Structural Silicone Adhesive Research Articles

Investigation of the structural performance of continuous adhesive glass-metal connections using structural silicone and hybrid polymer adhesives

Current design standards and technical guidelines regarding structural adhesive glass-metal connections demonstrate only a narrow applicability window, resulting in constraints regarding the geometry of the adhesive joint, e.g. width-to-thickness ratio, that are tight as well. Therefore, research is conducted considering continuous adhesive glass-metal connections with joint geometries beyond the design framework of existing technical design guidelines. The structural silicones DowSil™ 993 and Sikasil® SG-500, and the hybrid polymer adhesive Soudaseal 2K are used during the experiments as such types of adhesives have already demonstrated their durability over the last 50 years in structural sealant glazing systems. This paper reports on the mechanical performance of continuous adhesive glass-metal connections with varying width-to-thickness ratios. Numerical models are validated based on experimental data, which are then used for parametric studies. From the generated data basic relationships between joint stiffness and geometrical parameters are derived, such as width-to-thickness ratio. A mechanical model of continuous adhesive glass-metal joints as proposed in literature is used to perform analytical calculations to predict the deformations and stresses in the adhesive layer, which shows good agreement with the numerical results. The existing mechanical model predicts the stiffness of these continuous connections as function of geometrical and material parameters (here for DowSil™ 993) significantly accurate. Therefore, this research can serve as a basis for the expansion of current design methods to adhesive joint geometries outside their current framework.

Dimensioning of silicone adhesive joints: Eurocode-compliant, mesh-independent approach using the FEM

This paper deals with the application of the semi-probabilistic design concept (level I, DIN EN 1990) to structural silicone adhesives in order to calibrate partial material safety factors for a stretch-based limit state equation. Based on the current legal situation for the application of structural sealants in façades, a new Eurocode-compliant design concept is introduced and compared to existing design codes (ETAG 002). This is followed by some background information on semi-probabilistic reliability modeling and the general framework of the Eurocode for the derivation of partial material safety factors at Level I. Within this paper, a specific partial material safety factor is derived for DOWSIL 993 silicone on the basis of experimental data. The data were then further evaluated under a stretch-based limit state function to obtain a partial material safety factor for that specific limit state function. This safety factor is then extended to the application in finite element calculation programs in such a way that it is possible for the first time to perform mesh-independent static calculations of silicone adhesive joints. This procedure thus allows for great optimization of structural sealant design with potentially high economical as well as sustainability benefits. An example for the static verification of a bonded façade construction by means of finite element calculation shows (i) the application of EC 0 to silicone adhesives and (ii) the transfer of the EC 0 method to the finite element method with the result that mesh-independent ultimate loads can be determined.

Calibration of hyperelastic material models for structural silicone and hybrid polymer adhesives for the application of bonded glass

To enable the introduction of new developments and concepts of bonded glass in façades, finite element modelling is a frequently used method. The implementation of the complex material behaviour of suitable adhesives into such software, however, can be labour-intensive. Usually, hyperelastic material models need to be calibrated, i.e. theoretical mathematical expressions are fitted to experimentally obtained data. In this research, three two-component structural silicones and one two-component hybrid polymer adhesive were selected. Experimental tests on H-specimens in correspondence with ETAG 002 were performed for tension, compression and simple shear at different haul-off speeds of the testing machine, more specifically 1 mm/min, 5 mm/min, 50 mm/min and 500 mm/min. The stress–strain response of the adhesives altered with different test speeds. The material response under cyclic loading regime was investigated as well. These tests confirmed the presence of strain softening, also referred to as the Mullins effect, in all selected adhesives. Next, twenty isochoric hyperelastic material models were calibrated with the experimental data of the monotonic load tests at a speed of 5 mm/min for single, dual and triple deformation states. A theoretical estimator of the relative quality of the material models, more specifically the Akaike Information Criterion, allowed for the selection of the models that fit the experimental data the best. For all investigated adhesives, the polynomial based hyperelastic material models provided the best fit. This paper presents the results of the experimental programme that enable researchers to compare numerical and analytical analyses with these results. Designers and engineers can compare the structural performance of the adhesives in correspondence with ETAG 002 and assess the importance of cyclic loading regimes in the design of bonded glass. The calibrated hyperelastic material models allow engineers to numerically implement hyperelastic material behaviour of the studied adhesives in correspondence with ETAG 002. The best-performing material model for the considered application needs to be selected and the numerical model needs to be validated using the results of additional experiments on the considered bonded glass application. Especially the calibrated material models for multiple deformation states are valuable to model complex stress distributions in bonded glass applications subjected to multiple deformation states.

Stress whitening effects in transparent structural silicone adhesives

Stress whitening is a common effect in polymers where an increase in brightness or an increased opacity of the material can be observed under mechanical loading. Investigating the stress whitening effect for the transparent structural silicone adhesive (TSSA), this effect occurs in different forms depending on the applied deformation. The intensity and appearance of whitening in TSSA depends decisively on the type of loading, i.e. under isochoric deformation a spot-wise whitening can be observed, whereas under volumetric loading a very dense, cloud-like whitening becomes visible. In order to clarify why the stress whitening effect occurs at all, experimental investigations are carried out on uniaxial cyclic tensile tests and constrained tensile tests. The special feature of the uniaxial cyclic tensile tests is that they are performed in a miniature tensile testing machine, which is positioned under a light microscope. This makes it possible to observe stress whitening during cyclic deformation at a micro-scale. Furthermore, for the observation of whitening during constrained tensile tests, so-called pancake test are investigated and compared with the results of the uniaxial cyclic tensile tests. Since the whitening effect in the pancake test is in clear contrast to the uniaxial tensile tests, differences of both results are presented. Finally, the causes of both forms of whitening are defined and characterized.

Pseudo-elastic cavitation model—part II: extension to cyclic behavior of transparent silicone adhesives

This study investigates the cyclic structural behaviour of adhesive joints of glass and metal using thin, structural silicone adhesives in heavily constrained applications. Based on cyclic uniaxial tensile tests on dog-bone and pancake test samples, the pseudo-elastic cavitation model from part I of this publication will be extended to describe two phenomenons: stress softening due to Mullins effect, as well as a mechanical hysteresis occurring under hydrostatic loading of rubber-like materials. This mechanical hysteresis under hydrostatic loading is associated with the growth and shrinkage of microscopic voids in the materials structure, and shows distinctive differences to the mechanical hysteresis known from isochoric test samples. In order to transfer the already presented pseudo-elastic cavitation model to describe the cyclic material behaviour, the isochoric part of the cavitation model is extended according to the classical theory of pseudo-elasticity to numerically describe stress softening under isochoric deformations. In addition, the volumetric part is provided with a special material formulation so that it can numerically reproduce the void growth hysteresis under cyclic volumetric tests, e.g. pancake tests. For validation, three-dimensional simulations of both cyclic tensile tests (dog-bone and pancake tests) are carried out.

Mechanical behaviour of Transparent Structural Silicone Adhesive (TSSA) steel-to-glass laminated connections under monotonic and cyclic loading

Steel-to-glass laminated connections, which have recently been developed, limit stress intensifications on the glass and combine strength and transparency. Transparent Structural Silicone Adhesive (TSSA) connections have been used in several projects worldwide; however, the hyperelastic and viscoelastic nature of the material has to date not been fully investigated. In this work, the first objective is to investigate the mechanical response of TSSA connections under static and cyclic loading by means of experimental tests. Firstly, the shear behaviour of TSSA circular connections is characterized by means of monotonic and cyclic loading tests. The adhesive exhibits significant stress-softening under repeated cycles that becomes more severe as the maximum load increases. Secondly, TSSA circular connections are subjected to monotonic and cyclic tensile loading of increasing maximum load. The way whitening propagates on the adhesive surface shows some consistency comparing the cases of static and cyclic loading. The second objective is to analytically describe the deformation behaviour of the adhesive based on hyperelastic prediction models. Uniaxial and biaxial tension tests are combined with the simple shear tests, for the material characterization of TSSA. The hyperelastic material parameters are calibrated by a simultaneous multi-experiment-data-fit based on the nonlinear least squares optimization method. The softening behaviour observed in shear tests is modeled based on a simplified pseudo-elastic damage model proposed by Ogden–Roxburgh. A first attempt is also made to model the actual softening response of the adhesive. A less conservative approach proposed by Guo, also based on the theory of pseudo-elasticity, proved to give a good approximation of the actual cyclic response of the adhesive.

Preliminary Evaluation of the Mechanical Properties and Durability of Transparent Structural Silicone Adhesive (TSSA) for Point Fixing in Glazing

Abstract The paper reports on the preliminary evaluation of a transparent structural silicone adhesive (TSSA) developed for point fixing in glazing, which combines high transparency, strong adhesion performance, thermal stability, and excellent weatherability. The transparent film adhesive is a heat curing one-part material that shows strong bonding to glass, metals, ceramics, and even plastics typically without primer. The paper presents information on the durability and physical properties of the new material and suggests a methodology for deriving static and dynamic design strength values for the new material based on creep rupture experiments as well as nondestructive dynamic load experiments using the stress whitening phenomenon observed with this material as the limit state. The paper further discusses material characterization and hyperelastic modeling used in the finite element analysis based on finite strain theory.