Fracture Toughness:

Fracture toughness is an indication of the amount of stress needed to propagate a preexisting flaw.  A fracture toughness of a metal depends on the subsequent 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 along with the yield curve (Curve B) is known as the FTE (fracture transition elastic) point. The temperature corresponding to this point is generally about 60º F above the NDT temperature. That  temperature is also known as  the RTNDT  (Reference Temperature  - Nil-ductility  Transition) and is determined  in accordance  along with ASME Section III (1974  edition), NB  2300. An FTE is the temperature above that plastic deformation accompanies all fractures or the highest temperature at that fracture propagation could occur under purely elastic loads. The division of the crack arrest curve (Curve D) and the tensile strength or ultimate strength, curve (Curve A) is known as the FTP (fracture transition plastic) point. The temperature corresponding along with this point is normally about 120ºF above the NDT temperature. Above this temperature, just ductile fractures occur.