All structural materials contain some types of flaw in them, the size of which can range from microscopic to mesoscopic in scale; these defects promote stress to concentrate locally around them leading to premature failure of the structure. The toughness of structures, in presence of inherent defects, is evaluated through a fracture mechanics approach. While most of the codes use a linear elastic fracture mechanics (LEFM) approach, small structures and ductile materials require elastic-plastic fracture mechanics (EPFM) formulations. The validity of LEFM compared to EPFM depends on the plastic zone size as shown in Fig. 1.4 and, in general, LEFM is not applica­ble when the plastic zone size is too large compared to either the crack size, the uncracked ligament or the member height.9 In very large structures and relatively brittle materials where LEFM is valid, the stress fields are charac­terized by stress intensity factor, Kh given by

K^YoJm, [1.9]

where a is half-crack length, a is applied nominal stress and Y isa geometry factor which is a function of the ratio of crack length to its width (a/w). As long as Kj is lower than the plane strain critical fracture toughness KIC, the structure with the crack can withstand the applied loads. In cases where LEFM is not valid (Fig. 1.4) either crack tip opening displacement (CTOD) or elastic-plastic fracture toughness (J-integral) can be conveniently adopted.

Although fracture toughness is a fundamental parameter characterizing the fracture behaviour of cracked bodies, it is often more convenient to use the ductile to brittle transition temperature (DBTT) measured using the relatively simple Charpy impact tests, to study the effect of neutron

radiation exposure (Fig. 1.5).10 These effects are well defined in BCC met­als, as against FCC metals, which exhibit a clear transition from ductile to brittle fracture behaviour as test or operating temperature decreases. It is common practice to consider a reference transition temperature corre­sponding to a specific Charpy impact energy of 41J (50 ft-lb) in lieu of actual nil-ductility transition (RTndt) such that brittle fracture is expected to take place below this reference temperature. As we will note later, exposure of ferritic steels to neutron irradiation leads to decreased fracture energy and increased RTndt commonly referred to as radiation embrittlement of RPV steels. Charpy impact tests are very useful and are conveniently adopted for reactor pressure vessel surveillance programmes (RVSPs). The transition temperature is a function of various factors such as the chemi­cal composition, the temperature, the neutron flux and fluence as well as the microstructure (such as base material, heat-affected zone (HAZ) or weld metal). Validation of thermal annealing of radiation defects in RPV steels is also often established using the Charpy test method. It has been well recognized that other fracture parameters such as crack arrest frac­ture toughness (KIa), dynamic critical stress intensity factor (KId), etc., need to be considered in detailed analyses involving strain-rate effects that become important during a loss of coolant accident (LOCA) condition. It has also been found applicable to high-temperature crack growth, pre­sumably because the plastic stress zone is often relatively small and linear elastic fracture mechanics are considered valid. Another fracture mechan­ics based parameter used to describe creep crack growth is C*. While there are many advantages in using C* analyses in creep of cracked bodies, these types of studies are confined more to scientific curiosities than to techno­logical applications.