• Journal of Advanced Marine Engineering and Technology
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• 제30권4호
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• pp.474-482
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• 2006

Generally almost all fatigue crack growth is affected by mode I. For this reason a study on mode I has concentrated in the field of fracture mechanics. However the fatigue crack initiation and growth in machines and structures usually occur in mixed mode loading. If there is any relationship between the cause of fracture in mixed mode loading and fracture surface, fracture surface pattern will be the main mean explaining reasons of fatigue fracture and obtaining further information about fracture process. In this paper low point shear-fatigue test with Aluminum alloy hi 5083-O is carried out from this prospect and then the mixed mode distribution of fracture surface is examined from the result after identifying the generation of fatigue crack surface pattern. It was found from the experimental results that the fatigue crack surface pattern and the fatigue crack shear direction are remarkably consistent. Furthermore It is possible that the analysis of distribution of mixed mode through the fatigue crack surface pattern.

• 한국마린엔지니어링학회:학술대회논문집
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• 한국마린엔지니어링학회 2006년도 전기학술대회논문집
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• pp.43-44
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• 2006

Generally almost all fatigue crack growth is affected by model. For this reason a study on model has concentrated in the field of fracture mechanics. However the fatigue crack initiation and growth in machines and structures usually occur in mixed mode loading. If there is any relationship between the cause of fracture in mixed mode loading and fracture surface, fracture surface pattern will be the main mean explaining reasons of fatigue fracture and obtaining further information about fracture process. In this paper four point shear-fatigue test with Aluminum alloy Al 5083-O is carried out from this prospect and then the mixed mode distribution of fracture surface is examined from the result after identifying the generation of fatigue crack surface pattern. It was found from the experimental results that the fatigue crack surface pattern and the fatigue crack shear direction are remarkably consistent. Furthermore It is possible that the analysis of distribution of mixed mode through the fatigue crack surface pattern.

• 오토저널
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• 제14권2호
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• pp.65-75
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• 1992

An analytical scheme for predicting the crack aspect pattern of materials which contain twin surface cracks was developed. Fatigue tests were performed on twin surface cracked PMMA plate specimens to obtain the interaction factor accounting for the interference effect of adjacent cracks. Here, the interaction factor is defined as the ratio of the stress intensity factor for twin surface cracks to that for a single surface crack. From the analysis of the fatigue test result, the interaction factor was presented as the ninth-order polynomial expression having a function of dimensionless crack spacing ratio. Then the polynomial expression was incorporated into the prediction program of the crack aspect pattern for twin surface cracked materials. And, the interaction effect and the coalescence condition of adjacent cracks were simplified in the newly developed prediction scheme of the crack aspect pattern. The predicted crack growth pattern using the prediction scheme was compared with test data from PMMA specimen. The predicted pattern agreed well with the test data.

• 산업기술연구
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• 제19권
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• pp.25-29
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• 1999

Rotary bending fatigue tests were carried out to investigate the fatigue behavior of high performance ductile cast iron experienced super rapid induction heat treatment. The effect of super rapid induction treatment on fatigue limit was experimentally examined with the special focus on the variation surface microstructure and the fatigue crack initiation and propagation through fractography. Main results obtained are as follows. By super rapid induction treatment in FCD500, the martensite structure obtained through conventional quenching heat treatment was confirmed on the specimen surface. The fatigue crack initiation in the hardened surface layer was restricted by the martensite structure and compressive residual stress. Thus, it could be interpreted that the initiation stress would be increased by the improvement of surface structure. The fatigue crack propagation in the hardened layer was retarded by the presence of the globular shape martensite around the graphite nodule and compressive residual stress. The crack propagation path has shown zigzag pattern in the hardened surface layer.

• 대한기계학회논문집A
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• 제26권4호
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• pp.596-601
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• 2002

Rotary bending fatigue tests were carried out to investigate the fatigue characteristics of high performance ductile cast iron experienced super rapid induction treatment. The influence of super rapid induction treatment on fatigue limit was experimentally examined with the special focus on the variation of surface microstructure and the fatigue crack initiation and propagation through fractography. Main results obtained are as follows. By super rapid induction treatment in FCD500, the martensite structure obtained through conventional heat treatment was confirmed on the specimen surface. The fatigue crack initiation in the hardened surface layer was restricted by the martensite structure and compressive residual stress. Thus, it could be interpreted that the initiation stress would be increased by improved structure in the surface. The fatigue crack propagation in the hardened layer was retarded by the presence of the globular shape martensite around the graphite nodule and compressive residual stress and the crack propagation behavior has zigzag pattern in the hardened surface layer.

• 대한기계학회논문집A
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• 제27권12호
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• pp.2055-2060
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• 2003

Striation is a typical pattern observed on the fatigue fracture surface and the spacing is known to correspond to a macroscopic fatigue crack growth rate, and many models for the predict in the formation of such striation have been proposed. However, these theories and methods can't be applied under random loading spectrum. In this study, the fatigue tests were carried out on aluminum alloy under random loading spectrum. The fatigue fracture surfaces were observed in the scanning electron microscope (SEM) and great quantities of SEM micrographs were synthesized and saved in computer system. The space and morphology of several large-scale striations, which are expected to from at the relatively greater load range in loading block, were observed. The crack length for each loading blocks was decided in consideration of regularity and repetition of those striations. It is shown that the applicability of fractographic methods on the fatigue fracture surface under random loading spectrum.

• 대한기계학회논문집A
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• 제36권4호
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• pp.443-449
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• 2012

Ti-6Al-4V 재에 다양한 표면처리방법을 통하여 나노스킨화된 피로시험편을 제작하고 회전굽힘피로 시험과 축하중 인장압축피로시험을 수행하였다. 특히 S-N 곡선에서 초음파나노표면개질(UNSM)법은 국내특허기술로 다른 처리법들보다 우수한 결과를 보였고, UNSM 처리된 직경 5 mm 재는 6 mm 티타늄재를 대치 가능할 성능을 얻었다. UNSM 처리된 $10^6$ 사이클 이상의 장수명영역에서 피로강도가 크게 향상되는 시험결과를 얻었다. 균열발생 패턴의 분석을 통하여 Ti-6Al-4V 재는 응력의 작용형태에 따라 표면균열발생형(surface originating crack type)과 내부균열발생형(interior originating crack type)인 어안균열(fish eye crack)이 발생하는 메커니즘을 분석하였다.

• 동력기계공학회지
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• 제18권2호
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• pp.62-69
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• 2014

In this paper, in order to investigate the effects of marco and microscopic observations of fatigue crack growth in friction stir welded (FSWed) 7075-T651 aluminum alloy plates, fatigue crack growth tests were performed under constant amplitude loading condition at room temperature with three different pre-cack locations, namely base metal (BM-CL) and two kinds of pre-crack locations in welded joints, weld metal (WM-CL) and heat affected zone (HAZ-CL) specimens. The fatigue crack growth behavior of FSWed 7075-T651 aluminum alloy plates were discussed based on the marco and microscopic fractographic observations. The marcoscopic aspects of surface crack growth path for BM-CL and HAZ-CL specimens indicate relatively straight lines, however, the crack growth paths of WM-CL specimens grow first straight and by followed toward the TMAZ and HAZ. The microscopic aspects of fatigue fracture for BM-CL and HAZ-CL specimens indicate typical fatigue striation, but WM-CL showed intergranular fracture pattern by micro structural changes of FSW process.

• 한국해양공학회지
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• 제25권6호
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• pp.42-48
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• 2011

The present study makes three original contributions to nanoskinned Ti-6Al-4V materials. The nanoskins were fabricated on Ti-6Al-4V material using various surface treatments: deep rolling (DR), laser shot peening (LSP), and ultrasonic nanocrystal surface modification (UNSM). These surface treatments are newly developed techniques and are becoming more popular in industrial fields. A fatigue strength comparison at up to 106 cycles was conducted on these nanoskinned Ti-6Al-4V materials. Fatigue tests were carried out using MTS under axial loading tension-compression fatigue (R = -1, RT, 5 Hz, sinusoidal wave). The analysis of the crack initiation patterns in the nanoskinned Ti-6Al-4V materials found an interior originating crack pattern and surface originating crack type. Microscopic observation was mainly used to investigate the fatigue fractured sites. These surface modification techniques have been widely adopted, primarily because of the robust grade of their mechanical properties. These are mainly the result of the formation of a large-scale, deep, and useful compressive residual stress, the formation of nanocrystals by the severe plastic deformation (SPD) at the subsurface layer, and the increase in surface hardness.

• 대한치과보철학회지
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• 제31권4호
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• pp.482-505
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• 1993

The purpose of this study was to evaluate the fracture characteristics and the effect of resin bonding of laminate porcelain. In order to characterize the indentation-induced crack, Young's moduli and characteristic indentation dimensions were measured. The fatigue life under three point flexure test was measured using the electro-dynamic type fatigue machine, and the crack propagation with thermocycling was investigated on the condition of 15 second dwell time each in $5^{\circ}C\;and\;55^{\circ}C$ bath. The Vickers indentation pattern and the fracture surface were examined by an optical microscope and a scanning electron microscope (SEM). The results obtained were summarized as follows ; 1. Young's moduli(E) of the laminate porcelain and the resin cement used in this experiment were $62.56{\pm}3.79GPa$ and $15.01{\pm}0.12GPa$, respectively. 2. The initial crack size of the laminate porcelain was $69.19{\pm}5.94{\mu}m$ when an indentation load of 9.8N was applied, and the fracture toughness was $1.065{\pm}0.156MPa\;m^{1/2}$. 3. The fatigue life of laminate porcelain showed the constant fracture range at the stress level 27.46-35.30MPa. 4. When a cyclic flexure load was applied, the fatigue life of resin-bonded laminate porcelain was more decreased than that of laminate porcelain. 5. When a thermocycling was conducted, the crack growth rate of resin-bonded laminate porcelain was more increased than that of laminate porcelain. 6. Fracture surface showed the radial crack, the lateral crack, and the macroscopic crack branching region beneath the plastic deformation region when an indentation load of 9.8N was applied.