Saeid Ghouli

Last Name

Ghouli

First Name

Saeid

ORCID

0000-0002-0086-5560

Organisational unit

09697 - De Lorenzis, Laura / De Lorenzis, Laura

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Publications1 - 9 of 9

Crack tip fields in anisotropic planes: a review
Ayatollahi, Majid R.; Nejati, Morteza; Ghouli, Saeid (2022)
International Journal of Fracture
This paper reviews crack tip asymptotic fields in generic plane anisotropic media, rigorously categorising about 400 studies devoted to the determination of the asymptote coefficients, also referred to as crack tip parameters. These studies cover a broad spectrum of methodologies focused on determining crack tip parameters in anisotropic materials, from analytical, numerical and experimental perspectives, conducted over the last six decades. Brief descriptions are given for a selected number of approaches to shed light on their general concepts and their methodology to characterise crack tip parameters. Lastly, the influence of higher order coefficients in the crack tip stress field is demonstrated by a representative example from the literature.
The finite element over-deterministic method to calculate the coefficients of crack tip asymptotic fields in anisotropic planes
Ayatollahi, Majid R.; Nejati, Morteza; Ghouli, Saeid (2020)
Engineering Fracture Mechanics
Crack tip asymptotic field and K-dominant region for anisotropic semi-circular bend specimen
Nejati, Morteza; Ghouli, Saeid; Ayatollahi, Majid R. (2020)
Theoretical and Applied Fracture Mechanics
This study reports the coefficients of the crack tip asymptotic field for the semi-circular bend (SCB) specimen made of anisotropic rocks when subjected to pure mode loading. The finite element over-deterministic method is employed to determine the singular and higher order terms of the crack tip asymptotic field for a wide range of geometry and anisotropy parameters associated with the Mode I SCB test. These parameters are helpful when analysing the fracture path and failure mechanism of anisotropic rocks in cases where the process zone is large compared to the crack length. We also apply an energy-based criterion on the crack tip fields to evaluate the size of the K-dominant region as a function of geometry configuration and anisotropy. It is concluded that the ISRM-suggested size requirement for the SCB samples of rock can yield underestimated values of fracture toughness due to the constraints applied to the FPZ development.
On the validation of mixed-mode I/II crack growth theories for anisotropic rocks
Sakha, Mahsa; Nejati, Morteza; Aminzadeh, Ali; et al. (2022)
International Journal of Solids and Structures
We evaluate the accuracy of three well-known fracture growth theories to predict crack trajectories in anisotropic rocks through comparison with new experimental data. The results of 99 fracture toughness tests on the metamorphic Grimsel Granite under four different ratios of mixed-mode I/II loadings are reported. For each ratio, the influence of the anisotropy orientation on the direction of fracture growth is also analyzed. Our results show that for certain loading configurations, the anisotropy offsets the loading influence in determining the direction of crack growth, whereas in other configurations, these factors reinforce each other. To evaluate the accuracy of the fracture growth theories, we compare the experiment results for the kink angle and the effective fracture toughness with the predictions of the maximum tangential stress (MTS), the maximum energy release rate (MERR), and the maximum strain energy density (MSED) criteria. The criteria are first assessed in their classical forms employed in the literature. It is demonstrated that the energy-based criteria in their classical formulation cannot yield good predictions. We then present modified forms of the ERR and SED functions in which the tensile and shear components are decomposed. These modified forms give significantly better predictions of fracture growth paths. The evaluation of the three criteria illustrates that the modified MSED criterion is the least accurate model even in the modified form, while the results predicted by MTS and modified MERR are well matched with the experimental results.
Crack tip asymptotic fields in anisotropic planes: Importance of higher order terms
Nejati, Morteza; Ghouli, Saeid; Ayatollahi, Majid R. (2021)
Applied Mathematical Modelling
© 2020 Elsevier Inc. This paper introduces the series expansion of the crack tip asymptotic fields in an anisotropic plane. Using stress-based definitions of the fracture loading modes, we present the series solutions of stresses, strains and displacements associated with the modes I and II in a cracked anisotropic plane. Special care is given to the first non-singular terms of the stress field, deriving expressions of the T-stress and the crack rigid body rotation. We then present the elasticity solution of an infinite anisotropic plane containing an inclined central crack subjected to uniaxial or biaxial loading. Under such conditions, exact analytical solutions are obtained for all coefficients of the crack tip series expansion. Our results show that the T-term is the only stress parameter that depends on the elastic constants of the anisotropic material. Finally, we evaluate the dependency of the T-stress on the material properties, and show that how significantly the higher order terms, particularly the T-stress, can contribute to the stresses near the crack tip.
Introduction of a Scaling Factor for Fracture Toughness Measurement of Rocks Using the Semi-circular Bend Test
Ghouli, Saeid; Bahrami, Bahador; Ayatollahi, Majid R.; et al. (2021)
Rock Mechanics and Rock Engineering
This article discusses the scale dependence of the mode I fracture toughness of rocks measured via the semi-circular bend (SCB) test. An extensive set of experiments is conducted to scrutinise the fracture toughness variations with size for three distinct rock types with radii ranging from 25 to 300 mm. The lengths of the fracture process zone (FPZ) for different sample sizes are measured using the digital image correlation (DIC) technique. A theoretical model is also established that relates the value of fracture toughness to the sample size. This theorem is based on the strip-yield model to estimate the length of FPZ, and the energy release rate concept to relate the FPZ length to the fracture toughness. This theoretical model does not rely on any experimental-based curve-fitting parameter, but only on the tensile strength of the rock type as well as the fracture toughness at a specific sample size. The size effects predicted by the theoretical model is in a good agreement with the experimental data on both fracture toughness and the FPZ length. Finally, theoretical correction factors are introduced for various geometrical configurations of the SCB specimen, using which a scale-independent mode I fracture toughness of the rock material can be estimated from the results of experiments performed on small samples.
A topology optimisation framework to design test specimens for one-shot identification or discovery of material models
Ghouli, Saeid; Flaschel, Moritz; Kumar, Siddhant; et al. (2025)
Journal of the Mechanics and Physics of Solids
The increasing availability of full-field displacement data from imaging techniques in experimental mechanics is determining a gradual shift in the paradigm of material model calibration and discovery, from using several simple-geometry tests towards a few, or even one single test with complicated geometry. The feasibility of such a “one-shot” calibration or discovery heavily relies upon the richness of the measured displacement data, i.e., their ability to probe the space of the state variables and the stress space (whereby the stresses depend on the constitutive law being sought) to an extent sufficient for an accurate and robust calibration or discovery process. The richness of the displacement data is in turn directly governed by the specimen geometry. In this paper, we propose a density-based topology optimisation framework to optimally design the geometry of the target specimen for calibration of an anisotropic elastic material model. To this end, we perform automatic, high-resolution specimen design by maximising the robustness of the solution of the inverse problem, i.e., the identified material parameters, given noisy displacement measurements from digital image correlation. We discuss the choice of the cost function and the design of the topology optimisation framework, and we analyse a range of optimised topologies generated for the identification of isotropic and anisotropic elastic responses.
Size effect in true mode II fracturing of rocks: Theory and experiment
Bahrami, Bahador; Ghouli, Saeid; Nejati, Morteza; et al. (2022)
European Journal of Mechanics - A/Solids
The influence of specimen size on the measured values of true mode II fracture toughness (KIIc) of rocks have been investigated both experimentally and theoretically. In the experimental study, the double-edge notched Brazilian disk (DNBD) test is utilised to perform fracture toughness experiments with various specimen radii ranging from 27 to 101 mm. Test results demonstrate a significant increase of KIIc with the specimen size. The reason for this trend is explained by a novel mathematical model developed based on the fracture process zone and the energy release rate (ERR) concepts. Comparing the predictions of the theoretical model with the experimental data reveals that the proposed theory can accurately predict the size effect phenomenon in the DNBD test data. To compare the results of KIIc with KIc, similar experimental and theoretical analyses are also performed for mode I fracture toughness tests using the semi-circular bend specimens. Lastly, we present a set of correction factors as functions of specimen size, whereby making it possible to estimate the size independent value of KIIc from the test results obtained from laboratory size DNBD specimens.
On higher order parameters in cracked composite plates under far-field pure shear
Ghouli, Saeid; Ayatollahi, Majid R.; Nejati, Morteza (2020)
Fatigue & Fracture of Engineering Materials & Structures
This paper presents the analytical solution of the crack tip fields as well as the crack parameters in an infinitely large composite plate with a central crack subjected to pure shear loading. To this end, the complex variable method is employed to formulate an asymptotic solution for the crack tip fields in an anisotropic plane. Using a stress‐based definition of the crack tip modes of loading, only the mode II crack parameters are found to be non‐zero under pure shear load. Special focus is given to the determination of the higher order parameters of the crack tip asymptotic field, particularly the first non‐singular term, ie, the T‐stress. Unlike the isotropic materials, in which the T‐stress is zero under pure shear, it is found that the T‐stress is non‐zero for the case of anisotropic materials, being the only material‐dependent crack tip stress parameter. The veracity of our exact crack tip fields is assessed and verified through a comparison made with respect to the finite element (FE) solution. Finally, we demonstrate the significance of the T‐stress on stresses near the crack tip in composite plates under pure shear loads.

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Saeid Ghouli

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