Fracture propagation involves the coupling of many length scales ranging from the sample loading geometry to the molecular level. In brittle materials, the length scales of the damage process zone are reduced to a submicrometric scale and the coupling with the macroscopic scale is expected to be the domain of linear elastic fracture mechanics (LEFM). However, although 2D elastic analyses are generally adequate to describe the sample deformation at macroscopic scales, a micromechanical analysis requires the use of 3D mechanical tools due to the crack front local curvature and to the corner point singularities at the intersection between the crack front and the external surfaces of the sample.
In this lecture we will present a thorough investigation of the slow crack growth of a sharp crack in oxide glasses in the stress-corrosion regime, combining numerical and experimental analyses from the millimetre scale to the nanoscale range. The principal aim of the study is identifying the length and time scales of the mechanisms of damage and interaction between water and glass, which have been the subject of an extensive debate in last decades.
Subcritical crack propagation was performed on Double Cleavage Drilled Compression samples under controlled atmosphere. Post-mortem and in-situ observations were performed by optical techniques and atomic force microscopy (AFM). A 2D/3D LEFM analysis of this sample was realized to ensure the proper mechanical coupling of all length scales.
The mechanical effect of capillary condensation observed by AFM at the crack tip was modeled according to a cohesive zone model. This allowed notably to evaluate the negative Laplace pressure in the liquid and to explain the crack closure mechanism in glass. The analysis of AFM in-situ images of crack propagation by an integrated digital image correlation (DIC) technique reveals the adequacy of the proper elastic solutions to describe the surface displacement field down to a distance of 10 nm from the crack tip. A critical analysis of the height correlation functions performed on AFM images of the fracture surfaces does not reveal any process zone larger than 10 nm in agreement with the conclusions of DIC. In agreement with complementary recent observations in the literature, the length scales of damage in the stress-corrosion fracture of glass are confined to a range of few nanometres from the crack surface.
Matteo Ciccotti is Professor of Mechanics and Physics of Materials at École Supérieure de Physique et Chimie Industrielles de la Ville de Paris (ESPCI Paristech, France) since 2010. He graduated in Applied Physics at Università di Bologna (Italy) in 1996, where he also got his PhD in 2000 studying the mechanics of rocks and its relation to earthquake dynamics. He then worked as a CNRS researcher at Université de Montpellier 2 (France) on the nanomechanics of slow crack propagation in oxide glasses. The present research projects at the Laboratory of Soft Matter Science and Engineering at ESPCI Paristech concern the space and time scales of the dissipation mechanisms in the fracture mechanics of polymers and composite materials. Three main actual subjects are 1) the adherence energy of soft polymer adhesives; 2) the fracture energy of glassy polymers confined into glass fiber composites; 3) the dissipation mechanisms during the impact failure of a laminated windshield.