Define the ductile-to-brittle transition temperature (DBTT) concept.

• A. The temperature above which a material exhibits brittle failure; toughness increases with higher temperature
• B. The temperature below which a material exhibits brittle failure; toughness decreases with lower temperature
• C. The temperature at which a material becomes completely liquid
• D. The temperature where the material's melting point changes

Answer: (B)

The central idea here is how fracture behavior changes with temperature. The ductile-to-brittle transition temperature is the temperature below which a material tends to fail in a brittle manner rather than a ductile one; as temperature increases, the material generally becomes more ductile and can absorb more energy before fracturing.

This aligns with the statement: the temperature below which the material exhibits brittle failure, with toughness decreasing as temperature drops. It captures both the switch in fracture mode and the accompanying drop in energy-absorbing toughness when cooled. The other options describe melting or brittle behavior at the wrong temperature direction, which are not what DBTT refers to.