Cohesion fracture

Once it is recognized that particles adhere to a substrate so strongly that cohesive fracture often results upon application of a detachment force and that the contact region is better describable as an interphase [ 18J rather than a sharp demarcation or interface, the concept of treating a particle as an entity that is totally distinct from the substrate vanishes. Rather, one begins to see the substrate-particle structure somewhat as a composite material. To paraphrase this concept, one could, in many instances, treat surface roughness (a.k.a. asperities) as particles appended to the surface of a substrate. These asperities control the adhesion between two macroscopic bodies. 

In which we summarize the terms as CTq = cohesive fracture strength of A = modulus of A, h = thickness of A layer L = influx length to a maximum length Lc, I — influx coverage of the A-B interface (/ < 1) = fraction of elastic energy dissipated in the A layer ( < 1). 

Temperature Chewiness (°C) (mJ) Cohesiveness Fracturability (N) Gumminess (N) Hardness (N) Springiness (mm)... 

Liquid jet instability is only one possible cause of thread fracture during melt spinning. The order breaking mechanism is cohesive fracture. The importance of this phenomenon has been stressed by Ziabicki and Takserman-Krozer (1964). 

This is an order of magnitude that will permit spinning under normal conditions. A relatively small variation in the magnitude of the parameters involved, however, may lead to cohesive fracture. 

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

Above the glass transition temperature, thermosets are weak elastomers (because of their densely crosslinked structure) and are of no known practical use. Apparently, only King and Andrews Swetlin and LeMay have investigated the cohesive fracture or tear of thermosets above Tg, all using amine-linked epoxies. These studies have demonstrated that the rubbery fracture of epoxy thermosets is quite similar to that of more conventional crosslinked elastomers. 

The cohesive fracture of conventional, non-strain crystallizing, unfilled elastomers is sensitive to rate and temperature 32.4-1.48-53) exhibiting increased values of 2J with increasing rate and decreasing temperature. The basic viscoelastic nature of the fracture of these materials is evidenced by the fact that it can be described over wide ranges of temperature and rate by time-temperature superposition as described by the WLF Equation... 

Reactivity, independent 21 Relative rigidity 90, 94 P-Relaxation 139 Rubber-like elasticity theory 118 —modified epoxy resins 99 Rubbery cohesive fracture 125 — fracture energies 143, 146... 

The viscosities of polymer melts, calculated from the storage modulus and the loss modulus, have to be within a range to resist the applied forces, which act against the rheological forces. But they should not be as large as to prevent the necessary deformation before the start of sodification. The elastic part of deformation has to be small since an elastic deformation happens more rapidly than a viscous one. Therefore, a considerable elastic deformation can lead to a cohesive fracture of the fiber in the molten state. The ratio of the viscous to the elastic energy of the polymer melt may be seen as one of the most important factors for the spinnability of polymers. For the usual commercially used spinnable polymers, such as, for example, poly(ethylene terephthalate), the ratio is about G"/G >10... 

In most cases, adhesion fractures are a sign of insufficient surface pretreatment. In the case of mixed fractures, the reason is to be seen, with high probability, in an incomplete or uneven degreasing. The neglected degreasing after blasting can also be seen as a cause, if the compressed air has not been absolutely fat-free. A cohesion fracture may be the consequence of an adhesive layer not completely cured or of the application of an inadequate adhesive mix. 

Independent of the test methods applied and the respective results, a precise analysis of the failure source is required. Here, fracture-type time-diagrams are used in which an allocation of failure sources is made over the test period (adhesion fracture, cohesion fracture, adherend fracture, corrosion, q.v. Figure 7.8 and Figure 10.7). 

Cohesion fracture Failure of a bonded joint due to fracture in the adhesive layer. 

Figure 7. Experimentally determined time-temperature shift of the cohesive fracture energy for model solid propellent...

To be precise, cohesive fracture refers to crack growth within an homogeneous material, whereas adhesive fracture refers to crack... 

It has been mentioned previously that cohesive fracture occurred when a soft resin was used as the matrix of such dismantlable adhesives [5]. However, with... 

The generalized theory of fracture mechanics of Andrews (35) predicts that the cohesive fracture energy per unit surface area J is given by the energy required to break the bonds crossing the fracture plane, Jq, multiplied by a loss function, 6. 

The starting point of the fracture theory is the Griffith-Irwin theory of cohesive fracture. It has been extended to the adhesive fracture According to this theory the fracture strength a of an adhesive bond is related to the fracture energy e and the critical crack length 1. It is... 

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