Stress intensity factors are computed from J-Integral and Virtual Crack Extension. For a linear elastic material, there is a relation between the J-integral (or energy release rate) and the stress intensity factors. This relation is, however, only unique when the stress intensity factors are known, and not when the value the J-integral is known:

The split between KI, KII, and KIII is estimated using the displacement field on the crack faces at some distance behind the crack tip.

In Equation 2-30, Eeff is an effective Young’s modulus, defined as

At a free boundary, a state of plane stress prevails. Close to the boundary, there will be a transition zone where neither plane stress nor plane strain is applicable. In this region, the stress intensity factors, as computed from the J-integral, may be uncertain. It should be noted, however, that for most metals, the variation in Eeff between plane strain and plane stress is about 10%, leading to an error in the computed stress intensity factor in the transition region that is no more than 5%.

The SI unit for stress intensity factors is N/m3/2. Other common units are MPa·m1/2, psi·in1/2, and ksi·in1/2. The unit handling system in COMSOL Multiphysics only operates with integer powers of the base units, and can thus not handle stress intensity factors in a consistent manner.

Because of this, the expressions that define the stress intensity factors include a removal of the units, and the stress intensity factors appear as a unitless quantities. The value of a stress intensity factor should be interpreted as having an implicit unit, which is composed from the base units in the current unit system, for example, N/m3/2.