Interfibrillar cohesion

The modern silk sample not subjected to heat or light stress displayed fracture Types 2, 5, 7, and 11. The dominant fracture type exhibited by these fibers was Type 7 50 of the modern silk fractures were of this type. This finding indicates that surface flaws largely govern the fracture mechanism of modern silk. A smaller number of fracture Types 2, 5, and 11 were observed. This observation indicates that at least some of these fibers also exhibit ductility, a moderate loss of interfibrillar cohesion, and large internal voids, although their influence in fiber fracture is considerably less than that of surface flaws. 

Figure 4. SEM photomicrograph of a Type 5 (loss of interfibrillar cohesion) fracture (2600X).

The most common fracture event observed among the historic samples was Type 5 this observation indicated a greater loss of interfibrillar cohesion than that found in the modem silk samples. Because the modern samples stressed by heat at 60 °C also showed... 

For the partially oriented filament, interfibrillar cohesion is sufficiently high to cause relatively deep spontaneous cracking as the surface restructures by photooxidative cleavage, as reported for PPH films (14). Surface-core cohesion is sufficiently high to allow propagation of the surface cracks through the fiber core under tensile stress. 

The observed fact that chains break even after (20 minutes of) stress relaxation not only demands the integrity of the crystal blocks but also an intimate and persistent lateral cohesion between microfibrils within a fibril and between fibrils within a filament. In the same way as studied in detail in Chapter 5 for single chains the shear displacement of the ends of microfibrils against the interfibrillar cohesion permits the transfer onto the microfibrils of those forces which accumulate within the stress-transfer lengths to the axial stress a. This stress relaxes at constant filament elongation. The continued rupture of chains indicates that the axial strains of the microfibrils are maintained during such stress relaxation. Those strains, however, can only be maintained if large scale slip of microfibrils or fibrils does not occur. 

In some fibers failure in axial tension occurs by cracking, or splitting, along planes close to the fiber axis. Due to a nonhomogeneous fibrillar substructure there will be shear stresses in these planes. With reduced interfibrillar cohesion a break is produced characterized by multiple axial splitting over a long length (equal to many fiber diameters) (Fig. 8.22). 

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