Question 1

What is the Ductile-Brittle Transition Temperature (DBTT)?

Accepted Answer

DBTT is the midpoint of the temperature range where the impact absorption energy of BCC metals (such as ferritic steels) drops sharply. It is typically defined as the temperature corresponding to (USE+LSE)/2 energy, or the 50% fracture appearance transition temperature (FATT). It is an important design criterion for preventing low-temperature brittle fracture in structures.

Question 2

How does irradiation embrittlement shift the DBTT?

Accepted Answer

In steels subjected to neutron irradiation, such as reactor pressure vessel steels, the DBTT shifts toward higher temperatures (positive direction). The shift ΔT depends on the neutron fluence, irradiation temperature, and chemical composition of the steel (Cu, Ni, etc.). ASME codes manage RT_NDT and the shift for pressurized thermal shock (PTS) assessment.

Question 3

What is the tanh approximation for Charpy test curves?

Accepted Answer

The CVN impact energy is expressed as E(T) = (USE+LSE)/2 + (USE-LSE)/2 × tanh((T-DBTT)/C), where USE is the upper shelf energy (ductile fracture region), LSE is the lower shelf energy (brittle fracture region), and C is the slope parameter of the transition region.

Charpy Impact Test Simulator
Ductile-Brittle Transition Temperature (DBTT)

tanh Approximation

Charpy Impact Test SimulatorDuctile-Brittle Transition Temperature (DBTT)

tanh Approximation

Charpy Impact Test Simulator
Ductile-Brittle Transition Temperature (DBTT)