Peeling stresses

A meehanical analysis of the near surface stresses in a wedge test as a function of surface geometry shows that tan a is equal to the ratio of the shear stress to the peel stresses (Fig. 4). 

NR adhesives perform adequately under peeling stresses. The peel strength can vary from a few N/m in PSA formulations to substrate tear in vulcanized compounds used in hose, belting and tire products. 

An epoxy-polymercaptan reaction that is catalyzed with a tertiary amine is used in the standard two-component 5-min curing epoxy which can be found in the hardware stores. These fast-curing products, however, have a tendency to be somewhat brittle and may perform quite poorly under peel stress. The standard 5-min cure is obtained with the accelerated mercaptan, such as Capcure 3830-81 (Cognis Corporation). The fastest polymercaptan has a gel time of 40 s in a 25-g mass. 

The second important assumption in the analysis is that interfacial failure occurs only in shear, i.e. that any peeling stress, normal to the interface, is negligible. Analysis of an elastic bilayer (5) shows that, for the experimental parameters employed here, the peeling stress is, in fact, an order of magnitude less than the shear stress. Furthermore, finite element analysis (6) shows that the normal stress is compressive rather than tensile for the thicknesses of PET and Ni used here. Finally, it will be shown that the experimental results are consistent with the one-dimensional analysis presented above. 

Fig. 15 (a) Schematic representation of the stress profiles in an adherent portion of film at a distance x from a free surface of a through-thickness crack. Interfacial shear stress x. and peel stress p correspond to the action of the substrate on region (1 The normal stress parallel to the x axis, is supposed to remain constant through the film thickness h. 

Figure 4 shows typical failure surfaces obtained from tensile tests of the co-cured single and double lap Joint specimens. In the case of the co-cured single lap Joint, as the surface preparation on the steel adherend is better, a greater amount of carbon fibers and epoxy resin is attached to the steel adherend. Failure mechanism is a partial cohesive failure mode at the C ply of the composite adherend. In contrast with the co-cured single lap joint, failure mechanism of the co-cured double lap joint is the partial cohesive failure or interlaminar delamination failure at the 1 ply of the composite adherend because interfocial out-of-plane peel stress... 

It is important to consider the stacking sequence of the composite adherend of the co-cured single and double lap joints because the out-of-plane peel stress can be reduced through possible regulation of the stififhess difference between steel and composite adherends. 

This results in increased tensile stress ctz (normal stresses, peel stresses) for the adhesives layer in these areas caused by the bending moment Mb. 

Figure 11.2 Force transfer in the case of shear and peel stress.

In the case of drawing (b), that is, peel stress (the memo sheet is turned and bonded in an angle of 180°) the paper with the weight on it remains in its position, force transfer is not possible. 

From a design-engineering point of view, peel stress can be avoided by the possibilities shown in Figure 11.3. 

Figure 11.3 Design possibilities to avoid peel stress.

The stress state of a coating is closely interconnected with the adhesion of that elastomer to the concrete. If adhesion forces are not zero, peeling stresses arise in the coating, which is in a plain stress state. Such stresses can reach a critical value and concentrate in a zone of a concrete substrate where the cracks are formed. In the absence of adhesion, the coating functions independently of the substrate, and any increase in the exterior load will result in the development of a peeling area. Taking into consideration that the ultimate tension stress of any coating exceeds... 

Goland and Reissner(16) was limited because the peel and shear stresses were assumed constant across the adhesive thickness, the shear was assumed a maximum at the overlap end (and not zero as it must be at a free surface), and the shear deformation of the adherends was neglected. Later, several analysts including Renton and Vinson(17) and Allman) 18) produced solutions where the adherends were modelled to account for bending, shear and normal stresses. Adhesive shear stress was set to zero at the overlap ends. Allman additionally allowed for a linear variation of the peel stress across the adhesive thickness, although his adhesive shear stress was constant. 

When bonding elastic material, forces on the elastomer during cure should be carefully controlled, as too much pressure will cause residual stresses at the bond interface. Stress concentrations may also be minimized in rubber-to-metal joints by elimination of sharp comers and by the use of metal adherends sufficiently thick to prevent peel stresses that may arise with thinner-gauge metals. As with all joint designs, polymeric joints should avoid peel stresses. Figure 7.16 illustrates methods of bonding flexible substrates so that the adhesive will be stressed in its strongest direction. ... 

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