• Structural Engineering and Mechanics
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• v.44 no.4
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• pp.417-429
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• 2012

Taking the superficial temperature increment as the major fatigue damage indicator, the infrared thermography was used to predict fatigue parameters (fatigue strength and S-N curve) of welded joints subjected to fatigue loading with a high mean stress, showing good predictions. The fatigue damage status, related to safety evaluation, was tightly correlated with the temperature field evolution of the hot-spot zone on the specimen surface. An energetic damage model, based on the energy accumulation, was developed to evaluate the residual fatigue life of the welded specimens undergoing cyclic loading, and a good agreement was presented. It is concluded that the infrared thermography can not only well predict the fatigue behavior of welded joints, but also can play an important role in health detection of structures subjected to mechanical loading.

• Structural Engineering and Mechanics
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• v.67 no.6
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• pp.587-601
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• 2018

Elevated water tanks are inverted pendulum type structures where drift limit is an important criterion for seismic design and performance evaluation. Explicit drift criteria for elevated water tanks are not available in the literature. In this study, probabilistic approach is used to determine maximum drift limit for damage state of yielding and damage state of collapse for the elevated water tanks supported on RC frame staging. The two damage states are defined using results of incremental dynamic analysis wherein a total of 2160 nonlinear time history analyses are performed using twelve artificial spectrum compatible ground motions. Analytical fragility curves are developed using two-parameter lognormal distribution. The maximum allowable drifts corresponding to yield and collapse level requirements are estimated for different tank capacities. Finally, a single fragility curve is developed which provides maximum drift values for the different probability of damage. Further, for rational consideration of the uncertainties in design, three confidence levels are selected and corresponding drift limits for damage states of yielding and collapse are proposed. These values of maximum drift can be used in performance-based seismic design for a particular damage state depending on the level of confidence.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.20 no.12
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• pp.3892-3898
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• 1996

In this study, the prediction of the fatigue life as well as the extimation of the characteristics of fatigue cumulative damage on GFRP under random loading were performed. The constant amplitude tests and the ramdom loading test were carried on notched GFRP specimens with a circular hole. Random waves were generated with a micro-computer and had wide band spectra. Since it is useful that the prediction of fatigue life ot the given load sequences is based on S-N curves under constant amplitude loading, the estimation of equivalent stress is done on every random waves. The equivalent stress wasat first estimated by Miner's rule and then by the proposed model which was based on Hashin-Rotem's comulative damage theory regarding nonlinear fatigue cumulative damage behavior. The fatigue lives were predicted from each equivalent stress evaluated. And each predicted fatigue llife was compared with experimental results. The number of cycles of random loads were counted by mean-cross counting method. The reuslts showed that the fatigue life predicted by proposed model was correlated well with the experimental results in comparison with Miner's model.

• Computers and Concrete
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• v.27 no.2
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• pp.153-162
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• 2021

This paper aims to simulate the isotropic elastic damage theory of Liu Jun (2012) using the self-programmed UMAT subroutine in the interface of ABAQUS. Liu Jun (2012)'s method based on the mechanic theory can not be used interactively with the currently commonly used finite element software ABAQUS. The advantage of this method in the paper is that it can interact with ABAQUS and provide a constitutive program framework that can be modified according to user need. The model retains the two scalar damage variables and the corresponding two energy dissipation mechanisms and damage criteria for considering the tensile and compressive asymmetry of concrete. Taking C45 concrete as an example, the relevant damage evolution parameters of its tensile and compressive constitutive model are given. The study demonstrates that the uniaxial tensile stress calculated by the subroutine is almost the same as the Chinese Concrete Design Specification (GB50010) before the peak stress, but ends soon after the peak stress. The stress-strain curve of uniaxial compression calculated by the subroutine is in good agreement with the peak stress in Chinese Concrete Design Specification (GB50010), but there is a certain deviation in the descending stage. In addition, this paper uses the newly compiled subroutine to simulate the shear bearing capacity of the shear key in a new structural system, namely the open-web sandwich slab. The results show that the damage constitutive subroutine has certain reliability.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.33 no.4
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• pp.388-395
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• 2009

An accelerated fatigue test is essentially required to maintain the reliability of the actual structures of KTX under operation conditions. However, actual fatigue life cannot be obtained because the conventional fatigue tests are not adequate to the real load conditions. Moreover foreign component makers have not provided data of the loading stresses (S) versus cycles at the failure (N). In this study, we suggested a deduction method of the S-N curve for establishing an accelerating test under various load levels. Load history was acquired from the field tests. A Rainflow method was used on the cycle counting of the field load data. After that, an S-N curve was obtained through the iteration process under the condition that the damage index satisfies to 1 in the Miner's rule. The deduced S-N curve was applied to the performance evaluation of Korean-made sealed knuckles compared with imports.

• Geomechanics and Engineering
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• v.7 no.5
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• pp.569-587
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• 2014

Triaxial compression creep tests were performed on salt rock samples using cyclic confining pressure with a static axial pressure. The test results show that, up to a certain time, changes in the confining pressure have little influence on creep properties of salt rock, and the axial creep curve is smooth. After this point, the axial creep curve clearly fluctuates with the confining pressure, and is approximately a straight line both when the confining pressure decreases and when it increases within one cycle period. The slope of these lines differs: it is greater when the confining pressure decreases than when it increases. In accordance with rheology model theory, axial creep equations were deduced for Maxwell and Kelvin models under cyclic loading. These were combined to establish an axial creep equation for the Burgers model. We supposed that damage evolution follows an exponential law during creep process and replaced the apparent stress in creep equation for the Burgers model with the effective stress, the axial creep damage equation for the Burgers model was obtained. The model suitability was verified using creep test results for salt rock. The fitting curves are in excellent agreement with the test curves, so the proposed model can well reflect the creep behavior of salt rock under low-frequency cyclic loading. In particular, it reflects the fluctuations in creep deformation and creep rate as the confining pressure increasing and decreasing under different cycle periods.

• Journal of the Society of Disaster Information
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• v.7 no.3
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• pp.171-180
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• 2011

As a great earthquake hit east Japan recently, the interests for the necessity of earthquake resistant design and earthquake resistance ability of existent structures are much increased. The damage or collapse of a bridge, as a social overhead capital structure affects socially and economically. Thus the evaluation of earthquake resistance ability of these structures is very important. The reviewing methods for earthquake resistance ability are mostly deterministic. Although the deterministic methods are fit for the evaluation of safety of each member, they are not practical for the whole structure. For the evaluation of structural safety for earthquake, the method for the evaluation of fragility or damage is needed for some stages of damage. In this paper, fragility curves of steel box bridge using RFPB bearing for PGA, PGV, SA, SV, SI are constructed, and these are compared with the cases of FPB.

• Journal of the Korean Society of Hazard Mitigation
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• v.9 no.6
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• pp.57-68
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• 2009

Recently, damage scale of human-induced disaster is sharply increased but its occurrences and damages are so uncertain that it is hard to construct a resonable response & mitigation plan for infrastructures. Therefore, the needs for a advanced risk management technique based on a probabilistic and stochastic risk evaluation theory is increased. In this study, these evaluation methods were investigated and a advanced disaster risk evaluation method, which is based on the probabilistic or stochastic risk assessment theory and also is a quantitative evaluation technique, was suggested. With this method, the safety changes as the result of fire damage management for recent 40 years was analyzed. And the result was compared with that of Japan. Through the consilience of the traditional risk assessment method and this method, a stochastical estimation technique for the uncertainty of future disaster's damage could support a cost-effective information for a resonable decision making on disaster mitigation.

• Proceedings of the Korean Society for Technology of Plasticity Conference
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• 2008.10a
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• pp.292-296
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• 2008

A new method of evaluating critical damage values of commercial materials is presented in this paper. The method is based on the previous study of the methodology [1] of acquisition of true stress-strain curves or flow stress curves over large strain from the tensile test in which the flow stress is described by the Hollomon law-like form, that is, by the strain dependent strength coefficient and the strain hardening exponent. The strain hardening exponent is calculated from the true strain at the necking point to meet the Considere condition. The strength coefficient is assumed to be constant before necking and represented by a piecewise linear function of strain after necking. With the predicted flow stress, a tensile test is simulated by a rigid-plastic finite element method with higher accuracy of less than 0.5% error between experiments and predictions. The instant when the fracture begins and thus the critical damage is obtained is determined by observing the stress variation at the necked region. It is assumed that the fracture due to damage begins when the pattern of stress around the necked region changes radically. The method is applied to evaluate the critical damage of a low carbon steel.

• Journal of Aerospace System Engineering
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• v.1 no.2
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• pp.27-36
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• 2007

The Damage Tolerant Design is developed to help alleviate structural failure and cracking problems in aerospace structures. Recently, the Damage Tolerant Design is required and recommended for most of aircraft design. In this paper, the damage tolerant design is applied to tilt rotor UAV. First of all, the fatigue load spectrum for the tilt rotor UAV is developed and fatigue analysis is performed for the flaperon joint which has FCL (fatigue critical location). Tilt rotor UAV has two modes: helicopter mode when UAV is taking off and landing; fixed wing mode when the tilt rotor UAV is cruising. To make fatigue load spectrum, FELIX is used for helicopter mode. TWIST is used for fixed wing mode. Fatigue analysis of flaperon joint is performed using fatigue load spectrum. E-N curve approach is used for picking crack initiation point. The LEFM(Linear Elastic Fracture Method) is considered for analyzing crack growth or propagation. Finally, including the crack initiation and propagation, the fatigue life is evaluated. Therefore the Damage Tolerant Design can be done.