Adhesive joints design criteria

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. 

It has become evident that failure criteria such as maximum shear stress are inadequate since failure can also occur through other modes, such as tensile stresses in adhesive joints. Also, the uncertainty in the stress mode is magnified because properties of and stresses in adhesive joints are highly dependent on joint geometry. Therefore, complexities arise when deciding the proper criterion applicable to designing structural adhesive joints. 

When a complex joint is to be introduced in a structure, the ideal situation is to test that specific joint. However, this approach is very expensive. Before real joints or prototypes are built, the designer should first come up with a good prediction of the failure load based, among other things, on the basic mechanical properties of the adhesive. The basic properties can mean the elastic properties, such as the Young s modulus and the Poisson s ratio in case the analysis is linear elastic. However, for the more realistic theoretical methods that take into account the nonlinear behavior of the adhesive, the yield stress, the ultimate stress, and the failure strain are necessary. The stress-strain curve of adhesives is necessary for designing adhesive joints in order to compute the stress distribution and apply a suitable failure criterion based on continuum mechanics principles. 

An Annex to this ISO document provides a design criterion for adhesive bonding. In order to determine the bonded area, a simplified approach based on shear loading is proposed. Shear stresses should be limited to 20% of the nominal bond shear strength, that is, a safety factor of 5. This factor is intended to account for both stress concentrations in real joints and the viscoelastic (time and temperature dependent) nature of adhesives. Shear data values, to be used if test values are not available, are provided, O Table 48.3. 

Creep. The gradual extension of a lap-shear joint can be observed under conditions in which the adhesive is near or above its Tg, the stress is relatively high and its shear component much greater than the cleavage (hydrostatic) component. What little data is available refers to polyvinyl-P/F and to a modified epoxy of undisclosed composition. If the absence of creep is important in design then an adhesive with Tg well above the important working temperature must be chosen. In the absence of other data, the heat distortion temperature can be used as a criterion. 

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