Coating normal stresses

Fig. 6 Families of yield loci for lactose powder coated with 0.5 X 10 mol of light paraffin at 20 C. Pf A., 0.346 x, 0.282 , 0.279 o, 0.261 (A) normal stress (B) compound stress. (From Reft .)...

For the case of similar adherends bonded with an adhesive possessing a different CTE (or for the case of a coating), the adhesive will often have an equal biaxial normal stress present within the plane of the bond. For relatively stiff adherends made of the same material, the biaxial in-plane stress within the bondline may be estimated by... 

Consider a thin coating on a substrate having coefficients of thermal expansion Uj and Uj, respectively. Since the coating is thin, the isotropic normal stress in the coating plane can be considered approximately constant through the thickness and, in the case of a coating-substrate system, cooled from temperature 72 to T, is given by ... 

For most fluids, Ni N2 and, thus, the latter is often excluded in rheological discussions. Attempts to determine the value of secondary normal stress difference experimentally have been made by several rheologists, but without success. It is still a challenge to quantitatively determine this material function. Nevertheless, it is not very important in most hydrodynamic calculations baning, of course, wire coating [S] wherein the secondary normal stress difference helps provide the necessary restoring force for stabilizing the wire position whenever it becomes off-centered. 

Fracture mechanics (qv) affect adhesion. Fractures can result from imperfections in a coating film which act to concentrate stresses. In some cases, stress concentration results in the propagation of a crack through the film, leading to cohesive failure with less total stress appHcation. Propagating cracks can proceed to the coating/substrate interface, then the coating may peel off the interface, which may require much less force than a normal force pull would require. 

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