Bond Strength and Etching Pattern of Adhesive Systems to Enamel: Effects of Conditioning Time and Enamel Preparation

Statement of the problem: The performance of self-etch systems on enamel is controversial and seems to be dependent on the application technique and the enamel preparation. Purpose of the Study: To examine the effects of conditioning time and enamel surface preparation on bond strength and etching pattern of adhesive systems to enamel. Materials and Methods: Ninety-six teeth were divided into 16 conditions (N = 6) in function of enamel preparation and conditioning time for bond strength test. The adhesive systems OptiBond FL (Kerr, Orange, CA, USA), OptiBond SOLO Plus (Kerr), Clearfil SE Bond (Kuraray, Osaka, Japan), and Adper Prompt L-Pop (3M ESPE, St. Paul, MN, USA) were applied on unground or ground enamel following the manufacturers` directions or doubling the conditioning time. Cylinders of Filtek Flow (0.5-mm height) were applied to each bonded enamel surface using a Tygon tube (0.7 mm in diameter; Saint-Gobain Corp., Aurora, OH, USA). After storage (24 h/37 degrees C), the specimens were subjected to shear force (0.5 mm/min). The data were treated by a three-way analysis of variance and Tukey`s test (alpha = 0.05). The failure modes of the debonded interfaces and the etching pattern of adhesives were observed using scanning electron microscopy. Results: Only the main factor ""adhesive"" was statistically significant (p < 0.001). The lowest bond strength value was observed for OptiBond FL. The most defined etching pattern was observed for 35% phosphoric acid and for Adper Prompt L-Pop. Mixed failures were observed for all adhesives, but OptiBond FL showed cohesive failures in resin predominantly. Conclusions: The increase in the conditioning time as well as the enamel pretreatment did not provide an increase in the resin-enamel bond strength values for the studied adhesives. CLINICAL SIGNIFICANCE The surface enamel preparation and the conditioning time do not affect the performance of self-etch systems to enamel. (J Esthet Restor Dent 20:322-336, 2008)

Phenolic matrices and sisal fibers modified with hydroxy terminated polybutadiene rubber: Impact strength, water absorption, and morphological aspects of thermosets and composites

The aim of the present work was to investigate the toughening of phenolic thermoset and its composites reinforced with sisal fibers, using hydroxyl-terminated polybutadiene rubber (HTPB) as both impact modifier and coupling agent. Substantial increase in the impact strength of the thermoset was achieved by the addition 10% of HTPB. Scanning electron microscopy (SEM) images of the material with 15% HTPB content revealed the formation of some rubber aggregates that reduced the efficiency of the toughening mechanism. In composites, the toughening effect was observed only when 2.5% of HTPB was added. The rubber aggregates were found located mainly at the matrix-fiber interface suggesting that HTPB could be used as coupling agent between the sisal fibers and the phenolic matrix. A composite reinforced with sisal fibers pre-impregnated with HTPB was then prepared; its SEM images showed the formation of a thin coating of HTPB on the surface of the fibers. The ability of HTBP as coupling agent between sisal fibers and phenolic matrix was then investigated by preparing a composite reinforced with sisal fibers pre-treated with HTPB. As revealed by its SEM images, the HTPB pre-treatment of the fibers resulted on the formation of a thin coating of HTPB on the surface of the fibers, which provided better compatibility between the fibers and the matrix at their interface, resulting in a material with low water absorption capacity and no loss of impact strength. (C) 2009 Elsevier B.V. All rights reserved.

Koldiffusionens inverkan på utmattnings-hållfastheten hos varmvalsade sexkantsstänger;The influence of carbon diffusion on the fatigue strength of hot rolled hexagonal rods

Helstångsborr och konstång kallas bergborrverktyg, som tillverkas av sexkantiga stänger med inre hål för ledning av spolvatten. Dessa spolhål är klädda med ferritiskt rostfritt stål. Innan valsning borras ett centrerat hål i stålämnet som ska valsas. I det borrade hålet appliceras ett rör av ferritiskt rostfritt stål och i det röret skjuts en manganstålskärna in. Efter valsning dras kärnan ut och ett spolhål med invändigt foder av rostfritt stål bildas. Under uppvärmningen och valsningen av stålämnet sker normalt en koldiffusion från manganstålskärnan till det rostfria fodret. Fodrets syfte är att skydda spolhålets yta mot erosion och korrosion.Syftet med arbetet var att hitta en metod som garanterar uppkolning av det invändiga fodret, samt att utreda vad uppkolningen betyder för den mekaniska hållfastheten i stängerna. Ett laboratorieförsök visade att det var bra, med avseende på uppkolning, att blanda stearat i kärnbestrykningsmedlet. Kärnbestrykningsmedlet penslas på manganstålskärnan innan den förs in i stålämnet som ska valsas, dess syfte är att minska friktionen vid kärnutdragningen efter valsning.Ett försök utfördes i valsverket med stearat i kärnbestrykningsmedlet. En grundlig undersökning utfördes på två hetor som valsats med stearat-bestrykningsmedel, samt på en traditionellt tillverkad heta för att ha som referens. Hetorna kapades till provstänger om 1,5 m, totalt undersöktes 55 st.Utvärderingen av verksförsöket visade att stängernas kvalitet blir tillräckligt bra om ingenting oförutsett händer i produktionen, oavsett om koltillskott är tillsatt i bestrykningsmedlet eller inte. Arbetet gav inte en garanterande metod, men visade att stängerna normalt håller hög kvalitet.

Deregulation and The 2007-2008 Housing/Debt Crisis Analysis of the Housing/Debt Crisis of 2007-2008 and its impact on the Financial Strength and Vulnerability of the United States and Global Economy

One looming question has persisted in the minds of economists the world over in the aftermath of the 2007-2008 American Housing and Debt Crisis: How did it begin and who is responsible for making this happen? Another two-part question is: What measures were implemented to help end the crisis and what changes are being implemented to ensure that it will never happen again? Many speculate that the major contributing factor was the repeal of the Glass-Steagall Act in 1999 that prompted a virtual feeding frenzy among the banking community when new calls from Capitol Hill encouraged home ownership in America as well as the secondary mortgage market which skyrocketed thereafter. The Glass-Steagall Act will be among many of the topics explored in this paper along with the events leading up to the 2007-2008 housing/debt crisis as well as the aftermath.

A feasibility study for modelling tie strength with the facebook API

This thesis examines the concept of tie strength and investigates how it can be determined on the fly in the Facebook Social Network Service (SNS) by a system constructed using the standard developer API. We analyze and compare two different models: the first one is an adaptation of previous literature (Gilbert & Karahalios, 2009), the second model is built from scratch and based on a dataset obtained from an online survey. This survey took the form of a Facebook application that collected subjective ratings of the strength of 1642 ties (friendships) from 85 different participants. The new tie strength model was built based on this dataset by using a multiple regression method. We saw that the new model performed slightly better than the original adapted model, plus it had the advantage of being easier to implement. In conclusion, this thesis has shown that tie strength models capable of serving as useful friendship predictors are easily implementable in a Facebook application via standard API calls. In addition to a new tie strength model, the methodology adopted in this work permitted observation of the weights of each predictive variable used in the model, increasing the visibility of the factors that affects peoples’ relationships in online social networks.

Changes in peak expiratory flow and respiratory strength during the menstrual cycle

This study evaluated the spirometry and respiratory static pressures in 17 young women, twice a week for three successive ovulatory menstrual cycles to determine if such variables changed across the menstrual, follicular, periovulatory, early-tomid luteal and late luteal phases. The factors phases of menstrual cycle and individual cycles had no significant effect on the spirometry variables except for peak expiratory flow (PEF) and respiratory static pressures. Significant weak positive correlations were found between the progesterone:estradiol ratio and PEF and between estrogen and tidal volume (r = 0.37), inspiratory time (r = 0.22), expiratory time (r = 0.19), maximal inspiratory pressure (r = 0.25) and maximal expiratory pressure (r = 0.20) and for progesterone and maximal inspiratory pressure (r = 0.32) during the early-to-mid luteal phase. Although most parameters of the spirometry results did not change during the menstrual cycle, the correlations observed between sexual hormones and respiratory control variables suggest a positive influence of sexual female hormones controlling the thoracic pump muscles in the luteal phase

Modelagem numérica de estruturas de contenção atirantadas em areia

A numerical study on the behavior of tied-back retaining walls in sand, using the finite element method (FEM) is presented. The analyses were performed using the software Plaxis 2D, and were focused on the development of horizontal displacements, horizontal stresses, shear forces and bending moments in the structure during the construction process. Emphasis was placed on the evaluation of wall embedment, tie-back horizontal spacing, wall thickness, and free anchor length on wall behavior. A representative soil profile of a specific region at the City of Natal, Brazil, was used in the numerical analyses. New facilities built on this region often include retaining structures of the same type studied herein. Soil behavior was modeled using the Mohr-Coulomb constitutive model, whereas the structural elements were modeled using the linear elastic model. Shear strength parameters of the soil layers were obtained from direct shear test results conducted with samples collected at the studied site. Deformation parameters were obtained from empirical correlations from SPT test results carried out on the studied site. The results of the numerical analyses revealed that the effect of wall embedment on the investigated parameters is virtually negligible. Conversely, the tie-back horizontal spacing plays an important role on the investigated parameters. The results also demonstrated that the wall thickness significantly affects the wall horizontal displacements, and the shear forces and bending moments within the retaining structure. However, wall thickness was not found to influence horizontal stresses in the structure

Enhancement of thermal stability, strength and extensibility of lipid-based polyurethanes with cellulose-based nanofibers

Cellulose was extracted from lignocellulosic fibers and nanocrystalline cellulose (NC) prepared by alkali treatment of the fiber, steam explosion of the mercerized fiber, bleaching of the steam exploded fiber and finally acid treatment by 5% oxalic acid followed again by steam explosion. The average length and diameter of the NC were between 200-250 nm and 4-5 nm, respectively, in a monodisperse distribution. Different concentrations of the NC (0.1, 0.5, 1.0, 1.5, 2.0 and 2.5% by weight) were dispersed non-covalently into a completely bio-based thermoplastic polyurethane (TPU) derived entirely from oleic acid. The physical properties of the TPU nanocomposites were assessed by Fourier Transform Infra-Red spectroscopy (FTIR), Thermo-Gravimetric Analysis (TGA), Differential Scanning Calorimetry (DSC), X-Ray Diffraction (XRD), Dynamic Mechanical Analysis (DMA) and Mechanical Properties Analysis. The nanocomposites demonstrated enhanced stress and elongation at break and improved thermal stability compared to the neat TPU. The best results were obtained with 0.5% of NC in the TPU. The elongation at break of this sample was improved from 178% to 269% and its stress at break from 29.3 to 40.5 MPa. In this and all other samples the glass transition temperature, melting temperature and crystallization behavior were essentially unaffected. This finding suggests a potential method of increasing the strength and the elongation at break of typically brittle and weak lipid-based TPUs without alteration of the other physico-chemical properties of the polymer. (C) 2012 Elsevier Ltd. All rights reserved.

Aggregates in the strength and gravity regime: Particles sizes in Saturn's rings

Particles in Saturn's main rings range in size from dust to kilometer-sized objects. Their size distribution is thought to be a result of competing accretion and fragmentation processes. While growth is naturally limited in tidal environments, frequent collisions among these objects may contribute to both accretion and fragmentation. As ring particles are primarily made of water ice attractive surface forces like adhesion could significantly influence these processes, finally determining the resulting size distribution. Here, we derive analytic expressions for the specific self-energy Q and related specific break-up energy Q(star) of aggregates. These expressions can be used for any aggregate type composed of monomeric constituents. We compare these expressions to numerical experiments where we create aggregates of various types including: regular packings like the face-centered cubic (fcc), Ballistic Particle Cluster Aggregates (BPCA), and modified BPCAs including e.g. different constituent size distributions. We show that accounting for attractive surface forces such as adhesion a simple approach is able to: (a) generally account for the size dependence of the specific break-up energy for fragmentation to occur reported in the literature, namely the division into "strength" and "gravity" regimes and (b) estimate the maximum aggregate size in a collisional ensemble to be on the order of a few tens of meters, consistent with the maximum particle size observed in Saturn's rings of about 10 m. (c) 2012 Elsevier B.V. All rights reserved.

Influence of shot peening and hard chromium electroplating on the fatigue strength of 7050-T7451 aluminum alloy

Chromium electrodeposition is a technique for the production of functional coatings on engineering components. These coatings are extensively micro-cracked and present high level of hardness, resistance to corrosion and wear and low coefficient of friction. In this paper the shot peening influence on the fatigue strength of aluminum 7050-T7451 alloy chromium electroplated, was investigated.The shot peening process was carried out to create residual stresses using ceramic and glass shots. A hard chromium electroplated coating of 100 mu m thickness was performed on the base material and the shot peened base material surfaces. S-N curves were obtained in axial and bending fatigue tests and compared with the 7050-T7451 aluminum alloy. In order to study the influence of residual stresses on fatigue life, the behavior of compressive residual stress field was measured by an X-ray tensometry.An increase in the axial fatigue strength of 25% and 50% of ceramic and glass shots, respectively, was observed. The lower performance in fatigue life for ceramic-shot peening may be attributed to higher surface damage, as a consequence of the overpeening intensity performed. However, in bending fatigue the behavior was practically equivalent for both processes. Fracture surface analysis by scanning electron microscopy was used to observe crack origin sites from shot peened and chromium electroplated samples. (C) 2006 Elsevier Ltd. All rights reserved.

Influence of anodization on the fatigue strength of 7050-T7451 aluminium alloy

In recent years, with higher demand for improved quality and corrosion resistance, recovered substrates have been extensively used. Consequently residual stresses originated from these coatings reduce the fatigue strength of a component. Due to this negative influence occasioned by corrosion resistance protective coatings, an effective process like shot peening must be considered to improve the fatigue strength. The shot peening treatment pushes the crack sources beneath the surface in most of medium and high cycle cases due to the compressive residual stress field (CRSF) induced. The aim of this study was to evaluate the influence on the fatigue life of anodic films grown on 7050-T7451 aluminium alloy by sulphuric acid anodizing, chromic acid anodizing and hard anodizing. The influence on the rotating and reverse bending fatigue strength of anodic films grown on the aluminium alloy is to degrade the stress life fatigue performance of the base material.A consistent gain in fatigue life in relation to the base material was obtained through the shot peening process in coated specimens, associated to a residual stress field compressive near the surface, useful to avoid fatigue crack nucleation and delay or even stop crack propagation.

Effect of electroless nickel interlayer on the fatigue strength of chromium electroplated AISI 4340 steel

Deposition of wear-resistant hard chromium plating leads to a decrease in the fatigue strength of the base material. Despite the effective protection against wear and corrosion, fatigue life and environmental requirements result in pressure to identify alternatives or to improve conventional chromium electroplating mechanical characteristics. An interesting, environmentally safer and cleaner alternative for the replacement of hard chronic plating is tungsten carbide thermal spray coating, applied by high velocity oxyfuel (HVOF) process.To improve the fatigue strength of aeronautical steel chromium electroplated, shot peening is a successfully used method. Multiple lacer systems of coatings are considered to have larger resistance to crack propagation in comparison with simple layer.The aim of this study was to analyze the effect of nickel underplate on the fatigue strength of hard chromium plated AISI 4340 steel in two mechanical conditions: HRc 39 and HRc 52.Rotating bending fatigue tests results indicate that the clectroless nickel plating underlayer is responsible for the increase in fatigue strength of AISI 4340 steel chromium electroplated. This behavior may be attributed to the largest toughness/ductility and compressive residual stresses which, probably, arrested or delayed the inicrocrack propagation from the hard chromium external layer. The compressive residual stress field (CRSF) induced by the electroplating process was determined by X-ray diffraction method. The evolution of fatigue strength compressive residual stress field CRSF and crack sources are discussed and analyzed by SEM. (c) 2006 Elsevier Ltd. All rights reserved.

The effect of WC-17Co thermal spray coating by HVOF and hard chromium electroplating on the fatigue life and abrasive wear resistance of AISI 4340 high strength steel

One of the most interesting alternatives for replacement of hard chrome plating is tungsten carbide thermal spray coating applied by the high velocity oxy-fuel (HVOF) process which presents a safer, cleaner and less expensive alternative to chromium plating. The objective of this research is to compare the influence of the tungsten carbide-17cobalt (WC- 17Co) coating applied by high velocity oxy fuel (HVOF) process with that of hard-chromium electroplating on the fatigue strength and abrasive wear of AISI 4340 steel.

Effects of surface treatments on the fatigue strength of AISI 4340 aeronautical steel

Internal residual stresses significantly influence the fatigue strength of coated materials. It is well known that chromium plating is the most used electrodeposited coating for important industrial applications. However, pressure to identify alternatives or to improve the chromium electroplating process have increased in recent years, related to the reduction in fatigue strength of the base material and to environmental requirements. The high efficiency and fluoride free hard chromium electroplating there called accelerated) is an improvement to the conventional process. One environmentally safer and cleaner alternative to hard chromium plating is tungsten carbide thermal spray coating applied by the High Velocity Oxy-Fuel (HVOF) process. To increase the fatigue strength of chromium plated materials, coating thickness and microcracks density are important parameters to be controlled. Techniques as compressive residual stresses induced by shot peening and multilayers, are also used. The aim of this study was to analyse the effects on AISI 4340 steel, in the rotating bending fatigue behaviour, of the: tungsten carbide thermal spray coating applied by HP/HVOF process; chemical nickel underplate, and shot peening process applied before coating deposition, in comparison to hard chromium electroplatings. Rotating bending fatigue test results indicate better performance for the conventional hard chromium plating in relation to the accelerated hard chromium electroplating. Tungsten carbide thermal spray coating and accelerated hard chromium plate over nickel resulted in higher fatigue strength when compared to samples conventional or accelerated hard chromium plated. Shot peening showed to be an excellent alternative to increase fatigue strength of AISI 4340 steel hard chromium electroplated. (C) 2001 Elsevier B.V. Ltd. All rights reserved.

Evaluation of WC-17Co and WC-10Co-4Cr thermal spray coatings by HVOF on the fatigue and corrosion strength of AISI 4340 steel

It is known that chromium electroplating is related to the reduction in the fatigue strength of base metal. However, chromium results in protection against wear and corrosion combined with chemical resistance and good lubricity. Environmental requirements are an important point to be considered in the search for possible alternatives to hard chrome plating. Aircraft landing gear manufactures are considering WC thermal spray coating applied by the high-velocity oxygen-fuel (HVOF) process an alternative candidate, which shows performance at least comparable to results, obtained for hard chrome plating. The aim of this study is to compare the influence of WC-17Co and WC-10Co-4Cr coatings applied by HVOF process and hard chromium electroplating on the fatigue strength of AISI 4340 steel, with and without shot peening. S-N curves were obtained in axial fatigue test for base material, chromium plated and tungsten carbide coated specimens. Tungsten carbide thermal spray coating results in higher fatigue strength when compared to hard chromium electroplated. Shot peening prior to thermal spraying showed to be an excellent alternative to increase fatigue strength of AISI 4340 steel. Experimental data showed higher axial fatigue and corrosion resistance in salt fog exposure for samples WC-10Co-4Cr HVOF coated when compared with WC-17Co. Fracture surface analysis by scanning electron microscopy (SEM) indicated the existence of a uniform coverage of nearly all substrates. (C) 2004 Elsevier B.V. All rights reserved.