Cohesive break

This influx coverage parameter /, can be used to characterize both intra and inter-spherulitic influxes and combinations of the two.The maximum force Fj, required to cohesively break an influx is determined by... 

X-cut According to ASTM D 3359, method A, an X is cut into the film to the substrate, pressure tape (TESAPACK 4287) is applied over the X and then removed, and adhesion is evaluated by comparison with descriptions and pictures. The method is used to establish whether the adhesion failure of the coating to the substrate may occur between the coats (adhesive break) or in the coating (cohesive break). 

Relaxation phenomena arise already during the moment of a film drawing on a substrate when a shift pressure appears and possibilities for the cohesion break of an interface between the film and the substrate are created. During the drying, the coating should relax, forming a thin and smooth layer, thus the module of elasticity increases. Thus, the mechanical relaxation promotes decrease of the probability of fragile destruction and exfoliation of the film from the substrate [7],... 

Figure 38-7. Influence of the capillary break (—) and cohesion break ( ) on the spinnable length L of the fibers as a function of the fiber production rate v and viscosity rj. (—) Effect observed when capillary and cohesion breaks overlap. (After A. Ziabicki.)...

From the relation L=f(vij) it is obvious that the spinnability is governed by two processes, namely the cohesive break (or the swell effect) and the melt break (capillary break, melt fracture). According to Section 11.3.1, a certain amount of elastic energy can be stored in all viscoelastic fluids. This phenomenon leads, among others, to the Barus effect. 

This theory was first described by Bikerman. It states that bond failure at the interface is caused by either a cohesive break or a weak boundary layer.Weak boundary layers can originate from the adhesive, the adher-end, the environment, or a combination of any of these three factors. 

Weak boundary layer theory. According to the weak boundary layer theory, when bond failure seems to be at the interface, usually a cohesive break of a weak boundary layer is the real event. Weak boundary layers can originate from the adhesive, the adherend, the environment, or a combination of any of the three. When bond failure occurs, it is the weak boundary layer that fails, although failure seems to occur at the adhesive-adherend interface. Figure 9.16 shows examples of certain possible weak boundary layers for a metallic substrate. For plastic substrates there are many more opportunities for weak boundary layers, such as mold release, plasticizer migration, and moisture migrating to the interface. Certain weak boundary layers can he removed or strengthened by various surface treatments. 

E. Comet, Esso Abrasion Cohesion Test, A Description of the Cohesive Breaking of Emulsions for Chip Seals, Proc. Int. Symp. Asphalt Emulsion Technology, 1999, pp. 346 355. 

Most solid surfaces are marred by small cracks, and it appears clear that it is often because of the presence of such surface imperfections that observed tensile strengths fall below the theoretical ones. For sodium chloride, the theoretical tensile strength is about 200 kg/mm [136], while that calculated from the work of cohesion would be 40 kg/mm [137], and actual breaking stresses are a hundreth or a thousandth of this, depending on the surface condition and crystal size. Coating the salt crystals with a saturated solution, causing surface deposition of small crystals to occur, resulted in a much lower tensile strength but not if the solution contained some urea. 

Duetility (ASTMD113). The ductihty of an asphalt is expressed as the distance in cm which a standard briquet can be elongated before breaking. Ductihty is a combination of dow properties and redects both cohesion and shear susceptibiUty. 

Solid Dispersion If the process involves the dispersion of sohds in a liquid, then we may either be involved with breaking up agglomerates or possibly physically breaking or shattering particles that have a low cohesive force between their components. Normally, we do not think of breaking up ionic bonds with the shear rates available in mixing machineiy. 

For a crack, w cm wide, to advance by dx cm in a solid, a ribbon (or curtain) of this solid, dx cm thick and w cm wide, must be extended to its greatest possible elongation. If the curtain of length L can extend freely, the work of extension is, see Eq. (44), 2 Lw dx/2E ergs. The cohesion is equal to the breaking stress <7m (g/cm sec2) of the theoretical solid, otherwise identical with the real, but free of all defects. rather than am should be used here because dx is so small that no defect can find place in the curtain. The work per unit area corresponds to the hypothetical 7 . Hence,... 

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