Adhesive bonding in steel construction challenge and innovation

Adhesive Bonding in Steel Construction Challenge and Innovation Procedia Engineering 172 ( 2017 ) 186 – 193 1877 7058 © 2017 Published by Elsevier Ltd This is an open access article under the CC BY NC[.]

Procedia Engineering 172 ( 2017 ) 186 – 193

1877-7058 © 2017 Published by Elsevier Ltd This is an open access article under the CC BY-NC-ND license

(http://creativecommons.org/licenses/by-nc-nd/4.0/)

Peer-review under responsibility of the organizing committee of MBMST 2016

doi: 10.1016/j.proeng.2017.02.048

ScienceDirect

Modern Building Materials, Structures and Techniques, MBMST 2016

Adhesive bonding in steel construction - Challenge and innovation

Dilgerb

a Chair of Steel and Timber Structures, BTU, Konrad-Wachmann-Allee 2, 03046 Cottbus, Germany

b Institute of Joining and Welding, Technische Universität Braunschweig, Langer Kamp 8, 38106 Braunschweig, Germany

Abstract

Despite much advancement of typical joining techniques in steel construction, fundamental problems, like residual stresses for welds as well as weakening of the cross section for bolts and screws, still remain The application of bonded joints could improve the situation The automotive industry shows the potential of this certain joining technique since years Even in civil engineering and especially in steel construction, researchers are continuously establishing the bonding technology as a structural element On the one hand bonded joints mean a challenge, but on the other hand an innovation, what is shown in this paper

© 2016 The Authors Published by Elsevier Ltd

Peer-review under responsibility of the organizing committee of MBMST 2016

Keywords: Adhesive; joining technology; steel structure

1 Challenge

1.1 Design of bondline geometry

A great number of adhesives with different carrying and deformation behavior, as well as varying handling properties are available for the design of bonded structures Due to a general lack of experience the selection of suitable adhesive systems is a difficult task for a civil engineer General requirements for an adhesive are characterized by a specific viscosity for the manufacturing and a sufficient carrying and deformation capacity of the bondline Despite these parameters the properties of the adherent surface play an important role for the compound

* Corresponding author Tel.: +49-355-69-2255; fax: +49-355-69-2144

E-mail address: yvonne.ciupack@b-tu.de

© 2017 Published by Elsevier Ltd This is an open access article under the CC BY-NC-ND license

(http://creativecommons.org/licenses/by-nc-nd/4.0/)

Peer-review under responsibility of the organizing committee of MBMST 2016

Thus, cleaning and degreasing of steel surfaces are essential during the bonding process In addition, mechanical pre-treatments such as blasting can improve the adhesion properties

Fig 1 Influence of geometric design and loading of a bondline

Moreover, the kind of loading has a big influence on the carrying capacity of a bonded steel construction Uneven and combined loads as well as peel stresses should be avoided, as shown in Fig 1 But uniform distributed tension, pressure and shear impacts are suitable By slopping the adherent ends stress peaks can be reduced, which increases the carrying capacity of the bonded connection

1.2 Carrying and deformation behavior of bonded constructions

The carrying and deformation behavior of bondlines is nonlinear and time dependent According to the kind of adhesive basis distinct creep and relaxation effects can be observed The long-term mechanical behavior is influenced by environmental effects, e.g temperature, UV-radiation and humidity These impacts can lead to irreversible degradation of the mechanical bondline properties Particular attention is paid to the temperature dependency, because adhesives are characterized by a so-called glass transition temperature If a bondline is exposed to high thermal impacts, higher than the glass transition, it shows increased deformations and a decreased ultimate stress Low temperatures lead to a high stiffness and an increased notch sensitivity of the bondline These relations can be utilized in application, e.g elastic adhesives and sealing materials are used for temperatures higher than the glass transition to avoid thermal residual stresses and compensate thermal strains

Because of the complexity of the carrying and deformation behavior, an appropriate description of the stress distribution in a bondline means complex calculations, e.g numeric simulations Since simple and standardized concepts are necessary for the design in civil engineering, the development of suitable analytical models poses a special challenge

Fig 2 (a) Butt joint test; (b) Lap shear test

Another important challenge is the determination of characteristic material parameters for the design of bondlines Whereas the temperature dependency of material properties for steel adherents need only to be considered for fire design, it is not sufficient for adhesives The detection of time dependent material parameters, by consideration of damaging environment impacts, is imperative But currently, only simple test methods are available, which exclute the implementation of suchlike influence factors On the one hand the stress to shear-strain relation can be described with shear lap tests according to DIN EN 14869-2 [1] On the other hand evidence about the carrying behavior of bondlines under normal stress states can be obtained from butt joint tests according to DIN EN 15870 [2] The specimens and loading situation for these experimental investigations are shown in Fig 2

Uniform load Shear: "suitable" Pressure: "suitable" Tension: "less suitable"

Uneven and combinated load Shear and peel: "bad" peel: "bad" cleavage: "bad"

With these destructive tests, the bondline properties can be characterized, but initially without consideration of time and environment dependent effects Interaction relations can be deduced from multi-axial tests, such as arcan-tests The offhanded transfer of knowledge from these small scale specimen tests to typical steel construction dimension is not possible, so specimen component tests still remain necessary

2 Innovation

The Chair of Steel and Timber Structures of the Brandenburg University of Technology deals with two application examples of bonding technology in different research activities A bonded connection of a trapezoidal façade and a bonded reinforcement of a hollow section, for a typical transom-mullion-façade, are investigated Façades with trapezoidal sheets are deployed predominantly for light weight halls and industry constructions After the erection of a steel skeleton as the primary structure, a substructure of light weight steel profiles is installed, facilitates the connection of the trapezoidal sheets by screws or cartridge-fired pins This joining method with fasteners weakens the cross section and can lead to inadvertent dents, scratches or holes in the façade view In addition, the self cleaning effect is dysfunctional in the area of salient fastener heads Due to these problems of typical joints, the possibilities of indirect bonded façade connection have been developed, which are shown in Fig 3a The connection concepts are designed so that dead loads are carried by special load application points at the top

of the façade Thus, creep effects of the bondline are avoided Specially formed connection profiles in addition to a bolt connection, realized with an elongated hole, allow the deformation of the outer shell without introducing constrains into the bondline

Fig 3 (a) Structures for bonded façade connections with different shape of connection profile; (b) Bonded façade reinforcement

Principal requirements of architects and contractors for representative buildings are high side view transparency, structured and light filled façades In a lot of cases the transom-mullion-construction is realized, so in order to achieve the desired impression, it is necessary to put the primary structure in the background That requires a minimization of the outer dimensions by increasing the stiffness of the structured member at the same time An innovative solution is offered by the adhesive bonding technology With a steel reinforcement, bonded to the inside

of a typical hollow section, a new type of compound cross section with an increased stiffness and carrying capacity

is created (Fig 3b) The advantage of a reinforced transom-profile compared to a hollow section with higher thickness is the lesser slenderness of the webs, thus avoiding the buckling of webs under pressure The principle of façade hollow profiles with an inner reinforcement is similar to glued laminated timber, but in contrast to timber structure, a bondline design and evidence is necessary, due to the higher stiffness of the steel adherents

The presented constructions are developed so that all bonded connections can be realized in a laboratory under controlled conditions Non-reproducible bondlines and bonding on the side are avoided and the pre-prepared elements can be mounted on site The joining with adhesives is introduced as an additional process step, without changing the established principles and methods of façade construction

cover

outside

inside

isolation glass seal

beam screw

bondline

a

b

2.2 Experimental investigations of bonded steel constructions under static loading

Fig 4 (a) Test setup; (b) Test results

The application examples from Fig 2 were experimentally investigated, in order to determine the load and deformation behavior, as well as to deduce a Eurocode-based design concept [3] A strip coated trapezoidal profile with a thickness of 1 mm was used for the façade connection, and a simplified L-profile with a thickness of 2 mm was chosen to create the connection profile With a two-component copolymer based adhesive a bondline thickness

of 2 mm was set The surface was pretreated by blasting with rounded cut wire to improve the adhesion properties of the steel adherents The experimental set up, shown in Fig 4 accurately represents the conditions in the mounted state of the façade connection, because the general impact results from wind, acting perpendicular to the bondline During the conduction of the displacement-controlled test, the ends of the trapezoidal element were braced against the machine table All joints failed with a cohesive failure and behaved strongly non-linear (Fig 4b) The bondline allows large deformations, which is positive in regard to advance notice of failure

Specific experimental investigations were carried out, in order to evaluate the structural behavior of the façade reinforcement A typical hollow section with a width of 60 mm, height of 181.5 mm and a wall thickness of 2.5 mm was chosen Due to the interesting bondline failure, which should be investigated in the tests, the profile length was set to 1 m, based on pilot tests and analytical analysis With a special pneumatic method, which was developed for this application example, a sheet metal steel (width: 50 mm; height: 20 mm) was bonded to the inner side of the hollow profile The pre-treatment of the specimens was defined by blasting the reinforcement with rounded cut wire,

as well as cleaning and refining all surfaces with acetone To accurately reflect the install conditions, the load was transferred to the specimen by two double shear connections at every load application point and at the supports These points were designed to allow tension-free horizontal deformations and rotations

0 500 1000 1500 2000 2500

Deformation [mm]

0 50 100 150 200 250

0,0 2,0 4,0 6,0 8,0

Deformation in axis 0 [mm]

w

3

F/2

0

1

F F/2

2 w

w w

1

F F/2

w

2 w 3 w

F/2

1000

A

A

10

Cross section A-A

50

The evaluation of the experiment shows that the bondline of all specimens failed with a cohesive failure During the tests a distinct leveling of the ultimate load was observed, which develops with the bondline failure Compared

to a reference specimen without reinforcement (dashed line in Fig 5b), an improvement of stiffness and carrying capacity can be deduced

2.3 Design of bonded joints in steel structures

In a previous research project [3] Eurocode-based design rules for the presented application examples were developed This was realized by a juxtaposition of test results with solutions of analytical models, which are described in [4] The main idea of this procedure is to separate the bonded joint into theoretical components, which can be treated separately Based on the weakest-link-approach, the carrying capacity of the connection or compound can be expressed by the smallest value of calculated resistances of the bondline and the steel adherents Herein the bondline is modeled by spring elements, which can be characterized by small scale specimen tests (see chapter 1.2), representing the specific loading situation of the application example The principle of the component model is shown in Fig 6

Fig 6 Component spring model

Meinz [4] developed two analytical models to describe the carrying behavior of the presented applications, based

on the theory of elastic supported slabs He assumed the properties of a bondline to be expressed by characteristic values of tension strength (Vk), shear strength (Wk), Young’s modulus (Ek) and shear modulus (Gk) These parameters

were determined by butt joint and lap shear tests (see Fig 2) according to the Eurocode requirements and are summarized in table 1

Table 1 Characteristic material properties of different adhesives [N/mm²]

In accordance to the regulation of the Eurocode concept [5], semi-probabilistic studies for both models were carried out The main objective was to determine partial safety factors JM for the resistance models and to introduce

so-called conversion factors to take into account temperature dependent (Kt) and manufacturing dependent effects

(Km) Regarding the cohesive bondline failure mode, the design value of the resistance can be expressed with Eq

(1)

˜ ˜ ˜

k

M

R

The calibration process resulted in a partial safety factor for the bonded façade connection of 2.16 and for the façade reinforcement of 1.56 These values are based on defined reference conditions during the test, which are assumed to be 20°C and 2 mm bondline thickness for the elastic copolymer or 0.2 mm for the epoxy resin based adhesive To consider influences from temperature or changes of the thickness of the adhesive layer, small scale tests were repeated while varying certain test conditions To determine the conversion factors, probabilistic

conceptions with respect to probability functions are employed The distribution function of resistance must be formulated depending on the variables temperature and bondline thickness This procedure results in conversion factors, which are summarized in table 2 for temperature dependent effects and in table 3 for manufacturing dependent influences

Table 2 Conversion factors for temperature dependent effects

Kt

-20°C ≤ T ≤ 20°C 20°C < T ≤ 80°C -20°C ≤ T ≤ 20°C 20°C < T ≤ 80°C

Table 3 Conversion factors for manufacturing dependent effects

Km

2 mm ≤ d k ≤ 5 mm 0.2 mm ≤ d k ≤ 0.5 mm 0.5 mm < dk ≤ 2 mm

As shown within the results, the investigated effects depend on the type of loading Consequently different conversion factors for normal and shear stress conditions are recommended Due to the strong decrease of the material parameters at high temperatures and different bondline thicknesses, the conversion factors are declared for specific ranges

2.4 Adhesive bonded steel structures under cyclic loading

As mentioned before, the performance of adhesive bonded joints depends on time and environment influences The carrying and deformation behavior of a bondline is characterized by creep and relaxation effects, affected by temperature and permanently changed by ageing processes The objective of an ongoing research project is to carry out cyclic test procedures for the presented application examples, taking into account these polymer specific properties [6]

In a first step, representative load functions for the test sequences are deduced from real measured data Because the studied façade connection is mainly stressed by wind and temperature, this investigation focuses on environmental data from different weather stations in Germany The extreme temperatures for a specific period, as well as the variation of thermal impacts over time, are determined by statistical means For the deduction of the façade temperature based on measured air temperature, different thermo-physical effects, such as radiation and convection, are considered and shown in Fig 7a The obtained values from this simulation describe a temperature range from -20°C to 80°C, in agreement with Eurocode 1 [7] and the building code for external walls [8]

-20 -10 0 10 20 30 40 50 60 70 80 90 Jan Feb Mar Apr May June July Aug Sept Oct Nov Dec

Free convection

Heat radiation Heat passage

Heat radiation

Forced convection

Direct and diffuse

solar radiation

A suitable loading cycle (see Fig 7b) was determined based on statistical studies regarding an hourly approach The minimum temperature is assumed to be -20°C (dashed line in Fig 7b) according to the code requirements [7] Thus, for every month a so-called model day is provided, which ranges from the minimum to the maximum temperature The curve in Fig 7b shows the variation of thermal impacts during the monthly model day During the experimental investigations a model day needs to be repeated, according to the number of days a month

Moreover, a test procedure for simulation of wind loading is extrapolated from actual measured wind speed data Due to the combination of the typical wind load by a quasi-static mean and a turbulent time-dependent part, the test cycle was developed from measurements with various sampling rates To describe the time steady wind load, the 10 minute mean value of the windiest German weather station in Helgoland (from the archive of “Deutscher Wetterdienst”) is used This is combined with the wind turbulences measured in Karlsruhe [9] with a sampling rate

of 1 Hz The composite wind speed function is transferred to a load to number-of-cycles function for a 10 m wide quadratic building, as a model façade In the framework of the project, this specific function was analyzed by the rainflow method, which results in an amplitude collective for wind load shown in Fig 8

Fig 8 Amplitude collective for wind loads

The aim for the next step, the testing of application examples for cyclic loading, is to simplify the load collective for wind and reduce the testing time by means of endurance strength theory In order to describe the general behavior of bonded joints under cyclic loading, some pre-investigations on small scale specimens and the façade connection were carried out under constant amplitudes The stress ratio is chosen with a value of 0.1, which means a varying but permanent tensile loading for the bondline To prevent heating of the adhesive due to mechanical load, the tests were carried out with a frequency of 5 Hz If 2·106 load cycles are reached and no damage is detected, the value of the load is assigned within the fatigue strength The results of these examinations for a polyurethane adhesive are shown in Fig 9

0 200 400 600 800 1000 1200 1400

Number of cycles, cumulative [-]

10 -1

10 0

10 1

Cycles [-]

Adhesive basis: Polyurethan Substrat: Steel

sinusoidal Failure probability: 50 %

R = 0.1 1/Ts= 1.11

R = 0.1 1/Ts= 1.38

Without fracture

SkFatigue strength [kN]

Small scale specimen: Sk= 1.3 Specimen component: Sk= 1.1

Small scale specimen Specimen component

All specimens failed with a cohesive failure of the bondline The notch sensitivity (k) is calculated with a value of

13.74 for the butt joint tests and of 6.48 for the façade connection The threshold of the load, which the joint can

withstand permanently (fatigue limit Sk), is evaluated with a value of 1.3 kN for the small scale specimen and of

1.1 kN for the specimen component

3 Conclusion and Outlook

The bonding technology is more than just an alternative joining method for steel structures, which can be shown

by various applications and research projects in different fields Despite many advantages of bonded joints the civil engineering and especially steel construction community is reluctant in embracing this innovative technique Often

it is justified by doubts about the durability and a lack of experience Interpreting such doubts as open questions and challenges creates the possibility and the potential for exceptional innovations The general interest in developing standards as a basis for analysis and design of adhesive joints in steel, is expected to rise with a growing number of functional and calculable applications

The future work for the presented application examples for façade structures focuses on experimental investigations under variable amplitudes and the creation of a scientific based guideline for the design of bonded steel joints For the introduction of alternative innovative bonded steel joints, the effort is limited to the development

of engineering-models, planning and construction design General technical approvals or individual approvals can

be achieved more easily, thus sustainably increasing the innovative capacity of small and medium-sized enterprises

Acknowledgements

The IGF research project (IGF-No 18161 BG) of the Research Association for Steel Application (FOSTA), Sohnstraße 65, 40237 Düsseldorf, has been funded by the AiF within the program for sponsorship by Industrial Joint Research (IGF) of the German Federal Ministry of Economic Affairs and Energy, based on an enactment of the German Parliament

References

[1] DIN EN 14869-2, Structural adhesives - Determination of shear behaviour of structural bonds, –Part 2: Thick adherends shear test (ISO 11003-2:2001, modified), German version EN 14869-2:2004

[2] DIN EN 15870, Adhesives – Determination of tensile strength of butt joints (ISO 6922:1987 modified), German version EN 15870:2009

[3] K Dilger, H Pasternak et al., IGF Project No 16494 BG, Entwicklung eines Eurocode-basierten Bemessungskonzepts für Klebverbindungen

im Stahlbau (in Anlehnung an DIN 1990), Germany, 2012

[4] J Meinz, Kleben im Stahlbau – Betrachtungen zum Trag- und Verformungsverhalten und zum Nachweis geklebter Trapezprofilanschlüsse und verstärkter Hohlprofile in Pfosten-Riegel-Fassaden, Dissertation, Brandenburgische Technische Universität Cottbus, Germany, 2010

[5] DIN EN 1990, Eurocode 0: Basis of structural design; German version EN 1990:2002 + A1:2005 + A1:2005/AC:2010, 2010

[6] K Dilger, H Pasternak et al., IGF Project No 18161 BG, Untersuchungen zum Tragverhalten und der Lebensdauer von Klebverbindungen

im Stahlbau unter zyklischer Belastung, Germany, 01.04.2014 - 30.09.2016

[7] DIN EN 1991-1-5: Eurocode 1: Actions on structures – Part 1-5: General actions – Thermal actions; German version EN 1991-1-5:2003 + AC:2009, 2010

[8] DIN 18516-1, Cladding for external walls, ventilated at rear –Part 1: Requirements, principles of testing, 2010

[9] KIT, Institut für Meteorologie und Klimaforschung (2015): http://imkbemu.physik.uni-karlsruhe.de/~fzkmast/