Structural Silicone Sealants Research Articles

Cavitation and crack nucleation in thin hyperelastic adhesives

The present study investigates in the failure of adhesive bondings with structural silicone sealants. Point connectors of two circular metal adherends bonded with DOWSIL™ TSSA are subjected to tensile loading. We formulate and use a constitutive law that captures volumetric softening owing to the formation of cavities. Therein, cavitation is considered a process of elastic instability which is homogenized with a pseudo-elastic approach. Ultimate failure initiating from the free edges is predicted employing the framework of finite fracture mechanics. The concept requires both a strength-of-materials condition and a fracture mechanics condition to be satisfied simultaneously for crack nucleation. For the former, we use a novel multiaxial equivalent strain criterion. For the latter, we employ literature values of the fracture toughness of DOWSIL™ TSSA . The predicted onset of cavitation and ultimate failure loads are in good agreement with our experiments. The proposed model provides initial crack lengths that allow for the derivation of simple engineering models for both initial designs and proof of structural integrity while simultaneously extending the range of usability of the structural silicone compared to standardized approaches.

Assessment of the mechanical behavior of bonded glass-to-glass transparent epoxy adhesive joint at elevated temperatures for load-bearing elements

Glass is increasingly used as a structural material in buildings, not only for enclosing spaces but also for bearing loads. To meet architect requirements for a uniform structure without appearance disturbance, adhesively bonded joints are preferred over mechanical fasteners. However, in the construction industry, most technical guidelines cover only soft structural silicone sealants, while stiffer adhesives like epoxy resins are not considered. Alternatively, the specific transparent epoxy adhesives offer a high potential for bonded glass assemblies, enabling thinner joints and a fully transparent junction. Indeed, under real service conditions, elevated temperatures are one of the worst factors affecting the mechanical properties of structural bonded joints. Therefore, in this paper, an experimental program was conducted to evaluate the performance of transparent bonded glass joints with epoxy resin over a wide range of temperatures. Tensile and shear tests were performed on bulk-cured adhesive samples and bonded glass-to-glass block shear specimens respectively, at room temperature (RT), 40, 60, and 80 °C. The differences in surface characteristics such as roughness, wettability, and surface free energy (SFE) between both glass sides were examined. The results showed that tensile and shear strengths decreased with increasing temperatures. Furthermore, the shear strength was influenced by the surface properties of the glass sides. At all tested temperatures, the rougher bonded glass side specimens revealed higher shear strength coupled with a mixed failure while a loss of adhesion characterised the smoother glass surfaces.

Mechanical Properties of Prefabricated Cold-Formed Steel Stud Wall Panels Sheathed with Fireproof Phenolic Boards under Out-of-Plane Loading

In this paper, new prefabricated cold-formed light-gauge steel stud wall panels sheathed with fireproof phenolic boards, which are fabricated by connecting the steel studs and the boards by using structural silicone sealant, was proposed. The proposed prefabricated wall has a good fireproof performance and can be manufactured rapidly in a factory. Full-scale tests on the mechanical properties of the prefabricated wall system, consisting of the prefabricated wall and the connection between the wall and the surrounding steel structure under out-of-plane loading, were performed. A total of six specimens were tested considering the effects of the arrangement of the cold-formed light-gauge steel studs, the shape and thickness of the steel connector for jointing the prefabricated wall panel and the surrounding steel structure, and the number of self-tapping screws connecting the surrounding structure. The results show that the out-of-plane stiffness of the prefabricated wall system in the elastic stress state under out-of-plane loading can be increased by increasing the number of self-tapping screws, increasing the thickness of the steel connector, or adopting the symmetrical arrangement of the light-gauge steel studs. The out-of-plane stiffness of the prefabricated wall system and the stiffness contribution of a single special-shaped steel connector can both be increased by increasing the number of special-shaped steel connectors. Furthermore, the special-shaped steel connector is more beneficial to a greater out-of-plane stiffness than the L-shaped steel connector. In addition, the theoretical calculation methods for deflection of the proposed prefabricated wall and flexural stress of the CFS C-channel stud considering the fireproof phenolic board sheathing effect under elastic state were proposed. The predicted results using the proposed method are compared with test results and the predicted results by using other methods. It was found that the predicted results by using the proposed method agreed better with the test results compared with the predicted results using the transformed-section method or the reduced stiffness method, which demonstrates the acceptability and accuracy of the proposed mothed for calculating deflection of the proposed prefabricated wall and flexural stress of the CFS C-channel stud.

Structural silicone sealants after exposure to laboratory test for durability assessment

AbstractDuring the service life of structural sealant glazing (SSG) facades, the load‐bearing capacity of the silicone bonds needs to be guaranteed. Laboratory tests can assess the durability of SSG‐systems based on mechanical characteristics of the bond after simultaneous exposure to both climatic and mechanical loads. This article studies how the material characteristics of two common structural sealants are affected by laboratory and field exposure. Dynamic mechanical analysis (DMA) confirms a reduction in the dynamic modulus of exposed silicone samples. Results from thermogravimetric analysis, Fourier‐transform infrared spectroscopy, differential scanning calorimetry, and small‐angle X‐ray scattering/wide‐angle X‐ray scattering show differences between the two sealants and indicate no/minor changes in the composition and morphology of the laboratory and field exposed sealants. Mechanical characterization methods, such as DMA, and tensile and shear testing of the structural bond, are shown to be sensitive toward the combined climatic and mechanical loadings, and are hence suitable for studying degradation mechanisms of structural sealants.

Coupled distortional-dilatational failure mode concept for silicone adhesives

Nearly incompressible rubber-like materials, such as silicone adhesives, exhibit two failure modes: dilatational and distortional failure. The former corresponds to the formation and growth of cavities owing to triaxial deformation. The latter corresponds to fracture due to excessive isochoric deformation, i.e. deformation at constant volume. Both failure modes are typically assessed individually and corresponding failure criteria are often formulated in terms of stresses. However, stress-based descriptions of failure depend explicitly on the choice of constitutive law and the experimental measurement of critical stresses is challenging or even impossible. Therefore, recent studies proposed strain and stretch-based criteria for distortional failure of nearly incompressible hyperelastic materials, that can be obtained directly from optical measurements using digital image correlation.The present work exploits stretch-based descriptions of both failure modes to provide a unified coupled dilatational-distortional failure criterion which describes the complete failure surface of nearly incompressible silicone adhesives or rubber-like materials in general. For this purpose, we propose a stretch-based cavitation criterion which depends on a single parameter. Distortional failure is described by the Podgórski-criterion formulated in principal stretch space. The coupled dilatational-distortional failure criterion is validated using uniaxial tension, bulge and pancake tests of the nearly incompressible hyperelastic transparent structural silicone sealant DOWSIL™ TSSA. It is shown that the proposed failure criterion captures both the onset of cavitation and ultimate distortional failure in very good agreement with all experiments.

Semi-Probabilistic Calibration of a Partial Material Safety Factor for Structural Silicone Adhesives - Part II: Verification Concept

This paper deals with the application of the semi-probabilistic design concept (level I) to structural silicone sealants where a generally valid verification concept is proposed. The verification concept deals with the classification of silicone adhesive joints into cavitation-insensitive and cavitation-sensitive adhesive joints and at the same time advises which class of material models is necessary for the categorized adhesive joint systems. Furthermore, the static verification of a finite element based limit state analysis is shown within the design verification concept. The concept is elaborated and documented within this paper and illustrated by the help of an example. In the first part of this paper it was found, that the correct level I calibration of that partial material safety factors are significantly lower compared to currently existing estimates and thus allow for a great optimization of structural sealant design situations with potentially high economical as well as sustainability benefits.

Semi-Probabilistic Calibration of a Partial Material Safety Factor for Structural Silicone Adhesives - Part I: Derivation

This paper deals with the application of the semi-probabilistic design concept (level I) of Eurocode 0 in order to calibrate partial safety factors for structural silicone sealants. In the first part of this article the current legal situation for the application of structural sealants in facades is described, where a new Eurocode-conform design concept is introduced and compared to existing design codes (ETAG 002) or national design protocols (DIBt concept). Basic background on semi-probabilistic modelling and the general framework for deriving partial material safety factors at a level I stage for structural sealants is given then. The main part of this publication is concerned with determining the specific partial material safety factors for DOWSIL 993 silicone using existing experimental data, which were obtained under the ETAG 002 testing protocol. It is found, that the correct level I calibration of that partial material safety factors are significantly lower compared to currently existing estimates and thus allow for a great optimization of structural sealant design situation with potentially high economical as well as sustainability benefits.

Nonlinear elastic finite fracture mechanics: Modeling mixed-mode crack nucleation in structural glazing silicone sealants

Requiring both stress and energy conditions to be met simultaneously proved key to modeling brittle crack formation at singular and nonsingular stress concentrations in linear elastic materials. The present work extends this so-called coupled stress and energy criterion to brittle crack nucleation in hyperelastic media using the example of silicone adhesives. For this purpose, we provide a comprehensive constitutive as well as fracture mechanical characterization of the structural silicone adhesive DOWSIL™ 993 using a large set of experiments and propose a mixed-mode failure model for crack initiation in nonlinear elastic materials. Characterized in independent experiments, the model is used to determine critical loads of hyperelastic adhesive bonds in both shear and tension dominated configurations. For any of the examined adhesive joint configurations the model predicts and explains size effects and agrees well with experimental findings. We study stable and unstable crack propagation observed in video recordings of our experiments. It is shown that crack initiation, crack growth and crack arrest are caused by nonmonotonic energy release rates and can be predicted. Effects of excess energy available after crack nucleation and initial unstable crack growth are discussed.

Measuring mode I fracture properties of thick-layered structural silicone sealants

The failure hyperelastic structural sealant joints is a fracture mechanics problem. Its modeling requires knowledge of the material property fracture toughness. The present work describes a methodology for determining the mode I bulk material fracture toughness of structural silicone sealants. The concept is demonstrated using DOWSIL™ 993 as an example. In total we manufactured, tested and evaluated 13 DCB specimens of 6 mm and 12 mm adhesive thickness. The fracture toughness is identified from J-integral measurements. The theory for determining the J-integral at finite deformations is laid out and an automated data analysis procedure is suggested. The presented approach further allows for determining traction-separation laws to be used in cohesive zone models formulated with respect to the reference configuration. The theory and methodology presented are applicable to arbitrary hyperelastic structural sealants and adhesives. For DOWSIL™ 993 we determined a mode I bulk material fracture toughness of Gc=4.22±0.60 N/mm independent of adhesive layer thickness.

Crack nucleation in hyperelastic adhesive bonds

ABSTRACTAdhesively bonded glass‐to‐metal connections using structural silicone sealants are investigated in the present study of structural failure. Different failure modes are shown to be relevant depending on the geometry of the adhesive. Dilatational failure corresponding to abrupt growth of cavities in the bulk material may occur in heavily constraint applications where triaxial stress states are present. Distortional failure, i.e. failure owing to change in shape, can be observed when the adhesive can deform comparatively unconstrained. It typically originates from notches such as bi‐material points between adherend and adhesive. This work presents (i) a criterion for cavitation failure surfaces of materials with finite porosity such as Dow Corning® TSSA and (ii) an equivalent stretch concept capable of describing distortional bulk material failure surfaces of various rubber‐like materials. It is further shown that (iii) for notch‐induced crack nucleation a coupled stretch and energy criterion must be fulfilled simultaneously. The coupled criterion is capable of predicting size effects observed in tests of ETAG 002 shear specimen bonded using Dow Corning® 993.

Experimental and Numerical Investigation of the Buckling Performance of Tubular Glass Columns under Compression

This study investigates the buckling performance of tubular glass columns under axial compression. A total of two cases are considered (i) single glass tubular column and (ii) bundled column constructed using structural silicone sealants. A series of compression test were carried out on different geometrical dimensions of these two cases to determine their failure mechanism, load carrying capacity and to evaluate the buckling performance. Prior to the load carrying capacity (LCC) that was measured at the ultimate, a distinctive remaining structural capacity (RSC) was characterized especially for bundled glass based on the first crack. The shear connection in the bundled system was justified in comparison to the monolithic glass. This study showed that the failure mechanisms depended strongly on the slenderness ratio of the columns and that the failure occurred either by crushing or by buckling depending on the lengths of the column. The scatter in the failure load for specimens that had a higher slenderness ratio was much lower than for those that had a lower slenderness ratio. In order to justify the variability of the glass strength, a Weibull statistical distribution was used. Finite element modelling (FE) based on the simplified Riks method was performed using ABAQUS v6.10 to compliment the test results and to provide methods of analysis which could be used as a guideline for structural engineers to predict structural behaviour of tubular glass columns in general. An overestimation was predicted in the FE models which suggested modification of the imperfection factor.

Measuring the effective Young’s modulus of structural silicone sealant in moment-resisting glazing joints

Structural silicone sealants are synthetic rubber adhesives used in the construction industry to bond glass and other sheet infill materials to the frames of windows and curtain walls. In this paper, two different algebraic expressions are proposed to describe the way in which the rotational stiffness of the adhesive connection – resistance to moments acting about the axis of the joint – varies with the sealant’s cross-sectional dimensions and elastic modulus. Laboratory testing of DC-983, a two-component structural silicone sealant used widely in factory prefabricated glazing applications, has, with some caveats, validated the mathematical models.

Interfacial Adhesion–Strength Detection of Structural Silicone Sealant for Hidden Frame–Supported Glass Curtain Wall Based on Nonlinear Ultrasonic Lamb Wave

AbstractDegradation of interfacial adhesive strength is the main reason for failure of the hidden frame–supported glass curtain wall that is now widely used in the construction field. In this paper...

Study of Tensile Bond Strength for Structural Silicone Sealants after Artificial Accelerated Aging

Tensile bond strength is one of the most significant properties for structural silicone sealants used in the glass curtain walls. During the service process, aging of the silicone sealants shall be involved in comprehensive actions of environment factors, e.g. temperature, humidity, and ultraviolet light etc. In this study, artificial accelerated aging test was conducted to make clear the development of tensile bond strength, Shore hardness and elongation. The test results show that: (i) the specimens under damp-heat test have more degeneration than specimens under humidity-freeze test; (ii) the environment of high temperature and high humidity leads to the change of tensile bond, Shore hardness, elongation, and results in interfacial failure of samples.

Failure behaviour of silicone adhesive in bonded connections with simple geometry

In façade structures, adhesively bonded connections between glass panels and metallic substructures represent an attractive alternative to mechanical fixation devices. Apart from positive aspects regarding the construction's energy efficiency and aesthetics, the uniform load transfer reduces stress concentrations in the adherends, which is beneficial especially regarding brittle materials like glass. Structural silicone sealants are generally used for these kind of applications due to their excellent adhesion on glass and their exceptional resistance against environmental influences and ageing. For the verification of the bonded connection, non-linear numerical simulations, such as the Finite Element Method, are increasingly used. The resulting three-dimensional stress states need to be assessed with the help of an appropriate failure criterion. In this paper, an overview is given on available failure criteria for rubber-like materials. The applicability of these criteria on the silicone sealant is verified regarding three characteristic stress states: uniaxial tension, shear and compression. The proposed engineering failure criterion is the true strain magnitude, which is valid for bonded connections in form of linear beads for cohesive failure of the adhesive. For Dow Corning® 993 structural silicone sealant, the strain magnitude, evaluated using true strains, at failure could be determined as 1.6.

Influence of Structural Sealant Joints on the Blast Performance of Laminated Glass Panels

This paper investigates the influence of structural sealant joints on the blast performance of laminated glass (LG) panels, using a comprehensive numerical procedure. A parametric study was carried out by varying the width, thickness, and the Young’s modulus (E) of the structural silicone sealant joints, and the behavior of the LG panel was studied under two different blast loads. Results show that these parameters influence the blast response of LG panels, especially under the higher blast load. Sealant joints that are thicker, have smaller widths, and lower E values increase the flexibility at the supports, and hence increase the energy absorption of the LG panel while reducing the support reactions. Results also confirmed that sealant joints designed according to current standards perform well under blast loads. Modeling techniques presented in this paper could be used to complement and supplement the guidance in existing design standards. The new information generated in this paper will contribute toward safer and more economical designs of entire facade systems including window glazing, frames, and supporting structures.

Wooden window frames with structural sealants: manufacturing improvements and experimental validation of a finite element model

This paper proposes a finite element (FE) model of a novel wooden window frame typology and validates it by experimental tests on window frames and corner joint specimens. The innovation consists of the application of the structural silicone sealant only at the interface between wood and one of the thin glass layers of the double-glazing glass, so achieving a low application depth. This technique reduces the sealant quantity and allows the disassembly and substitution of the glass from the frame in case of breakage. In order to inform the FE model of the whole frame, the dowelled butt corner joint strength is measured by specific experimental tests. The results show a good correlation between the FE model and the experimental results in the range of small frame deformations, which are of interest for the proposed application. In particular, the FE curve overestimates the bottom corner displacement of about 17% at 1000 N load, 7% at 2000 N and less than 5% at higher applied force on the upper corner. The model is then applied to various wooden frame geometries in order to evaluate if the proposed silicone application mode and dowelled corner joints type allow an acceptable stiffness of the frame, and, in particular, low deformations of the bottom corner joint, in order to maintain the wooden frame functionality when subject to external loads. The approach could be useful when assessing optimal sealing application modes, wood-sealant-glass joint geometries, corner joint geometries and window frame geometries to limit the bottom corner joint deformation within the required technical specifications.

Structural silicone sealant modelling for wood frames: influence of adhesion on bonding strength

This paper provides a quantitative measure of the bonding strength of structural silicone sealant applied to wood–double glazing glass joints for wood frame applications. The joint strength is assessed by tensile and shear experimental tests. The paper aims to characterize the joint behaviour through experimental tests in order to implement and validate a finite element (FE) model of the joint that can be used for whole frame characterization. The experimental tests are carried out on three wood species (Meranti, White Oak and Pine), and two different FE models of the wood–silicone–glass joint are implemented: the first basic model assumes the modulus of elasticity and modulus of rupture of the silicone as provided by the manufacturer, while the second model assumes the results of the experimental tensile tests. The results of the first FE model do not fit well with the tests carried out, while the second FE model proves to be more reliable and is validated by experimental results. The results report that, when modelling wood–double glazing glass joints by FE methods, the equivalent structural sealant modulus of elasticity assigned in the model should be about 50% lower than what is declared by the manufacturer. This result can be useful when modelling whole wood frames and dimensioning sealant depth and thickness in wood–glass joint applications.

Experimental program and simplified nonlinear design expression for glass curtain walls with low-level blast resistance

A series of fi full-scale, nearly conventional, curtain wall specimens was tested in the UNC Charlotte Structures Laboratory. Specimens were subjected to quasi-static, uniform, out-of-plane loading to failure under displacement control. The tests were performed to obtain complete resistance curves, including the nonlinear behavior of the specimens up to ‘ultimate failure’. Ultimate failure was defi ned as mullion fracture or signifi cant breach of the curtain wall system when viewed as the protective barrier between building occupants and the external blast load. Representative load-defl ection and loadstrain resistance curves are presented. The energy absorbed by the curtain wall system up to three different limit states ‐ fi rst cracking of glass, fi rst yield of mullions, and fracture/breach of the system (ultimate failure) ‐ and maximum mullion end rotations are computed from the experimental results. Ultimate energy absorption capacity ‐ the recoverable linear strain energy plus the nonlinear energy due to formation of damage mechanisms ‐ and maximum mullion end rotations are essential for reliable and economical design of blast resistant curtain walls. To this end, a simplifi ed methodology is presented for analytically approximating curtain wall resistance functions that can be input to an energy expression that models nonlinear structural dynamic behavior due to an ‘impulsive’ loading. The blast resistance of a curtain wall can then be approximated using this procedure. It is shown that a nearly conventional curtain wall, a conventional system with two modifi cations ‐ use of laminated glass lites that are structurally glazed (wet-glazed) to a conventional framing system with structural silicone sealant ‐ had nearly 14 times the ultimate energy absorption capacity and nearly four times the blast resistance as the fully conventional system.

Safe and Sustainable for the Structural Silicone Sealant in Ultra High-Rise Building Curtains

The properties of the structural silicone sealant under standard condition and after aging conditions were characterized. The results showed that the structural silicone sealant had very good ability to resist the ultraviolet in the long time. In the other aging environment conditions, the different failure time for the structural silicone sealant were appeared. The value of the maximum strength under the different mechanical aging factors obviously existed in the structural silicone sealant with different compositions. The residual strength that the attenuation ratio is more than 75% standard strength would keep the ultra high-rise building curtains safe and sustainable.