Papers by Roberto Ballarini
Finite Element Modeling of Frictionally Restrained Composite Interfaces (3ASA-CB-182i81) P;IPfP% ... more Finite Element Modeling of Frictionally Restrained Composite Interfaces (3ASA-CB-182i81) P;IPfP% ELEBE14 IODBLIYG GP 189-239 18 PYICI.IOYALLI EES'ltiLXYlD CClIECSlFE IETBEPACES P i n a f Beport 4 ;
Volume 5: Manufacturing Materials and Metallurgy; Ceramics; Structures and Dynamics; Controls, Diagnostics and Instrumentation; Education; General, Jun 4, 1989
Local-Global Analysis of Crack Growth in Continuously Reinforced Ceramic Matrix Composites (YASI-... more Local-Global Analysis of Crack Growth in Continuously Reinforced Ceramic Matrix Composites (YASI-CR-18223 1) LOC dL-GLOE. AL ABIALYSIS OF I8 9-1 382 0 CliACK 6609T8 Ill CCITIYUOOSLP LEIPPGBCED CEBAtlIC HAPEIX CCEPOSILES P i o a l lieport (Case Western Reserve Onio.
MRS Proceedings, 1998
The development of polysilicon fracture mechanics specimens with characteristic dimensions compar... more The development of polysilicon fracture mechanics specimens with characteristic dimensions comparable to those of typical microelectromechanical systems (MEMS) devices is presented. The notched cantilever specimens are fully integrated with a simultaneously microfabricated electrostatic actuator, which allows on-chip testing of the specimens without the need of an external loading device, and without any possible influences from external sources. Under monotonic loading, the average maximum tensile stress (strength) and average nominal fracture toughness were measured as 4.2 GPa and 3.5 MPa-m½ for boron-doped specimens, and 5.0 GPa and 4.0 MPa-m½ for undoped specimens. An average modulus of rupture of 3.3 GPa and average nominal toughness of 2.7 MPa-m½ were measured for specimens cracked under cyclic resonance loading. The differences between the monotonic loading and cyclic loading data are attributed to fatigue initiation of a sharp crack from the 1 ýtm radius notch. The experimental data is consistent with a critical flaw size in the fabricated devices, a, that is related to the fracture toughness Klc by Klc/a1/2=4600 MPa.
Journal of Engineering Mechanics-asce, Sep 1, 2022
The results are presented of a stress intensity factor analysis of semicircular surface cracks in... more The results are presented of a stress intensity factor analysis of semicircular surface cracks in the inner raceway of an engine bearing. The loading consists of a moving spherical Hertzian contact load and an axial stress due to rotation and shrink fit. A 3-D linear elastic Boundary Element Method code was developed to perform the stress analysis. The element library includes linear and quadratic isoparametric surface elements. Singular quarter point elements were employed to capture the square root displacement variation and the inverse square root stress singularity along the crack front. The program also possesses the capability to separate the whole domain into two subregions. This procedure enables one to solve nonsymmetric fracture mechanics problems without having to separate the crack surfaces a priori. A wide range of configuration parameters was investigated. The ratio of crack depth to bearing thickness was varied from one-sixtieth to one-fifth for several different locations of the Hertzian load. The stress intensity factors for several crack inclinations were also investigated. The results demonstrate the efficiency and accuracy of the Boundary Element Method. Moreover, the results can provide the basis for crack growth calculations and fatigue life prediction.
This paper presents the effects of elastic mismatch and crack-tip position on the stress intensit... more This paper presents the effects of elastic mismatch and crack-tip position on the stress intensity factors of a long crack penetrating a circular inhomogeneity. The analysis relies on closed-form solutions, derived using complex variable techniques, for the stresses and displacements produced by dislocations positioned inside and outside the inhomogeneity. Dislocation distributions are introduced to express the traction boundary condition along the crack surfaces as a system of singular integral equations, whose solution is obtained through a numerical procedure. It is shown that if the elastic mismatch is interpreted correctly, then the stress intensity factors of this micromechanical model are very good approximations to those computed using a Monte Carlo finite element model of a long crack in a polycrystalline plate with compliant grain boundaries.
Volume 5: Manufacturing Materials and Metallurgy; Ceramics; Structures and Dynamics; Controls, Diagnostics and Instrumentation; Education; IGTI Scholar Award; General, Jun 3, 1991
The bridging of matrix cracks by fibers is an important toughening mechanism in fiber reinforced ... more The bridging of matrix cracks by fibers is an important toughening mechanism in fiber reinforced brittle matrix composites. This paper presents the results of a non-linear finite element analysis of the Mode-I propagation of a bridged matrix crack in a finite size specimen. The composite is modeled as an orthotropic continuum and the bridging due to the fibers is modeled as a distribution of tractions which resist crack opening. A critical stress intensity factor criterion is employed for matrix crack propagation while a critical crack opening condition is used for fiber failure. The structural response of the specimen (load-deflection curves) as well as the stress intensity factor of the propagating crack are calculated for various constituent properties and specimen configurations for both tensile and bending loading. By controlling the length of the bridged crack results are obtained which highlight the transition from stable to unstable behavior of the propagating crack.
Journal of Engineering Mechanics-asce, Oct 1, 2020
AbstractThere has been ever-increasing interest over the past decade in improving understanding o... more AbstractThere has been ever-increasing interest over the past decade in improving understanding of the mechanisms responsible for the progressive collapse of structures. Existing design recommendat...
Journal of Applied Mechanics
Thermomechanical buckling of slender and thin-walled structural components happens without warnin... more Thermomechanical buckling of slender and thin-walled structural components happens without warning and can lead to catastrophic failure. Similar phenomena are observed during plasmolysis (contraction of a plant cell’s protoplast) and rupture of viral capsids. Analytical formulas derived from stability analyses of elastic plates and shells that do not account for the effects of random geometric imperfections introduced during the manufacturing process or biological growth may vastly over-estimate buckling capacity. To ensure structural safety, the formulas must therefore be combined with empirical data to define “knockdown factors” which are in turn used to establish safety factors. Towards improved understanding of the role of imperfections on mechanical response, ingenious methods have been used to fabricate and test near-perfectly hemispherical shells and those containing dimple-like defects. However, a method of inducing imperfections in the form of randomly shaped surfaces remai...
The bridging of matrix cracks by fibers is an important toughening mechanism in fiber reinforced ... more The bridging of matrix cracks by fibers is an important toughening mechanism in fiber reinforced brittle matrix composites. This paper presents the results of a non-linear finite element analysis of the Mode-I propagation of a bridged matrix crack in a finite size specimen. The composite is modeled as an orthotropic continuum and the bridging due to the fibers is modeled as a distribution of tractions which resist crack opening. A critical stress intensity factor criterion is employed for matrix crack propagation while a critical crack opening condition is used for fiber failure. The structural response of the specimen (load-deflection curves) as well as the stress intensity factor of the propagating crack are calculated for various constituent properties and specimen configurations for both tensile and bending loading. By controlling the length of the bridged crack results are obtained which highlight the transition from stable to unstable behavior of the propagating crack.
Journal of the Mechanics and Physics of Solids, 2020
The mechanics of biological entities, from single molecules to the whole organ, has been extensiv... more The mechanics of biological entities, from single molecules to the whole organ, has been extensively analyzed during the last decades. At the smaller scales, statistical mechanics has fostered successful physical models of proteins and molecules, which have been later incorporated within constitutive models of rubber-like materials and biological tissues. At the macroscopic scale, the additive decomposition of energy functions i.e., a parallel arrangement of the tissue constituent, has been recurrently used to account for the internal heterogeneity of soft biological materials. However, it has not yet been possible to unite the mechanics at the tissue level with the actual response of the tissue components. Here, we exemplify our approach using cardiovascular tissue where the mechanical response at the tissue scale is in the range of kPa whereas the elastic modulus of collagen, the main component of the vascular tissue, is in the range of MPa GPa. In this work we develop a novel theoretical framework based on a complementary strain energy function that buildsup on a full network model. The complementary strain energy function introduces naturally an additive decomposition of the deformation gradient for the tissue constituents, i.e an arrangement in series of the constituents. We demonstrate that the macroscopic response of the tissue can be reproduced by just introducing the underlying mechanical and structural features of the micro-constituents, improving in a fundamental manner previous attempts in the mechanical characterization of soft biological tissues. The proposed theoretical framework unveils a new direction in the mechanical modeling of soft tissues and biological networks.
Microscopy and Microanalysis, 2016
Silicon has a rich history of technological importance, as well as serving as an ideal model mate... more Silicon has a rich history of technological importance, as well as serving as an ideal model material for studying mechanical behavior in semi-metallic materials. Of particular interest is the rapid transition in deformation mechanisms as a function of scale and temperature, where such concepts as dislocation character 1 , nucleation/propagation control 2 and possible core structure 3 changes may contribute. The result is that under certain loading conditions, sizes, temperatures and doping, silicon can display a wide variety of response from highly brittle cleavage to over 50% plastic strain 4 .
Scripta Materialia, 2017
We present novel in-situ scanning electron microscope experiments exploring the fracture of silic... more We present novel in-situ scanning electron microscope experiments exploring the fracture of silicon as a function of temperature at the microscale, from room temperature to 600°C. Clear post mortem TEM observations of dislocation activity at and above 450°C suggest that back stresses from crack-tip dislocation emission raise the applied stress intensity at initiation, as part of a brittle to ductile transition starting at 300°C. This is in agreement with other microscale measurements; however, these experiments are particularly noteworthy in their ability to directly observe crack advance and perform post-mortem analysis to investigate dislocation activity.
Journal of Engineering Mechanics, Feb 1, 2014
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Papers by Roberto Ballarini