p><p><b>Notation</b>:</p><p><img src="ContactForceT_Spring_eq15071274376249526572.png" alt="$\hat{t}$" style="width:6px;height:14px;">: Tangent direction between elements</p><p><img src="ContactForceT_Spring_eq17362946853508842186.png" alt="$\delta_{t}$" style="width:10px;height:14px;">: Tangent overlap</p><p><b>References</b>:</p><div><ul><li><a href="https://doi.org/10.1115/1.4009973">R.D. Mindlin. Compliance of elastic bodies in contact, <i>J. Appl. Mech.</i>, 16(3):259-268, 1949</a> (stiffness coefficie