1、 STP-NU-039 CREEP AND CREEP-FATIGUE CRACK GROWTH AT STRUCTURAL DISCONTINUITIES AND WELDS Prepared by: F. W. Brust G. M. Wilkowski P. Krishnaswamy K. Wichman Engineering Mechanics Corporation of Columbus (Emc2) STP-NU-039 Creep and Creep-Fatigue Growth ii Date of Issuance: June 30, 2011 This report w
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12、-5990 ISBN No. 978-0-7918-3363-6 Copyright 2011 by ASME Standards Technology, LLC All Rights Reserved Creep and Creep-Fatigue Crack Growth STP-NU-039 iii TABLE OF CONTENTS Foreword v Executive Summary vi 1 INTRODUCTION . 1 2 CREEP AND CREEP-FATIGUE CRACK GROWTH FUNDAMENTALS AND ENGINEERING METHODS 3
13、 2.1 High Temperature Damage Progression and Crack Growth: Theoretical Considerations 3 2.2 Currently Established Engineering Methods for Creep Fatigue Crack Growth 4 2.3 Creep Fatigue Crack Growth Methods for NH Code 5 3 FRACTURE MECHANICS BASIS FOR ENGINEERING CREEP-FATIGUE METHODS . 6 3.1 Elastic
14、 Fracture Considerations . 6 3.2 Fatigue Crack Growth . 7 3.3 Creep Crack Growth 8 4 REVIEW AND SUMMARY OF CURRENT ENGINEERING METHODS . 11 4.1 Overview of Engineering Creep Methods . 12 4.1.1 R5 Approach 12 4.1.2 RCC-MR (A16) 13 4.1.3 API-579 Approach . 13 4.2 Choice of Code Creep Crack Growth Proc
15、edure. 14 4.3 U.S. Nuclear Regulatory Commission (NRC) Interface . 15 5 THE R5 CREEP-FATIGUE CRACK GROWTH METHOD . 17 5.1 The R5 Method 17 5.2 The R5 Step-by-Step Approach. 17 5.2.1 STEP 1 - Establish the Expected or Actual Cause of Cracking and Characterize Initial Defect . 19 5.2.2 STEP 2 - Define
16、 Service Conditions for the Component . 19 5.2.3 STEP 3 - Collect Materials Data 19 5.2.4 STEP 4 - Perform Basic Stress Analysis 19 5.2.5 STEP 5 - Check Stability Under Time-Independent Loads . 19 5.2.6 STEP 6 - Check Significance of Creep and Fatigue . 20 5.2.7 STEP 7 - Calculate Rupture Life based
17、 on the Initial Defect Size . 20 5.2.8 STEP 8 - Calculate Crack Nucleation or Incubation Time 20 5.2.9 STEP 9 - Calculate Crack Growth for the Desired Lifetime 20 5.2.10 STEP 10 - Re-Calculate Rupture Life after Crack Growth 20 5.2.11 STEP 11 - Check Stability Under Time-Independent Loads after Crac
18、k Growth . 21 5.2.12 STEP 12 - Assess Significance of Results . 21 5.2.13 STEP 13 - Report Results . 21 5.3 Comments on R5 Application for ASME . 21 5.4 The R5 Material Data Requirements . 22 5.5 Summary of the R5 Material Data. 24 6 R5 VALIDATION AND EXAMPLE PROBLEMS . 25 STP-NU-039 Creep and Creep
19、-Fatigue Growth iv 6.1 Example Problem - Surface Crack Pipe . 25 6.1.1 Crack Growth Calculation 27 6.2 Theoretical Issues and Concerns with Engineering Creep Crack Growth Methods . 29 6.3 Validation and Creep Constitutive Laws . 30 7 DISCUSSION OF GEN IV AND R5 . 33 7.1 R5 as a Possible ASME NH Rule
20、 Set 33 7.2 Theoretical Issues with R5 Needing Resolution 34 7.3 Concluding Remarks on the R5 Approach 34 8 SUMMARY, CONCLUSION AND SUGGESTIONS FOR ADDITIONAL WORK 36 8.1 Summary 36 8.2 R5 Usage 37 8.3 Uncertainties in R5 and All Creep-Fatigue Crack Growth Methods . 38 8.4 Recommendations Regarding
21、Additional R with C* when the creep zone is larger during secondary creep; and with Ct(or C(t) when creep transients occur at the crack tip (C* and Ctare related); and with reference stress (which can also be related to C*). While reference stress methods are often used to estimate creep/fatigue cra
22、ck growth parameters within the current code approaches, there is some evidence that these methods are not accurate for all 1Practical engineering methods to account for diffusion based creep damage development and crack growth are in their infancy. Classical grain boundary cavitations only can be p
23、redicted properly in an engineering assessment. 2Department of Energy, Office of Basic Energy Sciences, DOE Grant No. DE-FG02-90ER14135 entitled, An Investigation of the Effects of History Dependent Damage In Time Dependent Fracture Mechanics, PI, F. W. Brust. Creep and Creep-Fatigue Crack Growth ST
24、P-NU-039 5 crack shapes. This is the topic of research at present. However, finite element methods can always be used to obtain the crack growth parameters although this may not always be practical. Most creep crack growth procedures used in worldwide codes are related to each other. The C*-integral
25、 is the creep analogue of the elastic-plastic J-Integral which is used extensively to predict elastic-plastic fracture. For this reason, C*/C(t) approach is a natural parameter to use in ASME NH code procedures. The U.S. NRC and utilities have developed a very large database of solutions used to est
26、imate the J-integral for through-wall and surface cracks in pipe, plate, vessels and other nuclear power plant components. Once the creep material constants in the form of power-law fits of creep data are available, these estimation schemes can be used directly to obtain C* and provide predictions o
27、f creep crack growth. Moreover, most commercial finite element codes permit the easy calculation of both C* and Ct, so obtaining this parameter for a creep-fatigue crack growth prediction for cases where compiled solutions are not available is not difficult. It is our view that extension of the J-in
28、tegral based methods for incorporation into NH based on C* is natural since NRC, contractors and utilities are well versed in these methods and, furthermore, J-based solutions are also in the ASME Boiler and Pressure Vessel code (e.g., Section XI flaw evaluation procedures). 2.3 Creep Fatigue Crack
29、Growth Methods for NH Code For conditions where time-dependent deformation does not occur, fatigue crack growth rates can be correlated with K using the Paris law, the Forman equation (including mean stress effects) and many other fatigue laws. When creep deformation can occur at the crack tip, the
30、fatigue crack growth rates are strongly affected. Hold times at load increase crack growth rates. A higher mean stress will increase crack growth rates, which can be important in and near welds or high-constraint cracks. The NH code has conservative procedures for combining the damage caused by fati
31、gue and creep in un-cracked structures. For crack-growth predictions, the separation of creep and fatigue crack growth damage is also the accepted procedure with well established engineering rules within R5 for materials where validation results are available. We anticipate that rules of this form w
32、ill serve as the basis of the new NH rules if and when they can be accepted for GEN IV conditions. It turns out that low-frequency creep conditions permit crack growth correlation with C*, and high-frequency fatigue correlates with K. In the transition regime, the current rules must be shown to be a
33、dequate for code use. However, the precise implementation into ASME code NH or other division should be delayed until validation is made for GEN IV materials. Alternatively, R5 rules should only be permitted for materials and conditions where validation has been made. These conditions are mainly tho
34、se experienced within the gas cooled reactors within UK. For low cycle fatigue, where there is non-negligible plasticity at the crack tip during reloading, the cyclic J-integral parameter may be more appropriate. Despite theoretical concerns with Dowling J based low cycle fatigue crack growth predic
35、tions, it has performed reasonably well in engineering predictions. STP-NU-039 Creep and Creep-Fatigue Growth6 3 FRACTURE MECHANICS BASIS FOR ENGINEERING CREEP-FATIGUE METHODS The engineering creep/fatigue crack growth methods depend on both elastic and creep fracture mechanics parameters. These par
36、ameters are summarized in this section. 3.1 Elastic Fracture Considerations Fracture mechanics began in the 1920s with the famous A. E. Griffith study of glass fracture. Griffith pondered the question as to why glass does not have the theoretical strength of the molecular bond and concluded that “cr
37、acking” was the cause. George Irwin is the father of modern fracture mechanics with his definition of the stress intensity factor needed for his famous studies of naval fractures in the 1950s and 1960s. Irwin identified three “modes” of fracture which are illustrated in Figure 2. Mode I type fractur
38、e is the opening mode defined by stresses which directly open the crack faces in the direction of the applied load. Modes II and III are shear modes with Mode III representing the “tearing” type analogous to ripping a sheet of paper. All three modes of fracture are possible at the same time however
39、mode I type fracture often dominates. In fact, all engineering creep crack growth methods available today require that Mode I crack growth dominates. Figure 2 - Elastic Crack Tip Fields Irwin applied the elasticity procedures of Westergaard to write the asymptotic solution of the crack tip stress fi
40、elds as (for Mode I type fracture) as seen in Figure 2, equation 1. Equation (1) then provides the stress field for every point (r, T) near the crack tip. The figure inserted above equation (1) illustrates the geometric definitions and “r” represents the radial distance from the crack tip and “T” Cr
41、eep and Creep-Fatigue Crack Growth STP-NU-039 7 represents the angular distance around the crack for the radial coordinate system centered at the crack tip3. f(T) is a known function of sine and cosine functions. KIis called the stress intensity factor (mode one hence the designation “I”) since, if
42、one knows its value (KIunits are psi-in1/2, Mpa-m1/2etc.), then one can determine if the crack will be stable or grow. If KI=Kcthen the crack grows, where Kcis obtained from tests on fracture specimen in the laboratory. KI depends on crack size, crack shape, material parameters and loading condition
43、s. Tables of K are available in all of the code methods, including R5 and A16. Alternatively, one can always calculate K using finite element methods for the geometry and load condition of interest. One can write similar equations for the other modes of fracture with the same conclusion: if one know
44、s the stress intensity factor(s), then one knows if the crack will grow or not. When time independent plasticity dominates near the crack tip, i.e., when the plastic zone at the crack tip is not embedded within the elastic crack tip fields, a nonlinear parameter called the J-integral is used to char
45、acterize fracture. As for the elastic case, J represents the strength of the asymptotic crack tip fields for a for a power law hardening material where the crack experiences proportional loading (replace C* in equation (3) of Figure 4 by “J”). For this case, the crack grows when J = JIC, where JIC i
46、s the measured fracture toughness. J-tearing theory applies for small amount of crack growth as well. The commercial nuclear industry in the U.S. (and in many other countries) bases crack growth assessment and leak before break rules on J-Theory. In practice, especially in the nuclear industry, J-te
47、aring theory is applied far beyond its theoretical basis into crack growth ranges and non-proportional load ranges that greatly violate the strict theoretical limits with success. The main reason it is accepted far beyond its theoretical limits is that extensive fracture test data in many geometries
48、 (specimens, pipe, vessels, elbows, etc.) and in many nuclear materials validates its use as a conservative predictive tool. This will be discussed later as well with regard to creep/fatigue fracture methods since the currently used methods violate the theory as well. 3.2 Fatigue Crack Growth Fatigu
49、e of metals became a concern in the early 1950s when the British de Havilland Comet, the worlds first commercial jet aircraft, experienced catastrophic service failures that were identified as metal fatigue. Structures are now designed to prevent fatigue failures throughout their expected life. There are two general philosophies of fatigue design, stress based and fracture mechanics based design. Stress Based Fatigue Design. The standard ASME NH procedure for the fatigue portion of life in hig
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