• Structural Engineering and Mechanics
• /
• v.38 no.6
• /
• pp.803-823
• /
• 2011

In this paper, the three dimensional Parallel Realistic Failure Process Analysis ($RFPA^{3D}$-Parallel) code based on micromechanical model is employed to investigate the bonding behavior in FRP sheet bonded to concrete in single shear test. In the model, the heterogeneity of brittle disordered material at a meso-scale was taken into consideration in order to realistically demonstrate the mechanical characteristics of FRP-to-concrete. Modified Mohr-coulomb strength criterion with tension cut-off, where a stressed element can damage in shear or in tension, was adopted and a stiffness degradation approach was used to simulate the initiation, propagation and growth of microcracks in the model. In addition, a Master-Slave parallel operation control technique was adopted to implement the parallel computation of a large numerical model. Parallel computational results of debonding of FRP-concrete visually reproduce the spatial and temporal debonding failure progression of microcracks in FRP sheet bonded to concrete, which agrees well with the existing testing results in laboratory. The numerical approach in this study provides a useful tool for enhancing our understanding of cracking and debonding failure process and mechanism of FRP-concrete and our ability to predict mechanical performance and reliability of these FRP sheet bonded to concrete structures.

• Corrosion Science and Technology
• /
• v.2 no.3
• /
• pp.155-160
• /
• 2003

Plasma spraying technique was used to fabricate functionally graded coating (FGC) of NiCrAIY/YSZ 8wt%$Y_2O_3-ZrO_2$ on a Co-base superalloy (HAYNES 188) substrate. Six layers were coated on the substrate for building up compositionally graded architecture. Conventional thermal barrier coating (TBC) of NiCrAIY/SZ with sharp interface was also fabricated. As-coated FGC and TBC samples were exposed at the temperature of $1100^{\circ}C$ for 10, 50, 100 hours in air. Microstructural change of thermally exposed samples was examined. Pores and microcracks were formed in YSZ layer due to evolution of thermal internal stress at high temperature. The amount of pores and microcracks in YSZ layer were increased with increasing exposure time at high temperature. High temperature oxidation of coatings occurred mainly at the NiCrAIY/YSZ interface. In comparison with the case of TBC. the increased area of the NiCrAIY/YSZ interface in FGC is likely to attribute to forming the higher amount of oxides.

• Journal of the Korea Concrete Institute
• /
• v.11 no.4
• /
• pp.107-116
• /
• 1999

The damage in concrete structures generally starts with microcracking and thus it is important to find and explore these microcracks in concrete in order to ensure appropriate safety and serviceability. The purpose of the present study is to identify the damage characteristics of concrete structures due to cracking by employing the acoustic emission techniques. A comprehensive experimental study has been done. The cracking damage under tensile and flexural loadings have been identified and the bond damage between steel and concrete have been also characterized. It is seen that the amplitudes and energy level of Acoustic Emission(AE) events are found to be relatively small for bond cracking damages and large for tensile cracking damages. The characteristic equations of the AE events for various cracking damages have been proposed based on the present test data. The internal microcracks are progressively developed ahead of a visible actual crack and the present study clearly exhibits these damage mechanism for various types of cracking in concrete. The present study provides useful data which can be used to identify the various types of cracking damages in concrete structures. This will allow efficient maintenance of concrete structures through monitoring of internal cracking based on acoustic emission.

• Proceedings of the KAIS Fall Conference
• /
• 2001.05a
• /
• pp.40-46
• /
• 2001

The thermal stability of $Al_2TiO_{5}$ ceramics was improved by formation of solid solution with MgO, such as $MgAl_2O_4$ spinel through electrofusion in an arc furnance, and by limitation of grain size and microcracks with $SiO_2$, $ZrO_2$ and ${\alpha}$-$Al_2O_3$. The low thermal expansion properties of $Al_2TiO_{5}$ composites show the thermal hysteresis curves due to the strong anisotropy of $Al_2TiO_{5}$. These phenomena are explained by the opening and closing of microcracks. The relation between thermal hysteresis, microstructures and sintering temperature were studied by dilatometry.

• 연구논문집
• /
• s.24
• /
• pp.161-174
• /
• 1994

The Ni-Al intermetallic compound that has the greatest potential to be commercialized shows the high ductility at room temperature with the addition of boron, but has extremely low ductility at high temperature and oxidation environment. On this research work, the changes of microstructure and compressive fracture properties were studied in ($NiAl+Ni_3Al$) two-phase alloys. The precipitation behavior of $Ni_3Al$ after solution treatment at $1300^\circC$ for 14hrs and aging treatment at $800^{\circ}C$ for 14hrs was varied with Al content in ($NiAl+Ni_3Al$) two-phase alloys. These microstructure could be modified dramatically by suitable heat treatments. Martensite or martensite plus $Ni_3Al$ microstructure was obtained upon oil quenching from $1300^\circC$. Aging of Martensite at $800^\circC$ resulted in the $Ni_3Al$ plus NiAl phase. The compressive fracture strength and compressive fracture strain were improved by the $Ni_3Al$ plus NiAl phase mixtures at room temperature and $1100^\circC$. Microcracks are observed mostly in the region of NiAl and the interface of $NiAl-Ni_3Al$ phase after compressive test at room temperature. In the case of high temperature compressive test, microcracks are formed in the region of $Ni_3Al$ phase.

• Journal of Welding and Joining
• /
• v.7 no.2
• /
• pp.35-48
• /
• 1989

This study is concerned with a correlation of microstructure and local brittle zone (LBZ) in offshore structural steel welds. The influence of the LBZ on fracture toughness was investigated by means of simulated heat-affected zone (HAZ) tests as well as welded joint tests. Micromechanical processes involved in void and cleavage microcrack formation were also identified using notched round tensile tests and subsequent SEM observations. The LBZ in the HAZ of a multiphase welded joint is the interstitially reheated coarse grained HAZ, which is influenced by metallurgical factors such as effective grain size, the major matrix structure and the amount of high-carbon martensite-austenite (M-A) constituents. The experimental results indicate that Chirpy energy was found to scale monotonically with the amount of M-A constituents, confirming that the M-A constituent is the major microstructural factor controlling the HAZ toughness. In addition, voids and microcracks are observed to initiate at M-A constituents by the shear cracking process. Thus, the M-A constituent played an important role in initiating the voids and microcracks, and consequently caused brittle fracture.

• Bulletin of the Society of Naval Architects of Korea
• /
• v.27 no.3
• /
• pp.100-106
• /
• 1990

Microcracking of type 304 stainless steel at $593^{\circ}C(1,100^{\circ}F)$ has been studied, in particular, initiation, growth, and coalescence of fatigue and creep microcracks on smooth specimens and small notch specimens via surface replicas and photomicrographs. Quantitative information, such as, initiation period, growth, and coalescence behavior, statistical distributions of crack length, density of cracks, distribution patterns and crack growth properties, were obtained. From this study, the fracture process, fatigue life, and creep life prediction characterized by the growth of surface microcracks have been analysed by a new approach unifying the conventional approaches based on the final fracture of materials with the fracture mechanics approach. Knowledge of these parameters is critical for the application of fracture mechanics to fatigue and creep life assessment, and the damage evaluation of structures at elevated temperature.

• The Journal of the Petrological Society of Korea
• /
• v.8 no.1
• /
• pp.24-33
• /
• 1999

Differential Strain Analysis (DSA) was performed to examine the characteristics of microcracks for the granites from two sites, Noeunri and Gunggiri. The results of the DSA are taken every 5 MPa for the first 50 MPa, then every 10 MPa to a pressure of 100 MPa, and then every 15 MPa to a pressure of 250 MPa. Differential strain was measured on core samples in three horizontal directions, using $45^{\circ}$ rosette strain gages, and one vertical direction. The gradients of cumulative crack strain curves in one vertical direction and three horizontal directions differed from one another, indicating anisotropic crack development in the sample. The magnitude of vertical cumulative crack strain was the highest, indicating that the microcracks from the studied rock are generally developed in horizontal direction. Under the pressure of 240 Mpa, vertical cumulative crack strains for samples N-1, N-2, G-1, and G-2 were $74{\times}l0^{-6}~820{\times}l0^{-6},\; 190{\times}l0^{-6}~460{\times}l0^{-6},\; 329{\times}l0^{-6}~836{\times}l0^{-6},\; 833{\times}10^{-6}~1,592{\times}l0^{-6}$, respectively. Under the pressure of 25O MPa, volumetric crack strains for Gunggiri and Noeunri ranged from $1,804{\times}10^{-6}\; to\; 3,936{\times}10^{-6}\; and \;from,\; 1, 125{\times}10^{-6}\; to\; 1,457{\times}10^{-6}$, respectively. Therefore, the amount of microcrackes produced were more distributed in Gunggiri than Noeunri. The ratio of a maximum crack strain to a minimum crack strain was calculated to find the orientations between microcracks and the rift plane of the granites. Generally, the ratio has very high values ranging from 2.42 to 3.43, which suggests most microcracks to be intragranular cracks with the regular orientations. These results indicate that the preferred orientations of microcracks in the granites were almost parallel to the rift plane of the granites.

• The Journal of the Petrological Society of Korea
• /
• v.18 no.4
• /
• pp.315-336
• /
• 2009

We have studied the orientational characteristics of microcrack frequency, it's length and density in Tertiary crystalline tuff from the northeastern part of the Gyeongsang Basin. 134 sets of microcracks on horizontal surfaces of 3 rock samples from Heunghae-eup were distinguished by enlarged photomicrographs of the thin sections. The variability in patterns among microcrack length-frequency histograms for three rock samples from different altitudes were derived. The pattern of histograms changes progressively from negative exponential form to log-normal form in proportion to altitude. The distribution pattern for rock sample no.1 from lower altitude shows the broad length distribution characterized by higher mean and median, and higher standard deviation. Meanwhile, this distribution pattern corresponds with characteristics of joint length distribution in sedimentary rocks of the lower part of the Gyeongsang Supergroup. The occurrence frequency of shorter microcracks increases toward both NW and NE directions from the $N0{\sim}10^{\circ}W$, with the dominant direction of $N80{\sim}90^{\circ}W$ and $N80{\sim}90^{\circ}E$, respectively. This distribution pattern represents the relative differences in formation timing among microcrack sets and the result of the new initiation of shorter microcracks. Meanwhile, the longest microcracks within $N60{\sim}70^{\circ}W$($L_{max}$:1.18 mm) and $N0{\sim}10^{\circ}W$($L_{max}$:0.80 mm) directions are seen, but this kind of microcracks are very limited in number. Whole domain of the directional angle($\theta$)-frequency(N), length(L) and density($\rho$) chart can be divided into five sections in terms of phases of the distribution of related curves. From the distribution chart, density curve shows five distinct peaks in the WNW-ESE($N70{\sim}80^{\circ}W$), NS~NNE-SSW($N0{\sim}10^{\circ}W$, $N10{\sim}20^{\circ}E$), ENE-WSW($N50{\sim}60^{\circ}E$), and nearly EW($N80{\sim}90^{\circ}E$) directions, respectively. Especially, main directions of faults correspond with the directional angle showing high density. Consequently, these distribution patterns of density curve reflect the representative maximum principal stress orientations suggested in previous studies.

• The Journal of Engineering Geology
• /
• v.16 no.3 s.49
• /
• pp.235-244
• /
• 2006

Granite cores were sampled within Korea Atomic Energy Research Institute and cyclic loadings up to 1550 cycles were applied. Microcrack development in samples due to cyclic loading was estimated using Acoustic Emission(AE) method. AE showed two different types depending on numbers of cycle. Type 1 appeared at low cycles and had low energy and diverse frequencies, while type 2 appeared at high cycles and had high energy and uniform frequency. AE property of type 1 indicates voids and pre-existing microcracks in samples may close or propagate up to certain length. Microcracks may be sheared or closed during loading and are recovered from shear or opened during unloading when AE of type 2 were measured. P wave velocities and Felicity ratios were measured at 50, 150, 350, 750, 1550 cycles. P wave velocities were almost the same regardless of number of cycles applied. However, Felicity ratios were much lower than 0.9, indicating that microcracks were developed within samples. This result indicates that Felicity ratio is a better tool than P wave velocity to estimate the damage of rock.