Based on the variational principle of potential energy

displacement jump condition which depends only on material strength is derived. Moreover

the connection of displacement jump and uniqueness of governing equations is built; and equivalent scopes are verified. Uniqueness of governing equations uses constitutive/acoustic tensor to deal with material stable problem caused by stiffness degeneration

which could be equal to displacement jump condition; but it is less straightforward than the displacement jump condition described by material strength

especially

those materials whose strength can be tested easily at lab.