Effect of Uncertainties on the Reliability of Fatigue Damaged Systems

2004 ◽  
Author(s):  
Francisco L. Silva-Gonza´lez ◽  
Ernesto Heredia-Zavoni
Keyword(s):  
Fracture Mechanics ◽  
Failure Modes ◽  
Crack Size ◽  
Initial Crack ◽  
Wave Forces ◽  
Scale Parameters ◽  
Uncertain Variables ◽  
Mechanics Model ◽  
Distribution Of Stress

Fluctuating stresses and strains due to wave forces cause accumulated fatigue damage in tubular joints of marine platforms. Considering the uncertainties in the loads, material properties, initial crack sizes, and stress intensity factors, etc., may affect significantly the reliability assessment of marine jacket platforms. In this paper, we assessed the effect of uncertainties about such fatigue variables on the time evolution of the reliability of series and parallel systems considering correlation between failure modes. The fracture mechanics Paris-Erdogan model is used to model crack growth and a FORM method is used for computing the safety index. The uncertain variables analyzed are: initial crack size, material parameters C and m in the fracture mechanics model and the shape and scale parameters of the Weibull density function used for the long-term distribution of stress range.

A Bayesian Model for the Probability Distribution of Fatigue Damage in Tubular Joints

2004 ◽  
Vol 126 (3) ◽  
pp. 243-249 ◽  
Author(s):  
Ernesto Heredia-Zavoni ◽  
Roberto Montes-Iturrizaga
Keyword(s):  
Fracture Mechanics ◽  
Probability Distributions ◽  
Error Variance ◽  
Crack Size ◽  
Offshore Structures ◽  
Density Functions ◽  
Time Varying ◽  
Tubular Joints ◽  
Mechanics Model ◽  
Distribution Of Stress

A Bayesian framework is used for updating the probability distributions of the parameters of a fracture mechanics model and of crack size in tubular joints using information from inspection reports of fixed offshore structures. An error model, defined as the logarithmic difference between measured crack size during inspection and crack size predicted by the fracture mechanics model, is assumed to have a normal distribution with known mean and uncertain variance. The distribution of the error variance is modeled by a conjugate distribution for samples of normal variables with known mean and uncertain variance. Based on these assumptions, an analytical model is obtained using a Bayesian approach for the updated distributions of the parameters of the fracture mechanics model and of crack size based. The capabilities of the model are illustrated by means of examples using the Paris-Erdogan formulation for crack growth. The examples illustrate the effects of inspection times, measured crack size, and the distribution of stress ranges on the updated density functions of crack size, time varying reliability and expected cost of failure.

A Novel High Cycle Fatigue Assessment of Small-Bore Side Branches: Tailor-Made Acceptable Vibration Levels Based on the Remaining Life of Existing Structures

2014 ◽  
Author(s):  
Pieter van Beek ◽  
Richard Pijpers ◽  
Kenneth Macdonald ◽  
Johan Maljaars ◽  
Knud Lunde ◽  
...  
Keyword(s):  
Fracture Mechanics ◽  
High Cycle Fatigue ◽  
Side Branch ◽  
International Standards ◽  
Screening Methods ◽  
Piping System ◽  
Cycle Fatigue ◽  
Fatigue Assessment ◽  
Mechanics Model

In the process systems of offshore installations, welded small-bore side branches can prove vulnerable to high-cycle fatigue failure due to vibrations. This is especially the case for welded connections at tie-in points to the main pipe which are often critical details. International standards and guidelines therefore provide maximum acceptable vibration levels to ensure long term safe operation. In some guidelines, however, these acceptable vibration levels are phrased in terms of screening levels and in practice can be unduly conservative. Process pipework might then unjustly be regarded as unsafe based on measured vibrations in the field. This is especially true for offshore systems, which are characterized by low mechanical damping in the structure. This may result in overdesigned piping or over-conservative operational limits in order to keep vibration levels within the acceptable range. Furthermore, the screening methods and any detailed fatigue assessments typically use established stress-life (S-N) based fatigue design methods where uncertainty exists in the very high-cycle regime. This paper describes a novel and advanced tailor-made fatigue assessment method whereby acceptable vibration levels are based on maximum acceptable stress ranges for individual side branches. The acceptable stress ranges for each critical welded connection are based on a fracture mechanics analysis of fatigue crack growth. This method also minimizes the cantilevered (overhung) mass of small-bore side branches, whilst remaining safe for long-term operation. To illustrate the strength of the assessment methodology in practice, this paper describes the application of the procedure to a 2″ side branch connected to a main piping system. A fracture mechanics model and a detailed 3D finite element model are made. By comparing the stress ranges from the fracture mechanics model with the normalized stress ranges obtained from the dynamic FE analysis, maximum acceptable vibration levels for this particular side branch have been derived. The method is validated with experimental modal analysis and strain gauge measurements.

scholarly journals Fracture Mechanical Behavior of Cracked Cantilever Roof with Large Cutting Height Mining

2020 ◽  
Vol 2020 ◽  
pp. 1-10 ◽  
Author(s):  
Zenghui Zhao ◽  
Wei Sun ◽  
Mingzhong Zhang ◽  
Xiaojie Gao ◽  
Shaojie Chen
Keyword(s):  
Fracture Mechanics ◽  
Cantilever Beam ◽  
Crack Size ◽  
Affected Area ◽  
Mechanics Model ◽  
Large Mining Height ◽  
Mining Disturbance ◽  
Seam Mining ◽  
Cutting Height ◽  
Mining Height

Accurately predicting the roof collapse span is crucial in ensuring the safe production of thick seam mining with large mining height, which is easy in forming a “cantilever beam” structure. Considering roof damage caused by roadway excavation and coal seam mining disturbance, the fracture mechanics model of large mining height roof cantilever beam with nonpenetrating cracks was established. The roof was divided into two parts: the crack-affected area and the crack-unaffected area. The analytical expression of the boundary between the two areas was established by fracture mechanics methods. Based on the boundary equation, the influences of crack size, crack inclination, roof lithology, and roof thickness on the roof crack-affected area were analyzed in detail. Finally, the accuracy of the theoretical model was verified by numerical experiments using the extended finite element method. The results demonstrate that the size of the area affected by the vertical crack increases with the increase of the crack size and the thickness of the roof. The influence of the crack decreases with the increase of roof lithology. The probability of early periodic collapse of a thin roof with the crack is increased. When the crack is completely located in the interior of the roof, the crack-affected area shrinks greatly with the decrease of the crack inclination. When the crack inclination is small, the crack will not cause the early collapse of the roof. Overall, the conclusions obtained are of great significance for predicting the collapse span of a cantilever roof with initial damage in large mining height.

The Application of Fracture Mechanics to Predict the Critical Initial Crack Length for Inspection

2003 ◽  
Author(s):  
C. T. Liu
Keyword(s):  
Composite Material ◽  
Fracture Mechanics ◽  
Crack Length ◽  
Particulate Composite ◽  
Crack Size ◽  
Initial Crack ◽  
Initial Crack Length ◽  
Particulate Composite Material

In this study, a method is developed based on fracture mechanics, for predicting the equivalent critical initial crack size, aic in a particulate composite material. The predicted aic is the crack size that should be used to develop an inspection criterion to determine the reliability of a structure made of the particulate composite material.

scholarly journals Comparing a Fracture Mechanics Model to the SN-Curve Approach for Jacket-Supported Offshore Wind Turbines: Challenges and Opportunities for Lifetime Prediction

2016 ◽  
Author(s):  
Lisa Ziegler ◽  
Michael Muskulus
Keyword(s):  
Crack Growth ◽  
Wind Turbines ◽  
Wind Farms ◽  
Crack Size ◽  
Initial Crack ◽  
Offshore Wind ◽  
Offshore Wind Turbines ◽  
Mechanics Model ◽  
Lifetime Extension ◽  
Sn Curve

Accurate lifetime predictions are needed for support structures of offshore wind turbines to optimize operation and maintenance and to decide about lifetime extension of aging wind farms. A comparison of a facture mechanics model to the SN-curve approach for jacket supported offshore wind turbines shows that it is attractive for lifetime extension decisions; however major challenges are calibration of material parameters and assumptions for initial crack size. Crack growths on a Y-joint connecting brace and jacket leg was analysed with simulations of structural response to aero- and hydrodynamic loading and Paris’ law for crack propagation. The model was calibrated to yield an identical fatigue life as obtained from the SN-curve analysis. The effect of weather seasonality on crack growth was evaluated with a Markov weather model and Monte Carlo simulations. Results show that crack growth is sensitive to parameter calibration and follows seasonal weather trends.

scholarly journals Random Outcome and Stochastic Analysis of Some Fatigue Crack Growth Data

2001 ◽  
Vol 17 (2) ◽  
pp. 61-68
Author(s):  
W. F. Wu ◽  
C. C. Ni ◽  
H. Y. Liou
Keyword(s):  
Fatigue Crack ◽  
Fracture Mechanics ◽  
Crack Size ◽  
Experimental Result ◽  
Exceedance Probability ◽  
Growth Data ◽  
Probabilistic Fracture Mechanics ◽  
Mechanics Model ◽  
Proposed Model ◽  
Analytical Result

ABSTRACTFatigue crack propagation data of a batch of AISI 4340 steel specimens are released in the present paper. The statistical nature of the data is specially emphasized, and a probabilistic fracture mechanics model is introduced to analyze the data. The stochastic differential equation associated with the model is then solved. The solution gives us the crack exceedance probability as well as the probability distribution of the random time to reach a specified crack size. These quantities are useful in the reliability assessment of structures made of the tested material. Comparing the analytical result with the experimental result, it is found that the proposed probabilistic fracture mechanics model can reasonably explain the experimental data. For those data that cannot be fitted well by the proposed model, methods of improvement are proposed in the present paper as well.

Duration of load effects in lumber. Part I: A fracture mechanics approach

1982 ◽  
Vol 9 (3) ◽  
pp. 502-514 ◽  
Author(s):  
Kenneth Johns ◽  
Borg Madsen
Keyword(s):  
Fracture Mechanics ◽  
Stress Ratio ◽  
Failure Time ◽  
Creep Function ◽  
Load Effect ◽  
Mechanics Model ◽  
Current Design ◽  
Load Effects ◽  
Duration Of Load

In Part I of this paper, the deterioration of the strength of lumber with continued application of constant stress, called the duration-of-load effect, is treated using a viscoelastic, limited ductility fracture mechanics model. The model is explained and developed in a general way, then modified for use with commercial lumber. The problems of assigning correct creep function parameters and values of stress ratio for use in calculations involving the model are discussed. The evident weakening of boards surviving a long-term test can be used to project a failure time that is longer than the test period. Numerical results are shown and compared with the Madison curve, the basis for current design codes. Parts II and III of this paper demonstrate experimental verification and discuss design implications.

Structural Integrity Evaluation of Reactor Pressure Vessels During PTS Events Using Deterministic and Probabilistic Fracture Mechanics Analysis

2013 ◽  
Author(s):  
Kazuya Osakabe ◽  
Hiroyuki Nishikawa ◽  
Koichi Masaki ◽  
Jinya Katsuyama ◽  
Kunio Onizawa
Keyword(s):  
Fracture Mechanics ◽  
Structural Reliability ◽  
Structural Integrity ◽  
Probabilistic Approach ◽  
Crack Size ◽  
Initial Crack ◽  
Pressure Vessels ◽  
Probabilistic Fracture Mechanics ◽  
Main Steam Line ◽  
Main Steam

To assess the structural integrity of reactor vessels (RVs) during pressurized thermal shock (PTS) events, a deterministic fracture mechanics (DFM) approach has been widely used such as the procedure in JEAC4206-2007. On the other hand, the application of a probabilistic fracture mechanics (PFM) analysis method for the structural reliability assessment of RV has become attractive recently because uncertainties related to input parameters can be incorporated rationally. The probabilistic approach has already been adopted as the regulation on fracture toughness requirements against PTS events in the U.S. In this paper, in order to verify the applicability of a PFM method to JEAC4206-2007, deterministic and probabilistic analyses have been performed, and the effects of initial crack size defined in JEAC4206-2007 on the temperature margin obtained from DFM and the probability of crack initiation obtained from PFM have been evaluated. With regard to the PTS event variation, a stuck open valve scenario (SO) has been considered in addition to large- and small-break loss of coolant accident (LBLOCA, SBLOCA) and main steam line break (MSLB).

Development of Probabilistic Fracture Mechanics Method for Fatigue Life Prediction Based on EIFS Concept

2017 ◽  
Author(s):  
Guang Zou ◽  
Kian Banisoleiman ◽  
Arturo González
Keyword(s):  
Fracture Mechanics ◽  
Crack Initiation ◽  
Crack Size ◽  
Initial Crack ◽  
Mean Value ◽  
Flaw Size ◽  
Crack Evolution ◽  
Initiation And Propagation ◽  
The Mean ◽  
Crack Initiation Life

A problem with fracture mechanics (FM) based fatigue analysis is that reliable information on initial crack/flaw size is often hard to obtain. Also, FM method can’t be applied directly to welded joints with relatively small initial flaws and long crack initiation life. This paper proposes a novel probabilistic FM method based on the equivalent initial flaw size (EIFS) concept. The initial crack size is substituted with EIFS to take both the crack initiation and propagation life into account. Three methods are tested to obtain mean value of EIFS: calibrating to S-N curves, Kitagawa-Takahashi (KT) diagram and fitting to test data. The obtained EIFSs are evaluated by comparing the predicted fatigue lives and crack evolutions with S-N curves and test crack evolution data. The suggested procedure is to derive the mean value of EIFS from S-N curves and the coefficient of variation from KT diagram.

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