As discussed earlier in this chapter, brittle failure generally occurs because a flaw or crack propagates throughout the material. The start of a fracture at low stresses is determined by the cracking tendencies at the tip of the crack. If a plastic flaw exists at the tip, the structure is not endangered because the metal mass surrounding the crack will support the stress. When brittle fracture occurs (under the conditions for brittle fracture stated above), the crack will initiate and propagate through the material at great speeds (speed of sound). It should be noted that smaller grain size, higher temperature, and lower stress tend to mitigate crack initiation. Larger grain size, lower temperatures, and higher stress tend to favor crack propagation. There is a stress level below which a crack will not propagate at any temperature. This is called the lower fracture propagation stress. As the temperature increases, a higher stress is required for a crack to propagate. The relationship between the temperature and the stress required for a crack to propagate is called the crack arrest curve, which is shown on Figure 2 as Curve D. At temperatures above that indicated on this curve, crack propagation will not occur.

Fracture Toughness

Fracture toughness is an indication of the amount of stress required to propagate a preexisting flaw. The fracture toughness of a metal depends on the following factors.

a. Metal composition

b. Metal temperature

c. Extent of deformations to the crystal structure

d. Metal grain size

e. Metal crystalline form

The intersection of the crack arrest curve with the yield curve (Curve B) is called the fracture transition elastic (FTE) point. The temperature corresponding to this point is normally about 60F above the NDT temperature. This temperature is also known as the Reference Temperature - Nil-ductility Transition (RT_NDT) and is determined in accordance with ASME Section III (1974 edition), NB 2300. The FTE is the temperature above which plastic deformation accompanies all fractures or the highest temperature at which fracture propagation can occur under purely elastic loads. The intersection of the crack arrest curve (Curve D) and the tensile strength or ultimate strength, curve (Curve A) is called the fracture transition plastic (FTP) point. The temperature corresponding with this point is normally about 120F above the NDT temperature. Above this temperature, only ductile fractures occur.

Figure 3 is a graph of stress versus temperature, showing fracture initiation curves for various flaw sizes.

Figure 3 Fracture Diagram

It is clear from the above discussion that we must operate above the NDT temperature to be certain that no brittle fracture can occur. For greater safety, it is desirable that operation be limited above the FTE temperature, or NDT + 60F. Under such conditions, no brittle fracture can occur for purely elastic loads.

As previously discussed, irradiation of the pressure vessel can raise the NDT temperature over the lifetime of the reactor pressure vessel, restricting the operating temperatures and stress on the vessel. It should be clear that this increase in NDT can lead to significant operating restrictions, especially after 25 years to 30 years of operation where the NDT can raise 200F to 300F. Thus, if the FTE was 60F at the beginning of vessel life and a change in the NDT of 300F occurred over a period of time, the reactor coolant would have to be raised to more than 360F before full system pressure could be applied.

Summarv

The important information in this chapter is summarized below.

Brittle Fracture Summary

Ductile fracture is exhibited when metals can sustain substantial plastic strain or deformation before fracturing.

Brittle fracture is exhibited when metals fracture with a relatively small or negligible amount of plastic strain.

Nil-Ductility Transition (NDT) temperature is the temperature above which a material is ductile and below which it is brittle.

Changes made to decrease NDT include:

Use of smaller grain size in metals

Small additions of selected alloying elements such as nickel and manganese to low-carbon steels

NDT decreases due to smaller grain size and increases due to irradiation

Brittle fracture requires three conditions:

Flaw such as a crack

Stress sufficient to develop a small deformation at the crack tip Temperature at or below NDT

Conditions to mitigate crack initiation:

Smaller grain size Higher temperature Lower stress levels

Factors determining fracture toughness of a metal include:

Metal composition Metal temperature Extent of deformations to the crystal structure Metal grain size

Metal crystalline form