• Journal of the Korean Society for Heat Treatment
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• v.4 no.4
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• pp.24-33
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• 1991

The effects of thermomechanical treatments on microstructure and fatigue properties of 7050 Al alloy were investigated. The precipitation kinetics changed to a faster rate due to cold deformation employed in this special TAHA thermomechanical treatments including pre-aging, plastic deformation and two step final-aging. The G.P. zones in the under-aged condition were cut by dislocations and dissolved during the plastic deformation. During the low cycle fatigue, the T6' condition showed cyclic hardening behavior whereas the TMT5, TMT27 and T76 conditions showed cyclic softening at above 0.7% total strain amplitudes. The ${\Delta}K_{th}$ value of TMT27 was improved more than two times, compared with that of T76 condition. The T6' with small shearable precipitates resulted in the markedly high ${\Delta}K_{th}$ value. This is thought to be resulted from dislocation reversibility and roughness-induced crack closure due to planarity of slip.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.26 no.3
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• pp.521-527
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• 2002

In this paper, tensile behavior and low cycle fatigue behavior of 316L stainless steel which is currently favored structural material for several high temperature components such as the liquid metal cooled fast breeder reactor (LMFBR) were investigated. Research was performed at 55$0^{\circ}C$, $600^{\circ}C$ and $650^{\circ}C$ since working temperature of 316L stainless steel in a real field is from 40$0^{\circ}C$ to $650^{\circ}C$. From tensile tests performed by strain controls with $1{\times}10^{-3}/s,\; l{\times}10^{ -4}/s \;and\; 1{\times}10/^{ -5}/ s $ strain rates at each temperature, negative strain rate response (that is, strain hardening decreases as strain rate increases) and negative temperature response were observed. Strain rate effect was relatively small compared with temperature effect. LCF tests with a constant total strain amplitude were performed by strain control with a high temperature extensometer at R.T, 55$0^{\circ}C$, $600^{\circ}C$, $650^{\circ}C$ and total strain amplitudes of 0.3%~0.8% were used and test strain rates were $1{times}10^{-2} /s,\; 1{times}10^{-3} /s\; and\; 1{times}10^{-4} /s$. A new energy based LCF life prediction model which can explain the effects of temperature, strain amplitude and strain rate on fatigue life was proposed and its excellency was verified by comparing with currently used models.

• Advances in Computational Design
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• v.8 no.1
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• pp.13-36
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• 2023

The Puebla-Morelos Earthquake (Mw 7.1) occurred in Mexico in 2017 causing 44 buildings to collapse in Mexico City. This work evaluates the non-linear response of a 6-story reinforced concrete (RC) frame prototype model with masonry infill walls on upper floors. The prototype model was designed using provisions prescribed before 1985 and was subjected to seismic excitations recorded during the earthquakes of 1985 and 2017 in different places in Mexico City. The building response was assessed through a damage index (DI) that considers low-cycle fatigue of the steel reinforcement in columns of the first floor, where the steel was modeled including buckling as was observed in cases after the 2017 earthquake. Isocurves were generated with 72 seismic records in Mexico City representing the level of iso-demand on the structure. These isocurves were compared with the location of 16 collapsed (first-floor column failure) building cases consistent with the prototype model. The isocurves for a value greater than 1 demarcate the location where fatigue failure was expected, which is consistent with the location of 2 of the 16 cases studied. However, a slight increase in axial load (5%) or decrease in column cross-section (5%) had a significant detrimental effect on the cumulated damage, increasing the intensity of the isocurves and achieving congruence with 9 of the 16 cases, and having the other 7 cases less than 2 km away. Including column special detailing (tight stirrup spacing and confined concrete) was the variable with the greatest impact to control the cumulated damage, which was consistent with the absence of severe damage in buildings built in the 70s and 80s.

• Aerospace Engineering and Technology
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• v.9 no.2
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• pp.90-97
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• 2010

A study about the fatigue life of copper alloy which is used in inner jacket of regenerative cooling chamber of liquid rocket engine has been performed. Mechanical properties of the material and fatigue life have been taken from tensile tests and low-cycle fatigue tests at room temperature and several elevated temperatures. Original universal slopes method, modified universal slopes method, Mitchell's method, Baumel and Seeger's method, and Ong's method have been used for predicting the fatigue data. It was found that the novel life prediction method should be developed for the copper alloys since almost all data have not been predicted well with the widely used methods.

• Journal of Industrial Technology
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• v.6
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• pp.33-38
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• 1986

The aim of this study is to find out the relationships between the microstructures of SM45C(AISI1045) steel and fatigue crack propagation behaviour. Three microstructures such as (i) as received (fully annealed). (ii) water quenched and tempered, and (iii) oil quenched and tempered were used for fundamental mechanical testing and fatigue crack proagation test. The microstructures of (ii) and (iii) showed superior in tensile strength to (i). Resistance against fatigue crack propagation was higher in structure (i), while tensile properties were better in structures (ii) and (iii). It is believed due to that the enhancement of roughness of fracture surface obsered in structure (i) increases ${\Delta}Kth$ and lowers fatigue crack growth rate. However it does not necessarily mean the quenched and tempered structures (ii) and (iii) are undesirable for the engineering component because fatigue limit in low cycle test appears usually higher in the microstructures of higher strength.

• Structural Engineering and Mechanics
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• v.66 no.2
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• pp.151-160
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• 2018

With the aim to evaluate the fatigue damage accumulation and predict the residual life of engineering components under variable amplitude loadings, this paper proposed a new strain energy-based damage accumulation model by considering both effects of mean stress and load interaction on fatigue life in a low cycle fatigue (LCF) regime. Moreover, an integrated procedure is elaborated for facilitating its application based on S-N curve and loading conditions. Eight experimental datasets of aluminum alloys and steels are utilized for model validation and comparison. Through comparing experimental results with model predictions by the proposed, Miner's rule, damaged stress model (DSM) and damaged energy model (DEM), results show that the proposed one provides more accurate predictions than others, which can be extended for further application under multi-level stress loadings.

• Journal of the Korean Institute of Gas
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• v.9 no.2 s.27
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• pp.34-37
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• 2005

The membrane for LNG storage tank behaves linearly in macroscopic view, but behaves elasto-plastically in some local areas, and has the structure undergoing both tension and bending. That is, the membrane is not able to be evaluated with the fatigue characteristics of the material, and it is so difficult to evaluate the membrane with a real big model because of the difficulty of imposing complex loads. Therefore, a prediction formula fur the fatigue life of the membrane is proposed to use for the design of LNG storage tank.

• Journal of Korean Society of Industrial and Systems Engineering
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• v.28 no.3
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• pp.75-86
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• 2005

To investigate the fatigue characteristic of upper limbs, this study analyzed RMS(root mean square) and MPF(mean power frequency) value between initial and terminal stages of each experiment condition. And the effect of intermittent endurance time was evaluated using the Borg's CR10 value that was measured for the parts of upper limb. According to the results of ANOVA on RMS value, there were significant difference on the %MVC about push, pull, and down force exertion. Particularly the ANOVA of up force exertion was significant difference on shoulder flexion, elbow flexion and rest time as well as %MVC. The results of ANOVA for MPF value were significant difference on the %MVC in regard of the push and up force exertion. In case of up force exertion, MPF value tended to shift low frequency at all of the experiment conditions. According to the analysis of duty cycle, RMS value considerably increased over 50% duty cycle and as the %MVC increased, the duty cycle affected the increase of RMS value. MPF value for up and down force exertion decreased at 33%, 50% and 67% duty cycle for all of %MVC. Borg CR10 value of hand and forearm were below the 3-point to the 40% of endurance time at 30%MVC and to the 20% of endurance time at 50%MVC with the exception of up force exertion. But Borg CR10 values of upper arm and shoulder at up force exertion were more than 3-point to the 20% of endurance time at 30%MVC and in the start point of endurance time at 50%MVC.

• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.31 no.3
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• pp.15-22
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• 2003

The fatigue crack growth model is derived and the retardation model is proposed. The fatigue crack growth model considers the residual plastic stretch on the crack surface which results from the plastic deformation at the tip of fatigue crack. The fatigue crack growth rate is calculated by using the cumulative fatigue damage and plastic strain energy in the material elements at the crack tip. This model gives the crack growth rate in reasonable agreement with test data for aluminum alloy AL6061-T651 and 17-4PH casting steel. The fatigue crack growth retardation model is based on the residual plastic stretch produced from a tensile overload which reduced the plastic strain range of the following load cycles. A strip-yield model of a crack tip plasticity is used for the calculation of a plastic zone size. The proposed retardation model characterized the observed features and delayed retardation of the fatigue crack growth under tensile overload.

• Composites Research
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• v.12 no.4
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• pp.8-16
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• 1999

In this study, when the applied directions of tensile loading is changed fatigue damage of quasi-isotropic composite laminates was discussed. Low cycle fatigue tests of $[0/-60/+60]_s$ laminates and $[+30/-30/90]_s$ laminates were carried out. Material systems used were AS4/Epoxy and AS4/PEEK. The fatigue damage of $[+30/-30/90]_s$ laminates differed from that of $[0/-60/+60]_s$ laminates. The position of delamination generated at AS4/Epoxy and AS4/PEEK $[+30/-30/90]_s$ laminates appeared differently according to the kind of matrix. Critical values of strain energy release rate were obtained by using the strain measured at the initiation of delamination. The experimental results agreed well with the results obtained by the proposed method for determining strain energy release rate.