Adhesion joint fracture energy

Relating Joint Fracture Energy to Intrinsic Adhesion... 

To generate a specific structure or texture on the adherend surface. Even though mechanical interlocking may not be responsible for primary bonding, the complex structures present on, for example, conversion-coated or anodized surfaces may help increase joint fracture energies by redistributing the stresses away from any interphasial polymer. These structures also create an extended surface area, which may be utilized if the adhesive is able to flow into, for example, the pores on anodic oxides. 

Usually tj/ is very much larger than Fq. This is why practical fracture energies for adhesive joints are almost always orders of magnitude greater than works of adhesion or cohesion. However, a modest increase in Fq may result in a large increase in adhesion as and Fo are usually coupled. For some mechanically simple systems where is largely associated with viscoelastic loss, a multiplicative relation has been found ... 

In recent years there has been a renewed appreciation of potential beneficial effects of roughness on a macroscale. For example Morris and Shanahan worked with sintered steel substrates bonded with a polyurethane adhesive [61]. They observed much higher fracture energy for joints with sintered steel compared with those with fully dense steel, and ascribed this to the mechanical interlocking of polymer within the pores. Extra energy was required to extend and break these polymer fibrils. 

Eqs. 1-5 hold whether failure is interfacial or cohesive within the adhesive. Furthermore, Eq. 5 shows that the reversible work of adhesion directly controls the fracture energy of an adhesive joint, even if failure occurs far from the interface. This is demonstrated in Table 5, which shows the static toughness of a series of wedge test specimens with a range of adherend surface treatments. All of these samples failed cohesively within the resin, yet show a range of static toughness values of over 600%. 

Daghyani, Fl.R, Ye. L. and Mai, Y.W. (1995a). Mode 1 fracture behaviour of adhesive joints, I, Relationship between fracture energy and bond thickness. J. Adhesion 53, 149-162. 

Example 13.8 Urea-formaldehyde (UF) adhesive used in bonded wood products was modified by oopolymerizing 10 mol% urea derivative of dodecanediamine (DDDU). The fracture energies of wood joints made with the unmodified and DDDU-modified adhesives were found to be 130 and 281 J/m, respectively. Explain the enhanced fracture energy of the wood joint bonded with the modified adhesive. 

While calculations like those discussed involve serious simplifications and idealizations, they do serve to show mechanisms by which surface roughness per se is capable of significantly increasing the fracture energy of an adhesive joint. 

Let us examine each term in turn, considering how it might contribute to the hypothetical fracture energy G of the adhesive joint ... 

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