Loading Rate Effect on the Double-K Fracture Parameters of Concrete

This work studies the effect of the loading rate on the double-K fracture parameters in concrete. These are the crack initiation toughness K!" !"! and the unstable toughness K!" !", which mark the two main stages of crack propagation and are calculated from a load-crack mouth opening curve, P-wM. However, the original methodology does not take into account the influence of the load velocity, which may not be negligible for certain testing configurations adopted to avoid an excessive test duration. The double-K method includes a third parameter, the cohesive toughness K!" ! , that relates K!" !"! and K!" !" by integrating the cohesive stresses in the fracture process zone. In this paper, the influence of the loading rate is introduced in the method through K!" ! by including in the softening law a viscous factor, function of the crack opening velocity. The idea arises from the viscous-cohesive approach implemented in a numerical model successfully used in a previous work (Engineering Fracture Mechanics 82:195-208, 2012). Rate-dependent double-K parameters are computed from P-wM curves obtained with the numerical model, analysing a wide range of specimens and crack opening rates. R-curves are also obtained. The results show that double-K parameters increase with the loading rate. The rate dependence is slightly greater for large specimens, which amplifies the size effect.

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