Personnel need to understand brittle fracture. This type of fracture occurs under specific conditions without warning and can cause major damage to plant materials.

EO 1.1DEFINE the following terms:

a. Ductile fracture c. Nil-ductility Transition

b. Brittle fracture (NDT) Temperature

EO 1.2DESCRIBE the two changes made to reactor pressure vessels to decrease NDT.

EO 1.3STATE the effect grain size and irradiation have on a material's NDT.

EO 1.4LIST the three conditions necessary for brittle fracture to occur.

EO 1.5STATE the three conditions that tend to mitigate crack initiation.

EO 1.6LIST the five factors that determine the fracture toughness of a material.

EO 1.7Given a stress-temperature diagram, IDENTIFY the following points:

a. NDT (with no flaw) c. Fracture transition elastic point

b. NDT (with flaw) d. Fracture transition plastic point

Brittle Fracture Mechanism

Metals can fail by ductile or brittle fracture. Metals that can sustain substantial plastic strain or deformation before fracturing exhibit ductile fracture. Usually a large part of the plastic flow is concentrated near the fracture faces.

Metals that fracture with a relatively small or negligible amount of plastic strain exhibit brittle fracture. Cracks propagate rapidly. Brittle failure results from cleavage (splitting along definite planes). Ductile fracture is better than brittle fracture, because ductile fracture occurs over a period of time, where as brittle fracture is fast, and can occur (with flaws) at lower stress levels than a ductile fracture. Figure 1 shows the basic types of fracture.

Brittle cleavage fracture is of the most concern in this module. Brittle cleavage fracture occurs in materials with a high strain-hardening rate and relatively low cleavage strength or great sensitivity to multi-axial stress.

Figure 1 Basic Fracture Types

Many metals that are ductile under some conditions become brittle if the conditions are altered. The effect of temperature on the nature of the fracture is of considerable importance. Many steels exhibit ductile fracture at elevated temperatures and brittle fracture at low temperatures. The temperature above which a material is ductile and below which it is brittle is known as the Nil-Ductility Transition (NDT) temperature. This temperature is not precise, but varies according to prior mechanical and heat treatment and the nature and amounts of impurity elements. It is determined by some form of drop-weight test (for example, the Izod or Charpy tests).

Ductility is an essential requirement for steels used in the construction of reactor vessels; therefore, the NDT temperature is of significance in the operation of these vessels. Small grain size tends to increase ductility and results in a decrease in NDT temperature. Grain size is controlled by heat treatment in the specifications and manufacturing of reactor vessels. The NDT temperature can also be lowered by small additions of selected alloying elements such as nickel and manganese to low-carbon steels.

Of particular importance is the shifting of the NDT temperature to the right (Figure 2), when the reactor vessel is exposed to fast neutrons. The reactor vessel is continuously exposed to fast neutrons that escape from the core. Consequently, during operation the reactor vessel is subjected to an increasing fluence (flux) of fast neutrons, and as a result the NDT temperature increases steadily. It is not likely that the NDT temperature will approach the normal operating temperature of the steel. However, there is a possibility that when the reactor is being shut down or during an abnormal cooldown, the temperature may fall below the NDT value while the internal pressure is still high. The reactor vessel is susceptible to brittle fracture at this point. Therefore, special attention must be given to the effect of neutron irradiation on the NDT temperature of the steels used in fabricating reactor pressure vessels. The Nuclear Regulatory Commission requires that a reactor vessel material surveillance program be conducted in watercooled power reactors in accordance with ASTM Standards (designation E 185-73).

Pressure vessels are also subject to cyclic stress. Cyclic stress arises from pressure and/or temperature cycles on the metal. Cyclic stress can lead to fatigue failure. Fatigue failure, discussed in more detail in Module 5, can be initiated by microscopic cracks and notches and even by grinding and machining marks on the surface. The same (or similar) defects also favor brittle fracture.