Threshold tear energy

Figure 3. Threshold tear energy T . Key O, A, , PDMS networks , A. PB networks , PI networks versus molecular weight Mc between cross-links calculated from Ct. O, , , random cross-linking A, A. trifunctional end-linking , tetrafunctional end-linking.

When the tearing (fracture) energy is measured under conditions at which energy is not dissipated in viscous processes, its value is termed the threshold tearing energy, T0 (30,31,32). If T0 for the TIPA elastomer is the same as that for a hydrocarbon elastomer whose 2Cj equals that for the... 

A theory due to Lake and Thomas appears to provide a satisfacto account of the origin of the parameter S in cross-linked systems. (In the literature the symbols Tq = 2S = S, are used, indicating minimum or threshold tearing energy referred respectively to unit area of fracture plane ie. two mrfaces, and unit area of fracture surface). 

To N mm threshold tearing energy from cyclic experiment... 

Fig. 5.12 Double-logarithmic and schematic representation of the crack propagation rate da/dN as a function of the fracture mechanics parameter AT (crack propagation curve) according to 94Blu, 75Cla with Tq as threshold value and Tc as critical tearing energy.

Next, we consider the relationship between Go, the minimum mechanical energy required to disrupt an interface, and W, the equilibrium, thermodynamic work of adhesion. Firstly, however, we discuss the corresponding quantities for the cohesive fracture of a lightly cross-linked rubber. These are Goc, the threshold tearing energy (114), and ITc, the reversible fracture energy of the bonds... 

The observed M dependence of arrest glassy fracture energies is curious because this is the same dependence predicted by Lake and Thomas for the threshold tearing of elastomers. It was previously shown that this dependence is exhibited by the rubber tear of the DGEBA/DDS epoxies over a wide range of temperatures, even though they were far removed from the threshold region. 

In eqttilibrium conditions and at rest, dWg = 0 and G = 2y if G crosses this critical threshold, the crack propagates slowly as long as G < Gg (see section 11.1) and then in a catastrophic manner for G = Gg. Gg is the critical energy of tearing per unit of area. If G < 2y, the crack should close (dS < 0, dWg > 0). In reality, different irreversible processes, and particularly surface hydrolysis, oppose the healing of the crack. 

It follows from this equation that there is a limiting frequency coj. below which the photo-effect in a given photocathode disappears completely. In fact, the quantum ho does not have enough energy to tear an electron out of the surface of electrode C. This happens when hco is lower than the work function. The so-called photo-electric threshold takes place at ho = A2. 

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