Random imperfection fields to model the size effect in laboratory wood specimens Sara Casciati * , Marco Domaneschi Department of Structural Mechanics, University of Pavia, via Ferrata 1, I 27100 Pavia, Italy Available online 26 September 2006 Abstract The composite nature of a wood continuum prevents one from extrapolating the results of laboratory tests on standard wood specimens to structural elements of significant size. Therefore, these elements are usually tested under standardized loading conditions in order to detect a sort of average material behaviour. In this paper, the initial step consists, instead, of testing the material specimens. The extension of the results to struc- tural elements is then pursued by introducing a random field, or, in a discretized model, a random array of imperfections. The calibration of the suitable spatial distribution of the imperfections is then investigated by a mixed experimental– numerical approach, for a reference beam. The analyses on the relative finite elements model are iterated to match the response of the full scale laboratory tests. Ó 2006 Elsevier Ltd. All rights reserved. Keywords: Biaxial tests; Finite element model; Imperfections; Laboratory tests; Random field; Wood specimens 1. Introduction ‘‘Black locust’’ specimens of square section were carefully prepared and visually graded by Prof. Uzielli of the University of Florence [1]. The specimens were cut along the likely principal directions of an orthotropic model consistent with the fibres orientation [2]. The wood presents its fibres organized in parallel planes, so that the three reference directions will be: (i) along the fibres, (ii) orthogonal to the fibres and parallel to their planes, and (iii) orthogonal to the fibres and to their planes. The identification of the parameters of an orthotropic material was achieved by performing several static tests on different specimens in each of the above directions. This material characterization is the initial step for a further study. A beam of square section, made of the same material of the specimens, is used to undergo a classical bending test. Its finite element model is also gen- erated within the general purpose software described in Ref. [3]. Following the idea originally formulated in [4], a random array of imperfections is added to the model to make the numerical response match the experimental results. The correlation structure of such an array is also discussed. 0167-4730/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.strusafe.2006.07.014 * Corresponding author. Tel.: +39 0382 985 787. E-mail address: SaraCasciati@msn.com (S. Casciati). Structural Safety 29 (2007) 308–321 www.elsevier.com/locate/strusafe STRUCTURAL SAFETY