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Dive into the research topics where Antonia Pajares is active.

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Featured researches published by Antonia Pajares.


Journal of Materials Research | 2002

Overview: Damage in brittle layer structures from concentrated loads

Brian R. Lawn; Yan Deng; Pedro Miranda; Antonia Pajares; Herzl Chai; Do Kyung Kim

In this article, we review recent advances in the understanding and analysis of damage initiation and evolution in laminate structures with brittle outerlayers and compliant sublayers in concentrated loading. The relevance of such damage to lifetime-limiting failures of engineering and biomechanical layer systems is emphasized. We describe the results of contact studies on monolayer, bilayer, trilayer, and multilayer test specimens that enable simple elucidation of fundamental damage mechanics and yet simulate essential function in a wide range of practical structures. Damage processes are observed using post mortem (“bonded-interface”) sectioning and direct in situ viewing during loading. The observations reveal a competition between damage modes in the brittle outerlayers—cone cracks or quasiplasticity at the top (near-contact) surfaces and laterally extending radial cracks at the lower surfaces. In metal or polymeric support layers, yield or viscoelasticity can become limiting factors. Analytical relations for the critical loads to initiate each damage mode are presented in terms of key system variables: geometrical (layer thickness and indenter radius); material (elastic modulus, strength and toughness of brittle components, hardness of deformable components). Such relations provide a sound physical basis for the design of brittle layer systems with optimal damage thresholds. Other elements of the damage process—damage evolution to failure, crack kinetics (and fatigue), flaw statistics, and complex (tangential) loading—are also considered.


Acta Biomaterialia | 2010

Improving the compressive strength of bioceramic robocast scaffolds by polymer infiltration

Francisco J. Martínez-Vázquez; Fidel Hugo Perera; Pedro Miranda; Antonia Pajares; Fernando Guiberteau

The effect of polymer infiltration on the compressive strength of β-tricalcium phosphate (TCP) scaffolds fabricated by robocasting (direct write assembly) is analyzed in this work. Porous structures consisting of a tetragonal three-dimensional mesh of interpenetrating rods were fabricated from concentrated TCP inks with suitable viscoelastic properties. Biodegradable polymers (polylactic acid (PLA) and poly(ε-caprolactone) (PCL)) were infiltrated into selected scaffolds by immersion of the structure in a polymer melt. Infiltration increased the uniaxial compressive strength of these model scaffolds by a factor of three (PCL) or six (PLA). It also considerably improved the mechanical integrity of the structures after initial cracking, with the infiltrated structure retaining a significant load-bearing capacity after fracture of the ceramic rods. The strength improvement in the infiltrated scaffolds was attributed to two different contributions: the sealing of precursor flaws in the ceramic rod surfaces and the partial transfer of stress to the polymer, as confirmed by finite element analysis. The implications of these results for the mechanical optimization of scaffolds for bone tissue engineering applications are discussed.


Materials Science and Engineering A-structural Materials Properties Microstructure and Processing | 1996

Mechanical characterization of plasma sprayed ceramic coatings on metal substrates by contact testing

Antonia Pajares; Lanhua Wei; Brian R. Lawn; Nitin P. Padture; Christopher C. Berndt

Abstract Hertzian indentation testing is used to generate contact damage in plasma sprayed ceramic coatings on metal substrates. Two basic ceramic/metal coating/substrate systems are examined: alumina on steel and zirconia on superalloy. Macroscopic mechanical responses are measured via indentation stress-strain curves, which quantify the relative role of the coating and substrate in the net deformation and facilitate evaluations of elastic moduli and yield stresses. Micromechanical damage processes within the coating and substrate subsurface layers are studied using a “bonded-interface” specimen. Degradation occurs primarily by delamination and other cracking at the coating/substrate interface or in the coating, but plastic deformation of the metal substrate contributes importantly to the crack driving force.


Journal of Materials Research | 2001

Contact fracture of brittle bilayer coatings on soft substrates

Pedro Miranda; Antonia Pajares; Fernando Guiberteau; F.L. Cumbrera; Brian R. Lawn

Contact-induced fracture modes in trilayers consisting of a brittle bilayer coating on a soft substrate were investigated. Experiments were performed on model transparent glass/sapphire/polycarbonate structures bonded with epoxy adhesive, to enable in situ observation during the contact. Individual layer surfaces were preferentially abraded to introduce uniform flaw states and so allowed each crack type to be studied separately and controllably. Fracture occurred by cone cracking at the glass top surface or by radial cracking at the glass or sapphire bottom surfaces. Critical loads for each crack type were measured, for fixed glass thickness and several specified sapphire thicknesses. Finite element modeling (FEM) was used to evaluate the critical load data for radial cracking, using as essential input material parameters evaluated from characterization tests on constituent materials and supplemental glass/polymer and sapphirse/polymer bilayer structures. The FEM calculations demonstrated pronounced stress transfer from the applied contact to the underlying sapphire layer, explaining a tendency for preferred fracture of this relatively stiff component. Factors affecting the design of optimal trilayer structures for maximum fracture resistance of practical layer systems were considered.


Advanced Engineering Materials | 2000

Damage-Resistant Brittle Coatings

Brian R. Lawn; Kee Sung Lee; Herzl Chai; Antonia Pajares; Do Kyung Kim; Sataporn Wuttiphan; Irene Mona Peterson; Xiaozhi Hu

Laminate structures consisting of hard, brittle coatings andsoft, tough substrates are important in a wide variety of engi-neering applications (cutting tools, electronic multilayers, la-minated windscreens), biological structures (teeth and dentalcrowns, shells, bones), and traditional pottery (ceramicglazes). A hard outerlayer variously offers increased load-bearing capacity, wear resistance, thermal and corrosion pro-tection, electrical insulation, and aesthetics; a compliant un-derlayer offers stress redistribution and damage tolerance.But hard layers are susceptible to cracking, especially in sur-face-concentrated loads from static or cyclic contacts. In nat-ural or restorative tooth structures, for instance, forces in ex-cess of 100 N operate at contacts between opposing cusps ofcharacteristic radii 2–4 mm over 10


Acta Materialia | 2003

Designing damage-resistant brittle-coating structures: II. Trilayers

Pedro Miranda; Antonia Pajares; Fernando Guiberteau; Yan Deng; Hong Zhao; Brian R. Lawn

A FEA study of coating/substrate bilayers is conducted as a foundation for damage analysis. Attention is focused on the stresses along the contact axis immediately adjacent to the bilayer interface, where radial cracking or yield in the coating, or yield in the substrate, tend to occur. The stress analysis is used to determine critical loads to initiate each damage mode in terms of basic material properties and coating thickness. Controlling material parameters are strength (brittle mode) and yield stress or hardness (plastic mode). The critical loads are shown to have a simple quadratic dependency on coating thickness, but more complex dependencies on elastic modulus mismatch ratio. Simplified explicit modulus functions afford a route to prediction of the critical loads for design purposes. Implications concerning the design of bilayers for specific applications are discussed.


Acta Materialia | 2001

Role of flaw statistics in contact fracture of brittle coatings

Pedro Miranda; Antonia Pajares; Fernando Guiberteau; F.L. Cumbrera; Brian R. Lawn

Abstract A flaw statistics analysis is here developed to account for systematic differences between experimentally observed and theoretically predicted critical loads for the initiation of contact-induced radial cracks in brittle coatings on compliant substrates. Specific attention is drawn to deviations in critical load ( P R ) data from ideal quadratic dependence on coating thickness ( d ), i.e. P R ∝ d 2 , especially at low d values. It is postulated that these deviations are attributable to the existence of distributions in flaw size and location, in relation to the bell-shaped tensile stress fields responsible for initiation of the radial cracks at the coating lower surface. A statistics-based expression is derived for the mean values of P R in terms of flaw density and size distribution. Data from model bilayers consisting of glass plates of different thicknesses d bonded to polycarbonate substrates are used as an illustrative case study. Controlled pre-abrasion flaws are introduced into the lower glass surfaces before joining into the bilayer configuration, to enable a priori characterization of distribution parameters by image analysis. Finite element modelling is used to determine the tensile stress distribution at the coating lower surface. The predicted statistics-based P R ( d ) function is shown to fit the data within uncertainty bounds. Implications concerning the continued usefulness of the ideal, P R ∝ d 2 relation for designing ceramic coatings for failure resistance are considered.


Acta Biomaterialia | 2008

Finite element modeling as a tool for predicting the fracture behavior of robocast scaffolds

Pedro Miranda; Antonia Pajares; Fernando Guiberteau

The use of finite element modeling to calculate the stress fields in complex scaffold structures and thus predict their mechanical behavior during service (e.g., as load-bearing bone implants) is evaluated. The method is applied to identifying the fracture modes and estimating the strength of robocast hydroxyapatite and beta-tricalcium phosphate scaffolds, consisting of a three-dimensional lattice of interpenetrating rods. The calculations are performed for three testing configurations: compression, tension and shear. Different testing orientations relative to the calcium phosphate rods are considered for each configuration. The predictions for the compressive configurations are compared to experimental data from uniaxial compression tests.


Thin Solid Films | 1997

Effect of substrate and bond coat on contact damage in zirconia-based plasma-sprayed coatings

Sataporn Wuttiphan; Antonia Pajares; Brian R. Lawn; Christopher C. Berndt

Abstract This paper reports a Hertzian indentation study of damage modes in zirconia-based plasma-sprayed coatings on metal substrates, with and without bond coats. The structure of the study is as follows: (i) measurement of Hertzian indentation stress-strain curves, first on individual bulk material components (controls) and then on the composite layer structures, to quantify the degree of plasticity, (ii) micrographic analysis of the corresponding subsurface damage modes, particularly of the yield zones, in both coatings and underlayers; (iii) finite clement modelling of the clastic-plastic stress fields in the adjacent layers, again with a focus on the yield zones, It is demonstrated that the substrate can have a profound influence on the damage distribution, depending on the degree of clastic-plastic mismatch relative to the coating. The bond coal, by virtue of its relative thinness, plays a lesser role in the damage intensity, notwithstanding an apparent improvement in substrate adhesion. Indentation variables followed are applied load, to examine the evolution of damage, and number of cycles, to examine fatigue. The results indicate the power and simplicity of the Hertzian technique as a route to mechanical characterization of coating structures: for identifying damage modes, especially yield (but also fracture, in the present case delamination fracture); for evaluating damage parameters, such as Youngs modulus and the yield stress, from FEM analysis or stress-strain curves and yield zone microscopy; and for quantifying design concepts, e.g. maximum sustainable bearing stress and damage tolerance.


Materials | 2014

Effect of Polymer Infiltration on the Flexural Behavior of β-Tricalcium Phosphate Robocast Scaffolds

Francisco J. Martínez-Vázquez; Antonia Pajares; Fernando Guiberteau; Pedro Miranda

The influence of polymer infiltration on the flexural strength and toughness of β-tricalcium phosphate (β-TCP) scaffolds fabricated by robocasting (direct-write assembly) is analyzed. Porous structures consisting of a tetragonal three-dimensional lattice of interpenetrating rods were impregnated with biodegradable polymers (poly(lactic acid) (PLA) and poly(ε-caprolactone) (PCL)) by immersion of the structure in a polymer melt. Infiltration increased the flexural strength of these model scaffolds by a factor of 5 (PCL) or 22 (PLA), an enhancement considerably greater than that reported for compression strength of analogue materials. The greater strength improvement in bending was attributed to a more effective transfer of stress to the polymer under this solicitation since the degree of strengthening associated to the sealing of precursor flaws in the ceramic rod surfaces should remain unaltered. Impregnation with either polymer also improved further than in compression the fracture energy of the scaffolds (by more than two orders of magnitude). This increase is associated to the extraordinary strengthening provided by impregnation and to a crack bridging toughening mechanism produced by polymer fibrils.

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Pedro Miranda

University of Extremadura

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Brian R. Lawn

National Institute of Standards and Technology

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Siamak Eqtesadi

University of Extremadura

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Yan Deng

National Institute of Standards and Technology

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