Adhesion fracture theory

The fracture theory is the most widely applied theory in studying mucoadhesion mechanisms. It accormts for the forces required to separate two sttrfaces after adhesion. The maximttm tensile stress (a) produced dttring detachment can be determined by Eq. (6) by dividing the maximiun force of detachment by the total surface area A ) involved in the adhesive interaction ... 

The same theories relevant to adhesion, developed to explain and predict the performance of glues, adhesives, and paints, have also been applied to bioadhesive systems [44], These include the electronic, absorption, wetting, diffusion, and fracture theories. 

Several mechanisms by which mucoadhesives adhere to biological surface have been suggested, including the electronic, adsorption, wetting, diffusion, and fracture theories. It is likely that water movement from the mucosa to the polymer and physical entanglement of the adhesive polymer in the mucus glycoprotein chains are important in obtaining adherence. 

The thickness of the TDCB specimens (S = 10 mm) is sufficient to ensure plain strain conditions. It should be noted that during the test the arms remain within their elastic limit. Therefore, from simple beam theory [7], and by the use of linear elastic fracture mechanics, the strain energy release rate of the adhesive can be obtained using Eqn. 2, where P is the load at failure and E, is the substrate modulus. The calculated adhesive fracture energy was employed in the simulation of the TDCB and impact wedge-peel (IWP) tests. 

The starting point of the fracture theory is the Griffith-Irwin theory of cohesive fracture. It has been extended to the adhesive fracture According to this theory the fracture strength a of an adhesive bond is related to the fracture energy e and the critical crack length 1. It is... 

The random roughness of surfaces can be modeled by a statistical distribution, as first shown by Johnson and later much expanded by others. Using such a statistical theory, Fuller and Tabor defined an adhesion parameter which was the asperity height divided by the maximum extension an asperity could withstand before adhesive fracture. This adhesion parameter increased with roughness and elastic modulus but decreased with work of adhesion and asperity... 

The section on design of structural adhesive joints will describe and cite advantages and disadvantages of joint geometries, such as butt, lap, scarf, strap, and combined versions of these. A general design criterion will also be included. Another section of the chapter will pertain to fracture mechanics. General theories on fracture mechanics and test techniques used to characterize structural adhesives fracture behavior will be discussed. 

All the studies conducted on fracture of bulk polymers are certainly relevant to the adherence of polymers, the mechanisms of losses at a crack tip being the same viscoelastic losses due to moving stresses, work to extract chains or fibrils, and viscous drag in the presence of a liquid. It is probable that the various theories of adhesion, namely, theory of wetting, theory of the rheological factor, theory of the chemical bond, theory of the weak boundary layer, and theory of interdiffusion, are all valid, each corresponding to an emphasis on a dominant mechanism. 

This equation for lap joint failure is surprising in a number of ways. It is equivalent to Griffith s brittle fracture theory for glass [4], Moreover, it fits the puzzling historic results for lap joint failure which showed that the overlap length was not important for long joints, and the strength increased with sheet thickness d and stiffness E. Additionally, it is now clear why chemical environment can weaken the Joint because the failure depends on work of adhesion W, which decreases markedly with surface contamination. 

The most-often cited theoretical underpinning for a relationship between practical adhesion energy and the work of adhesion is the generalized fracture mechanics theory of Gent and coworkers [23-25] and contributed to by Andrews and Kinloch [26-29]. This defines a linear relationship between the mechanical work of separation, kj, , and the thermodynamic work of adhesion ... 

The term aK2v", derived from reptation theory, describes the velocity-dependent energy necessary to fracture the bulk adhesive. K2 is the consistency which relates the viscosity to the shear rate for a non-newtonian fluid. a = TtraL fh", with r being the chain radius, L the chain length, a the density of chains crossing over the fracture plane, and h is the distance between the chain and reptation tube. 

It is noted that attempts to apply composites theory to the materials investigated have not been entirely successful. While upper and lower bounds on, e g., moduli can be established there is little quantitative ediction of the impact strei th or fracture toughness parameters of the composites. Hence, the systems cannot be considered as optimized, for example, with regard to impact strength versus particle size, shape, or distribution or matrix-particle adhesion. The complexity is, of course, due to the statistical structure of the dispersed phase and the resultant uncertainties in the calculations of local stress fields, which in turn imply uncertainty in the local mode of yielding or rate of yielding. 

The fracture-based approach derives from continuum fracture mechanics theory, which claims the strength of most real solids is governed by flaws within the material [2]. To help predict this type of behavior, many test methods have been developed to determine fracture properties of adhesives. These tests are used to characterize the mode I, II, and III fracture properties of many types of material systems. In this study, the focus will be on the mode I and II characteristics of bonded joints for automotive applications. 

In 1983, an important review volume on crazing was published ( ) and a number of other relevant reviews on polymer fracture O) and assemblages of papers on adhesion (4-6) have been published recently. These and other developments enable us, now, to propose a theory of the separation process that will make it possible to predict and correlate experimental results, e.g., as to when strong or weak adhesion of a polymer to a hard, strong solid will occur. 

Research by Kyokong and others [28] lent credibility to Ilcewicz and Wilson s hypothesis. They applied Eringen s nonlocal theory to solid poplar (Populus tremuloides) joints bonded with resorcinol adhesive, substituting the average vessel lumen diameter of aspen (100 pm) as the characteristic dimension. They were able to show that the nonlocal theory using this dimension correlated very closely with the fracture toughness of the joints as determined by classic (local) theory. 

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