Adhesive material behavior

In optimized commercial materials, the interface functions as an efficient transmitter of forces between fiber and matrix. As such, as long as the interface is intact, composite material behavior can be adequately described by models which assume ideal adhesion between fiber and matrix and consider the interface to be a two-dimensional boundary. 

Now assume that in the example of interest here, preliminary analysis of the motor case shows that at the most likely points of debonding. Mode I loading dominates adhesive crack behavior. Two test methods that could appropriately be used to determine Ya Mode I are the blister test and the double cantilever beam (26). If one assumes that the first is chosen, then a layer of the adhesive (the solid propellant) is cast on a plate of the material from which the motor case is made. This plate contains a hole that has been fitted with a pressure connection. This connection is used to pressurize (with air, inert gases, fluids, etc.) the region between the plate and propellant, arid thereby produces a blister. The value of Ya can be determined from measurement of the pressure at which this blister grows as a function of its diameter (26). In our case, let us assume a series of tests yields a "least value" for Ya of J/m2. 

Generally, multiple experimental test data of adhesive material are necessary for an adequate representation of the joint behavior under loading. Uniaxial tension, compression and single lap shear tests were therefore performed. 

In this paper, we present test results of two methods for surface pretreatment which are generally applicable under atmospheric conditions and which can be integrated in the production line. Pragmatic approaches for the numerical description of the material behavior of adhesives according to specific loading conditions are given. Furthermore, we present model parameters for some commercial adhesive systems which were tested in relevant conditions. A concept of knock-down factors and characteristic values which is widely used in component design will be discussed. Experimental results were used to manufacture a fuUy bonded structural component of a rail vehicle. Test results are compared with FE-model predictions. 

Rix, C. and C. Eastland, Cryogenic and thermal cycling behavior of adhesive materials, Defense Technical Information Center, stinet.dtic.mil/oai/oai verb=getRecord metadataPreflx=html identiiier=ADD852483,1989. 

There are many relative viscometric methods, i.e., those which require calibration with a fluid of known viscosity. Although they may be useful for viscometry of Newtonian fluids and for process and product control, the relative methods are inappropriate for adhesive materials, since concentrated colloidal dispersions and concentrated solutions of macromolecules show non-Newtonian behavior. While macromolecular systems usually exhibit viscoelasticity, colloidal dispersions can often be considered to be purely viscous. Non-Newtonian behavior can be characterized in steady shear, and is usually expressed as the functional dependence of shear stress on shear rate, or of viscosity on shear rate. 

An adhesive is often subjected to a rupture test, in which the stress response of the material is measured in order to determine the utility of the adhesive. In such a test, one or a combination of several different modes of deformation—shear, extension, compression, torsion, or flexure—can be important. While one of these modes may resemble the application of interest more closely than the other modes, the knowledge obtained regarding material behavior from the different tests is similar in some cases, the information is the same. In other words, the information gathered in one experiment can often be predicted from the results of the other experiments. Although this is a gross simplification, one can, for purposes of illustration, cite the behavior of linearly elastic solids and purely viscous Newtonian liquids. While the former material is characterized by its elastic modulus, the behavior of the latter is determined by the (shear) viscosity. In the case of incompressible Hookean solids, the modulus of elasticity is three times the shear modulus. (See also Chapter 2 by Krieger.)... 

Block copolymer/homopolymer mixtures have been of considerable interest because they can be easily modified to yield desired properties in polymeric materials such as pressure sensitive adhesives. Phase behavior of these mixtures is generally quite complicated because two different natures of transition can occur at the same time macrophase and microphase separation. If the molecular weight of a... 

Modem finite element analysis or other numerical methods have no problem in treating non-linear behavior. Our physical understanding of material behavior at such levels is lacking, however, and effective numerical analysis depends to a large extent on the experimental determination of these properties. Despite these limitations, many researchers have shown that elastic analyses of many adhesive systems can be very Informative and useful. A number of adhesive systems are sufficiently linear, such that it is adequate to lump the plastic deformation and other dissipative mechanisms at the crack tip into the adhesive fracture energy (critical energy release rate) term. 

Usually, sealants and adhesive materials for construction applications are evaluated by looking at the engineering side, butnotthe chemistry of the material. As a result, only tests that measure the mechanical properties are used. Most of the studies on the viscoelastic properties use traditional tests such as tensile testing to obtain data, which can be used in complicated mathematical equations to obtain information on the viscoelastic properties of a material. For example, Tock and co-workers studied the viscoelastic properties of stmctural silicone rubber sealants. According to the author, the behavior of silicone mbber materials subjected to uniaxial stress fields carmotbe predicted by classical mechanical theory which is based on linear stress-strain relationship. Nor do theories based on ideal elastomers concepts work well when extensions exceed... 

Abstract Multicomponent materials based on synthetic polymers were designed and used in a wide variety of common and hi-tech applications, including the outdoor applications as well. Therefore, their response to the UV radiation and complex weathering conditions (temperature, seasonal or freeze—thaw cycles, humidity, pH, pollutants, ozone, microorganisms) is a matter of utmost importance in terms of operational reliability and lifetime, protection of the environment and health safety. This chapter offers an overview of this subject and a critical assessment of more particular topics related to this issue. Thus, various types of multicomponent systems based on thermoplastic and thermosetting polymer matrices were subjected to natural and/or simulated UV radiation and/or weathering conditions. Their behavior was evaluated in correlation with their complex formulation and taking into consideration that the overall effect is a sum of the individual responses and interactions between components. The nature and type of the matrix, the nature, type and size distribution of the filler, the formation of the interphase and its characteristics, the interfacial adhesion and specific interfacial interactions, they all were considered as factors that influenced the materials behavior, and, at the same time, were used as classification criteria for this review. 

Most adhesives are polymer-based materials and exhibit viscoelastic behavior. Some adhesives are elastomer materials and also exhibit full or partial rubberlike properties. The word elastic refers to the ability of a material to return to its original dimensions when unloaded, and the term mer refers to the polymeric molecular makeup in the word elastomer. In cases where brittle material behavior prevails, and especially, when inherent material flaws such as cracks, voids, and disbonds exist in such materials, the use of the methods of fracture mechanics are called for. For continuum behavior, however, the use of damage models is considered appropriate in order to be able to model the progression of distributed and non-catastrophic failures and/or irreversible changes in material s microstructure, which are sometimes described as elastic Hmit, yield, plastic flow, stress whitening, and strain hardening. Many adhesive materials are composite materials due to the presence of secondary phases such as fillers and carriers. Consequently, accurate analysis and modeling of such composite adhesives require the use of the methods of composite materials. 

Due to their viscoelastic nature, most adhesives exhibit rate-dependent material behavior, which can be modeled, for example, by using mechanical model characterization. 

If the adhesive material exhibits viscoelastic behavior, then the high stress magnitudes observed at the overlap end region of bonded joints are expected to diminish in time due to the creep process. For example, Weitsman (1981) utilized the nonlinear viscoelastic power-law response, which describes a stress-enhanced creep process to illustrate time-dependent... 

Since most adhesives contain secondary phase materials such as fillers and carrier cloth, they should be treated as composite materials in constitutive modeling. The interesting aspect in such treatment is the fact that such secondary phase constituents form adhesive bonds with the adhesive matrix in small scale. Consequently, localized non-catastrophic failures within the bulk adhesive consisting of interfacial separations are expected to cause constitutive behavior changes for the adhesive bulL Such behavior can be properly described by using damage mechanics models, some of which will be described later in this chapter within the context of adhesive materials. 

Wei (1995) and Sancaktar and Wei (1998) applied the elastic moduli and thermal expansion coefficient relations developed by Chow in describing the material behavior of electronically conductive (filled) adhesives in order to be able to determine, accurately, the thermal mismatch stresses which occur between the adhesive and the substrates in the bonded form. Both finite element analysis and closed form solutions were used for stress analysis. 

Using Eqs. 23.94 and 23.95 and Hooke s law to represent adhesive and adherend s material behavior, the following expression can be obtained for the interphase shear modulus ... 

The over-stress idea can be incorporated into the viscoelastic mechanical models by means of adding sliding elements to describe yield or termination points. With such an application, equations containing viscosity terms are obtained in a form similar to (24.114). For example, Brinson et al. (1975), Renieri et al. (1976), Sancaktar and Brinson (1979, 1980), Sancaktar (1981), Sancaktar and Padgilwar (1982), Sancaktar et al. (1984), and Sancaktar and Schenck (1985) used the modified Bingham model developed by Brinson (1974) to describe the shear and tensile material behavior of structural adhesives in the bulk and bonded forms ... 

Another important characteristic of adhesives is their time-dependent response to loads. Adhesives may exhibit viscoelastic or visco-plastic material behavior, such as creep and relaxation, resulting in time-dependent stresses and strains. In adhesives with a high glass transition temperature relative to the operating temperature it maybe acceptable to model the adhesive with a time-independent material model. However, as temperature, absorbed moisture, stress level, and time under load increase, there is an increased likelihood of errors in using such a model. Selection of a time-dependent material model will depend on a number of... 

The term viscoelasticity combines viscous and elastic stress-strain flow characteristics. If materials behavior is dominated by viscous flow it is generally referred to as a fluid, whereas if the elastic properties dominate the mechanical properties of a material it is considered to be solid. Most adhesives are applied in a liquid or pasty condition to allow wetting and promote spreading and then are required to phase change into a solid. In the liquid state, rheology provides the methods to differentiate between elastic and viscous flow characteristics while, for example, dynamic mechanical analysis of cured adhesive polymers uses similar principles to access elastic and viscous parameters of the stress-strain response. 

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