Why is a strong ductile metal usually tougher than a hard brittle material?

In the context of materials testing, a strong, ductile metal is generally tougher than a hard, brittle material because of its ability to accommodate stress through plastic deformation.12

Toughness is defined as the amount of energy a material can absorb before it fractures. While a hard, brittle material lacks the capacity to deform permanently, it cannot effectively manage local stress concentrations, such as those found at a notch or crack. Consequently, these materials tend to fracture easily and suddenly when overloaded.231

In contrast, a ductile metal can undergo significant permanent (plastic) deformation after reaching its yield point. This ability to "neck" or deform plastically allows the material to absorb substantial energy and redistribute stress, which prevents the rapid propagation of cracks. By permitting a component to deform rather than crack, ductility serves as a critical factor in ensuring structural integrity and preventing sudden, catastrophic failure.243