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Tesi etd-11242012-201516

Thesis type
Tesi di dottorato di ricerca
Crack propagation in brittle materials
Settore scientifico disciplinare
Corso di studi
tutor Ing. Royer-Carfagni, Gianni
Parole chiave
  • porous crack plasticity
  • ESPI
  • crack
  • cohesive crack
  • brittle materials
  • process zone
Data inizio appello
Riassunto analitico
The aim of this research has been the study of the mechanisms that control the crack<br>propagation in brittle materials, in a region close to the crack tip. Two different kind<br>of materials have been considered: glass float and glass ceramic. The basic idea to<br>develop this investigation has been the accurate measurement of the Crack Opening<br>Displacement (COD) during the crack propagation. In fact, the shape of the tip profile<br>is a consequence of the phenomena which rule the stress distribution at the crack tip<br>and, the consequent macroscopic behaviour of the material.<br>To allow a precise measurement of the COD, an interferometric technique called<br>Electronic Speckle Pattern Interferometry (ESPI) has been adopted. With respect<br>to other optical techniques, ESPI system works on the phase changes between two<br>distinct laser beams deriving by the same laser source and reflected by the specimen<br>surface, providing a precision higher than the wavelength of the laser source. To<br>avoid an instantaneous failure due to the brittle nature of the investigated materials,<br>every specimen has been naturally pre-cracked and then tested under strain-driven<br>three point bending, pausing opportunely the loading to stabilize the crack evolution.<br>The ESPI results have been compared with the classical solutions of the Linear Elastic<br>Fracture Mechanics (LEFM). Since in this solution the COD directly depends by<br>the square root of the distance from the crack tip, it’s particularly instructive to report<br>the same graphs in bi-logarithmic scales, obtaining a linear trend for LEFM, with<br>slope 1/2 . This procedure permitted to note a considerable difference in the crack<br>behaviour of the materials investigated. In glass, all the crack profiles presented a<br>slope lower than 1/2 at the crack tip, and consequently a “wider” COD with respect to<br>the LEFM profile. In glass ceramic instead, the profiles showed a higher slope than 1/2 at the tip, and so a “tighter” COD at the tip, similar to a cusp.<br>By these observations, an accurate FEM model has been studied to reproduce the<br>experimental results. In particular, two different models have been considered. The<br>first one was the cohesive model `a la Barenblatt, where the presence of cohesive<br>forces at the neighbourhood of the crack tip provides the typical cusp profile. The<br>second one was a local damage model, where different phenomena are able to develop<br>a degradation of the material in a process zone, providing a wider COD profile<br>at the tip with respect to LEFM trend.<br>Comparing the experimental results with the numerical profiles obtained by a specific<br>calibration of the fem models, we found the following conclusions. In glass,<br>nucleation and coalescence of microvoids seem to be the main cause which determines<br>the COD profile observed. This is well described through a model of porous<br>plasticity `a la Gurson-Tvergaard. In glass ceramic instead, the ESPI COD was reproduced<br>only assuming a distribution of cohesive forces far behind the crack tip.<br>These actions would derive by crack bridging phenomena due to the intergranular<br>microstructure of glass ceramic.