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Proposal of a Failure Criterion of Adhesively Bonded Connections with Silicone

Staudt, Yves (2018):
Proposal of a Failure Criterion of Adhesively Bonded Connections with Silicone.
Darmstadt, Technische Universität, [Online-Edition: http://tuprints.ulb.tu-darmstadt.de/7287],
[Ph.D. Thesis]

Abstract

In the field of façade engineering, structural silicone sealants have been used in adhesively bonded connections since the 1960s. The low strength and stiffness of silicone rubber compared to other types of adhesives are compensated by the excellent adhesion properties and the good resistance against ageing and environmental influences, like UV radiation. Silicone sealants show a pronounced nonlinear material behaviour. The applicable design concepts in civil engineering propose simplified design equations, which are based on the assumption of a linear material law. Due to the current state of knowledge and to compensate the simplified model assumptions in the design concept, high reduction factors on the material strength and many restrictions on applications are defined. In order to overcome these drawbacks, the stress state within the sealant is increasingly described using the Finite Element Method. Considering the results of these analysis, the assessment of both the complex stress states with a suitable failure criterion and the influence of stress singularities on the failure behaviour constitute inevitable questions.

The present work addresses these two questions. In the first step, the strain magnitude has been determined as a suitable failure criterion for the defect-free bulk material of the considered Dow Corning® 993 structural silicone sealant, subjected to a quasi-static loading. The failure criterion has been calibrated using the results of uniaxial tension as well as circular shear tests and validated with the results of compression tests. The strain magnitude is a strain-based failure criterion, which can be seen as a measure for the distortion of the molecular chains.

In a second step, the stress distribution of the single-lap shear joint has been investigated in detail. For the assessment of the singular stresses and strains at the edge area of the interface between the adhesive and the substrate, referred to as two-material wedge, the so-called coupled stress and energy criterion, a concept of Finite Fracture Mechanics, was extended to nonlinear elastic material behaviour. Based on results of conduced simple shear tests on small bonded connections with varying adhesive thicknesses and overlap lengths, the coupled criterion was used to predict the crack initiation loads and a good agreement with the experimentally recorded values was obtained. Knowing the strength and the critical energy release rate of the material, the crack initiation load and the corresponding crack length are determined in the coupled criterion by solving an optimisation problem based on a Finite Element Analysis. The stress partial criterion has been modified to consider the strain magnitude as a failure criterion. In order to analyse the energy partial criterion, the critical energy release rate of Dow Corning® 993 structural silicone sealant has been determined in Double Cantilever Beam tests using the evaluation method based on the J-integral approach. Furthermore, concepts of the Theory of Critical Distances were used to determine the failure loads of the small scale tests. Unlike as for the notched circular shear tests and the tension tests on small scale bonded connections, good predictions were found in the Finite Element Analysis for the simple shear specimens, when a constant element size and formulation at the vicinity of the two-material wedge were used. Similar results were obtained with the point method and the control volume approach. These findings indicate that the characteristic material length for silicone rubber is not constant.

Item Type: Ph.D. Thesis
Erschienen: 2018
Creators: Staudt, Yves
Title: Proposal of a Failure Criterion of Adhesively Bonded Connections with Silicone
Language: English
Abstract:

In the field of façade engineering, structural silicone sealants have been used in adhesively bonded connections since the 1960s. The low strength and stiffness of silicone rubber compared to other types of adhesives are compensated by the excellent adhesion properties and the good resistance against ageing and environmental influences, like UV radiation. Silicone sealants show a pronounced nonlinear material behaviour. The applicable design concepts in civil engineering propose simplified design equations, which are based on the assumption of a linear material law. Due to the current state of knowledge and to compensate the simplified model assumptions in the design concept, high reduction factors on the material strength and many restrictions on applications are defined. In order to overcome these drawbacks, the stress state within the sealant is increasingly described using the Finite Element Method. Considering the results of these analysis, the assessment of both the complex stress states with a suitable failure criterion and the influence of stress singularities on the failure behaviour constitute inevitable questions.

The present work addresses these two questions. In the first step, the strain magnitude has been determined as a suitable failure criterion for the defect-free bulk material of the considered Dow Corning® 993 structural silicone sealant, subjected to a quasi-static loading. The failure criterion has been calibrated using the results of uniaxial tension as well as circular shear tests and validated with the results of compression tests. The strain magnitude is a strain-based failure criterion, which can be seen as a measure for the distortion of the molecular chains.

In a second step, the stress distribution of the single-lap shear joint has been investigated in detail. For the assessment of the singular stresses and strains at the edge area of the interface between the adhesive and the substrate, referred to as two-material wedge, the so-called coupled stress and energy criterion, a concept of Finite Fracture Mechanics, was extended to nonlinear elastic material behaviour. Based on results of conduced simple shear tests on small bonded connections with varying adhesive thicknesses and overlap lengths, the coupled criterion was used to predict the crack initiation loads and a good agreement with the experimentally recorded values was obtained. Knowing the strength and the critical energy release rate of the material, the crack initiation load and the corresponding crack length are determined in the coupled criterion by solving an optimisation problem based on a Finite Element Analysis. The stress partial criterion has been modified to consider the strain magnitude as a failure criterion. In order to analyse the energy partial criterion, the critical energy release rate of Dow Corning® 993 structural silicone sealant has been determined in Double Cantilever Beam tests using the evaluation method based on the J-integral approach. Furthermore, concepts of the Theory of Critical Distances were used to determine the failure loads of the small scale tests. Unlike as for the notched circular shear tests and the tension tests on small scale bonded connections, good predictions were found in the Finite Element Analysis for the simple shear specimens, when a constant element size and formulation at the vicinity of the two-material wedge were used. Similar results were obtained with the point method and the control volume approach. These findings indicate that the characteristic material length for silicone rubber is not constant.

Place of Publication: Darmstadt
Divisions: 13 Department of Civil and Environmental Engineering Sciences > Institute für Structural Mechanics and Design > Structural Engineering
13 Department of Civil and Environmental Engineering Sciences > Institute für Structural Mechanics and Design
13 Department of Civil and Environmental Engineering Sciences
Date Deposited: 06 May 2018 19:55
Official URL: http://tuprints.ulb.tu-darmstadt.de/7287
URN: urn:nbn:de:tuda-tuprints-72872
Referees: Odenbreit, Prof. Dr. Chrsitoph and Schneider, Prof. Dr. Jens
Refereed / Verteidigung / mdl. Prüfung: 17 November 2017
Alternative Abstract:
Alternative abstract Language
Im Bereich Fassadenbau werden Silikone bereits seit den 1960er Jahren in geklebten lastabtragenden Verbindungen eingesetzt. Die im Vergleich zu anderen Strukturklebstoffen geringe Steifigkeit und Festigkeit werden durch die hervorragende Hafteigenschaften sowie die gute Alterungs- und Witterungsbeständigkeit ausgeglichen. Silikone zeigen ein stark nichtlineares Materialverhalten. Das anzuwendende Bemessungskonzept im Bauwesen schlägt vereinfachte Bemessungsformeln vor, die auf der Annahme eines linear elastischen Materialverhaltens beruhen. Aufgrund des aktuellen Wissensstands und zur Kompensation der vereinfachten Modellannahmen bei der Bemessung werden sehr hohe Abminderungsfaktoren auf der Materialfestigkeit sowie Einschränkungen hinsichtlich der Nutzung vorgeschrieben. Um diese Nachteile zu überwinden wird die Beanspruchung des Silikons zunehmend mit der Finite Elemente Methode ermittelt. Die Bewertung sowohl der komplexen Spannungszustände mittels eines geeigneten Versagenskriteriums als auch der Einfluss von Singularitäten auf das Versagensverhalten stellen hinsichtlich der Ergebnisse einer solchen Berechnung unausweichliche Fragestellungen dar. Die vorliegende Arbeit behandelt diese beiden Fragen. In einem ersten Schritt wurde der Dehnungsbetrag als geeignetes Versagenskriterium für das defektfreie Grundmaterial des Zweikomponentensilikonklebstoffs Dow Corning® 993 unter quasi statischen Belastungen ermittelt. Das Versagenskriterium wurde anhand der Ergebnisse von uniaxialen Zug- sowie Kreisschubversuchen kalibriert und mit den Ergebnissen von Druckversuchen validiert. Der Dehnungsbetrag ist ein dehnungsbasiertes Versagenskriterium, welches als ein Maß für die Verzerrung der Molekülketten gesehen werden kann. In einem zweiten Schritt wurde die Spannungsverteilung in der einschnittig überlappten schubbeanspruchten Klebeverbindung detailliert untersucht. Zur Bewertung der singulären Spannungen und Dehnungen im Eckbereich der Grenzschicht zwischen Fügeteil und Klebstoff, der sogenannten Bimaterialkerbe, wurde das gekoppelte Spannungs- und Energiekriterium, eine Methode der Finiten Bruchmechanik, auf nichtlinear elastisches Materialverhalten erweitert. Auf der Grundlage der Ergebnisse von durchgeführten Schubversuchen an kleinen Bauteilproben mit unterschiedlichen Klebschichtdicken und Überlappungslängen wurde die Rissinitiierungslast mithilfe des gekoppelten Kriteriums ermittelt und eine gute Übereinstimmung mit den experimentellen Daten erzielt. Mit Kenntnis der Festigkeit und der kritischen Energiefreisetzungsrate des Materials werden im Rahmen des gekoppelten Kriteriums auf Grundlage von Finite Elemente Berechnungen in einem Optimierungsproblem sowohl die Rissinitiierungslast als auch die entsprechende Risslänge ermittelt. Das Spannungsteilkriterium wurde dabei auf ein dehnungsbasiertes Kriterium, den Dehnungsbetrag, umformuliert. Zur Anwendung des Energiekriteriums wurden Double Cantilever Beam Versuche am Dow Corning® 993 durchgeführt und die kritische Energiefreisetzungsrate mit der J-Integral Methode ausgewertet. Darüber hinaus wurden Methoden der Theorie der kritischen Distanzen zur Ermittlung der Rissinitiierungslast bei den Kleinteilversuchen herangezogen. Anders als bei den gekerbten kreisförmigen Schubproben und den Zugproben an geklebten Verbindungen konnte eine gute Übereinstimmung mit den experimentellen Ergebnissen bei den Schubversuchen festgestellt werden, wenn in der Finite Elemente Simulation eine konstante Elementgröße und Formulierung gewählt wurden. Ähnliche Ergebnisse wurden mit der Punktmethode und dem Ansatz eines Kontrollvolumens erzielt. Die Untersuchungen zeigen, dass der charakteristische Längenparameter keine Materialkonstante ist.German
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