BRITTLE FRACTURE MECHANISMDOE-HDBK-1017/2-93Brittle FractureBrittle cleavage fracture is of the most concern in thisFigure 1 Basic Fracture Typesmodule. Brittle cleavage fractureoccurs in materialswith a high strain-hardening rate and relatively lowcleavage strength or great sensitivity to multi-axialstress.Many metals that are ductile under some conditionsbecome brittle if the conditions are altered. The effectof temperature on the nature of the fracture is ofconsiderable importance. Many steels exhibit ductilefracture at elevated temperatures and brittle fracture atlow temperatures. The temperature above which amaterial is ductile and below which it is brittle is knownas the Nil-Ductility Transition (NDT) temperature. Thistemperature is not precise, but varies according to priormechanical and heat treatment and the nature and amounts of impurity elements. It isdetermined 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 grainsize tends to increase ductility and results in a decrease in NDT temperature. Grain size iscontrolled by heat treatment in the specifications and manufacturing of reactor vessels. TheNDT temperature can also be lowered by small additions of selected alloying elements such asnickel and manganese to low-carbon steels.Of particular importance is the shifting of the NDT temperature to the right (Figure 2), whenthe reactor vessel is exposed to fast neutrons. The reactor vessel is continuously exposed to fastneutrons that escape from the core. Consequently, during operation the reactor vessel issubjected to an increasing fluence (flux) of fast neutrons, and as a result the NDT temperatureincreases steadily. It is not likely that the NDT temperature will approach the normal operatingtemperature of the steel. However, there is a possibility that when the reactor is being shutdown or during an abnormal cooldown, the temperature may fall below the NDT value whilethe internal pressure is still high. The reactor vessel is susceptible to brittle fracture at thispoint. Therefore, special attention must be given to the effect of neutron irradiation on the NDTtemperature of the steels used in fabricating reactor pressure vessels. The Nuclear RegulatoryCommission requires that a reactor vessel material surveillance program be conducted in water-cooled 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/ortemperature 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 andeven by grinding and machining marks on the surface. The same (or similar) defects also favorbrittle fracture.MS-04Page 2Rev. 0
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