astoPlasticLinear_eq01053953801552351832.png" alt="$\hat{n}$" style="width:9px;height:11px;">: Normal direction between elements</p><p><img src="ContactForceN_ElastoPlasticLinear_eq09065624731070659725.png" alt="$\delta_{n}$" style="width:13px;height:14px;">: Normal overlap</p><p><img src="ContactForceN_ElastoPlasticLinear_eq05345183001595785157.png" alt="$\dot{\delta_{n}}$" style="width:13px;height:17px;">: Time rate of change of normal overlap</p><p><img src="ContactForceN_ElastoPlasticLinear_eq0410163221