Mitigation of Fatigue Damage in Self-Healing Vascular Materials

The fatigue response of an epoxy matrix containing vasculature for the delivery of liquid healing agents is investigated. The release of a rapidly curing, two-part epoxy healing chemistry into the wake of a propagating crack reduces the rate of crack extension by shielding the crack tip from the full range of applied stress intensity factor. Crack propagation is studied for a variety of loading conditions, with the maximum applied stress intensity factor ranging from 62 to 84% of the quasi-static fracture toughness of the material. At the highest level of applied load, the rate of mechanical damage is so fast that the healing agents do not fully mix and polymerize, and the effect of healing is minimal. The self-healing response is most effective at impeding the slower propagating cracks, with complete crack arrest occurring at the lowest level of applied load, and reductions of 79–84% in the rate of crack extension at intermediate loads.

Granular anchors under vertical loading - axial pull

Granular anchors are a relatively new concept in ground engineering with relatively little known regarding their load–displacement behaviour, failure modes, ultimate pullout capacity and also potential applications. A granular anchor consists of three main components: a base plate; tendon and compacted granular backfill. The tendon is used to transmit the applied load to the base plate which compresses the granular material to form the anchor. A study of the load–displacement response and ultimate pullout capacity of granular anchors constructed in intact lodgement till and made ground deposits is reported in this paper. Parallel tests were also performed on cast insitu concrete anchors which are traditionally used for anchoring purposes. A new method of analysis for the determination of the ultimate pullout capacity of granular anchors is presented and verified experimentally, with the dominant mode of failure controlled by the column length to diameter ratio. Granular anchors with L/D > 7 principally failed on bulging whereas short granular anchors failed on shaft resistance, with the latter mobilising similar pullout capacities as conventional concrete anchors.

Thermomechanical analyses of ultrasonic welding process using thermal and acoustic softening effects

Ultrasonic welding process is a rapid manufacturing process used to weld thin layers of metal at low temperatures and low energy consumption. Experimental results have shown that ultrasonic welding is a combination of both surface (friction) and volume (plasticity) softening effects. In the presented work, a very first attempt has been made to simulate the ultrasonic welding of metals by taking into account both of these effects (surface and volume). A phenomenological material model has been proposed which incorporates these two effects (i.e. surface and volume). The thermal softening due to friction and ultrasonic (acoustic) softening has been included in the proposed material model. For surface effects a friction law with variable coefficient of friction dependent upon contact pressure, slip, temperature and number of cycles has been derived from experimental friction tests. Thermomechanical analyses of ultrasonic welding of aluminium alloy have been performed. The effects of ultrasonic welding process parameters, such as applied load, amplitude of ultrasonic vibration, and velocity of welding sonotrode on the friction work at the weld interface are being analyzed. The change in the friction work at the weld interface has been explained on the basis of softening (thermal and acoustic) of the specimen during the ultrasonic welding process. In the end, a comparison between experimental and simulated results has been presented showing a good agreement. © 2008 Elsevier Ltd. All rights reserved.

Theoretical and FE analysis of ultrasonic welding of aluminum alloy 3003

Ultrasonic welding (consolidation) process is a rapid manufacturing process that is used to join thin layers of metal at low temperature and low energy consumption. Experimental results have shown that ultrasonic welding is a combination of both surface (friction) and volume (plasticity) softening effects. In the presented work, an attempt has been made to simulate the ultrasonic welding of metals by taking into account these effects (surface and volume). A phenomenological material model has been proposed, which incorporates these two effects (i.e., surface and volume). The thermal softening due to friction and ultrasonic (acoustic) softening has been included in the proposed material model. For surface effects, a friction law with variable coefficient of friction that is dependent on contact pressure, slip, temperature, and number of cycles has been derived from experimental friction tests. The results of the thermomechanical analyses of ultrasonic welding of aluminum alloy have been presented. The goal of this work is to study the effects of ultrasonic welding process parameters, such as applied load, amplitude of ultrasonic oscillation, and velocity of welding sonotrode on the friction work at the weld interface. The change in the friction work at the weld interface has been explained on the basis of softening (thermal and acoustic) of the specimen during the ultrasonic welding process. In the end, a comparison between experimental and simulated results has been presented, showing a good agreement. Copyright © 2009 by ASME.

Numerical modelling of laboratory tests for TBM cutter indentation process analysis

Considering that TBMs are nowadays used for long Trans-Alpine tunnels, the
understanding of rock breaking and chipping due to TBM cutter disks mechanism, for deep tunnelling operations, becomes very interesting. In this paper, the results from carried out laboratory tests that simulate the disk cutter action at the rock tunnel face by means of an indentation tool, acting on a rock
specimen with proper size, and the related three-dimensional and two-dimensional numerical modelling are proposed. The developed numerical models simulate the different test conditions (applied load, boundary conditions) allowing the analysis of the stresses distributions along possible breaking planes.
The influence of a confinement-free area on one side of the specimen, simulating the formation of a groove near the tool, is pointed out.
The obtained results from numerical modelling put in evidence a satisfactory agreement with the experimental observations.

Influence of test methodology and probe geometry on nanoscale fatigue mechanisms of diamond-like carbon thin film

The aim of this paper is to investigate the mechanism of nanoscale fatigue using nano-impact and multiple-loading cycle nanoindentation tests, and compare it to previously reported findings of nanoscale fatigue using integrated stiffness and depth sensing approach. Two different film loading mechanism, loading history and indenter shapes are compared to comprehend the influence of test methodology on the nanoscale fatigue failure mechanisms of DLC film. An amorphous 100 nm thick DLC film was deposited on a 500 μm silicon substrate using sputtering of graphite target in pure argon atmosphere. Nano-impact and multiple-load cycle indentations were performed in the load range of 100 μN to 1000 μN and 0.1 mN to 100 mN, respectively. Both test types were conducted using conical and Berkovich indenters. Results indicate that for the case of conical indenter, the combination of nano-impact and multiple-loading cycle nanoindentation tests provide information on the life and failure mechanism of DLC film, which is comparable to the previously reported findings using the integrated stiffness and depth sensing approach. However, the comparison of results is sensitive to the applied load, loading mechanism, test-type and probe geometry. The loading mechanism and load history is therefore critical which also leads to two different definitions of film failure. The choice of exact test methodology, load and probe geometry should therefore be dictated by the in-service tribological conditions, and where necessary both test methodologies can be used to provide better insights of failure mechanism. Molecular dynamics (MD) simulations of the elastic response of nanoindentation is reported, which indicates that the elastic modulus of the film measured using MD simulation was higher than that experimentally measured. This difference is attributed to the factors related to the presence of material defects, crystal structure, residual stress, indenter geometry and loading/unloading rate differences between the MD and experimental results.

Wear analysis and prediction of the life of a riveting die used in the automotive industry

Knowledge on the life span of the riveting dies used in the automotive industry is sparse. It is often the case that only when faulty products are produced are workers aware that their tool needs to be changed. This is of course costly both in terms of time and money. Responding to this challenge, this paper proposes a methodology which integrates wear and stress analysis to quantify the life of a riveting die. Experiments are carried out to measure the applied load required to split a rivet. The obtained results (i.e. force curves) are used to validate the wear mechanisms of the die observed using scanning electron microscopy. Sliding, impact, and adhesive wears are observed on the riveting die after a certain number of riveting cycles. The stress distribution on the die during riveting is simulated using a finite element (FE) approach. In order to confirm the accuracy of the FE model, the experimental force results are compared with the ones produced from FE simulation. The maximum and minimum von Mises' stresses generated from the FE model are input into a Goodman diagram and an S-N curve to compute the life of the riveting die. It is found that the riveting die is predicted to run for 4 980 000 cycles before failure.

Cyclic nanoindentation and nano-impact fatigue characterisation of functionally graded TiN/TiNi shape memory film

This paper investigates the mechanism of nanoscale fatigue of functionally graded TiN/TiNi films using nano-impact and multiple-loading-cycle nanoindentation tests. The functionally graded films were deposited on silicon substrate, in which TiNi films maintain shape memory and pseudo elastic behavior, while a modified TiN surface layer provides tribological and anti-corrosion properties. Nanomechanical tests were performed to comprehend the localized film performance and failure modes of the functionally graded film using NanoTestTM equipped with Berkovich and conical indenter between 100 μN to 500 mN loads. The loading mechanism and load history are critical to define film failure modes (i.e. backward depth deviation) including the shape memory effect of the functionally graded layer. The results are sensitive to the applied load, loading type (e.g. semi-static, dynamic) and probe geometry. Based on indentation force-depth profiles, depth-time data and post-test surface observations of films, it is concluded that the shape of the nanoindenter is critical in inducing the localized indentation stress and film failure, including shape recovery at the lower load range. Elastic-plastic finite element (FE) simulation during nanoindentation loading indicated that the location of subsurface maximum stress near the interface influences the backward depth deviation type of film failure. A standalone, molecular dynamics simulation was performed with the help of a long range potential energy function to simulate the tensile test of TiN nanowire with two different aspect ratios to investigate the theory of its failure mechanism.

Synthesis of polyelectrolyte brushes on silica-based substrates through surface-initiated polymerization : brush characterization and responsiveness to variation in pH and ionic strength

Les brosses de polyélectrolytes font l’objet d’une attention particulière pour de nombreuses applications car elles présentent la capacité de changer de conformation et, par conséquent, de propriétés de surface en réponse aux conditions environnementales appliquées. Le contrôle des principaux paramètres de ces brosses telles que l'épaisseur, la composition et l'architecture macromoléculaire, est essentiel pour obtenir des polymères greffés bien définis. Ceci est possible avec la Polymérisation Radicalaire par Transfert d’Atomes - Initiée à partir de la Surface (PRTA-IS), qui permet la synthèse de brosses polymériques de manière contrôlée à partir d’une couche d'amorceurs immobilisés de manière covalente sur une surface. Le premier exemple d’une synthèse directe de brosses de poly(acide acrylique) (PAA) par polymérisation radicalaire dans l’eau a été démontré. Par greffage d’un marqueur fluorescent aux brosses de PAA et via l’utilisation de la microscopie de fluorescence par réflexion totale interne, le dégreffage du PAA en temps réel a pu être investigué. Des conditions environnementales de pH ≥ 9,5 en présence de sel, se sont avérées critiques pour la stabilité de la liaison substrat-amorceur, conduisant au dégreffage du polymère. Afin de protéger de l’hydrolyse cette liaison substrat-amorceur sensible et prévenir le dégreffage non souhaité du polymère, un espaceur hydrophobique de polystyrène (PS) a été inséré entre l'amorceur et le bloc de PAA stimuli-répondant. Les brosses de PS-PAA obtenues étaient stables pour des conditions extrêmes de pH et de force ionique. La réponse de ces brosses de copolymère bloc a été étudiée in situ par ellipsométrie, et le changement réversible de conformation collapsée à étirée, induit par les variations de pH a été démontré. De plus, des différences de conformation provenant des interactions du bloc de PAA avec des ions métalliques de valence variable ont été obtenues. Le copolymère bloc étudié semble donc prometteur pour la conception de matériaux répondant rapidement a divers stimuli. Par la suite, il a été démontré qu’un acide phosphonique pouvait être employé en tant qu’ amorceur PRTA-IS comme alternative aux organosilanes. Cet amorceur phosphonate a été greffé pour la première fois avec succès sur des substrats de silice et une PRTA-IS en milieux aqueux a permis la synthèse de brosses de PAA et de poly(sulfopropyl méthacrylate). La résistance accrue à l’hydrolyse de la liaison Sisubstrat-O- Pamorceur a été confirmée pour une large gamme de pH 7,5 à 10,5 et a permis l’étude des propriétés de friction des brosses de PAA sous différentes conditions expérimentales par mesure de forces de surface. Malgré la stabilité des brosses de PAA à haute charge appliquée, les études des propriétés de friction ne révèlent pas de changement significatif du coefficient de friction en fonction du pH et de la force ionique.

Influence of Interface Surface Geometries In The Tensile Characteristics Of Friction Welded Joints From Aluminium Alloys

Friction welding is a solid state joining process that produces coalescence in materials, using the heat developed between surfaces through a combination of mechanical induced rubbing motion and applied load. In rotary friction welding technique heat is generated by the conversion of mechanical energy into thermal energy at the interface of the work pieces during rotation under pressure. Traditionally friction welding is carried out on a dedicated machine because of its adaptability to mass production. In the present work, steps were made to modify a conventional lathe to rotary friction welding set up to obtain friction welding with different interface surface geometries at two different speeds and to carry out tensile characteristic studies. The surface geometries welded include flat-flat, flat-tapered, tapered-tapered, concave-convex and convex-convex. A comparison of maximum load, breaking load and percentage elongation of different welded geometries has been realized through this project. The maximum load and breaking load were found to be highest for weld formed between rotating flat and stationary tapered at 500RPM and the values were 19.219kN and 14.28 kN respectively. The percentage elongation was found to be highest for weld formed between rotating flat and stationary flat at 500RPM and the value was 21.4%. Hence from the studies it is cleared that process parameter like “interfacing surface geometries” of weld specimens have strong influence on tensile characteristics of friction welded joints

Composition and cure temperature: The influence on properties of final flexible PU cold cure foam parts

Foams are cellular structures, produced by gas bubbles formed during the polyurethane polymerization mixture. Flexible PU foams meet the following two criteria: have a limited resistance to an applied load, being both permeable to air and reversibly deformable. There are two main types of flexible foams, hot and cold cure foams differing in composition and processing temperatures. The hot cure foams are widely applied and represent the main composition of actual foams, while cold cure foams present several processing and property advantages, e.g, faster demoulding time, better humid aging properties and more versatility, as hardness variation with index changes are greater than with hot cure foams. The processing of cold cure foams also is attractive due to the low energy consumption (mould temperature from 30 degrees to 65 degrees C) comparatively to hot cure foams (mould temperature from 30 degrees to 250 degrees C). Another advantage is the high variety of soft materials for low temperature processing moulds. Cold cure foams are diphenylmethane diisocyanate (MDI) based while hot cure foams are toluene diisocyanate (TDI) based. This study is concerned with Viscoelastic flexible foams MDI based for medical applications. Differential Scanning Calorimetry (DSC) was used to characterize the cure kinetics and Dynamical Mechanical Analisys to collect mechanical data. The data obtained from these two experimental procedures were analyzed and associated to establish processing/properties/operation conditions relationships. These maps for the selection of optimized processing/properties/operation conditions are important to achieve better final part properties at lower costs and lead times.

Tratamento de efluente de curtume por contactores biológicos rotatórios com opção de pré-tratamento por reator acidogênico

Esta pesquisa teve por objetivo avaliar a performance de um sistema de tratamento baseado no processo de Contactores Biológicos Rotatórios (CBR), com a opção de prétratamento por Reator Acidogênico (RA), para o tratamento do efluente de um curtume que realiza as operações de recurtimento e acabamento. Os estudos experimentais foram desenvolvidos em escala de bancada, onde foram testadas duas configurações para a estação de tratamento. Na Configuração I o Reator Acidogênico precede o sistema de CBR, na Configuração II apenas um decantador precede os CBR. Para cada configuração foram testados três Tempos de Detenção Hidráulica (TDH) de 90, 45 e 22 horas no sistema de CBR correspondentes as Etapas 1, 2 e 3 respectivamente. Deste modo foi possível avaliar a influência do RA no desempenho dos CBR, sendo que o RA teria a função de promover a hidrólise e solubilização da matéria orgânica dificilmente degradável, facilitando a ação dos microrganismos aeróbios nos CBR. Foi avaliada a remoção de matéria orgânica nas três etapas das duas configurações estudadas, tendo como resultados eficiências (remoção de DQOt) de 60%, 61% e 63% para as etapas 1,2 e 3 respectivamente da Configuração I e 81%, 83% e 66% para as etapa 1, 2 e 3 da Configuração II. Também foram avaliadas as remoções de cromo além da nitrificação e denitrificação no sistema de tratamento. A comparação entre as duas Configurações estudadas ficou prejudicada devido a mudanças nas características do efluente do curtume que na Configuração II apresentou uma maior degradabilidade (maior relação DBO/DQO) que na Configuração I. Mesmo assim, foi obtida uma correlação entre os dados de carga de DQOsol aplicada e removida indicando que o Reator Acidogênico promoveu uma otimização no sistema de CBR. Assim foi possível avaliar as características operacionais e a proposição de modelo matemático referente ao desempenho de CBR tratando um efluente real de lenta degradação.

Descontinuidade na seção transversal em laminados compósitos poliméricos: efeitos e propriedades

The growing demand in the use of composite materials necessitates a better understanding its behavior to many conditions of loading and service, as well as under several ways of connections involved in mechanisms of structural projects. It is know that most of the structural elements are designed with presence of geometric discontinuities (holes, notches, etc) in their longitudinal sections and / or transversals, and that these discontinuities affect the mechanical response of these elements. This work has aims to analyze a study of the mechanical response, when in the presence geometric discontinuity, of polymer matrix composite laminates (orthophthalic polyester) to the uniaxial tensile test. The geometric discontinuity is characterized by the presence of a center hole in the transversal section of the composite. In this study, different kinds of stacking sequences are tested, with and without the presence of the hole, so as to provide better understanding of the mechanical properties. This sense, two laminates were studied: the first is only reinforced by with seven layers short mats of fiberglass-E (CM) and the second where the reinforcement of fiberglass-E comes in the form of bidirectional fabric (CT), with only four layers. The laminate CT has the presence of anisotropy (sense of continuous fibers with respect to the applied load) as the main parameter influencing its mechanical behavior, behavior this, not observed for the CM. In addition to the mechanical properties was also studied the fracture characteristics developed in each composite laminated. The results also showed that the presence of the hole in the transversal section decreased the ultimate strength of laminates and changed the final characteristic of fracture in all kinds of composite laminated studied

Fractography analysis and fatigue strength of carbon fiber/RTM6 laminates

Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)

Deposição de filmes por plasma eletrolítico em ligas de alumínio

Pós-graduação em Ciência e Tecnologia de Materiais - FC