BRITTLE FRACTURE MECHANISM
DOE-HDBK-1017/2-93
Brittle Fracture
Brittle cleavage fracture is of the most concern in this
Figure 1 Basic Fracture Types
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.
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 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/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.
MS-04
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