Fracture of elastomers

The standard methods of measuring the strength of elastomers are tensile tests and tear measurements. Uniaxial extension tests give ultimate stress values and the area under the stress-strain curve represents the toughness or elastic energy per unit 

FIGURE 10.15 Time-dependent strength for polymeric and nonpolymeric materials 1, silver chloride 2, poly(vinyl chloride) 3, aluminum 4, PMMA 5, zinc 6, celluloid 7, rubber 8, nitro-ceUuloid, 9, platinum 10, silver 11, phosphoric bronze 12, nylon 6 (oriented). (Data from Hsaio, C. C Phys. Today, 19, 49, March 1966.) 

A classic theory by Lake and Thomas [20] has guided many of the analysis of correlating the threshold fracture energy of an elastomer with the length of the 

M is the molar mass of the elastic strand is a prefactor that depends on the polymer properties and is given by [20] 

U is the dissociation energy of a bond along the polymer chain q is the chain stiffness / is the bond length 

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The nominal tensile stress is given by the tensile force divided by the unstrained cross-sectional area of the specimen. It has been commonly used in the literature dealing with deformation and fracture of elastomers.) This reduction scheme is clearly quite successful in dealing with a wide range of crosslinking (see Figure 10.25) (Smith, 1969). 

Although relatively few studies have been made of the fracture of elastomers under complex stress conditions, some general conclusions can be drawn regarding fracture under specific combined-stress states, as follows. 

Rubber Chemistry and Technology 68, No.2, May-June 1995, p.197-211 FRACTURE OF ELASTOMERS BY GAS DECOMPRESSION... 

The limited viscoelastic extensibility of rubber strands determines according to the theory of Bueche and Halpin [69] the fracture of elastomers. The authors assume... 

New sections on fracture of elastomers, diffusion in polymers, and membrane formation... 

I.H. Gregory and A.H. Muhr, Stiffness and fracture analysis of bonded rubber blocks in simple shear, in Finite Element Analysis of Elastomers, ed. by D. Boast and V.A. Coveny, Professional Engineering Publications, Bury St. Edmunds, United Kingdom, 1999, pp. 265-274. 

Such soft-touch materials are usually TP Vs or thermoplastic elastomers (TPEs) which combine the moldability of thermoplastics in the melt state with elasticity, lower hardness, fracture resistance, and surface characteristics of elastomers. However, plastics and elastomers respond differently to mechanical stress. Hence, both rheological behavior and mechanical strength will to a large extent depend on the morphology of the blend which may change with change in the composition. 

Bascom, W.D., Cottington, R.L., Jones, R.L. Peyser, P. (1975). The fracture of epoxy and elastomer modified epoxy polymers in bulk and as adhesives. J. Appl. Polym. Sci. 19 2545-2562. 

A new class of elastomers uses block copolymers, which contain segments of different monomers. These form crystalline and noncrystalline regions in the polymer, and these polymers have elastomeric properties, as well as high fracture toughness. 

So far, we have considered the elasticity of filler networks in elastomers and its reinforcing action at small strain amplitudes, where no fracture of filler-filler bonds appears. With increasing strain, a successive breakdown of the filler network takes place and the elastic modulus decreases rapidly if a critical strain amplitude is exceeded (Fig. 42). For a theoretical description of this behavior, the ultimate properties and fracture mechanics of CCA-filler clusters in elastomers have to be evaluated. This will be a basic tool for a quantitative understanding of stress softening phenomena and the role of fillers in internal friction of reinforced rubbers. 

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... 

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. 

The energy necessary to propagate the crack within the model described above can be obtained using the classic Lake-Thomas [65] model of elastomer failure. Within this model Qc = 2n 17 where n is the number of main-chain bonds between crosslink or entanglement points, and 17 is the energy needed to break a main-chain bond. This relationship predicts a fracture energy of 6 J/m2 when 2 is 0.075 chains/nm2,in reasonable agreement with the experimental results. 

Elongation at break Parameter indicating the elongation in relation to the original length (in %) of a material up to its fracture. In the case of elastomers also called ultimate elongation. 

Recent work has shown that the addition of epoxy-terminated butadiene-acrylonitrile (ETBN) liquid rubbers to elastomer-modified VER gives a fivefold increase in fracture energy over that of elastomer-modified VER (3). 

This chapter describes our work in determining the impact fracture energies of elastomer-modified VER that has been further blended with various amounts of ETBN, and measuring the fatigue resistance of these materials. 

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