• 한국복합재료학회:학술대회논문집
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• 한국복합재료학회 2001년도 춘계학술발표대회 논문집
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• pp.60-64
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• 2001

In this paper, the tension-compression fatigue test method and the fatigue life characteristics of carbon fabric/epoxy laminate coupon are presented. To avoid the buckling during the compression, a proper design for the test coupons is essential. The critical buckling loads for the coupons are calculated by assuming the coupons as columns under two types of fixed conditions. The first is that both ends of each coupon are perfectly clamped, the second is that both ends of each coupon are simply supported. The strain-load curves are obtained by compressing the representative coupons, on each surface of which a strain gage is attached. The buckling loads obtained from the tests are all between the two calculated critical buckling loads. All the coupons are broken by the compression during the fatigue tests. It is estimated to be the reason that the fatigue load causes delamination before the eventual failure of each coupon, and sequentially the micro-buckling in the delaminated region drives each coupon into fatigue failure during the compression. The S-N curve, the fatigue life characteristics of carbon fabric/epoxy is obtained.

• 대한기계학회논문집A
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• 제20권11호
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• pp.3415-3423
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• 1996

The compact tension specimen geometry has been widely used for measuring fatigue crack growth rates at elevated temperature when the fatigue load is under tension/tension condition. However, most of the elevated temperature components which have significant crack growth life experience fatigue load under tension/compression conditions. Thus test techniques are required since the compact tension specimen cannot be used for tension/compression loading. In this paper, a simplified test procedure for measureing fatigue crack growth rates is proposed, which employs a round bar specimen with a small surface crack. Fatigue crack growth rates under tension/ tension loading conditions at elevated temperature were measured according to the proposed procedure and compared with those previously measured by C/(T) specimens. Since both the measured crack growth rates were comparable, the fatigue crack growth rates under tension/ compression load can be reliably measured by the proposed procedure. For monitoring crack depth. DC electric potential method is employed and an optimal probe location and current input conditions were proposed.

• Composites Research
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• 제23권5호
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• pp.22-29
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• 2010

본 논문은 철도차량 경량화 재질로 적용된 유리섬유/에폭시 4-매 주자직 적층 복합재료의 인장-압축 피로특성을 평가하였다. 유리섬유/에폭시 4-매 주자직 적층 복합재료의 인장-압축 피로시험은 경사, 위사 그리고 ${\pm}45^{\circ}$ 방향으로 적층된 시험편에 대하여 수행하였다. 인장-압축 피로시험은 5Hz의 주파수를 갖으며, 응력비(R)는 -1로 수행하였다. 인장-압축 피로시험 수행 시 압축하중에 의한 시험편의 좌굴을 방지하기 위하여 좌굴방지지그를 설계하고 이를 시험에 적용하였다. 또한, Goodman 선도는 유리섬유/에폭시 4-매 주자직 적층 복합재의 피로특성과 수명을 평가하기 위해 사용하였다. 유리섬유/에폭시 4-매 주자직 적층 복합재료의 인장-압축 피로시험결과 경사방향 적층 복합재의 피로특성이 기존 금속재 대차에 적용되는 SM490에 비하여 우수한 것으로 나타났다.

• 대한기계학회:학술대회논문집
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• 대한기계학회 2000년도 춘계학술대회논문집A
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• pp.206-211
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• 2000

Fatigue test was conducted on a S45C steel using hour-glass shaped smooth tubular specimen under biaxial loading in order to investigate the crack formation and growth at room temperature. Three types of loading system, i.e fully reserved cyclic torsion without a superimposed static tension or compression, fully reserved cyclic torsion with a superimposed static tension and fully reserved cyclic torsion with a superimposed static compression were employed. The test results show that a superimposed static tensile mean stress reduced fatigue lifetime. however a superimposed static compressive mean stress increased fatigue lifetime. Experimental results indicated that cracks were initiated on planes of maximum shear strain with either a superimposed mean stresses or not. A biaxial mean stress had an effect on the direction which cracks nucleated and propagated at stage I (mode II).

• 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.

• 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.

• Composites Research
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• 제36권1호
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• pp.42-47
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• 2023

In this paper, we present a fatigue life prediction methodology using multiple S-N curves according to the different stress states of laminated composites. The stress states of the plies of the laminated composites are classified into five modes: longitudinal tension or compression and transverse tension or compression, and shear according to the maximum stress criterion and Puck's criterion with a scaling factor K. This methodology has advantages in computational cost, and it can also consider microstructural characteristics of the composites by applying different S-N curves. The S-N curves for the fatigue analysis are obtained by experimental fatigue test. The proposed methodol is implemented into commercial software, ABAQUS user material subroutine and therefore, the fatigue analysis is conducted using the structural analysis results. The finite element (FE) simulation results are presented for unidirectional composites with and without open-hole. The FE simulation results show that the stress condition is different depending on the fiber orientation of the unidirectional composite, so the fatigue life is calculated with different S-N curves.

• Steel and Composite Structures
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• 제4권2호
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• pp.149-167
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• 2004

Offshore platforms have to serve in harsh environments and hence are likely to be damaged due to wave induced fatigue and environmental corrosion. Welded tubular joints in offshore platforms are most vulnerable to fatigue damage. Such damages endanger the integrity of the structure. Therefore it is all the more essential to assess the capacity of damaged structure from the point of view of its safety. Eight internally ring stiffened fatigue damaged tubular joints with nominal chord and brace diameter of 324 mm and 219 mm respectively and thickness 12 mm and 8 mm respectively were tested under axial brace compression loading to evaluate the reserve capacity of the joints. These joints had earlier been tested under fatigue loading under corrosive environments of synthetic sea water and hence they have been cracked. The extent of the damage varied from 35 to 50 per cent. One stiffened joint was also tested under axial brace tension loading. The residual strength of fatigue damaged stiffened joint tested under tension loading was observed to be less than one fourth of that tested under compression loading. It was observed in this experimental investigation that in the damaged condition, the joints possessed an in-built load-transfer mechanism. A bi-linear stress-strain model was developed in this investigation to predict the reserve capacity of the joint. This model considered the strain hardening effect. Close agreement was observed between the experimental and predicted results. The paper presents in detail the experimental investigation and the development of the analytical model to predict the reserve capacity of internally ring stiffened joints.

• 한국정밀공학회지
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• 제19권8호
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• pp.71-76
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• 2002

In this study low cycle fatigue (LCF) behavior of 12Cr steel at high temperature are described. Secondly, comparisons between predicted lives and experimental lives are made for the several sample life prediction models. Two minute hold period in either tension or compression reduce the number of cycles to failure by about a factor of two. Twenty minute hold periods in compression lead to shorter lives than 2 minute hold periods in compression. Experiments showed that life predictions from classical phenomenological models have limitations. More LCF experiments should be pursued to gain understanding of the physical damage mechanisms and to allow the development of physically-based models which can enhance the accuracy of the predictions of components. From a design point-of-view, life prediction has been judged acceptable for these particular loading conditions but extrapolations to thermo-mechanical fatigue loading, for example, require more sophisticated models including physical damage mechanisms.

• 한국공작기계학회:학술대회논문집
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• 한국공작기계학회 1999년도 추계학술대회 논문집 - 한국공작기계학회
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• pp.275-280
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• 1999

This study is object to program development of fatigue test for universal testing machine. Fatigue program is consist of test simulation, data analysis and print report by control fatigue testing program which expansively applies tension-compression tests with using oil pressure mechanism by Visual Basic software running under windows 98.