When a specimen is pulled in a tensile test, the machine records load P against elongation δ. The area under a load–deformation curve is energy (force × distance), so the area under the P–δ diagram represents the total mechanical energy the specimen absorbs.

If that area is taken all the way to the point of fracture, it represents the total energy the material can absorb before it breaks — both the recoverable elastic energy and the much larger energy dissipated in plastic deformation. This total energy-absorbing capacity is called toughness. A ductile material, which yields and stretches considerably before rupturing, encloses a large area and is therefore tough.

It helps to separate the related terms carefully:

The distinction between the two "toughness" answers is subtle but decisive. Modulus of toughness is defined as energy per unit volume and is the area under the stress–strain (σ–ε) diagram, not the load–elongation diagram. Resilience and modulus of resilience refer only to the elastic portion and therefore represent a much smaller area that stops at yield, so neither describes energy up to fracture. Since the question specifically names the load versus elongation curve taken up to fracture, the total area corresponds to toughness.