Materials Science Seminar - SCALING PROPERTIES OF FRACTURE SURFACES by Elisabeth Bouchaud

SCALING PROPERTIES OF FRACTURE SURFACESElisabeth Bouchaud,
Fracture Group, Division of Physics & Chemistry of Surfaces and Interfaces
CEA-Saclay, FranceFor materials as different as metallic alloys and silicate glasses, the morphology of fracture surfaces has revealed anisotropic scale invariance properties which can be described with two sets of parameters: roughness exponents and characteristic length scales, measured either along the direction of crack propagation, or perpendicularly to it. If characteristic length scales depend on the material, its microstructure, and the external loading, roughness exponents, on the contrary, are "universal". The same roughness exponents are indeed observed for metallic alloys and for glasses, for example, albeit at length scales three orders of magnitude smaller in the latter case.Statistical models describing the quasi-static propagation of an elastic line (the crack front) through an array of randomly distributed obstacles (the microstructure) can reproduce qualitatively these observations. We conjecture that the lack of quantitative agreement is due to the fact that these models do not take damage into account.Damage formation at the crack tip is however a general phenomenon, which is well documented for metallic materials. More recently, our Atomic Force Microscopy experiments on silicate glasses show that damage also forms ahead of a stress corrosion crack tip in glass. This mechanism has several implications at the macro-scale, in terms of nonlinear elastic deformation and sample lifetime.The extension of the process zone is estimated, and it is shown that fracture surface roughness is evaluated within this region. It is argued that the observed exponents reflect in fact damage screening occurring at length scales smaller than the process zone size. "Line models", however, lead to good predictions when measurements are performed at length scales larger than the process zone size.Dr. Elisabeth Bouchaud is the Director of the Division of Physics and Chemistry of Surfaces and Interfaces at CEA-Saclay, France and currently a visiting professor at Caltech. Dr. Bouchaud has started a whole new research area involving the study of the scaling properties of fracture surfaces. Her group has performed seminal experiments on crack propagation and damage evolution in glasses. For her pioneering work on fracture, Dr. Bouchaud recently received the prestigious Ancel award for Condensed Matter Physics from the French Physical Society, and in 1997, she was awarded the European Materials Science Society's Lecturer Award.First Year MASC students are required to attend.

Location: Vivian Hall of Engineering (VHE) - 217

Audiences: Everyone Is Invited

Contact: Petra Pearce