In many fields there are excellent choices for publishing open access (even without costing all that much). For computational mechanics that doesn't seem to be the case.
Looking through DOAJ I could mostly only find rather small and obscure journals. What I would like is at least something that gets listed on ScienceDirect (and other common places such as Google Scholar and such). I would write more requirements, but I dont think I am in a position to be picky here. A decent homepage, ability to comment, automatic statistics, online versions, creative commons licensing, decent ranking, etc. would also be on my requirements if I could wish.
From what I could find, these seemed most promising to what I had in mind (some are a bit too mathematically oriented, but perhaps some articles could fit the subject);
Advances in Mechanical Engineering http://www.hindawi.com/journals/ame/
Advances in Mathematical Physics http://www.hindawi.com/journals/amp/
Boundary value problems http://www.boundaryvalueproblems.com/
International Journal of Differential Equations http://www.hindawi.com/journals/ijde/
Feel free to suggest related themes as well (mathematics, pde, plasticity, continuum mechanics).