BRITTLE FRACTURE MECHANISM
When a small flaw is present, the tensile strength follows the dashed Curve C. At elevated
temperatures, Curves A and C are identical. At lower temperatures, approximately 50F above the NDT
temperature for material with no flaws, the tensile strength curve drops to the yield curve and then
follows the yield curve to lower temperatures. At the point where Curves C and B meet, there is a new
NDT temperature. Therefore, if a flaw exists, any failure at a temperature equal or below the NDT
temperature for flawed material will be brittle.
Crack Initiation and Propagation
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 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.
Extent of deformations to the crystal structure
Metal grain size
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 (RTNDT) 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.