Reliability-Based Derivation of Partial Safety Factor for Structural Integrity Assessment

2008 ◽  
Author(s):  
Shuo Pan ◽  
Jianping Zhao
Keyword(s):  
Safety Factor ◽  
Fracture Mode ◽  
Structural Integrity ◽  
Research Work ◽  
Random Variables ◽  
Assessment Procedure ◽  
Safety Factors ◽  
Integrity Assessment ◽  
Partial Safety Factor ◽  
Partial Safety Factors

When there are uncertainties in the input random variables, or scatter in the material properties, probabilistic assessment is a useful tool for decision making in the field of safety analysis. The partial safety factor (PSF) method was aimed on ensuring that the failure probability did not exceed a target value. In order to be conservative the input value for each random variable during the assessment procedure should be multiplied by the partial safety factors. So it is essentially a deterministic assessment using conservative values of the input random variables and a relatively simple and independent method of assessing failure probabilities using R6 failure assessment diagram. The application of partial safety factors is an important breakthrough of assessment in structures containing defects. In recent years, sets of PSFs for load, defect size, fracture toughness and yield stress had been given in two standards, BS7910 and API579. However, the recommended PSFs in both standards were larger than the original PSFs in PD6493 which was replaced by BS7910. It is therefore a new method of calculating PSFs should be found to prove which is more appropriate and convenient for engineering application. In the case of the partial safety factor method target reliabilities in the range from 0.001 to 0.00001 were considered and new series of PSFs were derived from the results of reliability analysis for the linear elastic fracture mode and elastic-plastic fracture mode. After comparing with the PSFs in BS7910 and API 579, it is concluded that the partial safety factors were generally conservative compared to our research work.

Reliability Analysis of Defect-Containing Structures Using Partial Safety Factors

2009 ◽  
Author(s):  
Liwu Wei
Keyword(s):  
Failure Probability ◽  
Structural Integrity ◽  
General Trend ◽  
Safety Factors ◽  
Integrity Assessment ◽  
Fracture Region ◽  
Failure Assessment ◽  
Elastic Fracture ◽  
Partial Safety Factors ◽  
Collapse Region

Some standards of structural integrity assessment such as BS 7910 and API 579-1/ASME FFS-1 recommend values of partial safety factor (PSF) applied to the deterministic engineering critical assessments of flaw-containing structures to achieve certain reliability levels. However, it is still uncertain as to whether the use of the PSFs can achieve the target reliability level specified in the codes, or excessively exceed the targets (un-conservative) or under-reach the targets (too conservative). This work was undertaken to make investigations into these issues raised from the use of PSFs through case studies involving deterministic fitness-for-service analysis incorporating PSFs and probabilistic fracture mechanics analysis. Two cases, a through-thickness crack and a surface-breaking elliptical crack in a plate subjected to tension, were considered. The results in terms of failure probability from the studied cases have shown a general trend that for each of the four PSFs recommended in BS 7910, the failure probability decreased as the assessments changed from the elastic fracture region to the plastic collapse region in the failure assessment diagram. Some over-conservatism has been found in certain situations from the use of PSFs recommended in BS 7910:2005. Cautions are given for application of the PSFs for integrity assessment of the structures and components containing flaws.

Partial safety factors for designing and assessing flexible pavement performance

2004 ◽  
Vol 31 (3) ◽  
pp. 397-406 ◽  
Author(s):  
S S Wang ◽  
H P Hong
Keyword(s):  
Safety Factor ◽  
Load Resistance ◽  
Flexible Pavement ◽  
Pavement Performance ◽  
Single Factor ◽  
Safety Factors ◽  
State Highway ◽  
Partial Safety Factor ◽  
Reliability Method ◽  
Partial Safety Factors

In designing and assessing pavement performance, the uncertainty in material properties and geometrical variables of pavement and in traffic and environmental actions should be considered. A single factor is employed to deal with these uncertainties in the current American Association of State Highway and Transportation Officials (AASHTO) guide for design of pavements. However, use of this single factor may not ensure reliability-consistent pavement design and assessment because different random variables that may have different degrees of uncertainty affect the safety and performance of pavement differently. Similar problems associated with structural design have been recognized by code writers and dealt with using partial safety factors or load resistance factors. The present study is focused on evaluating a set of partial safety factors to be used in conjunction with the flexible pavement deterioration model in the Ontario pavement analysis of cost and the model in the AASHTO guide for evaluating the flexible pavement performance or serviceability. Evaluation and probabilistic analyses are carried out using the first-order reliability method and simple simulation technique. The results of the analysis were used to suggest factors that could be used, in a partial safety factor format, for designing or assessing flexible pavement conditions to achieve a specified target safety level.Key words: deterioration, reliability, pavement, serviceability, stochastic process, performance, partial safety factor.

A Comparison of Fracture Assessment Methods in Several Structural Integrity Assessment Procedures

2009 ◽  
Author(s):  
Tiecheng Yang ◽  
Xuedong Chen ◽  
Zhichao Fan
Keyword(s):  
Structural Integrity ◽  
Limit Load ◽  
Assessment Method ◽  
Assessment Methods ◽  
Safety Factors ◽  
Specific Calculation ◽  
Integrity Assessment ◽  
Fracture Assessment ◽  
Partial Safety Factors ◽  
Assessment Procedures

For the fracture assessment method internationally used in different structural integrity assessment procedures, such as R6, BS 7910, FITNET API 579 and GB/T 19624, this paper gives the results of analytical comparisons in combination with specific calculation examples by comparing different assessment options or levels, the partial safety factors (PSFs), limit load solutions, stress intensity factor solutions, residual stress distribution and treatment methods, secondary stresses and ρ factor solutions etc., which provide a basis for improvement of fracture assessment methods.

Safety factors in structural integrity assessment of components with defects

2013 ◽  
Vol 4 (4) ◽  
pp. 457-476 ◽  
Author(s):  
Yury Matvienko
Keyword(s):  
Fracture Mechanics ◽  
Safety Factor ◽  
Structural Integrity ◽  
Plastic Collapse ◽  
Safety Factors ◽  
Content Type ◽  
Nonlinear Fracture Mechanics ◽  
Integrity Assessment ◽  
Nonlinear Fracture ◽  
Linear And Nonlinear

Purpose – The purpose of this paper is to develop basic principles of deterministic structural integrity assessment of a component with a crack- or notch-like defect by including safety factors against fracture and plastic collapse in criteria equations of linear and nonlinear fracture mechanics. Design/methodology/approach – The safety factors against fracture are calculated by demanding that the applied critical stress should not be less than the yield stress of the material for a component with a crack or a notch of the acceptable size. Structural integrity assessment of the engineering components damaged by crack- or notch-like defects is discussed from view point of the failure assessment diagram (FAD). The methodology of the FAD has been employed for the structural integrity analysis and assessment of acceptable sizes of throw-thickness notch in a plate under tension and surface longitudinal notch-like defects in a pressure vessel. Findings – Basic equations have been presented to calculate the safety factor against fracture for critical values of the stress intensity factor, crack tip opening displacement (CTOD), the J-integral and the FAD as well as to estimate an acceptable (safe) region for an engineering component with a crack- or notch-like defect of the acceptable size. It was shown that safety factors against fracture depend on both the safety factor against plastic collapse and employed fracture mechanics criterion. The effect of crack/notch tip constraint is incorporated into criteria equations for the calculation of safety factors against fracture. Originality/value – The deterministic method of fracture mechanics is recommended for structural integrity assessment of a component with a crack- or notch-like defect by including safety factors against fracture and plastic collapse in criteria equations of linear and nonlinear fracture mechanics.

Structural Integrity for Nuclear Power Plant Against Excessive Load on Design Extension Condition

2013 ◽  
Author(s):  
Daigo Watanabe ◽  
Kiminobu Hojo
Keyword(s):  
Nuclear Power ◽  
Structural Integrity ◽  
Safety Factors ◽  
Design Code ◽  
Acceptance Criteria ◽  
Integrity Assessment ◽  
Current Design ◽  
Factor Design ◽  
Excessive Load ◽  
Load And Resistance Factor

This paper introduces an example of structural integrity evaluation for Light Water Reactor (LWR) against excessive loads on the Design Extension Condition (DEC). In order to assess the design acceptance level of DEC, three acceptance criteria which are the stress basis limit of the current design code, the strain basis limit of the current design code and the strain basis limit by using Load and Resistance Factor Design (LRFD) method were applied. As a result the allowable stress was increased by changing the acceptance criteria from the stress basis limit to the strain basis limit. It is shown that the practical margin of the LWR’s components still keeps even on DEC by introducing an appropriate criterion for integrity assessment and safety factors.

Applying Partial Safety Factors in Mooring System Design

2005 ◽  
Author(s):  
Siril Okkenhaug ◽  
Bjo̸rn Sogstad ◽  
Jan Mathisen
Keyword(s):  
Safety Factor ◽  
Low Frequency ◽  
Offshore Structures ◽  
Mooring Line ◽  
Safety Factors ◽  
Safety Level ◽  
Partial Safety Factor ◽  
Account Due ◽  
Almost All ◽  
Partial Factor

The DNV offshore standard for position mooring, DNV-OS-E301 [1], was issued June 2001 based on the result from a joint industry project. A new revision was issued in October 2004. The consequences for mobile units when applying the new standard, compared to the old class rules, have been a major concern for operators of mobile units. A comparison study has therefore been initiated, where four relevant units are considered. We have applied the mooring design for existing mobile units that operate in Norwegian waters. Two different water depths are covered. The new standard, DNV-OS-E301, applies a partial safety factor format. However, the main difference when applying DNV-OS-E301 compared to the old POSMOOR [2] rules is that low frequency (LF) motions will have to be taken into account when calculating the line tensions. The results for the four mobile units are compared also to other relevant codes, i.e. the Draft International Standard ISO 19901-7 [4] and the present Norwegian regulations for offshore structures, NMD [5 & 6]. It should be noted that the present NMD regulations still do not require that LF motions are taken into account. Due to the partial safety factor format in DNV-OS-E301, more or less all of the units fulfill the requirements even though LF motion is accounted for. However, when comparing the results to the NMD regulations, the introduction of LF motion is crucial for almost all of the mobile units studied, as they have problems in fulfilling the requirements when this response is accounted for. Simply including LF motion in design would tend to increase the required strength of the resulting mooring line designs, and thereby raise the safety level if nothing else is done with the NMD regulations. Thus, provided that the present safety level for mobile units is sound, the present NMD safety factors could either be reduced or the partial factor format in DNV-OS-E301 could be adopted in order to maintain the safety level for mobile units when LF motion is taken into account.

Estimation of Through-Wall Cracking Frequency of RPV Under PTS Events Using PFM Analysis Method for Identifying Conservatism Included in Current Japanese Code

2014 ◽  
Author(s):  
Kazuya Osakabe ◽  
Koichi Masaki ◽  
Jinya Katsuyama ◽  
Genshichiro Katsumata ◽  
Kunio Onizawa
Keyword(s):  
Fracture Toughness ◽  
Fracture Mechanics ◽  
Crack Initiation ◽  
Structural Integrity ◽  
Probabilistic Approach ◽  
Pressure Vessels ◽  
Assessment Procedure ◽  
Analysis Method ◽  
Integrity Assessment ◽  
The U.S

To assess the structural integrity of reactor pressure vessels (RPVs) during pressurized thermal shock (PTS) events, the deterministic fracture mechanics approach prescribed in Japanese code JEAC 4206-2007 [1] has been used in Japan. The structural integrity is judged to be maintained if the stress intensity factor (SIF) at the crack tip during PTS events is smaller than fracture toughness KIc. On the other hand, the application of a probabilistic fracture mechanics (PFM) analysis method for the structural reliability assessment of pressure components has become attractive recently because uncertainties related to influence parameters can be incorporated rationally. A probabilistic approach has already been adopted as the regulation on fracture toughness requirements against PTS events in the U.S. According to the PFM analysis method in the U.S., through-wall cracking frequencies (TWCFs) are estimated taking frequencies of event occurrence and crack arrest after crack initiation into consideration. In this study, in order to identify the conservatism in the current RPV integrity assessment procedure in the code, probabilistic analyses on TWCF have been performed for certain model of RPVs. The result shows that the current assumption in JEAC 4206-2007, that a semi-elliptic axial crack is postulated on the inside surface of RPV wall, is conservative as compared with realistic conditions. Effects of variation of PTS transients on crack initiation frequency and TWCF have been also discussed.

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