Some Fundamental Aspects of High Temperature Defect Assessment

2005 ◽  
Vol 297-300 ◽  
pp. 428-434 ◽  
Author(s):  
Shan Tung Tu ◽  
Fu Zhen Xuan
Keyword(s):  
High Temperature ◽  
Assessment Method ◽  
Damage Effect ◽  
Assessment Procedure ◽  
Continuum Damage ◽  
Creep Failure ◽  
Failure Assessment Diagram ◽  
Defect Assessment ◽  
Failure Assessment ◽  
Dependent Failure

Current research efforts in the development of high temperature defect assessment procedure are summarized. Creep exemption criteria are proposed for the assessment of defective structures at high temperature in consideration of the effects of loadings, operating temperature and service time. Time-dependent failure assessment diagram (TDFAD) is developed that covers major failure mechanisms of defective high temperature structures. Challenges due to the welding effect are discussed. TDFAD for weldments is derived for various combinations of materials. In order to develop a unified assessment method to cope with material and loading complexity, a new failure assessment diagram based on continuum damage concept is proposed to reflect the damage effect on ductile creep failure and brittle creep fracture.

Integrity Assessment of Defected Structure at High Temperature With a Material Database

2010 ◽  
Author(s):  
Zhengdong Wang ◽  
Fu-Zhen Xuan ◽  
Shan-Tung Tu
Keyword(s):  
High Temperature ◽  
Crack Size ◽  
Pressure Vessels ◽  
Time Dependent ◽  
Integrity Assessment ◽  
Failure Assessment Diagram ◽  
Base Materials ◽  
Axial Tension ◽  
Failure Assessment ◽  
Dependent Failure

The basic theory of Time Dependent Failure Assessment Diagram (TDFAD) is introduced in this paper to be modified for cracks in mismatched welds at high temperature. A time-dependent failure assessment curve is constructed using the method of R6 Option 2. This curve depends not only on the geometry and crack size but also on the mechanical properties of both weld and base materials. The application of such an approach is given to an internal circumferentially defected welded-cylinder under axial tension. An engineering tool of integrity assessment for defected structure at high temperature has been developed for a decade based on the Time Dependent Failure Assessment Diagram (TDFAD) technology for pressure vessels. The software is illustrated in this paper by framework and interfaces and some industrial cases are introduced to show the procedure of the assessment by using the software. A high temperature material database (HTMD) of main properties for typical high temperature steels and alloys is constructed to meet the needs of the safety assessment of the pressure vessels and other structures. The structure of the database is shown in the paper with some tables and charts.

Modification of the Failure Assessment Diagram for Non-Sharp Defects

2010 ◽  
Author(s):  
Anthony J. Horn ◽  
Andrew H. Sherry
Keyword(s):  
Fatigue Cracks ◽  
Safety Margin ◽  
Assessment Method ◽  
Assessment Procedure ◽  
J Integral ◽  
Element Analysis ◽  
Failure Assessment Diagram ◽  
Failure Assessment ◽  
Notch Tip ◽  
Failure Predictions

Current defect assessment procedures assume all flaws to be sharp. While this assumption may be appropriate for fatigue cracks, in other cases such as voids, mechanical dents or welding defects it can be an over-conservative assumption that can lead to pessimistic assessment of structures and significant under-estimation of their safety margin against fracture. This study has developed an assessment procedure for predicting the cleavage fracture resistance of structures containing non-sharp defects. The new assessment method was developed using the Weibull stress based toughness scaling model and an approach based on a modification of the Failure Assessment Diagram (FAD). In the new assessment procedure, the notch driving force is described by the notch J-integral, the notch tip loading severity by the elastic notch tip opening stress σN, the notch geometry by a load-independent parameter βN, and the sensitivity of the material toughness to the notch effect by the material parameters γ and l. Finite element analysis of SE(B) specimens containing U-notches was used to demonstrate that the notch J-integral can be estimated using existing expressions in fracture toughness testing standards intended for pre-cracked specimens. A test programme of SE(B) specimens containing U-notches was used to validate the new assessment procedure. Failure predictions of the SE(B) specimens using the notch-modified FAD approach result in significantly reduced conservatism compared to the standard FAD assessment approach for sharp cracks.

Elastic-Plastic J-Estimation for Circumferentially Cracked Pipes Under Combined Mechanical and Thermal Loadings

2013 ◽  
Author(s):  
Chang-Young Oh ◽  
Yun-Jae Kim ◽  
R. A. Ainsworth
Keyword(s):  
Finite Element ◽  
Thermal Gradient ◽  
Order Effects ◽  
Assessment Procedure ◽  
Element Analysis ◽  
Elastic Plastic ◽  
Defect Assessment ◽  
Failure Assessment ◽  
Two Parameters ◽  
J Estimation

This paper addresses load order effects on elastic-plastic J estimation under combined mechanical and thermal loads for circumferentially cracked pipes. The load order effects, for various thermal gradient types and mechanical loading, are evaluated for a range of magnitudes of the loadings, crack sizes and material hardening. Variations of elastic-plastic J obtained by finite element analysis are compared with existing and proposed methods for use with the R6 defect assessment procedure. The load order effects are presented on the R6 failure assessment diagram (FAD) by calculating the two parameters Kr and Lr from the finite element results. It is shown that there are significant load order effects at large secondary stress cases but these are successfully treated by simplified methods proposed for use with R6.

Numerical Validation of a Strain-Based Failure Assessment Diagram Approach to Fracture

2009 ◽  
Author(s):  
Peter J. Budden ◽  
Michael C. Smith
Keyword(s):  
Finite Element ◽  
Plastic Strain ◽  
Present Author ◽  
Plastic Relaxation ◽  
Failure Assessment Diagram ◽  
Defect Assessment ◽  
Failure Assessment ◽  
Basic Approaches ◽  
Assessment Procedures ◽  
Diagram Approach

The basic approaches in defect assessment procedures such as R6 consider the stresses on the section containing the flaw. Such approaches can be overly conservative and lead to unacceptably small estimates of limiting defect sizes for cases where the applied loads are due to displacements or strains well in excess of yield, when significant plastic relaxation of stress occurs. The potential for over-conservative assessments has led to a renewed interest in recent years in strain-based assessment methods, in both the power and pipeline industries. Significant levels of plastic strain can be imposed across the flawed section in some cases. Recently, the present author has published a general approach to strain-based fracture that uses a strain-based failure assessment diagram (SB-FAD). This includes a range of Options similar to that of the basic R6 approach. The present paper describes some validation of the SB-FAD approach based on elastic-plastic cracked-body finite element data for plates and cylinders.

Defect Assessment Procedure for Low to High Temperature Range

2003 ◽  
Author(s):  
Bilal Dogan ◽  
Robert Ainsworth
Keyword(s):  
High Temperature ◽  
Growth Conditions ◽  
Life Assessment ◽  
Assessment Procedure ◽  
High Temperature Range ◽  
Temperature Range ◽  
Defect Assessment ◽  
Creep Fatigue ◽  
Recent Developments ◽  
Assessment Procedures

There are many similarities between available procedures used for defect assessment. They have been developed as a result of experience gained from material-specific programs and have often been verified using the same data. One recently updated document covering life assessment procedures under creep and creep/fatigue crack growth conditions is BS 7910. This document takes into account some of the most recent developments in the subject, including some from the British Energy R5 Procedure. Future developments in defect assessment procedures will follow the route of simplified and unified codes covering defect behaviour in the low to high temperature range. In this paper, the relevance of the insignificant creep curves in RCC-MR for defect free structures and the creep exemption criteria in BS7910 are examined. Then, an overview is given of some European developments in defect assessment methods for Fitness-for-Service assessment, based on recent and current projects such as the EC thematic network FITNET.

Assessment Procedure for Multiple Cracklike Flaws in Failure Assessment Diagram (FAD)

2009 ◽  
Vol 131 (4) ◽  
Author(s):  
Shinji Konosu
Keyword(s):  
Body Force ◽  
Pressure Vessels ◽  
The Body ◽  
Assessment Procedure ◽  
Failure Assessment Diagram ◽  
Reference Stress ◽  
Body Force Method ◽  
Failure Assessment ◽  
Common Problems ◽  
Shielding Effects

Assessment of multiple discrete cracklike flaws is one of the most common problems relating to pressure vessels and piping components. Under the current fitness for service (FFS) rules, such as ASME, BS, and so on, multiple cracklike flaws are usually recharacterized as an enveloping crack (defined as a single larger crack), following their assessment rules. The procedure, however, varies significantly in these FFS codes. In this paper, the interaction between nonaligned multiple unequal cracks is clarified by applying the body force method. Based on the interaction that indicates the magnification and shielding effects and the reference stress solutions, a newly developed assessment procedure for multiple discrete cracklike flaws in the failure assessment diagram is proposed.

scholarly journals Blunt defect assessment in the framework of the failure assessment diagram

2016 ◽  
Vol 146 ◽  
pp. 39-54 ◽  
Author(s):  
Jae-Jun Han ◽  
Nicolas Larrosa ◽  
Yun-Jae Kima ◽  
Robert A. Ainsworth
Keyword(s):  
Failure Assessment Diagram ◽  
Defect Assessment ◽  
Failure Assessment
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