Two parameters are extrapolated: pre-failure index (Ipf) and post-failure index (Ipstf). Ipf gives informations about the brittleness of material and can be
Figure 7. Apf and Apstf compute.
evaluated as the ratio between the subtended area Apf under the rising branch until ultimate load and the total area Atot (Fig. 7):
to increase the value of such index the brittleness will increase. Such characteristic corresponds to the aptitude of the material to arrive to failure in a sudden way. Ipstf represents, on the contrary, the tendency to have a ductile behaviour, and it's expressed by the ratio between the area Apstf subtended under post-failure curve and the total area Atot (Fig. 4):
to increase Ipstf the ductility will increase, that is the capacity to distribute the strains, to happened failure, deforming plastically.
However just referring to these ratio it's not possible to consider the real value of failure load, since materials with different strength could have the same indexes. In order to consider the rapidity by the material loses the capacity to resist to further strains once the collapse, it's possible considering the slope of the unloading branch of load-displacement diagram: to increase the slope the brittleness will increase, of against, a slope less accentuated will be index of ductility. It's possible to express brittleness index Ib as:
where Ccs is the angular coefficient of the descending branch which interpolate the values of the post-failure curve when there is a precipitous loss of performance. By means of index Ib has been possible evaluating the brittle or ductile mortars behaviour during their maturation (tables 5-6).
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