New way of evaluating the fracture toughness of materials

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Civil and Environmental Engineering, 2011.Cataloged from PDF version of thesis.Includes bibliographical references (p. 126-130).This thesis develops, validates and implements a fracture mechanics model for the assessment of the fracture toughness of materials from scratch tests. Dimensional Analysis highlights two major processes at work during scratch tests: plastic yielding and fracture dissipation. An original set-up of controlled laboratory tests on paraffin wax allows us to identify fracture processes as predominant. An analytical model for scratch tests with a rectangular blade and a back-rake angle is then developed. This model applies to linear elastic isotropic brittle materials and links the fracture toughness to the average horizontal and vertical forces recorded in the scratch test, and to the width and depth of the scratch. Finite Element simulation show that the model is highly accurate for back-rake angles smaller than 25'. From the model, an inverse technique to predict the fracture toughness is developed and implemented. This technique is validated for scratch tests on cement paste, Jurassic limestone, red sandstone and Vosges sandstone. and applied to oil cements hydrated at high temperature and pressure. The application shows that the scratch tests is highly reproducible. almost non-destructive, and not more sophisticated than classical strength-of-materials tests; which makes this *old' technique highly attractive for both materials research and industrial applications.by Ange-Therese Akono.S.M

Assessment of fracture properties and rate effects on fracture of materials by micro scratching: application to gas shale

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Civil and Environmental Engineering, 2013.Cataloged from PDF version of thesis.Includes bibliographical references (pages 196-208).Since 1921, several experimental methods have been implemented to measure the Griffith fracture energy. The challenge lies in providing a measure that is intrinsic and invariant with respect to external factors such as specimen geometry, loading conditions and prescribed rates. In this thesis, by combining multi-scale experiments and advanced theoretical modeling, we provide a means to characterize the intrinsic fracture toughness using microscopic scratch tests. The scratch test consists in plowing and cutting with a scratch device the surface of a weaker material and it is relevant in many fields of science and engineering, ranging from thin films and coatings, to wear of metals and polymers, and strength of rocks. In this thesis, Dimensional Analysis and Advanced Imaging are employed to demonstrate the predominance of fracture processes in scratch tests with a Rockwell C diamond probe. Based on experimental observations, Linear Fracture Mechanics models are developed that utilize an energy-based approach in order to link the scratch forces to the scratch probe geometry and the fracture properties of the scratched material. The analytical models are implemented into inverse experimental methods for the calibration of the scratch probe geometry and for the determination of the fracture toughness. In particular, the method for fracture toughness determination is shown to be precise, accurate and reproducible. This method is then extended to rate-dependent materials in order to decouple creep and fracture and assess the intrinsic fracture toughness. In particular, for homogeneous materials, a handshake is achieved between macroscopic and microscopic scratch tests. Finally this method is applied to gas shale materials, which exhibit a higher degree of complexity, including heterogeneity, anisotropy and rate-dependence. In particular, a strong directionality of the fracture behavior is observed at the microscopic scale, which is also confirmed at the macroscopic scale. Thus, throughout this work, we elucidate the physical mechanisms of failure underlying scratch tests and build a method for the multi-scale assessment of intrinsic fracture properties, which is robust, accurate, precise and reproducible, and which is applicable to a wide range of material behaviors. This in turn opens additional venues of application for scratch tests.by Ange-Therese Akono.Ph. D

Rate-dependent toughness in soft materials via microscopic scratch testing

Although fracture processes are rate-sensitive in a wide variety of biological and engineering systems, such as protein materials and bulk metallic glasses, the exact contribution of the prescribed mechanical loading rate to the measured fracture resistance is not fully understood. In this study, we formulate a novel energy-based framework for crack propagation in nonlinear viscous solids, using microscopic scratch tests. The scratch test consists in pushing a tool across the surface of a weaker material at a given penetration depth, and is relevant to several fields of science and engineering ranging from quality control of thin films and coatings to fracture characterization of cementitious materials. A hybrid experimental and theoretical study on amorphous and semicrystalline polymers shows that the apparent fracture toughness increases with the prescribed scratching speed up to an asymptotic value that is independent on the prescribed loading rate. Nonlinear viscoelastic fracture mechanics reveals that, because of the bulk viscous dissipation, the crack propagation processes can inhibited or delayed, resulting in a coupling between the intrinsic fracture energy and the material viscoelastic properties. Moreover, by combining indentation and scratch tests to decouple creep and fracture, it becomes possible to represent with a single master curve the evolution of the apparent fracture toughness for three loading rates, 0, 45, and 90 N/min, and for scratching speeds ranging from 0 to 20 mm/min. Overall, by considering a dual dissipation mode, viscous and fracture dissipation, we can capture the scaling of the scratch forces over a wide range of loading rates and scratching speeds in order to assess the intrinsic rate-independent and geometry independent fracture toughness of the tested material. Given the scalability of scratch tests, this new development open new venues for the characterization of the fracture toughness of soft materials at the microscopic scale