Browsing by Author "Degrande, Geert"

Now showing 1 - 3 of 3

A micromechanics-based variational phase-field model for fracture in geomaterials with brittle-tensile and compressive-ductile behavior
(2022) Francois, Stijn
This paper presents a framework for modeling failure in quasi-brittle geomaterials under different loading conditions. A micromechanics-based model is proposed in which the field variables are linked to physical mechanisms at the microcrack level: damage is related to the growth of microcracks, while plasticity is related to the frictional sliding of closed microcracks. Consequently, the hardening/softening functions and parameters entering the free energy follow from the definition of a single degradation function and the elastic material properties. The evolution of opening microcracks in tension leads to brittle behavior and mode I fracture, while the evolution of closed microcracks under frictional sliding in compression/shear leads to ductile behavior and mode II fracture. Frictional sliding is endowed with a non-associative law, a crucial aspect of the model that considers the effect of dilation and allows for realistic material responses with non-vanishing frictional energy dissipation. Despite the non-associative law, a variationally consistent formulation is presented using notions of energy balance and stability, following the energetic formulation for rate-independent systems. The material response of the model is first described, followed by the numerical implementation procedure and several benchmark finite element simulations. The results highlight the ability of the model to describe tensile, shear, and mixed-mode fracture, as well as responses with brittle-to-ductile transition. A key result is that, by virtue of the micromechanical arguments, realistic failure modes can be captured, without resorting to the usual heuristic modifications considered in the phase-field literature. The numerical results are thoroughly discussed with reference to previous numerical studies, experimental evidence, and analytical fracture criteria.
Performance and seismic vulnerability of a typical confined masonry house in Cuenca-Ecuador
(Association of Seismology and Earthquake Engineering, ACHISINA, The International Association for Earthquake Engineering, IAEE, 2017) García Erazo, Hernán Alfredo; Degrande, Geert
The extent of damage in a city, in case of strong earthquakes, can be estimated from the results of the seismic vulnerability of its most common construction type. The aim of the present contribution is to determine the seismic vulnerability of a confined masonry building typical of Cuenca, Ecuador. Firstly, the macroscopic properties of masonry are derived from mesoscopic finite analyses of masonry piers. Secondly, the proposed structure is modeled using an equivalent frame method, in which masonry walls are defined as piers or spandrels using a macroelement model. Thirdly, a pushover analysis is performed on the structure; the results of this analysis are used to define an equivalent SDOF system. The properties of the SDOF are used to calibrate a single-macroelement model that characterizes the cyclic behavior of the MDOF model. Both models are capable to reproduce in-plane shear and flexural failure modes. Finally, nonlinear dynamic analyses are performed on the single-macroelement system for different ground motions, which are obtained from natural records (SIMBAD data base) compatible with the Ecuadorian design spectrum for several PGA levels. The maximum displacement of each analysis is compared with defined limit states, the exceedance of a limit state is recorded and then fitted to a fragility function using the maximum likelihood procedure. The proposed methodology presents an option for seismic vulnerability, to use in scenarios where little data is available.
Seismic vulnerability analysis of a two-story family dwelling in confined masonry in Cuenca, Ecuador
(2017) García Erazo, Hernán Alfredo; Degrande, Geert
This paper presents a seismic vulnerability assessment of a single-family dwelling in confined masonry, a common construction type in the city of Cuenca, Ecuador. The macroscopic properties of hollow and solid brick masonry are derived from mesoscopic finite element analyses of masonry piers. The structure is modeled using an equivalent frame method, in which masonry walls are defined as piers or spandrels using a macro-element model; reinforced columns and beams and orthotropic floor slabs are also included. Several models, considering a deterministic geometry and stochastic properties of masonry, are analyzed using a quasi-static pushover method. The results of these analyses are used to define a single equivalent macro-element that incorporates in-plane shear and flexural failure modes and is capable of reproducing the cyclic behavior of the 3D model. Non-linear dynamic time history analyses are finally performed on the single equivalent macro-element for different earthquake ground motions that are selected from the PEER Ground Motion Database and that are compatible with the Ecuadorian design spectrum for several PGA levels. The maximum response is compared with different performance levels and the fragility curves are determined using a maximum likelihood analysis. The proposed methodology can be used for seismic vulnerability analysis when few data are available, but is subject to high levels of uncertainty, so further analysis and experiments on materials and structural components are needed.