• Journal of Ocean Engineering and Technology
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• v.2 no.2
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• pp.104-111
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• 1988

This paper deals with the micro-surface-cracks behavior on the unnotched smooth specimens of Type 304 stainless steel at $593^{\circ}C$ in air under creep and creep-fatigue conditions that have 10 mim and 1 min load holding times respectively. The behaviors of the micro-surface-cracks have been visualized by means of surface replica method and optical micro-photography. The quantitative characteristics of initiation, growth and coalescence of micro-surface-cracks have been investigated by observing and measuring the crack growth behaviors. some of the important results are as follows: Main crack initiates at grain boundary in the early stage(10 to 20%)of its life time and grows through coalescence and finally leads to fracture. The distribution of micro-surface-crack length, 2a, can be plotted against the composite Weibull distribution. The growth rate of the main crack can be plotted against the stress intensity factor, crack tip opering displacement and J integral.

• Journal of the Korean Society of Manufacturing Process Engineers
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• v.11 no.6
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• pp.107-114
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• 2012

Fracture of brittle material due to dynamic load such a particle impact has been reported by many researchers as the fracture behavior by variation of stress for a short minute. Especially, the brittle material, such a ceramic, applied to the structural component of machine, is considered as the important project. In order to evaluate the improvement of impact resistance, the particle impact test for the $Al_2O_3-TiO_2$ coated glass is practiced. And then, the damage variation according to the impact energy of steel ball was evaluated. There was a large improvement by the ceramic coating on the surface of a glass substrate. The damage volume was especially imported to evaluate damage behavior in quantity. These data were plotted on logarithmic coordinate and experimental equations were induced by data analysis based on test results. And the variation of critical energy for crack initiation was analyzed with critical impact energy when each crack occurs.

• Transactions of the Korean Society of Mechanical Engineers
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• v.13 no.3
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• pp.472-478
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• 1989

Using the SMC(sheet moulding compound) composite material consisting of E-glass chopped strand mat and unsaturated polyester resin, three-point bending tests are carried out to evaluate the elasto-plastic fracture toughness by means of R curve. The crack extension is experimentally observed with the ink staining method. The point of stable crack growth is discussed in consideration of the load-load point displacement curve, the damage behavior of the notch vicinity, and the R-curves. The damage zone of the notch vicinity was composed of the initiation and growth of subcracks as well as those of the main crack. The point of stable crack growth can be defined as the inflection point in the R curves and its point also concurrs with the proportional limit on the load-load point displacement curve.

• Transactions of the Korean Society of Mechanical Engineers
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• v.13 no.3
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• pp.451-460
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• 1989

This paper dealt with the effect of the ratios N2 to H2 gas on the corrosion fatigue failure behavior of ion-nitrided SM45C steel specimens. The specimens were water cooled after ion-nitriding at 500.deg. C for 3hrs in 5 Torr, 0.8N$_{2}$ and 0.5N$_{2}$ atmospheres. As the nitrogen concentration increases, the higher compressive residual stresses developed in the surface layer and the depth of nitrided layer increased, which in turn gave rise to increases in fatigue strength and corrosion fatigue life. In the region less than 1.5 * 10$^{5}$ cycles, fatigue failure initiated at the brittle nitrided case, whereas in the region higher than 1.5 * 10$^{5}$ cycles crack initiated from the non-metallic inclusions in the subsurface. The initiation of corrosion fatigue failure was mainly attributed to pitting of case hardened surface layer.

• Journal of Power System Engineering
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• v.6 no.1
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• pp.61-67
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• 2002

15Cr-5Ni 석출강화 스테인리스강 3종류의 피로균열 발생과 성장 특성 및 파괴인성에 대하여 노치함수로서 연구하였다. 3종류강의 열처리 조건은 $482\;^{\circ}C,\;579\;^{\circ}C$ 및 $621\;^{\circ}C$이다. $621\;^{\circ}C$에서 4시간동안 열처리한 시험편 C는 약 $280\;MPa\;\sqrt{m}$의 가장 높은 파괴인성을 보였으며, 3종류에서 피로균열 성장이 가장 늦었다. $482\;^{\circ}C$에서 1시간 열처리한 시험편 A에서, 피로균열발생한계, ${\Delta}k{\rho}$, 는 노치반경0.3 mm에서 약 $280\;MPa\;\sqrt{m}$의 가장 높은 값을 보였다. 시험편 A는 시험편 B와 C보다 피로균열 성장이 빨랐지만, 피로균열 발생이 늦었다. 예 하중에 의한 노치선단의 압축잔류응력은 노치 시험편의 피로강도 향상에 유용한 방법이었다.

• Structural Engineering and Mechanics
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• v.73 no.5
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• pp.585-598
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• 2020

We develop design model within which nucleation and propagation of crack in a fibrous composite is described. It is assumed that under loading, crack initiation and fracture of material happens in the composite. The problem of equilibrium of a composite with embryonic crack is reduced to the solution of the system of nonlinear singular integral equations with the Cauchy type kernel. Normal and tangential forces in the crack nucleation zone are determined from the solution of this system of equations. The crack appearance conditions in the composite are formed with regard to criterion of ultimate stretching of the material's bonds. We study the case when near the fiber, the binder has several arbitrary arranged rectilinear prefracture zones and a crack with interfacial bonds. The proposed computational model allows one to obtain the size and location of the zones of damages (prefracture zones) depending on geometric and mechanical characteristics of the fibrous composite and applied external load. Based on the suggested design model that takes into account the existence of damages (the zones of weakened interparticle bonds of the material) and cracks with end zones in the composite, we worked out a method for calculating the parameters of the composite, at which crack nucleation and crack growth occurs.

• Journal of the Korean Fracture Society
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• v.30 no.4
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• pp.186-191
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• 2017

Purpose: The purpose of this study was to investigate the radiological and epidemiological characteristics, as well as the clinical course of pelvic insufficiency fractures in the elderly population. Materials and Methods: At a Haeundae Paik Hospital, we retrospectively reviewed patients with pelvic insufficiency fractures between March 2010 and May 2017. The demographic data of patients were analyzed, and bone mineral density and bone turnover markers were evaluated to estimate the metabolic status of the bone. The radiological characteristics were evaluated by comparing the simple x-ray images with the computed tomography images, and the types of fractures were classified via computed tomography images. For clinical course evaluation, we investigated comorbid complications, and compared the walking ability scale before and 6 months after the fracture. Results: A total of 42 patients were included, with an average age of 76.5 years. All were female except one case. In 5 cases where the initial medical examination was from another institution, the fracture was not found in 3 cases. All cases received conservative treatment. After the diagnosis of pelvic bone fracture using a simple x-ray imaging, additional fractures were found in 81.0% of the study population using a computed tomography. Initiation of gait occurred at an average of 2.8 weeks, and every case except 1 (97.6%) fully recovered their gait ability. Conclusion: We concluded that there was a limitation with diagnosing pelvic insufficiency fracture using only a simple x-ray imaging technique. In general, cases in this study showed conservative treatment yielded favorable clinical outcome with relatively less critical complications.

• Journal of the Korea institute for structural maintenance and inspection
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• v.10 no.3
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• pp.185-196
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• 2006

Fracture toughness is a material property for crack initiation and propagation in fracture mechanics. For mode I fracture toughness measurement in concrete, RILEM committees 89-FMT proposed three-point bend tests based on the two-parameter fracture model. But, there is no proposed test method as a standard for mixed mode test for now. And RILEM three-point bend test procedure is complicate. Therefore, in this study, brazilian disks of various size were designed as the concrete with a similar specified concrete strength and maximum size of coarse aggregate($G_{max}$) were respectively 20mm and 40mm. And mode I fracture toughness of brazilian disks was compared with that of RILEM three-point bend test. As a result, it was suggested appropriate size(thickness, diameter) and notch length ratio of brazilan disk on the $G_{max}$. And it was verified that stress intensity factors for mixed mode can be easily calculated with the disk specimen. Stress intensity factors of a concrete brazilian disk were evaluated with finite element analysis and five terms approximation for comparison.

• Journal of Welding and Joining
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• v.35 no.2
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• pp.23-29
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• 2017

Mechanical properties and hardness distributions in arc brazed joints of Dual phase steel using Cu-Al insert metal were investigated. The maximum tensile shear load was 10.4kN at the highest brazing current. It was about 54% compared to tensile load of base metal. This joint efficiency is higher than that of joint of DP steel using Cu-based filler metals which are Cu-Si, Cu-Sn. Fracture positions can be divided into two types. Crack initiation commonly occurred at three point junction among upper sheet, lower sheet and the fusion zone. However crack propagations were different with increasing the brazing current. In case of the lower current, it instantaneously propagated along with the interface between fusion zone and upper base material. On the other hand, in case of higher current, a crack propagation occurred through fusion zone. When the brazing current is low (60, 70A), the interface shape is flat type. However the interface shape is rough type, when the brazing current is high (80A). It is thought that the interface shapes were the reason why the crack propagations were different with brazing current. The interface was the intermetallic compounds which consisted of $(Fe,Al)_{0.85}Cu_{0.15}$ IMC formed by crystallization at $1200^{\circ}C$during cooling. Therefore the maximum tensile shear load and the fracture behavior were determined by a interface shape and effective sheet thickness of the fracture position.

• Geomechanics and Engineering
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• v.30 no.1
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• pp.93-105
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• 2022

With the exploitation of natural resources in China, underground resource extraction and underground space development, as well as other engineering activities are increasing, resulting in the creation of many defective rocks. In this paper, uniaxial compression tests were performed on rocks with double holes and fractures at different angles using particle flow code (PFC2D) numerical simulations and laboratory experiments. The failure behavior and mechanical properties of rock samples with holes and fractures at different angles were analyzed. The failure modes of rock with defects at different angles were identified. The fracture propagation and stress evolution characteristics of rock with fractures at different angles were determined. The results reveal that compared to intact rocks, the peak stress, elastic modulus, peak strain, initiation stress, and damage stress of fractured rocks with different fracture angles around holes are lower. As the fracture angle increases, the gap in mechanical properties between the defective rock and the intact rock gradually decreased. In the force chain diagram, the compressive stress concentration range of the combined defect of cracks and holes starts to decrease, and the model is gradually destroyed as the tensile stress range gradually increases. When the peak stress is reached, the acoustic emission energy is highest and the rock undergoes brittle damage. Through a comparative study using laboratory tests, the results of laboratory real rocks and numerical simulation experiments were verified and the macroscopic failure characteristics of the real and simulated rocks were determined to be similar. This study can help us correctly understand the mechanical properties of rocks with defects and provide theoretical guidance for practical rock engineering.