4.14.1 Introduction

4.14.1.1 Historical Review of Fracture Toughness Determination for Ferritic Steels

Fracture mechanics is an engineering discipline which concerns the behavior of crack-like defects in struc­tures or components and their effect on integrity. Initially conceived by Griffith during World War I, early applications were limited to the study of frac­ture of highly brittle materials (e. g., glass).1 Interest in the discipline languished until World War II, when ^25% of the all-welded US Liberty ships experi­enced brittle fracture, exposing the urgent need to understand failure in ferritic structural steels and weldments. The earliest development in fracture mechanics of metals was focused on linear-elastic theory for understanding the fracture behavior of primarily high-strength steels and aluminum alloys. Application to brittle cleavage fracture in structures

made of welded ferritic steels of low-moderate strength evolved later.

One of the basic quantities of fracture mechanics is the stress-intensity factor, K, which is used to describe the loading condition of a cracked structure as a function of crack depth, a, and the applied stress, a. In the simplest case, a wide plate containing a central crack (2a), the loading condition can be expressed using the stress-intensity factor in the form:

K = af%a [1]

Increasing the stress eventually results in a situation where the crack starts to propagate. Depending on the material and loading condition, this can occur by a ductile, cleavage, intergranular, or some mixed­mode mechanism. Once the crack starts to propagate, the critical stress intensity (i. e., the fracture tough­ness) of the material has been exceeded. The fracture toughness is thus a material property, but it may be strongly affected by many environmental factors like temperature and air humidity. Using fatigue pre­cracked test specimens, the fracture toughness of a material can be determined experimentally. This frac­ture toughness quantity can be used to evaluate the integrity of real structures with real or postulated flaws. The fracture mechanism can be different depending on the material, and for ferritic steels, different fracture modes are possible at different tem­peratures due to the ductile-brittle transition. There­fore, different approaches for the characterization of fracture mechanics are needed.

The fracture toughness in body-centered cubic (bcc) ferritic steels exhibits a temperature depen­dence characterized by: (1) a low toughness cleavage initiation shelf at low temperatures; (2) an increasing transitional rise in toughness (going from cleavage to a mixture of cleavage and ductile-tearing fracture) with increasing temperature defined arbitrarily as a ductile-brittle transition temperature; and (3) an upper shelf characterized by fully ductile initiation fracture (Figure 1). The lower shelf and the region around the ductile-brittle transition temperature can be characterized using linear-elastic fracture mechanics (LEFM). LEFM considers material defects (flaws and cracks) and the effects of those defects on brittle cleavage crack behavior. LEFM is based on elastic stress analysis of the stress-strain field in the vicinity of the crack tip and a singularity called the stress-intensity factor, K. The linear-elastic theory was soon followed by elastic-plastic fracture mechanics (EPFM), which involved a different type of singular­ity parameter called the У-integral. Determination of

Temperature

Figure 1 Schematic description of the fracture toughness transition region and parameters used to characterize fracture toughness in the lower shelf, over the transition region, and in and near the upper shelf where ductile cracking gradually becomes the predominant fracture mode.

material У-integral fracture resistance (J-R) curves expanded the scope of application to also include stable crack growth characterized by ductile tearing. Regardless of which theory is being used, it is neces­sary to know the material resistance to fracture, that is, the fracture toughness of the material being eval­uated. Standardized test methods for determining material fracture toughness properties have been developed. In LEFM, fracture toughness is charac­terized by the parameter KIc; in EPFM, the initiation toughness parameter yc (often converted to an approximate equivalent K value termed Kjc) is used to characterize the onset of unstable crack growth under significant crack-tip plastic deformation con­ditions. (The statistical size effect and the elastic — plastic parameter Kjc are associated with the Master Curve methodology discussed in Section 4.14.1.3.3. The linear-elastic parameter KIc is not presently recommended to characterize the transition region, as shown in Figure 1, due to the inherently large scatter of data in this region.) The J-R curve deter­mination and parameters for the onset of stable crack growth are described in separate standards or sec­tions of standards (not discussed here in detail).2

Historically, LEFM concepts for determining the fracture toughness of ferritic, bcc steels have been used, often together with conventional Charpy V-notch impact tests, to characterize the lower shelf and the transition fracture toughness region. There have been few alternatives to the LEFM meth­odology when combined with Charpy V-notch tran­sition temperature results, as this is the current
methodology applied for irradiated reactor pressure vessel (RPV) integrity following the ASME Boiler and Pressure Vessel Code. The LEFM approach itself is simple, because only the load record and specimen dimensions are needed for KIc determination, that is, due to the qualification requirements, the test tends to be invalid if there is any significant plastic area under the load versus displacement record. This restriction imposes a major disadvantage in that the amount of test material needed is often large, even if only one large specimen is tested. In this respect, the J-integral EPFM concepts using the parameter JIc are more applicable, as they make possible testing with smaller specimens due to less severe size requirements.

Although current KIc and crack opening displace­ment (COD) testing standards better correspond to the latest fracture mechanics understanding (e. g., size restrictions relative to test specimen ligament and thickness dimensions), these standards generally are no longer recommended for characterizing the tran­sition behavior of ferritic steels; they are more appli­cable to cases where the fracture mode is known to be ductile or possibly quasicleavage, and the material shows predominantly elastic behavior. The reason is that these older standards do not account for the statistical nature of the brittle fracture process in ferritic steels. More recently, a statistical assessment methodology, called the Master Curve procedure, has been developed as an improved method for char­acterizing the material fracture toughness (both LEFM and EPFM) of ferritic bcc materials, and for characterizing the temperature dependence of the transition temperature fracture toughness curve. It is the purpose of this chapter to provide a summary review of the Master Curve methodology.

The following summary review of the Master Curve fracture toughness approach provides the basis and general framework for the methodology, but it also focuses on some key technical details that are often misunderstood. The reader should consult several of the references for greater detail regarding the various considerations needed in applying the Master Curve methodology for structural integrity assessments. In this chapter, the discussion is first devoted to LEFM involving the standard methods for experimentally determining the value of KIc; then, the more advanced approach based on EPFM and the approximate equiv­alent Kjc is reviewed. Finally, the Master Curve proce­dure is discussed in depth and is the primary focus of this chapter.