Optimal estimation of gradient damage parameters from localization phenomena in quasi-brittle materials

Non-local and gradient enhanced damage models are able to properly model lo calization phenomena in quasi-brittle materials. By the introduction of an internal length scale they avoid mesh-size sensitivity in finite element ca lculations and are capable of describing size effects. To experimentally de termine the internal length scale and other damage model parameters, indire ct identification methods have to be used. The method presented is based on the Markov estimation procedure, which also permits to evaluate the accura cy and the well-posedness of the identification problem, characterized by t he uniqueness and identifiability of the estimated values. This information offers the possibility to compare and optimize the designs of laboratory t ests in order to minimize the effect of errors on the estimated values. The indirect determination of the gradient damage parameters based on global r esponse measurements far from the crack on one-size specimens is sensitive to the problem of ill-posedness due the high correlation between the gradie nt damage parameters, which dominate the localization process. The size eff ect method can be used as an accurate identification method for the gradien t damage parameters, when information on the tensile behaviour is included and a sufficient size range is considered. Local displacement analysis clos e to the crack provides sufficient information with respect to the identifi cation problem independent of the size of the specimen and gives the most a ccurate parameter estimations. Copyright (C) 1999 John Wiley & Sons, Ltd.