Carini, A., and G. Maier. “Extremum and Saddle-Point Theorems for Elastic Solids With Dissipative Displacement Discontinuities”. Archives of Mechanics, vol. 52, no. 4-5, Sept. 2000, pp. 523-45, http://am.ippt.gov.pl/index.php/am/article/view/v52p523.