Tensile stress during adhesion

As mentioned earlier, the degree of realization of the gelation process determines the quality of physical properties of plasticized products such as resistance to tensile stress, elasticity, adhesion, migration of plasticizer, and so on. To accelerate the gelation process, pregelation can be produced during the mixing step. 

The separation of two surfaces in contact is resisted by adhesive forces. As the nonnal force is decreased, the contact regions pass from conditions of compressive to tensile stress. As revealed by JKR theory, surface tension alone is sufficient to ensure that there is a finite contact area between the two at zero nonnal force. One contribution to adhesion is the work that must be done to increase surface area during separation. If the surfaces have undergone plastic defonnation, the contact area will be even greater at zero nonnal force than predicted by JKR theory. In reality, continued plastic defonnation can occur during separation and also contributes to adhesive work. 

Tyres are very definitely fatigued during use and, as mentioned for fabric/rubber adhesion above, it is very important to carry out dynamic tests to assess bond efficiency. Methods have not apparently been standardised but a variety of procedures have been reported71 79 Some workers have used the same or a similar test piece as in static tests and applied a cyclic tensile stress or strain, whilst others have used some form of fatigue tester operating in compression/shear to repeatedly stress or strain cord/rubber composite, or even to flex samples in the form of a belt. Khromov and Lazareva80 describe a method using test pieces cut from tyres. 

The fracture theory is the most widely applied theory in studying mucoadhesion mechanisms. It accounts for the forces required to separate two surfaces after adhesion. The maximum tensile stress (a) produced during detachment can be determined by Eq. (6) by dividing the maximum force of detachment /v by the total surface area (Aq) involved in the adhesive interaction ... 

The primary structural role of the face/core interface in sandwich construction is to transfer transverse shear stresses between faces and core. This condition stabilizes the faces against rupture or buckling away ftom the core. It also carries loads normally applied to the panel surface. They resist transverse shear and normal compressive and tensile stress resultants. For the most part, the faces and core that contain all plastics can be connected during a wet lay-up molding or, thereafter, by adhesive bonding. In some special cases, such as in a truss-core pipe. 

For example, Christenson et al. [3,19] performed a detailed study of polyisobutylene-based pressure-sensitive adhesives. Although these authors did not postulate a specific detachment criterion, they did extensive work characterizing the linear viscoelastic properties, the tensile stress-strain properties, and the peel force. In addition, they conducted detailed visualization of the deformation of the adhesive during peel and therefore, could assess the ability to predict the peel force from the mechanical properties of the adhesive and the visually observed detachment strain. In this work, the adhesive consisted of a blend of high and low molecular weight polyisobutylene. They showed that when they used the Giesekus model as the constitutive equation for the adhesive, they could accurately describe the stress-strain curves of the adhesive and the peel force was well predicted by the integral of the stress-strain curve up to the measured detachment strain. Their results are summarized in Table 1. 

The introduction of stress during the climatic exposure markedly influences strength retention. Figures 121-3 illustrate this and refer, in numerical order, to adhesives F, G and H of Table 29 at the hot, dry and hot, wet tropical exposure sites over 4 years. The stress applied was tensile and the level is expressed as a percentage of the proof ultimate tensUe breaking stress. It was continuously applied by dead load. 

Attempts to project results from short-term creep tests to a longer time scale often fail because the increase of shear strain during a static load creep experiment will lead to a change of the state of stress in the adhesive joint and will eventually cause an excessive amount of superimposed tensile stress near the end of the overlap area. Therefore, the dotted lines in O Fig. 34.13 merely represent a guide to the eye projection rather than a mathematically confirmed extrapolation. 

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