• Transactions of the Korean Society of Mechanical Engineers A
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• v.26 no.9
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• pp.1897-1904
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• 2002

Monotonic torsional and pure torsional low cycle fatigue(LCF) test with artificial degradation were performed on duplex stainless steel(CF8M). CF8M is used in pipes and valves in nuclear reactor coolant system. It was aged at 430$^{\circ}C$ for 3600hrs. Through the monotonic and LCF test, it is found that mechanical properties(i.e., yield strength, strain hardening exponent, strength coefficient etc.) increase and fatigue life(N$\sub$f/) decreases with degradation of material. The relationship between shear strain amplitude(${\gamma}$$\sub$a/)and N$\sub$f/ was proposed.

• Proceedings of the KSME Conference
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• 2008.11a
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• pp.344-349
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• 2008

Ratcheting behavior of IN 718 was investigated at $649^{\circ}C$ under various proportional and non-proportional loading conditions with stress control. The material response was initially elastic but substantial plastic strain was developed as the material softened cyclically. Ratcheting strain was measured to near fatigue life, and is found to have three stages of development - primary, secondary (steady-state) and tertiary. The secondary stage dominates for most cases. Under the same equivalent stress amplitude and mean stress, it was revealed that circular path loading gives higher ratcheting rates and shorter lives than linear paths and that the more ratcheting occurs when the cyclic load is in the same direction as the mean stress. The ratcheting strain at failure depends not only on its rate but also on fatigue life itself, and it is not a primary life-determining factor.

• Transactions of the Korean Society of Automotive Engineers
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• v.15 no.1
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• pp.139-145
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• 2007

This paper presents the low cycle thermal fatigue of the engine exhaust manifold subject to thermomechanical cyclic loadings. The analysis includes the FE model of the exhaust system, temperature dependent material properties, and thermal loadings. The result shows that at an elevated temperature, large compressive plastic deformations are generated, and at a cold condition, tensile stresses are remained in several critical zones of the exhaust manifold. From the repetitions of thermal shock cycles, plastic strain ranges could be estimated by the stabilized stress-strain hysteresis loops. The method was applied to assess the low cycle thermal fatigue for the engine exhaust manifold. It shows a good agreement between numerical and experimental results.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.30 no.4 s.247
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• pp.343-349
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• 2006

Systematic experiments in $CH_4/Air$ counterflow diffusion flames diluted with He have been undertaken to study the oscillatory instability in which lateral flame size was less than burner nozzle diameter and thus lateral heat loss could be remarkable at low global strain rate. The oscillatory instability arises for Lewis numbers greater than unity and occurs near extinction condition. The oscillation is the direct outcome from the advancing and retreating edge flame. The dynamic behaviors of extinction in this configuration can be classified into three modes; growing, harmonic and decaying oscillation mode near extinction. As the global strain rate decreases, the amplitude of the oscillation becomes larger. This is caused by the increase of lateral heat loss which can be confirmed by the reduction of lateral flame size. Oscillatory edge flame instabilities at low global strain rate are shown to be closely associated with not only Lewis number but also heat loss (radiation and lateral heat loss).

• Transactions of the Korean Society of Mechanical Engineers A
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• v.37 no.7
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• pp.891-897
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• 2013

In this study, the compressive deformation behaviors of aluminum alloy under high strain rates were investigated by means of a SHPB test. An acoustic emission (AE) technique was also employed to monitor the signals detected from the deformation during the entire impact by using an AE sensor connected to the specimen with a waveguide in real time. AE signals were analyzed in terms of AE amplitude, AE energy and peak frequency. The impacted specimen surface and side area were observed after the test to identify the particular features in the AE signal corresponding to the specific types of damage mechanisms. As the strain increased, the AE amplitude and AE energy increased whereas the AE peak frequency decreased. It was elucidated that each AE signal was closely associated with the specific damage mechanism in the material.

• Advances in materials Research
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• v.3 no.3
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• pp.163-174
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• 2014

In this study, fatigue properties and crack growth characteristics of a polycarbonate (PC) were examined during cyclic loading at various mean stress (${\sigma}_{amp}$) and stress amplitude (${\sigma}_{mean}$) conditions. Different S vs. N and da/dN vs. ${\Delta}K$ relations were obtained depending on the loading condition. The higher fatigue strength and the higher resistance of crack growth are seen for the PC samples cyclically loaded at the higher mean stress and lower stress amplitude due to the low crack driving force. Non-linear S - N relationship was detected in the examination of the fatigue properties with changing the mean stress. This is attributed to the different crack growth rate (longer fatigue life): the sample loaded at the high mean stress with lower stress amplitude. Even if the higher stress amplitude, the low fatigue properties are obtained for the sample loaded at the higher mean stress. This was due to the accumulated strain energy to the sample, where severe plastic deformation occurs instead of crack growth (plasticity-induced crack closure). Shear bands and discontinuous crack growth band (DGB) are observed clearly on the fracture surfaces of the sample cyclically loaded at the high stress amplitude, where the lower the ${\sigma}_{mean}$, the narrower the shear band and DGB. On the other hand, final fracture occurred instantly immediately after the short crack growth occurs in the PC sample loaded at the high mean with the low ${\sigma}_{amp}$, i.e., tear fracture, in which the shear bands and DGB are not seen clearly.

• Journal of the Korean Geotechnical Society
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• v.26 no.3
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• pp.5-12
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• 2010

In this study, in order to clarify the effect of the direction of cyclic shear on the post-earthquake settlement the multi-directional shear tests were carried out for Toyoura Japan standard sand, Genkai natural sand, Kaolinite and the Granulated Blast Furnace Slag (GBFS). The diameter and the height of the specimen are 75 mm and 20 mm, respectively. In a series of tests, the number of strain cycles was adjusted as n=5, 20, 30, 100, 200 and the shear strain amplitudes were varied from 0.1% to 1.0%. The relative densities of each samples were also adjusted as Dr=50, 60 and 70%. From the test results for Toyoura sand and GBFS, it is clarified that the post-earthquake settlement is relatively large at the small relative density and becomes large with the shear strain amplitude. When the influence of difference on the direction of cyclic shear decreases, the post-earthquake settlement strain for Toyoura sand is converged to a constant value, but the GBFS increases with the number of strain cycles. In addition, the post-earthquake settlement is in the order of Kaolinite > Toyoura sand > Genkai sand > GBFS.

• Journal of Welding and Joining
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• v.9 no.1
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• pp.40-47
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• 1991

The most fatigue tests carried out under the either stress or strain control, but machines and structures had taken variable stress. This variable stress was treated as statistics based on p-type distributions. In this paper, the cumulative fatigue damage of SM45C round bar specimens having a center hole resulting from block loading with p-distributions in rotating bending conditions, is presented. The value of p was changed in the range from 0.25 to 1; 0.25, 0.5, 0.75, 1. The following conclusions were obtained through the constant stress amplitude experiments and the block loading experiments. (1) In constant loading test, fatigue life was affected by cyclic rate. From experimental data, N$_{f}$ (100cpm)/N$_{f}$(3000cpm)equal to 0.56. (2) In case of the cyclic rate 100cpm and 3000cpm, at the high stress amplitude level the crack propagation life N$_{*}$f is longer than the low stress amplitude level. (3) Miner's hypothesis may be valid for p=0.75 and prediction of fatigue life by Haibach's method agree with experimental data well for the case p=0.5, while the modified Miner's method agree with experimental data well for the case p=0.25.5.

• Korea-Australia Rheology Journal
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• v.19 no.1
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• pp.35-42
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• 2007

The effects of particle size distribution on the magnetorheological response of inverse ferrofluids was investigated using controlled mixtures of two monodisperse non-magnetisable powders of sizes $4.6\;{\mu}m\;and\;80{\mu}m$ at constant volume fraction of 30%, subjected to large amplitude oscillatory shear flow. In the linear viscoelastic regime (pre-yield region), it was found that the storage and loss moduli were dependent on the particle size as well as the proportion of small particles, with the highest storage modulus occurring for the monodisperse small particles. In the nonlinear regime (post yield region), Fourier analysis was used to compare the behaviour of the $1^{st}\;and\;3^{rd}$ harmonics ($I_{1}\;and\;I_{3}\;respectively$) as well as the fundamental phase angle as functions of the applied strain amplitude. The ratio of $I_{3}/I_{1}$ was found to become more pronounced with decreasing particle size as well as with increasing proportion of small particles in the bidisperse mixtures. Furthermore, the phase angle was able to clearly show the transition from solid-like to viscous behaviour. The results suggested that the nonlinear response of a bidisperse IFF is dependent on particle size as well as the proportion of small particles in the system.

• Structural Engineering and Mechanics
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• v.59 no.3
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• pp.431-454
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• 2016

In this paper, the nonlinear static and free vibration analysis of Euler-Bernoulli composite beam model reinforced by functionally graded single-walled carbon nanotubes (FG-SWCNTs) with initial geometrical imperfection under uniformly distributed load using finite element method (FEM) is investigated. The governing equations of equilibrium are derived by the Hamilton's principle and von Karman type nonlinear strain-displacement relationships are employed. Also the influences of various loadings, amplitude of the waviness, UD, USFG, and SFG distributions of carbon nanotube (CNT) and different boundary conditions on the dimensionless transverse displacements and nonlinear frequency ratio are presented. It is seen that with increasing load, the displacement of USFG beam under force loads is more than for the other states. Moreover it can be seen that the nonlinear to linear natural frequency ratio decreases with increasing aspect ratio (h/L) for UD, USFG and SFG beam. Also, it is shown that at the specified value of (h/L), the natural frequency ratio increases with the increasing the values amplitude of waviness while the dimensionless nonlinear to linear maximum deflection decreases. Moreover, with considering the amplitude of waviness, the stiffness of Euler-Bernoulli beam model reinforced by FG-CNT increases. It is concluded that the R parameter increases with increasing of volume fraction while the rate of this parameter decreases. Thus one can be obtained the optimum value of FG-CNT volume fraction to prevent from resonance phenomenon.