<Dd> U (=) N m m </Dd> <Dd> U (=) J m </Dd> <P> Toughness can also be defined with respect to regions of a stress - strain diagram . Toughness is related to the area under the stress - strain curve . In order to be tough, a material must be both strong and ductile . For example, brittle materials (like ceramics) that are strong but with limited ductility are not tough; conversely, very ductile materials with low strengths are also not tough . To be tough, a material should withstand both high stresses and high strains . Generally speaking, strength indicates how much force the material can support, while toughness indicates how much energy a material can absorb before rupturing . </P>

Which of the following is not the unit of a stress