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Experimental and numerical modeling-based characterization of fracture process zone in quasi-brittle materials
Garg, Prasoon
Garg, Prasoon
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2023
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2024-10-18
Abstract
Fracturing in quasi-brittle materials such as rocks involves the development of a significant size of the inelastic zone around the pre-crack tip, also known as the fracture process zone (FPZ). A comprehensive characterization of the FPZ is crucial for predicting the associated macroscopic cracking process, as it plays a vital role in the estimation of various crack properties, such as fracture toughness and tensile strength of the material. This thesis focused on investigating FPZ development in quasi-brittle material under different loading conditions using a combination of experimental testing and numerical modeling.
The work conducted in this thesis begins with developing a novel methodology for characterizing the FPZ evolution in pre-cracked Barre granite specimens under mode I loading. This novel methodology was based on the Digital image correlation (DIC) approach and facilitated the identification of a suitable constitutive model for material softening inside the FPZ. This is followed by an investigation of FPZ development under mode II loading using DIC in two different testing geometries. The analysis of these two geometries showed that novel methodology is reliable for characterizing the FPZ evolution in various loading conditions, and most mode II geometries don’t result in a pure mode II fracture until a high level of confinement is applied to the specimen. This thesis identifies the short beam in compression (SBC) as a better alternative for estimating mode II fracture toughness than existing punch through shear testing configuration. The main advantage of the SBC geometry lies in the fact that it can be monitored using optical techniques such as DIC, which provide explicit evidence of fracture mode along with detailed information of about deformation characteristics of crack geometry.
The XFEM-based numerical models were used in this work to simulate the FPZ evolution in quasi-brittle materials due to their ability to model the multiple crack propagation without the need for remeshing. In this research, a novel XFEM-based user element was developed with the capability to model FPZ development in various crack types, especially in compression-induced shear cracks. This was done by implementing a new cohesive zone model that accounts for both cohesion degradation and frictional sliding in propagating shear cracks. The XFEM-based user element, through the implementation of advanced branching algorithms, is able to simulate multiple crack types from the body of pre-existing cracks, thereby removing the limitation of traditional XFEM models. As a result, the XFEM-based user element can reliably predict paths of secondary cracks along with crack coalescence mechanisms in pre-cracked rock-like specimens under compression loading.
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