The durability of building joint sealants is generally assessed using a descriptive methodology involving visual inspection of exposed specimens for defects. It is widely known that this methodology has inherent limitations, including that the results are qualitative. A new test method is proposed that provides more fundamental and quantitative information about changes occurring in a sealant during durability testing. This test method utilizes a stress relaxation experiment to evaluate the non-linear viscoelastic behavior of sealants. In particular, changes in the time dependence of the apparent modulus can be observed and related to molecular changes in the sealant. Such changes often precede the formation of cracks and theultimate failure of the sealant. This paper compares results obtained from the new test method and the currently used descriptive methodology.
{"title":"A Test Method for Monitoring Modulus Changes during Durability Tests on Building Joint Sealants","authors":"C. White, D. Hunston, K. Tan","doi":"10.1520/JAI104090","DOIUrl":"https://doi.org/10.1520/JAI104090","url":null,"abstract":"The durability of building joint sealants is generally assessed using a descriptive methodology involving visual inspection of exposed specimens for defects. It is widely known that this methodology has inherent limitations, including that the results are qualitative. A new test method is proposed that provides more fundamental and quantitative information about changes occurring in a sealant during durability testing. This test method utilizes a stress relaxation experiment to evaluate the non-linear viscoelastic behavior of sealants. In particular, changes in the time dependence of the apparent modulus can be observed and related to molecular changes in the sealant. Such changes often precede the formation of cracks and theultimate failure of the sealant. This paper compares results obtained from the new test method and the currently used descriptive methodology.","PeriodicalId":15057,"journal":{"name":"Journal of Astm International","volume":null,"pages":null},"PeriodicalIF":0.0,"publicationDate":"2012-07-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"88110251","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
M. Feldmann, Björn Abeln, Eva Preckwinkel, A. Wolf, S. Dean
In the recent past there have been calls for transparent and filigree structures in the building industry. Therefore, glass plays an increasing role not only in a classic way, as a space enclosing element, but also more and more in terms of offering load carrying functions. Glass beams for facade elements or floor girders, glass columns, or bracing facade elements are examples of this. To realize such architecturally attractive structures, bonded hybrid steel-glass elements have been developed in which each material is used in an optimized way according to its material properties. Promising examples for such bonded structures are I-beams in which steel flanges and glass webs are connected by linear adhesive bonds. The shear force is carried by the glass web, whereas the bending capacity of the hybrid beam is significantly increased by slender steel flanges compared to the pure glass pane. The shear forces between steel and glass are sustained only by the adhesive between them. In order to maximize the exploitation of both steel and glass, the adhesive on the one hand has to ensure an adequate stiffness but on the other hand has to be flexible enough to allow for a reduction or redistribution of local stress peaks, as well as other constraints such as thermal dilatation. However, the load-bearing capacity of such beams is governed, besides by the mechanical and geometrical characteristics of the adhesive joint, by aging, temperature, and creeping. In this contribution, an approach is shown for characterizing the adhesive joints for hybrid steel-glass beams by means of simplified small-scale tests. Standardized specimens (block shear and tension bulk specimens) and small-scale push-out tests are used to derive adequate mechanical values for analytical and numerical calculations, allowing one to draw conclusions regarding the general load carrying behavior of large-scale hybrid steel-glass beams. The results show that full-scale hybrid steel-glass beams, especially those with butt splice bonded and U-bonded geometries, are feasible using new structural adhesives, predominantly elasto-plastics such as polyurethanes or epoxy resins.
{"title":"Characterization of Adhesive Joints for Hybrid Steel-Glass Beams by Means of Simplified Small Scale Tests","authors":"M. Feldmann, Björn Abeln, Eva Preckwinkel, A. Wolf, S. Dean","doi":"10.1520/JAI104079","DOIUrl":"https://doi.org/10.1520/JAI104079","url":null,"abstract":"In the recent past there have been calls for transparent and filigree structures in the building industry. Therefore, glass plays an increasing role not only in a classic way, as a space enclosing element, but also more and more in terms of offering load carrying functions. Glass beams for facade elements or floor girders, glass columns, or bracing facade elements are examples of this. To realize such architecturally attractive structures, bonded hybrid steel-glass elements have been developed in which each material is used in an optimized way according to its material properties. Promising examples for such bonded structures are I-beams in which steel flanges and glass webs are connected by linear adhesive bonds. The shear force is carried by the glass web, whereas the bending capacity of the hybrid beam is significantly increased by slender steel flanges compared to the pure glass pane. The shear forces between steel and glass are sustained only by the adhesive between them. In order to maximize the exploitation of both steel and glass, the adhesive on the one hand has to ensure an adequate stiffness but on the other hand has to be flexible enough to allow for a reduction or redistribution of local stress peaks, as well as other constraints such as thermal dilatation. However, the load-bearing capacity of such beams is governed, besides by the mechanical and geometrical characteristics of the adhesive joint, by aging, temperature, and creeping. In this contribution, an approach is shown for characterizing the adhesive joints for hybrid steel-glass beams by means of simplified small-scale tests. Standardized specimens (block shear and tension bulk specimens) and small-scale push-out tests are used to derive adequate mechanical values for analytical and numerical calculations, allowing one to draw conclusions regarding the general load carrying behavior of large-scale hybrid steel-glass beams. The results show that full-scale hybrid steel-glass beams, especially those with butt splice bonded and U-bonded geometries, are feasible using new structural adhesives, predominantly elasto-plastics such as polyurethanes or epoxy resins.","PeriodicalId":15057,"journal":{"name":"Journal of Astm International","volume":null,"pages":null},"PeriodicalIF":0.0,"publicationDate":"2012-07-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"73010279","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
This paper summarizes long-term research on fatigue crack closure in several labs of the author. Various experimental techniques were used in different combinations. Primarily, a sophisticated dynamic compliance technique based on highly sensitive resonance vibration measurements and crack tip diffracted ultrasonic wave analysis is applied. Whereas the first method reflects the global through thickness behavior, the second method enables spatial resolution, which is achieved with a reduction of sensitivity yet has the advantage that the signal generation is independent on the stress state ahead of the crack tip. The results allow a coherent view into the effects of the stress state, material plasticity, fracture surface roughness, and oxidation tendency on the closure behavior and, therefore, on both the threshold stress intensity and near-threshold crack propagation. Specifically, the different load ratio, load sequence, and the environmental effects on the threshold values of ferritic, martensitic, and austenitic steels are addressed.
{"title":"Fatigue Crack Closure at Near-Threshold Growth Rates in Steels, Effects of Microstructure, Load Sequence and Environment","authors":"M. Schaper, S. Daniewicz, S. Dean","doi":"10.1520/JAI103968","DOIUrl":"https://doi.org/10.1520/JAI103968","url":null,"abstract":"This paper summarizes long-term research on fatigue crack closure in several labs of the author. Various experimental techniques were used in different combinations. Primarily, a sophisticated dynamic compliance technique based on highly sensitive resonance vibration measurements and crack tip diffracted ultrasonic wave analysis is applied. Whereas the first method reflects the global through thickness behavior, the second method enables spatial resolution, which is achieved with a reduction of sensitivity yet has the advantage that the signal generation is independent on the stress state ahead of the crack tip. The results allow a coherent view into the effects of the stress state, material plasticity, fracture surface roughness, and oxidation tendency on the closure behavior and, therefore, on both the threshold stress intensity and near-threshold crack propagation. Specifically, the different load ratio, load sequence, and the environmental effects on the threshold values of ferritic, martensitic, and austenitic steels are addressed.","PeriodicalId":15057,"journal":{"name":"Journal of Astm International","volume":null,"pages":null},"PeriodicalIF":0.0,"publicationDate":"2012-07-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"79145536","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
This paper shows the evolution of the surface crack front in round bars constituted of different materials (determined by the exponent m of the Paris law), subjected to fatigue tension loading (with free ends) or fatigue bending loading. To this end, a numerical modeling was developed on the basis of a discretization of the crack front (characterized with elliptical shape) and the crack advance at each point perpendicular to such a front, according to a Paris-Erdogan law, using a three-parameter stress intensity factor (SIF). Each analyzed case was characterized by the evolution of the semielliptical crack front, studying the progress with the relative crack depth a/D of the following three key variables: (i) crack aspect ratio a/b (relation between the semiaxes of the ellipse which defines the crack front); (ii) maximum dimensionless SIF; and (iii) minimum dimensionless SIF.
{"title":"Modeling of Surface Crack Advance in Round Wires Subjected to Cyclic Loading","authors":"J. Toribio, J. Matos, B. González, J. Escuadra","doi":"10.1520/JAI103996","DOIUrl":"https://doi.org/10.1520/JAI103996","url":null,"abstract":"This paper shows the evolution of the surface crack front in round bars constituted of different materials (determined by the exponent m of the Paris law), subjected to fatigue tension loading (with free ends) or fatigue bending loading. To this end, a numerical modeling was developed on the basis of a discretization of the crack front (characterized with elliptical shape) and the crack advance at each point perpendicular to such a front, according to a Paris-Erdogan law, using a three-parameter stress intensity factor (SIF). Each analyzed case was characterized by the evolution of the semielliptical crack front, studying the progress with the relative crack depth a/D of the following three key variables: (i) crack aspect ratio a/b (relation between the semiaxes of the ellipse which defines the crack front); (ii) maximum dimensionless SIF; and (iii) minimum dimensionless SIF.","PeriodicalId":15057,"journal":{"name":"Journal of Astm International","volume":null,"pages":null},"PeriodicalIF":0.0,"publicationDate":"2012-07-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"90812804","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
H. Miyauchi, M. Lacasse, N. Enomoto, Shigeki Murata, Kyoji Tanaka
In Japan acrylic sealants are traditionally the sealant products of choice when specified for use between autoclaved lightweight concrete (ALC) panels. Although, in general terms, the mechanisms of the deterioration of acrylic sealants are well known its long-term durability to outdoor exposure has not, however, been fully investigated. The research described in this paper focuses on the change in the properties and deterioration of acrylic sealant products when exposed to outdoor testing. The two stage project consisted of (i) on-site investigations of deteriorated acrylic sealants that had been placed in external joints of ALC-clad buildings; and (ii) outdoor exposure testing of different types of acrylic sealant in three climate regions located in Japan. The results of the work from the first stage of the study revealed the following. Two-sided adhesion joint configurations installed in deep panel ALC cladding were more reliable than three-sided adhesion joints used for thin panel ALC cladding from the viewpoint of the durability of the sealed joint installed in actual buildings. Most fractures of the sealed joint could be characterized as failure in peel (or thin layer cohesive failures), in which the sealant ruptured at the interface with the ALC substrate to which it was applied. Additionally, in 47 of 62 locations surveyed, surface cracks were apparent on the coating that had been applied to protect the sealant. The second stage of the project focused on the degree of deterioration of coated and non-coated acrylic sealants subjected to outdoor exposure testing in a cold, a warm, and a subtropical climate. Results from this stage showed that aging of the sealant, as determined by the degree of surface cracking, expectedly depended on the local temperature and the respective degree of exposure to solar radiation. It was determined that the longer the exposure period, the lower the tensile performance of the acrylic sealants. The elongation of three-sided adhesive joint configurations after 5 years exposure testing decreased remarkably and their maximum elongation was less than 50 %. A significant number of sealed joints after 5 years ofexposure had ALC substrate failure.
{"title":"Durability of Acrylic Sealants Applied to Joints of Autoclaved Lightweight Concrete Walls: Evaluation of Exposure Testing","authors":"H. Miyauchi, M. Lacasse, N. Enomoto, Shigeki Murata, Kyoji Tanaka","doi":"10.1520/JAI104063","DOIUrl":"https://doi.org/10.1520/JAI104063","url":null,"abstract":"In Japan acrylic sealants are traditionally the sealant products of choice when specified for use between autoclaved lightweight concrete (ALC) panels. Although, in general terms, the mechanisms of the deterioration of acrylic sealants are well known its long-term durability to outdoor exposure has not, however, been fully investigated. The research described in this paper focuses on the change in the properties and deterioration of acrylic sealant products when exposed to outdoor testing. The two stage project consisted of (i) on-site investigations of deteriorated acrylic sealants that had been placed in external joints of ALC-clad buildings; and (ii) outdoor exposure testing of different types of acrylic sealant in three climate regions located in Japan. The results of the work from the first stage of the study revealed the following. Two-sided adhesion joint configurations installed in deep panel ALC cladding were more reliable than three-sided adhesion joints used for thin panel ALC cladding from the viewpoint of the durability of the sealed joint installed in actual buildings. Most fractures of the sealed joint could be characterized as failure in peel (or thin layer cohesive failures), in which the sealant ruptured at the interface with the ALC substrate to which it was applied. Additionally, in 47 of 62 locations surveyed, surface cracks were apparent on the coating that had been applied to protect the sealant. The second stage of the project focused on the degree of deterioration of coated and non-coated acrylic sealants subjected to outdoor exposure testing in a cold, a warm, and a subtropical climate. Results from this stage showed that aging of the sealant, as determined by the degree of surface cracking, expectedly depended on the local temperature and the respective degree of exposure to solar radiation. It was determined that the longer the exposure period, the lower the tensile performance of the acrylic sealants. The elongation of three-sided adhesive joint configurations after 5 years exposure testing decreased remarkably and their maximum elongation was less than 50 %. A significant number of sealed joints after 5 years ofexposure had ALC substrate failure.","PeriodicalId":15057,"journal":{"name":"Journal of Astm International","volume":null,"pages":null},"PeriodicalIF":0.0,"publicationDate":"2012-07-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"90980267","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Conventional methods to estimate variable-amplitude fatigue life revolve either around cumulative damage analysis using the local stress-strain approach, or, around one of the crack growth load interaction models. Despite advances in modeling the mechanics of fatigue, none of these methods can faithfully reproduce the near-threshold variable amplitude fatigue response that determines the durability of machines and structures primarily because they fail to model the science behind the residual stress effect. Residual stress effects have a strong bearing on metal fatigue and owe their influence to the moderation of crack-tip surface chemistry and surface physics. This demands the treatment of threshold stress intensity as a variable, sensitive to load history. The correct estimation of crack closure is also crucial to determining the variable amplitude fatigue response and demands assessment of the cyclic plastic zone stress-strain response.
{"title":"Unraveling the Science of Variable Amplitude Fatigue","authors":"R. Sunder, S. Daniewicz, S. Dean","doi":"10.1520/JAI103940","DOIUrl":"https://doi.org/10.1520/JAI103940","url":null,"abstract":"Conventional methods to estimate variable-amplitude fatigue life revolve either around cumulative damage analysis using the local stress-strain approach, or, around one of the crack growth load interaction models. Despite advances in modeling the mechanics of fatigue, none of these methods can faithfully reproduce the near-threshold variable amplitude fatigue response that determines the durability of machines and structures primarily because they fail to model the science behind the residual stress effect. Residual stress effects have a strong bearing on metal fatigue and owe their influence to the moderation of crack-tip surface chemistry and surface physics. This demands the treatment of threshold stress intensity as a variable, sensitive to load history. The correct estimation of crack closure is also crucial to determining the variable amplitude fatigue response and demands assessment of the cyclic plastic zone stress-strain response.","PeriodicalId":15057,"journal":{"name":"Journal of Astm International","volume":null,"pages":null},"PeriodicalIF":0.0,"publicationDate":"2012-07-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"84391828","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
The weatherability of construction sealants is a highly important performance criterion for the prediction of their service lives. Currently, the evaluation of a sealant’s surface weatherability is carried out mainly via qualitative visual judgment. One objective of this paper is to establish a quantitative method for the assessment of surface degradation. A further objective is the evaluation of the relationship between outdoor and accelerated exposure with regard to surface degradation. The understanding of this relationship is important for the shortening of the development cycle and the prediction of a sealant’s service life. The paper presents information on the weatherability of construction sealants based on a recently developed test specimen design that allows simultaneous exposure of the sealant to forced compression and extension movement in a single specimen with cyclic movement and weathering carried out simultaneously. A quantitative method for the assessment of surface cracks is employed, and the relationship between outdoor and accelerated weathering exposure is evaluated by using metrics that indicate the degree of surface cracking as a new semi-quantitative criterion of surface degradation.
{"title":"Attempt at Quantification of Surface Degradation and Evaluation of Relationship between Outdoor and Accelerated Exposure of Construction Sealants","authors":"N. Enomoto, A. Ito, Kyoji Tanaka","doi":"10.1520/JAI104048","DOIUrl":"https://doi.org/10.1520/JAI104048","url":null,"abstract":"The weatherability of construction sealants is a highly important performance criterion for the prediction of their service lives. Currently, the evaluation of a sealant’s surface weatherability is carried out mainly via qualitative visual judgment. One objective of this paper is to establish a quantitative method for the assessment of surface degradation. A further objective is the evaluation of the relationship between outdoor and accelerated exposure with regard to surface degradation. The understanding of this relationship is important for the shortening of the development cycle and the prediction of a sealant’s service life. The paper presents information on the weatherability of construction sealants based on a recently developed test specimen design that allows simultaneous exposure of the sealant to forced compression and extension movement in a single specimen with cyclic movement and weathering carried out simultaneously. A quantitative method for the assessment of surface cracks is employed, and the relationship between outdoor and accelerated weathering exposure is evaluated by using metrics that indicate the degree of surface cracking as a new semi-quantitative criterion of surface degradation.","PeriodicalId":15057,"journal":{"name":"Journal of Astm International","volume":null,"pages":null},"PeriodicalIF":0.0,"publicationDate":"2012-07-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"76625536","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
G. Schueneman, C. Hunt, Steven J. Lacher, C. White, D. Hunston
The durability of sealants is dictated by many factors such as joint design, surface preparation, application, formulation, joint movement, and weather. Among these factors the link between formulation (material behavior) and weathering durability is difficult to assess in short term tests. We attempt to address this challenge by monitoring changes in apparent modulus during exposure to outdoor weathering and cyclic strain. This is accomplished via custom built systems that apply cyclic strain to 16 samples simultaneously via programs that simulate wood (cold compression) and concrete=metal (hot compression) construction materials. A key finding of the research presented here is that changes in apparent modulus are primarily driven by underlying changes in the compression set, a potentially critical contributor to stress in structures during rapid temperature changes. Detection of the compression set is made possible by the in situ material property assessments used in this research. Aging tests that rely on offline evaluation of property changes may miss or underestimate this effect on the sealant's stiffness due to time delay and=or losing track of the original zero stress-zero strain state.
{"title":"In Situ Measurement of Compression Set in Building Sealants During Outdoor Aging","authors":"G. Schueneman, C. Hunt, Steven J. Lacher, C. White, D. Hunston","doi":"10.1520/JAI104142","DOIUrl":"https://doi.org/10.1520/JAI104142","url":null,"abstract":"The durability of sealants is dictated by many factors such as joint design, surface preparation, application, formulation, joint movement, and weather. Among these factors the link between formulation (material behavior) and weathering durability is difficult to assess in short term tests. We attempt to address this challenge by monitoring changes in apparent modulus during exposure to outdoor weathering and cyclic strain. This is accomplished via custom built systems that apply cyclic strain to 16 samples simultaneously via programs that simulate wood (cold compression) and concrete=metal (hot compression) construction materials. A key finding of the research presented here is that changes in apparent modulus are primarily driven by underlying changes in the compression set, a potentially critical contributor to stress in structures during rapid temperature changes. Detection of the compression set is made possible by the in situ material property assessments used in this research. Aging tests that rely on offline evaluation of property changes may miss or underestimate this effect on the sealant's stiffness due to time delay and=or losing track of the original zero stress-zero strain state.","PeriodicalId":15057,"journal":{"name":"Journal of Astm International","volume":null,"pages":null},"PeriodicalIF":0.0,"publicationDate":"2012-07-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"81715655","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
This paper presents the experimental outcome of a study of the pool boiling heat transfer characteristics of alumina and CuO nanofluid in distilled water using a 0.19 mm diameter NiCr wire. A series of experiments were conducted in order to visualize the flow, critical heat flux (CHF) enhancement, and transient characteristics of nanofluid. The boiling phenomenon was visualized using a 0.1 g/l concentration of alumina nanofluid. The average bubble diameter was measured and was found to increase with increased heat flux. The average bubble contact angle decreased from 69° during the initial stages of boiling to 33° at CHF. Massive vapour bubbles were observed on the test heater surface near the CHF, inducing vapour blankets and forming hot/dry spots. The increase in the CHF could be well explained by the hot/dry spot theory. Pool boiling experiments conducted using low volume concentrations of CuO-water nanofluid at atmospheric pressure in distilled water showed an increase in the CHF by 30 % at a 0.3 g/l concentration. The transient behaviour of nanofluid, examined by exposing the heater surface at a constant heat flux of 700 kW/m2, indicated CHF enhancement of 5.21 % to 6.77 % for the two time durations. Based on the experimental investigations, it was concluded that the CHF enhancement is due to nanoparticle coating, which changes the thickness of the surface as a function of time and surface wettability and corroborates the hot/dry spot theory.
{"title":"Flow Visualization, Critical Heat Flux Enhancement, and Transient Characteristics in Pool Boiling Using Nanofluids","authors":"R. Hegde, S. Rao, R. Reddy","doi":"10.1520/JAI104443","DOIUrl":"https://doi.org/10.1520/JAI104443","url":null,"abstract":"This paper presents the experimental outcome of a study of the pool boiling heat transfer characteristics of alumina and CuO nanofluid in distilled water using a 0.19 mm diameter NiCr wire. A series of experiments were conducted in order to visualize the flow, critical heat flux (CHF) enhancement, and transient characteristics of nanofluid. The boiling phenomenon was visualized using a 0.1 g/l concentration of alumina nanofluid. The average bubble diameter was measured and was found to increase with increased heat flux. The average bubble contact angle decreased from 69° during the initial stages of boiling to 33° at CHF. Massive vapour bubbles were observed on the test heater surface near the CHF, inducing vapour blankets and forming hot/dry spots. The increase in the CHF could be well explained by the hot/dry spot theory. Pool boiling experiments conducted using low volume concentrations of CuO-water nanofluid at atmospheric pressure in distilled water showed an increase in the CHF by 30 % at a 0.3 g/l concentration. The transient behaviour of nanofluid, examined by exposing the heater surface at a constant heat flux of 700 kW/m2, indicated CHF enhancement of 5.21 % to 6.77 % for the two time durations. Based on the experimental investigations, it was concluded that the CHF enhancement is due to nanoparticle coating, which changes the thickness of the surface as a function of time and surface wettability and corroborates the hot/dry spot theory.","PeriodicalId":15057,"journal":{"name":"Journal of Astm International","volume":null,"pages":null},"PeriodicalIF":0.0,"publicationDate":"2012-05-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"86466918","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
The use of Electron Paramagnetic Resonance (EPR) spectroscopy with materials such as alanine is well established as a technique for measurement of ionizing radiation absorbed dose in photon and electron fields such as Co-60, high-energy bremsstrahlung and electron-beam fields [1]. In fact, EPR/Alanine dosimetry has become a routine transfer standard for national standards bodies such as NIST and NPL. In 1992 the Radiation Metrology Laboratory (RML) at Sandia National Laboratories implemented EPR/Alanine capabilities for use in routine and calibration activities at its Co-60 and pulsed-power facilities. At that time it also investigated the usefulness of the system for measurement of absorbed dose in the mixed neutron/photon environments of reactors such as the Sandia Pulsed Reactor and the Annular Core Research Reactor used for hardness testing of electronics. The RML concluded that the neutron response of alanine was a sufficiently high fraction of the overall dosimeter response that the resulting uncertainties in the photon dose would be unacceptably large for silicon-device testing. However, it also suggested that non-hydrogenous materials such as polytetrafluoroethylene (PTFE) would exhibit smaller neutron response and might be useful in mixed environments. Preliminary research with PTFE in photon environments indicated considerable promise, but further development was not pursuedmore » at that time. Because of renewed interest in absorbed dose measurements that could better define the individual contributions of photon and neutron components to the overall dose delivered to a test object, the RML has re-initiated the development of an EPR/PTFE dosimetry system. This effort consists of three stages: 1) Identification of PTFE materials that may be suitable for dosimetry applications. It was speculated that the inconsistency of EPR signatures in the earlier samples may have been due to variability in PTFE manufacturing processes. 2) Characterization of dosimetry in photon-only environments. This is necessary to establish requirements for sample preparation, operating parameters and limitations for use in well-defined and predictable environments prior to deployment in the less well-defined mixed environments of test reactors. 3) Characterization of the EPR responses obtained with PTFE in mixed neutron/photon fields. This includes evaluation of the neutron and photon contributions to response, determination of applicable of neutron fluence and photon dose ranges. This paper presents a summary of the research, a description of the EPR/PTFE dosimetry system, and recommendations for preparation and fielding of the dosimetry in photon and mixed neutron/photon environments. (authors)« less
{"title":"EPR/PTFE Dosimetry for Test Reactor Environments","authors":"D. Vehar, P. Griffin, T. Quirk","doi":"10.1520/JAI104051","DOIUrl":"https://doi.org/10.1520/JAI104051","url":null,"abstract":"The use of Electron Paramagnetic Resonance (EPR) spectroscopy with materials such as alanine is well established as a technique for measurement of ionizing radiation absorbed dose in photon and electron fields such as Co-60, high-energy bremsstrahlung and electron-beam fields [1]. In fact, EPR/Alanine dosimetry has become a routine transfer standard for national standards bodies such as NIST and NPL. In 1992 the Radiation Metrology Laboratory (RML) at Sandia National Laboratories implemented EPR/Alanine capabilities for use in routine and calibration activities at its Co-60 and pulsed-power facilities. At that time it also investigated the usefulness of the system for measurement of absorbed dose in the mixed neutron/photon environments of reactors such as the Sandia Pulsed Reactor and the Annular Core Research Reactor used for hardness testing of electronics. The RML concluded that the neutron response of alanine was a sufficiently high fraction of the overall dosimeter response that the resulting uncertainties in the photon dose would be unacceptably large for silicon-device testing. However, it also suggested that non-hydrogenous materials such as polytetrafluoroethylene (PTFE) would exhibit smaller neutron response and might be useful in mixed environments. Preliminary research with PTFE in photon environments indicated considerable promise, but further development was not pursuedmore » at that time. Because of renewed interest in absorbed dose measurements that could better define the individual contributions of photon and neutron components to the overall dose delivered to a test object, the RML has re-initiated the development of an EPR/PTFE dosimetry system. This effort consists of three stages: 1) Identification of PTFE materials that may be suitable for dosimetry applications. It was speculated that the inconsistency of EPR signatures in the earlier samples may have been due to variability in PTFE manufacturing processes. 2) Characterization of dosimetry in photon-only environments. This is necessary to establish requirements for sample preparation, operating parameters and limitations for use in well-defined and predictable environments prior to deployment in the less well-defined mixed environments of test reactors. 3) Characterization of the EPR responses obtained with PTFE in mixed neutron/photon fields. This includes evaluation of the neutron and photon contributions to response, determination of applicable of neutron fluence and photon dose ranges. This paper presents a summary of the research, a description of the EPR/PTFE dosimetry system, and recommendations for preparation and fielding of the dosimetry in photon and mixed neutron/photon environments. (authors)« less","PeriodicalId":15057,"journal":{"name":"Journal of Astm International","volume":null,"pages":null},"PeriodicalIF":0.0,"publicationDate":"2012-05-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"88771603","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
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