Zur Bemessung struktureller Klebungen im Stahlbau

  • Design of structural adhesive bondings in steel construction

Abeln, Björn; Feldmann, Markus (Thesis advisor); Ummenhofer, Thomas (Thesis advisor)

1. Auflage. - Aachen : Mainz (2019)
Book, Dissertation / PhD Thesis

In: Schriftenreihe des Instituts für Stahlbau 86
Page(s)/Article-Nr.: xv, 365, XLIII Seiten : Illustrationen, Diagramme

Dissertation, RWTH Aachen University, 2019

Abstract

The permissible area of application for bonded steel and glass constructions has been severely restricted so far and is limited to a few individual cases with high building supervisory obstacles, which is due to a lack of regulations and insufficient and non-uniform material standardization of the adhesives. The aim of this work is therefore to formulate approaches for a design concept for structural, hyperelastic and elastoplastic bonded joints based on simple parameter tests and existing material laws in FEM for applications in steel construction and steel-glass façade construction. A system based on elastic limit extensibility will be developed for this purpose, which will make it possible to design bonded structural joints in the future. First, the mechanical load-bearing behaviour of adhesives and adhesive composites is derived from their polymer structure as an essential basis and an extended classification is established which divides adhesives into energy-elastic and entropy-elastic systems and thus describes them mechanically more accurately. In the following, the building law framework of adhesive joints at national and European level with a focus on ETAG 002 will be explained and the clear weaknesses of the current building law situation will be pointed out. The next step is to explain suitable material models for adhesive bonds, separately for hyperelastic (entropyelastic) and elasto-plastic (energyelastic) adhesives. Viscoelastic models for the description of creep and relaxation are presented, which allow an estimation of creep and relaxation processes in bondings. Subsequently, the basic systematics for determining the material parameters are described on the basis of characteristic value tests. Results of tensile, tensile shear, compression and compression tests are presented and the influence of the substrate is described. The mechanical parameters serve directly as (un-aged) input values for the material models of the numerical calculations as part of the design concept. In order to determine the long-term behaviour of bonded composites, large-format torsion tests are carried out on bonded glass-glass elements under creep and relaxation loading and temperature, whereby an integral shear modulus can be determined for bonded composites as far as possible independently of boundary fault defects. Using short-term creep and relaxation tests on thick tensile shear specimens, isochronous stress-strain curves can be created as limit curves for a design. The ageing tests on thick tensile shear samples show the extent of the degradation and the fact that hard, accelerated ageing methods lead to comparable results and to the formation of plateau areas, which allow a conservative estimation of lower limit values and make a strain-based design approach appear sensible. Using the example of strap joints as a common bonding geometry, the load-bearing behaviour of single- and double-strap joints is investigated with the aid of analytical solutions and the essential influencing factors are worked out. In the course of the following numerical investigations butt-joint tensile tests as validation and component samples will be investigated. The strong influence of the hydrostatic stress level, which becomes visible by the stress multiaxiality, is particularly evident in very thin bondings. A numerical estimation of the maximum adhesive elongation in the course of a design, except in the case of strongly constrained bondings or multi-flank adhesions, appears conceivable in this way, but this fact must be further validated. In the following, methods for the calculation of yield stresses are presented and yield strain curves are determined with the help of the Johnson-Cook model. The investigations culminate in an approach for a concept for the design of structural adhesive connections: on the basis of simple characteristic value and component-like tests, the elastic limit strain curves can be determined for both hyper-elastic and elastoplastic adhesives. This elastic limit strain curve is an adhesive-specific tensile-shear interaction relationship that already includes significant influences from multi-axiality, strain rate, temperature and creep and also guarantees a high material reserve. By reducing the elastic limit strain curve by ageing effects, the long-term behaviour is also recorded. With an FE calculation, the maximum strains in the component can now be determined and delimited from the elastic limit strain curve in the sense of a design. In the future, the concept must be embedded in the framework of the semi-probabilistic safety concept of the Eurocodes. Here a basic procedure and approaches for the safety-theoretical recording of ageing degradation are presented.

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