# Why are stress intensity units MPa√m

The units of stress intensity are usually taken as MPa√m. Cracks are usually measured in (mm), could someone please explain why this convention is used an not consistent SI units

• because the inverse root of the crack length is what matters. We measure height in meters but gravity is a function of the inverse height squared. it's just the wqay it works. Jan 3, 2023 at 14:50
• This unit is surely one of the weirdest, but that is how nature works. If you have a specimen with a crack which is under stress, the crack can enlarge, thus creating new surfaces which have higher energy than interior. The energy for that comes from stress relaxation during crack propagation. If you equate these energies, you will end up with units like this. For more information, check out Griffith's criterion. Jan 3, 2023 at 18:26
• As I remember Griffith and others calculated stress at the crack tip and the result was in units of psi X square root inches. The material property is fracture toughness. Wikipedia has a good explanation as LEFM ( linear elastic fracture mechanics.) Jan 4, 2023 at 13:49
• The other primary author is Irwin. Jan 4, 2023 at 15:59