Issue 33

A.Spagnoli et alii, Frattura ed Integrità Strutturale, 33 (2015) 80-88; DOI: 10.3221/IGF-ESIS.33.11 80 Focussed on characterization of crack tip fields Interpreting experimental fracture toughness results of quasi-brittle natural materials through multi-parameter approaches Andrea Spagnoli, Andrea Carpinteri, Sabrina Vantadori Dipartimento di Ingegneria Civile, dell’Ambiente del Territorio e Architettura, Università di Parma, Italy spagnoli@unipr.it A BSTRACT . Natural stones like marbles are often employed as façade panels to externally cover buildings. These natural materials tend to exhibit a quasi-brittle nonlinear fracture behaviour which, conversely to concrete counterpart, has much less been studied in the literature. In the present paper, an experimental campaign on the so-called red Verona marble is carried out, and the results are discussed together with some previously published results on the white Carrara marble. The analysis of the two marbles as a whole allows us to discuss size effect and to point out the need for additional parameters in order to describe their fracture behaviour. The study focuses on a two-parameter model which accounts for a characteristic material length. A quantitative correlation between material microstructure features, obtained from thin sections technique, and the characteristic material length is proposed. K EYWORDS . Fracture process zone; Marble; Quasi-brittle behaviour; Size effect; Two-parameter model. I NTRODUCTION uasi-brittle materials are typically characterized by a stable nonlinear load-deflection response followed by an unstable fracture. Such a behaviour is generally attributed to the development of a finite-size process zone ahead of the crack tip. The quasi-brittle behaviour has largely been examined for concrete both experimentally and theoretically [1]. When fracture toughness in quasi-brittle materials has to be determined, an effective crack length (spanning a traction-free segment and a stressed segment embedded in the process zone) is needed to be defined, with respect to which the critical stress intensity factor has to be computed. Accordingly, in the realm of concrete material testing, Shah and co-workers [2] proposed a two-parameter method to calculate the effective crack length in the verge of unstable cracking. Other theories are available in the literature to describe experimental size-dependent fracture data. These theories (e.g. theory of critical distances [3,4], finite fracture mechanics [5], cohesive crack models [6], size effect laws [7-10], non-local stress failure criteria [11], and crack extension resistance curves [12]) are characterized by a fil rouge, that is to say, the near-crack tip stress field and/or the fracture toughness is described by means of multi-parameter approaches. A size-dependence discussion of the RILEM work-of-fracture method can be found in Ref. [13]. Natural materials, like calcite marbles used for building claddings [14, 15], often exhibit a quasi-brittle mechanical behaviour [16, 17] which, conversely to concrete counterparts, has much less been investigated despite the fact that, under different deteriorating in-service actions (temperature cycles, wind and seismic loads, chemical actions, humidity changes, etc.), fracture behaviour might significantly affect structural failure. Q