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The finite element modelling of discrete fracture in quasi-brittle materials

An effective methodology for discrete fracture in quasi-brittle material is presented within an explicit finite discrete element framework. Simple pragmatic models are envisaged that reflect the data deficiency of the quasi-brittle material and recover the observed physical response within engineering accuracy. Phenomenological strain-softening constitutive models are adopted for the modelling of micromechanical processes in an average sense. An extensional basis for fracture is assumed in both tensile and compressive stress fields, with only the mechanism with which inelastic strain is realised differing between the two stress states. To overcome the mesh dependence introduced by local softening constitutive relationships, the socalled localisation limiters are adopted in the form of the tensile crack band, nonlocal and viscous smeared crack models. Effective localisation lengthscales introduced by these regularisation methods ensure mesh objective failure localisation a priori to discrete crack insertion. A nonlocal map of failure indicators initiates fracture, with discrete cracks inserted into the finite element continuum by the splitting of the discretisation. An isotropic, non-associative Mohr-Coulomb model is derived in principal stress space as a first order approximation to the quasi-brittle response in compression. A model for discrete fracture in tensile and compressive stress fields is proposed, defined by a composite yield surface consisting of the fully anisotropic rotating crack band model coupled with the isotropic, non-associative Mohr-Coulomb model. The novel inclusion of an explicit coupling between the extensional inelastic dilation strain accrued during compressive failure and tensile strength degradation in the dilation directions permits the realisation of discrete fracturing in purely compressive stress fields. The so-called continuum-discrete transition introduces additional degrees of freedom into quasi-brittle systems and permits large deformation to be realised through the process of cataclastic flow. This advancement is considered significant and necessary in the recovery of the observed quasi-brittle response. The effectiveness of the proposed constitutive fracture models is verified by application to a number of physical quasi-brittle fracture systems, including borehole breakout, fracturing around excavations, strip punch tests, dynamic spalling and anchor pullout tests, amongst others.
Date January 2000
CreatorsKlerck, Paul Alexander
PublisherSwansea University
Source SetsEthos UK
Detected LanguageEnglish
TypeElectronic Thesis or Dissertation

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