Uniaxial tensile test and fractal evaluation of softening damage in concrete

In a previous work (Carpinteri et al. 1998) the authors showed how to acquire the meso-structural characteristics of undamaged concrete-like materials by a peculiar laser equipment (Carpinteri et al. 1999). In order to extend the analysis to damaged disordered materials, a new direct tension test equipment has been developed, that minimizes flexural effects by freely rotating boundary conditions. Increasing levels of damage are obtained, after reaching the peak load, by proceeding along the descending strain-softening curve. After the desired damage level is reached, the load is removed and the specimen is cut to permit the laser acquisition of the most damaged zone. The progressive rarefaction of the effective stress-carrying cross section is described by means of fractal concepts. It is worth noting that both the fractal dimension and the measure of the stress carrying cross section decrease after the peak load, and vanish when the specimen is broken apart. Finally, a powerlaw relation is proposed for the fractal dimension of the effective cross section as a function of damage.