Microestructura, propagación y emisión acústica de grietas en dos papeles de celulosa

Abstract

We report the comparative study of crack propagation on two types of paper by monitoring both the crack advance and the acoustic emision signal while considering the possible effect of the structure of the materials.The papers studied were a type of Bond paper identified as ``Alrey paper'' and a chemically treated paper known as ``tracing paper''. The microstructure was characterized and it was found that in the first case it exhibited relatively long microfibers whereas in the second case, as a result of the chemical treatment during its maufacture, the microfibers were shorter and appeared arranged in a continous way, the Alrey paper exhibited more porosity and was less homogeneous. Image analysis of the tension fracture for both paper using notched specimens resulted in two scenarios related to the microstructural differencies: In the Alrey, microcraking was produced and ocassionally this resulted in crack branching whereas in the tracing paper secondary cracks were nearly absent. In both cases, the crack growth dynamics starts with a steep acceleration followed by a steady stage, finishing with a prominent acceleration. This behavior is consistent with the Fiber Bundle Model, though a behavior with more dispersion was observed for the Alrey paper, this can be associated to the effect of the microstructure, since in the tracing paper the structure is more continuous and less random, thus preventing significant secondary cracks. The statistical analysis of waiting times between the acoustic emission events, $wt_{EA}$, and the energy of these events, $s_{EA}$, generated by the deformation and fracture of both papers corroborated that the behaviors correspond to power laws: Omori-type for the waiting times, with $\alpha = 1.46 \pm 0.05$ and $\alpha = 1.27 \pm 0.05$ for the Alrey and tracing papers, respectively and Gutenberg-Richter-type for the energy of the AE events, with $\beta = 1.8 \pm 0.2$ and $\beta = 2 \pm 0.2$, for the Alrey and tracing papers, respectively. The statistically different $ \alpha $ exponents for the Omori law appear to reflect the effect of the different fracture mechanism as a result of the microstructural differences on the distributions of waiting times. On the other hand, the effect of the microstructure on the $ \beta $ exponents in the Gutenberg-Richter law is not clear.