groups of 15 each. Group 1 contained new HEROShapers without any surface defects. Group 2 contained HEROShapers with manufacturing defects such as metal rollover and machining marks. Croup 3 contained HEROShapers that had been clinically used for the canal preparation of 4-6 molars A fatigue-testing device was designed to allow cyclic tension and compressive stress on the tip of the instrument whilst maintaining similar conditions to those experienced in a clinic. The level of fatigue fracture time was measured using a computer connected the system. Statistical analysis was performed using a Tukey's test. Scanning electron microscopy (SEM) was used for fractographic analysis of the fractured surfaces. The fatigue fracture time between groups 1 and 2, and between groups 1 and 3 was significantly different (p<0.05) but there was no significant difference between groups 2 and 3 (p>0.05). A low magnification SEM views show brittle fracture as the main initial failure mode At higher magnification, the brittle fracture region showed clusters of fatigue striations and a large number of secondary cracks. These fractures typically led to a central region of catastrophic ductile failure. Qualitatively, the ductile fracture region was characterized by the formation of microvoids and dimpling. The fractured surfaces of the HEROShapers in groups 2 and 3 were always associated with pre-existing surface defects. Typically, the fractured surface in the brittle fracture region showed evidence of cleavage (transgranular) facets across the grains, as well as intergranular facets along the grain boundaries. These results show that surface defects on cutting blades of Ni-Ti rotary files might be the preferred sites for the origin of fatigue fracture under experimental conditions. Furthermore this work demonstrates the utility of fractography in evaluating the failure of Ni-Ti rotary flies.

• Smart Structures and Systems
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• v.8 no.3
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• pp.321-341
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• 2011

Acoustic emission (AE) monitoring was conducted for mortar specimens under three types of static loading patterns (cubic-splitting, direct-shear and pull-out). Each of the applied loading patterns was expected to produce a particular fracture process. Subsequently, the AEs generated by various fracture or damage processes carried specific information on temporal micro-crack behaviors of concrete for post analysis, which was represented in the form of detected AE signal characteristics. Among various available characteristics of acquired AE signals, frequency content was of great interest. In this study, cement-based piezoelectric sensor (as AE transducer) and home-programmed DEcLIN monitoring system were utilized for AE monitoring on mortar. The cement-based piezoelectric sensor demonstrated enhanced sensitivity and broad frequency domain response range after being embedded into mortar specimens. This broad band characteristic of cement-based piezoelectric sensor in frequency domain response benefited the analysis of frequency content of AE. Various evaluation methods were introduced and employed to clarify the variation characteristics of AE frequency content in each test. It was found that the variation behaviors of AE frequency content exhibited a close relationship with the applied loading processes during the tests.

• Proceedings of the Korean Society of Precision Engineering Conference
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• 2002.05a
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• pp.852-855
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• 2002

In order to investigate the fracture behaviors(penetration modes) and resistance to penetration during ballistic impact of surface hardening treated Aluminum, Titanium alloy laminates, ballistic tests were conducted. In this paper, Anodized Al 5083-H131 alloy laminates and nitrified Ti (Gr.2) alloy laminates were used to achieve higher surface hardness. Surface hardness test were conducted using a Micro victor's hardness tester and thickness of surface hardening treated specimens was measured by video microscope. Resistance to penetration is determined by the protection ballistic limit($V_50$), a statistical velocity with 50% probability far complete penetration. Fracture behaviors and ballistic tolerance, described by penetration modes, are respectfully observed at and above ballistic limit velocities, as a result of $V_50$ test and Projectile Through Plates (PTP) test methods. PTP tests were conducted with $0^{\circ}$ obliquity at room temperature using 5.56mm ball projectile. $V_50$ tests with $0^{\circ}$ obliquity at room temperature were conducted with projectiles that were able to achieve near or complete penetration during PTP tests.

• Journal of Ocean Engineering and Technology
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• v.1 no.1
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• pp.127-137
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• 1987

The evaluation of the elastic-plastic fracture toughness $J_{1C}$ was performed on the center of weld metal(CWM), the heat affected zone (HAZ) and the base metal (BM) of API 5LB steel pipes welded by the high frequency electric resistance welding. The $J_{1C}$ was evaluated by the JSME R-Curve and JSME SZW methods using the smooth and side-grooved specimens. The results are as follows; (1) The $J_{1C}$ values by the SZW method are overestimated as compared with those by the R-curve method, because the micro-crack is formed as SZW increase with the deformation at SZ after initiation of the ductile crack. (2) The everage of $J_{1C}$ values by the the R-curve and the SZW methods in side-grooved specimens tended to decrease in comparison with smooth specimens 9.42% at CWM, 4.2% at HAZ, 23.2% at BM, respectively. (3) The boundary of the fatigue pre-crack, stretched zone, and dimple regions appeared more clearly in side-grooved specimens, for the slight change of SZW in the direction of the plate thickness, as compared with smooth specimens.