Fracture energy adhesive joints

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. 

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 ... 

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 ... 

Whereas the fracture of bonded joints is due to the presence of critical discontinuity in a field of stress, the stress intensity or crack initiation energy of bonded joints and materials at which fracture occurs is also a function of the properties of the wood and the adhesive, the environment at a given time, changes in the environment, and external forces on the joint or bonded material. These relationships are explored in the following sections. 

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