<Li> Toughness: toughness is the amount of energy that a material can absorb before fracture . </Li> <P> The technical definition is: the ratio of the stress (force per unit area) along an axis to the strain (ratio of deformation over initial length) along that axis in the range of stress in which Hooke's law holds . </P> <P> Young's modulus is the ratio of stress (which has units of pressure) to strain (which is dimensionless), and so Young's modulus has units of pressure . Its SI unit is therefore the pascal (Pa or N / m or kg m s). The practical units used are megapascals (MPa or N / mm) or gigapascals (GPa or kN / mm). </P> <P> In United States customary units, it is expressed as pounds (force) per square inch (psi). The abbreviation ksi refers to kips per square inch . A "kip" is an Imperial unit of force and is equal to 1000 pounds - force, making a ksi equal to 1000 psi . </P>

What are the units of stress strain and young’s modulus