Лако-красочные материалы — производство

There are obvious similarities between the polymer which has solidified within the pores of a microfibrous surface and fibers embedded in the matrix of a composite material (cf. Fig. 1). Standard treatments of fiber composites (e. g., [72]) draw attention to the signifi­cance of the critical length of fiber. When short fibers are stressed axially, shear failure at, or close to the fiber/matrix interface is considered to occur, and the fibers may be pulled out of the matrix. Fibers greater than the critical length, with a consequently larger fiber matrix interfacial area, fail in tension, and only the broken ends are pulled out. This, of course, is one of the points that Gent and Lin were demonstrating. The fracture toughness of the composite may be enhanced by energy terms associated with fiber fracture, with fiber matrix adhesion, and with fiber pull-out. By assuming that the fiber is linearly elastic and equating the interfacial shear force to the tensile force for a fiber of critical length l, it immediately follows that

2l_ a a x

where a is the fiber radius, a its tensile strength, and x the interfacial shear strength. As in Eq. (15), the l/a ration is significant.

Arslanov and Ogarev [73] use Eq. (16) to argue that the critical length of a filament of adhesive in a microporous anodic film is very small, so the filaments will fail in tension and most of the pore length is irrelevant to adhesion. Application of the simple model of Eq. (15) to this situation shows that even with a short length of elastic adhesive filament a useful increase in peel strength might be expected. For polyethylene embedded in a film formed by anodising in phosphoric acid, a ratio G0a/Ga of three to four times is obtained.

In a realistic situation the adhesive filament will not act as a perfect elastic body uniformly stressed up to fracture. Uneven stress distributions and plastic yielding would be expected to increase the energy dissipation observed beyond that calculated for the ideal elastic model. It will be very interesting to see whether in the future auxetic materials can be developed to an extent that they can be used as coatings for such porous substrates. Even greater increases in fracture energy can then be anticipated.

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.