Minimum theorems in 3D incremental linear elastic fracture mechanics

The crack propagation problem for linear elastic fracture mechanics has been studied by several authors exploiting its analogy with standard dissipative systems theory (see e.g. Nguyen in Appl Mech Rev 47, 1994, Stability and nonlinear solid mechanics. Wiley, New York, 2000; Mielke in Handbook of differential equations, evolutionary equations. Elsevier, Amsterdam, 2005; Bourdin et al. in The variational approach to fracture. Springer, Berlin, 2008). In a recent publication (Salvadori and Carini in Int J Solids Struct 48:1362-1369, 2011) minimum theorems were derived in terms of crack tip "quasi static velocity" for two-dimensional fracture mechanics. They were reminiscent of Ceradini's theorem (Ceradini in Rendiconti Istituto Lombardo di Scienze e Lettere A99, 1965, Meccanica 1:77-82, 1966) in plasticity. Following the cornerstone work of Rice (1989) on weight function theories, Leblond et al. (Leblond in Int J Solids Struct 36:79-103, 1999; Leblond et al. in Int J Solids Struct 36:105-142, 1999) proposed asymptotic expansions for stress intensity factors in three dimensions-see also Lazarus (J Mech Phys Solids 59:121-144, 2011). As formerly in 2D, expansions can be given a Colonnetti's decomposition (Colonnetti in Rend Accad Lincei 5, 1918, Quart Appl Math 7:353-362, 1950) interpretation. In view of the expression of the expansions proposed in Leblond (Int J Solids Struct 36:79-103, 1999), Leblond et al. (Int J Solids Struct 36:105-142, 1999) however, symmetry of Ceradini's theorem operators was not evident and the extension of outcomes proposed in Salvadori and Carini (Int J Solids Struct 48:1362-1369, 2011) not straightforward. Following a different path of reasoning, minimum theorems have been finally derived.