In Paper 3 adhesive glass joints were studied. In the first part of the study, which de-scribes the work of [30], the shear-capacity of various adhesives for connecting glass was determined experimentally. The tests were performed on small specimens and the shear-capacity was evaluated for a short-term load.
The test equipment was designed to obtain a state of pure shear in the joint. The test equipment consisted of two steel-parts that transmit the forces from the testing machine to the specimen.
The specimens in the tests consisted of two pieces of glass with dimensions 20× 20 mm2 joined together with an adhesive layer.
R2
Figure 21: Beam model applied to a balustrade without holes.
The tested adhesives can be grouped into softer and stiffer adhesives. The softer adhe-sives contained four types of silicone based adheadhe-sives, three types of SMP (Silyl Modi-fied Polymer) based adhesives and Bostik Multifog 2640. The stiff adhesives consisted of polyurethane adhesive, HBM Rapid Adhesive X 60, strong epoxy adhesive and UV-hardening glass-glue. The adhesive products were chosen so that a wide span of different adhesive characteristics was obtained.
Test specimens had different thickness of the adhesive layer. For the silicone glues, the thickness was 6 mm, for the SMP based adhesives and Bostik Multifog 2640, the thick-nesses were 2 and 0.3 mm. The polyurethane glue and HBM Rapid adhesive X 60 had an adhesive thickness of 0.2 mm, whereas the strong epoxy and the UV-hardening glue had 0.3 mm as thickness of the adhesive.
In the tests, data on force versus displacements were collected. The shear-capacity was obtained through dividing the ultimate measured shear force by the initial surface area of the adhesive.
The results for one softer adhesive and one stiffer adhesive are presented. For the softer adhesive, one SMP based adhesive, the obtained shear-capacity was 2.3 MPa. For the stiffer adhesive, polyurethane glue, the shear-capacity was determined to 3.8 MPa.
Later, as a part of this work and the work of [30], a finite element model of the test arrange-ment was developed to determine the material models of the adhesives. The evaluation of the results consisted of plotting the measured data of the shear-force versus the deforma-tion of the test series. The data was fitted to a polynomial curve and compared with the data extracted from the finite element simulations. For each adhesive, different material models were tested until a satisfying agreement was obtained. For the softer adhesives, the hyperelastic material models Neo-Hooke and Mooney-Rivlin were tested whereas the stiffer adhesives were modeled as linear elastic. For an overview of the resulting material models it is referred to Paper 3. From the respective matching material model, data on the shear-deformation was extracted and relationships between shear-stress and shear-strain were established. An initial shear modulus, G, was calculated from the shear-stress versus shear-strain diagrams and an ultimate shear-stress was determined.
Results showing the mechanical characteristics of two groups of adhesives (softer, 2 mm specimens and stiffer) are shown in Table 11. In general, the stiffer adhesives had greater stiffness (G) and ultimate shear-strength (τavg,u) than the softer adhesives.
Finally, the work of [30] is described, in which a large-scale experimental test was made to determine the shear-capacity of an adhesive joint in a large dimension glass beam.
Five adhesives from the small-scale tests were chosen to be tested in the large-scale tests.
The five adhesives were chosen considering the results from the finite element simula-tions of the corresponding test set-up. The two strongest SMP based adhesives and the
Table 11: Mechanical characteristics of adhesives.
Quantity Softer adhesives Stiffer adhesives
G (MPa) 0.5-1.2 83-500
τavg,u(MPa) 1.3-2.3 4-20
γu(%) 200-300 4-10
stiffer adhesives polyurethane glue, UV-hardening glass-glue and the strong epoxy adhe-sive were chosen.
The test performed was a four-point bending test of a beam that consisted of three beams made of flat-glass joined together by two adhesive joints. The test arrangement is shown in Figure 20 and creates a symmetrical beam in order to obtain pure shear- stresses in the joint. The beam had a span of three meters and every flat-glass element had dimensions 250× 2000 mm and a width of 12 mm. The adhesive joints had dimensions 250 × 250 mm.
During the measurements, data on load and deformation of the mid-point of the beam were collected.
To investigate whether the derived material models are valid for larger joints, a finite element model was made of the large-scale test set-up. The adhesives were modeled ac-cording to the material models from the tests with the small specimens. Load-deformation graphs were determined for the tested adhesives and maximum load values were deter-mined.
For the third SMP based adhesive, the measured data showed a less stiff behavior than the data from the simulations. After an initial deviation, the stiffness of the measured data for the polyurethane adhesive compared well with experiments. It is referred to Paper 3 for result graphs.
Looking at the principal pattern of shear-stresses in the joints, in the stiffer adhesives stress-concentrations occurred at the corners of the joints. In the softer adhesives, the stresses were more evenly distributed. This result is presented in Paper 3.
Table 12 summarizes the ultimate loads and deformations for a selection of the softer ad-hesives. The results were obtained from the finite element simulations. The corresponding results for the stiffer adhesives are shown in Table 13.
Overall, the softer adhesives had ultimate loads ranging between 28 and 50 kN and the stiffer adhesives had ultimate loads in the interval 10-30 kN.
Table 12: Ultimate loads and deformations at the mid-point of the beam for softer adhe-sives.
Adhesive Ultimate load (kN) Ultimate deformation (mm)
Bostik Multifog 2640 28.8 53
SMP Based Adhesive 1 49.3 51
SMP Based Adhesive 2 38.2 43
SMP Based Adhesive 3 48.8 50
Table 13: Ultimate loads and deformations at the mid-point of the beam for stiffer adhe-sives.
Adhesive Ultimate load (kN) Ultimate deformation (mm)
Strong Epoxy 30.3 10.0
Polyurethane Glue 10.3 3.5
UV-hardening Glue 22.3 7.5
HBM Rapid Adhesive X 60 20.3 7.0