Adhesives viscoelastic behavior

Fig. 22. Nomialized pull-off energy measured for polyethylene-polyethylene contact measured using the SFA. (a) P versus rate of crack propagation for PE-PE contact. Change in the rate of separation does not seem to affect the measured pull-off force, (b) Normalized pull-off energy, Pn as a function of contact time for PE-PE contact. At shorter contact times, P does not significantly depend on contact time. However, as the surfaces remain in contact for long times, the pull-off energy increases with time. In seinicrystalline PE, the crystalline domains act as physical crosslinks for the relatively mobile amorphous domains. These amorphous domains can interdiffuse across the interface and thereby increase the adhesion of the interface. This time dependence of the adhesion strength is different from viscoelastic behavior in the sense that it is independent of rate of crack propagation.

Thus, fundamentally the interest is in testing the limits and theory of polymer behavior in end-tethered systems, e.g., viscoelastic behavior, wetting and surface energies, adhesion, shear forces relevant to tribology, etc. It should be noted that relevant surfaces and interfaces can also refer to polymers adsorbed in liquid-liquid, liquid-gas, solid-gas, and solid-liquid interfaces, which makes these polymer systems also of prime importance in interfacial science and colloidal phenomena (Fig. 2). Correspondingly, a wide number of potential applications can be enumerated ranging from lubrication and microelectronics to bioimplant surfaces. 

The following illustration (Figure 10.9) shall explain the basic coherences for the dimensioning of a bonded shaft-to-hub joint. Here, a limitation to the essential geometrical and mechanical parameters is required, since the reduction factors, viscoelastic adhesive layer behavior, stress development, surface geometry of the adherends and the like, do not allow for a detailed consideration at this point (Figure 10.9). 

Marcinko, J. J., Parker, A. A., and DiPietrantonio, B., Block Shear Analysis, Viscoelastic Behavior, and Structural Adhesive Development, Biographies and Abstracts of the Forest Products Society 57" Annual Meeting, Forest Products Society, Madison, WI, 2003, p.21, http i/Zw vw. forestprod. org/confpast.html. 

A New Low Cost Method For Measuring the Viscoelastic Behavior of Adhesives... 

Dissipation phenomena generally occur during measurement of the adherence of polymer materials, leading to an adherence energy function of both the number and nature of interfacial interactions (adhesion) and dissipative properties, mainly due to viscoelastic behavior [1-5]. Friction properties of polymers are also governed by interfacial interactions and dissipation mechanisms. Common phenomena (interfacial interaction and dissipation) therefore control adherence and friction behaviors. However, the relationship between the two phenomena is still vague or undefined. The first objective of this experimental work is then to compare adherence and friction of polydimethylsiloxane (PDMS) networks in order to establish relationships between these two properties. 

Main Features of the Viscoelastic Behavior of the Pure Components, Blends, and Full Adhesive Formulations... 

Generally PSAs are well known for their very viscoelastic behavior, which is necessary for them to function properly. It was therefore important to characterize first the effect of the presence of diblocks on the linear viscoelastic behavior. Since a comprehensive study on the effect of the triblock/diblock ratio on the linear viscoelastic properties of block copolymer blends has recently been reported [46], we characterized the linear viscoelastic properties of our PSA only at room temperature and down to frequencies of about 0.01 Hz. Within this frequency range all adhesives have a very similar behavior in terms of elasticity, as can be seen in Fig. 22.10. The differences appear at low frequency, a regime where the free iso-prene end of the diblock chain is able to relax. This relaxation process is analogous to the relaxation of an arm of a star-like polymer [47], and causes G to drop to a lower plateau modulus, the level of which is only controlled by the density of triblock chains actually bridging two styrene domains [46]. 

However, one important energy loss which was explained was the effect of the viscoelastic behavior of the polymer. This was studied by varying the crosslink density of the rubber, to alter the loss of elastic energy as the material relaxed. As the viscoelastic loss increased, so did the adhesive hysteresis, as shown in Fig. 8.13. 

G. Krause, F.B. Jones, O.L. Marrs, and K.W. Rollmann, "Morphology and Viscoelastic Behavior of Styrene-Diene Block Copolymers in Pressure Sensitive Adhesives", J. Adhesion, 8, pp. 235-258 (1977). 

There are numerous examples of the application of fracture mechanics to structural adhesive systems. Most notable are those of Mostovoy and his coworkers which have already been mentioned. " Bascom and coworkers have made significant contributions to the understanding of the effect of bondline thickness on fracture toughness. Kinloch and Shaw extend the work of Bascom to include rate effects and to develop mathematical models of the fracture resistance of adhesives. Hunston et al have used these methods to study viscoelastic behavior in the fracture process of structural adhesives.Mostovoy and Ripling used these techniques to determine the flaw tolerance of several adhesives,while Bascom and Cottington have studied the effect of flaws caused by air entrapment in structural adhesives." Finally it must be mentioned that one of the most simple, most widely used tests for strucural adhesives, the peel test, is actually a version of the double cantilever beam test. 

Dynamic mechanical analysis measures changes in mechanical behavior, such as modulus and damping as a function of temperature, time, frequency, stress, or combinations of these parameters. The technique also measures the modulus (stiffness) and damping (energy dissipation) properties of materials as they are deformed under periodic stress. Such measurements provide quantitative and qualitative information about the performance of materials. The technique can be used to evaluate reinforced and unreinforced polymers, elastomers, viscous thermoset liquids, composite coating and adhesives, and materials that exhibit time, frequency, and temperature effects or mechanical properties because of their viscoelastic behavior. 

The presence of a confined interfacial layer, with specific rheological behavior, is proposed to explain this complex behavior. The low stiffness of PDMS allows a competition between the (low) cohesion and the confined chain layer at the PDMS surface and the adhesion level (interfacial interactions between PDMS and substrates). At low speeds, interfacial interactions have a significant effect and partly govern the friction, and at high speeds the influence of the substrate surface becomes negligible and friction is then governed by the polymer s intrinsic viscoelastic behavior. Experimental results underline the subtle competition between interfacial interactions and polymer rheological properties, especially for PDMS samples. Comparison... 

The stresses in an adhesive joint depend, once a constitutive model is chosen, on the geometry, boundary conditions, the assumed mechanical properties of the regions involved, and the type and distribution of loads acting on the joint. In practice, most adhesives exhibit, depending on the stress levels, nonlinear-viscoelastic behavior, and the adhetends exhibit elastoplastic behavior. Most theoretical studies conducted to date on the stress analysis of adhesively bonded joints have made simplifying assumptions of linear and elastic and/or viscoelastic behavior in the interest of tracking solutions. 

The present discussion has a twofold objective First, to review the literature in the stress analysis of adhesive joints using the finite-element method. Second, to present a finite-element computational procedure for adhesive joints experiencing two-dimensional deformation and stress fields. The adherends are linear elastic and can undergo large deformations, and the adhesive experiences linear strains but nonlinear viscoelastic behavior. Following these general comments, a review of the literature is presented. The technical discussion given in the subsequent sections comes essentially from the authors research(i 2> conducted for the Oifice of Naval Research. 

The surface forces, of van der Waals type for rubber-like materials, are able to grandly modify the stress tensor provided by the contact of a blunt asperity applied against the flat and smooth surface of a rubber sample. It will be shown how the coupling of surface adhesion properties and bulk viscoelastic behavior of rubber-like material allows us to solve adherence problems. This will be illustrated through three examples the spontaneous peeling due to the intervention of internal stresses the no-rebound of balls on the smooth surfoce of a soft elastomer and the adhesive contact and rolling of a rigid cylinder under a smooth-surfaced sheet of rubber. 

When a driver jams on the brakes he thinks usually that the deceleration of the vehicle results from this action, but the true reason why the vehicle stops is in fact the great force induced by friction at the tire-road interface. It will be shown that this fiiction force is the direct result of the coupling of surface adhesion properties and bulk viscoelastic behavior of rubber-like materials. 

Figure 7. Master curve, in log-log coordinates, regrouping all tfie adhesive rebound critical heights (balls and projectiles), which allows one to determine, firom its slope P, the index n characterizing the viscoelastic behavior of the natural rubber sample tested.

One suggestion for an interfacial detachment failure criterion can be found in the work of Derail et al. [10,11]. The adhesives used in this work were blends of polybutadiene and tackifying resin. These authors also assume that the dominant deformation mode in the adhesive is elongational deformation. They employ a non-linear integral constitutive model for the adhesive stress-strain behavior of the KBKZ type [25]. This model describes fairly well the viscoelastic behavior of... 

The results presented in the previous sections assume that the contacting materials have well-defined elastic constants. In fact, most materials have at least some viscoelastic character, and it is important to understand how these effects should be taken into account. Viscoelastic effects enter into our analysis in two ways. First, it is possible that the overall elastic response of the system, described by the effective elastic constant, , is time-dependent. In the case where adhesion is present, the stress near the crack tip will be defined by stress intensity factors, K and K that are themselves time-dependent. A unique energy release rate cannot be defined in this case. We refer to this macroscopic manifestation of viscoelastic behavior as large-scale viscoelasticity . In this case one needs a procedure for determining the stress intensity factor that describes the current state of stress in the vicinity of the contact perimeter. Appropriate expressions for K are an essential result of treatments of large scale viscoelasticity, and these expressions are provided in Section 5.1. 

Light and Harvey [24] presented a paper that analyzed the pulse-echo ultrasonic squirter technique for the first layer delamination in a composite structure. Parikh and Achenbach [25] established a framework for studying the nonlinear viscoelastic behaviors of adhesive layers in an attempt at advancing the state of the art in a nondestructive evaluation of adhesively bonded structures. 

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