• Journal of Korean Society of Coastal and Ocean Engineers
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• v.32 no.3
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• pp.147-160
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• 2020

A stochastic probabilistic model for harbor structures such as rubble-mound breakwater has been formulated by using the generalized Wiener process considering the nonlinearity of damage drift and its nonlinear uncertainty, by which the damage path with real-time can be tracked, the residual useful lifetime at some age can also be analyzed properly. The formulated stochastic model can easily calculate the probability of failure with the passage of time through the probability density function of cumulative damage. In particular, the probability density functions of residual useful lifetime of the existing harbor structures can be derived, which can take into account the current age, its present damage state and the future damage process to be occurred. By using the maximum likelihood method and the least square method together, the involved parameters in the stochastic model can be estimated. In the calibration of the stochastic model presented in this paper, the present results are very well similar with the results of MCS about tracking of the damage paths as well as evaluating of the density functions of the cumulative damage and the residual useful lifetime. MTTF and MRL are also evaluated exactly. Meanwhile, the stochastic probabilistic model has been applied to the rubble-mound breakwater. The related parameters can be estimated by using the experimental data of the cumulative damages of armor units measured as a function of time. The theoretical results about the probability density function of cumulative damage and the probability of failure are very well agreed with MCS results such that the density functions of the cumulative damage tend to move to rightward and the amounts of its uncertainty are increased as the elapsed time goes on. Thus, the probabilities of failure with the elapsed time are also increased sharply. Finally, the behaviors of residual useful lifetime have been investigated with the elapsed age. It is concluded for rubble-mound breakwaters that the probability density functions of residual useful lifetime tends to have a longer tail in the right side rather than the left side because of the gradual increases of cumulative damage of armor units. Therefore, its MRLs are sharply decreased after some age. In this paper, the special attentions are paid to the relationship of MTTF and MRL and the elapsed age of the existing structure. In spite of that the sum of the elapsed age and MRL must be equal to MTTF deterministically, the large difference has been shown as the elapsed age is increased which is due to the uncertainty of cumulative damage to be occurred in the future.

• Journal of Korean Society of Coastal and Ocean Engineers
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• v.34 no.2
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• pp.37-45
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• 2022

A simplified model using the lifetime distribution has been presented to estimate the Mean Residual Life (MRL) of rubble-mound breakwaters, which is not like a stochastic process model based on time-dependent history data to the cumulative damage progress of rubble-mound breakwaters. The parameters involved in the lifetime distribution can be easily estimated by using the upper and lower limits of lifetime and their likelihood that made a judgement by several experts taking account of the initial design lifetime, the past sequences of loads, and others. The simplified model presented in this paper has been applied to the rubble-mound breakwater with TTP armor layer. Wiener Process (WP)-based stochastic model also has been applied together with Monte-Carlo Simulation (MCS) technique to the breakwater of the same condition having time-dependent cumulative damage to TTP armor layer. From the comparison of lifetime distribution obtained from each models including Mean Time To Failure (MTTF), it has found that the lifetime distributions of rubble-mound breakwater can be very satisfactorily fitted by log-normal distribution for all types of cumulative damage progresses, such as exponential, linear, and logarithmic deterioration which are feasible in the real situations. Finally, the MRL of rubble-mound breakwaters estimated by the simplified model presented in this paper have been compared with those by WP stochastic process. It can be shown that results of the presented simplified model have been identical with those of WP stochastic process until any ages in the range of MTT F regardless of the deterioration types. However, a little of differences have been seen at the ages in the neighborhood of MTTF, specially, for the linear and logarithmic deterioration of cumulative damages. For the accurate estimation of MRL of harbor structures, it may be desirable that the stochastic processes should be used to consider properly time-dependent uncertainties of damage deterioration. Nevertheless, the simplified model presented in this paper can be useful in the building of the MRL-based preventive maintenance planning for several kinds of harbor structures, because of which is not needed time-dependent history data about the damage deterioration of structures as mentioned above.

• Journal of the Korea Institute of Military Science and Technology
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• v.2 no.1
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• pp.101-109
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• 1999

The objective of this research is to study effects of the residual stresses on the crack growth and the life of the structure, caused by cold working around the hole of the aircraft structure which will be jointed by rivets and bolts, etc. The compensated Morrow's equation, by experimental data from the materials AL7075-T6 and AL2024-T3, is suggested to calculate the values of the fatigue life prediction of the structure. Also, the compensated Forman's equation, by experimental data from a material AL7075-T6, is suggested to calculate the values of the crack growth prediction of the structure. It is founded that the calculated values from the suggested equations are almost close to the known values of the fatigue life prediction and the crack growth prediction. It is shown that this paper, associated with an initial research on the effects of residual stresses around hole, gives a direction to study the problem at the aircraft maintenance field.

• Proceedings of the KSME Conference
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• 2005.11a
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• pp.141.1-141.1
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• 2005

• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.31 no.2
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• pp.35-40
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• 2003

Cold expansion method is an effective process to retard the crack initiation at a hole and to improve fatigue life by developing the compressive residual stress around the hole. Several researches have been treated for the crack propagating toward a hole around which the residual stress by cold expansion exists. The objective of this paper is to evaluate the fatigue characteristics under the residual stress by cold expansion. Fatigue tests are performed for the plate with a central hole to which cold expansion has been applied, and the effects of the residual stress on the fatigue life are evaluated. Also the fatigue crack growth tests are conducted for the CT specimen in which the residual stress by cold expansion has been generated. The change of fatigue crack growth characteristics by the residual stress is investigated and the effects of cold expansion ratio are evaluated.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.41 no.9
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• pp.845-851
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• 2017

Fatigue failure of an autofrettaged pressure vessel with a groove at the outside surface occurs owing to the fatigue crack initiation and propagation at the groove root. In order to predict the fatigue life of the autofrettaged pressure vessel, residual stresses in the autofrettaged pressure vessel were evaluated using the finite element method, and the fatigue properties of the pressure vessel steel were obtained from the fatigue tests. Fatigue life of a pressure vessel obtained through summation of the crack initiation and propagation lives was calculated to be 2,598 cycles for an 80% autofrettaged pressure vessel subjected to a pulsating internal pressure of 424 MPa.

• Proceedings of the Korean Society For Composite Materials Conference
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• 2002.10a
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• pp.75-78
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• 2002

A simplified methodology is presented to predict fatigue life and residual strength of composite structures. To avoid excessive amount of tests that are required for model characterization, strength degradation parameter is assumed as function of fatigue life. S-N curve is used to extract fatigue life that is required to characterize the stress levels comprising a randomly-ordered load spectrum. And different stress ratios are handled with Goodman correction approach(fatigue envelope). It is assumed that the residual strength is a function of the number of loading cycles and applied fatigue stress amplitude. And the residual strength distribution after an arbitrary load cycles is represented by two parameter Weibull functions.

• Journal of the Korea Academia-Industrial cooperation Society
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• v.11 no.9
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• pp.3148-3154
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• 2010

This paper presents the probabilistic analysis for fatigue life of Glass/Epoxy laminates with impact-induced damage. For this, a series of impact tests were perfomed on the Glass/Epoxy laminates using instrumented impact testing machine. Then, tensile and fatigue tests carried out so as to generate post-impact residual strength and fatigue life. Two Parameter Weibull distribution was used to fit the residual strength and fatigue life data of Glass/Epoxy composite laminates. The residual strength was affected by impact energy and their variance decreased with increasing of impact energy. The fatigue life of impacted laminates was greatly reduced by impact energy and this trend depended on applied stress amplitude. Additionally, the variation of fatigue life was gradually decreased with the applied stress amplitude.

• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.39 no.12
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• pp.1107-1114
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• 2011

In this paper, the fatigue life of a laser peened structure was predicted. In order to calculate residual stress induced by laser peening finite element simulation was carried out. Modified Goodman equation was used to consider the effect of compressive residual stress induced by laser peening in fatigue analysis. In addition, additive adjustment factor approach was applied to consider S-N curve model uncertainty. Consequently, the reliable bounds of the predicted fatigue life of the laser peened structure was determined.

• Journal of Korean Society of Steel Construction
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• v.21 no.1
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• pp.55-62
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• 2009

Blast cleaning has been applied in steel bridges for cleaning forged surface and increasing adhesive property of applied coating systems. Blasting is the operation of cleaning or preparing a surface by forcible propelling a stream of abrasive metals against it. Blast cleaning may improve surface geometry and induce compressive residual stress, and eventually may increase fatigue life of weld joints. In this paper, fatigue tests were carried out on three types of non-load-carrying fillet welded cruciform joints, as-welded joints, blast-treated joints, and stress-relieved joints after blasting, in order to investigate effect of blast cleaning on fatigue behavior of the weld joints. By Blast cleaning, the weld toe radius was increased by 29% and compressive residual stress was induced near weld toes. Blast cleaning increased fatigue life and fatigue endurance limit of the weld joints. When the applied stress ranges decreased, the increment in fatigue life became larger. About a 150% increase in fatigue limit could be realized by using blast cleaning.