Fracture of Epoxy Polymers

At present a large number of new polymers is synthesised in scientific laboratories around the world, from which only a small part reaches the industrial production stage [48]. Naturally, such work requires large expenditures of time and means. These expenditures can be reduced essentially by the development of methods of the prediction of the properties of new polymers, proceeding from their chemical constitution [49]. 

Mechanical properties of polymers are among the most important, since a certain level of these properties is always required even for polymers of different special-purpose functions [50]. In papers [38, 51] it has been shown that the curing process of the chemical network of epoxy polymers with the formation of nodes of various density results in a change in the molecular characteristics, particularly the characteristic ratio C. If such an effect actually exists, then it should be reflected in the deformation-strength characteristics of crosslinked epoxy polymers. Therefore the authors [49] offered methods of prediction of the limiting properties (properties at fracture), based on the notions of fractal analysis and the cluster model of the amorphous state structure of polymers, with reference to a series of sulfur-containing epoxy polymers [52, 53] (see also Section 5.4). 

The authors [49] carried out predictions of two deformation-strength characteristics strain up to fracture and fracture stress For the value of two methods of theoretical estimation can be used. The first does not include molecular characteristics in its calculation and, hence, does not account for their change, at least directly [54]. This method is based on the notions of the cluster model of amorphous state structure polymers [8, 9] and the limiting drawing ratio value in this case is given by Equation 1.6. 

The second method is based on the notions of fractal analysis and the final formula has the form [55]  

For calculation of the strength (failure stress) o,the following equation was used [56]  

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Thermosetting epoxy polymers are widely employed in structural engineering applications and thus a knowledge of the mechanics and mechanisms of the fracture of such materials is of vital importance. The present Chapter discusses the fracture of epoxy polymers, concentrating on the use of a continuum fracture mechanics approach for elucidating the micromechanisms of crack growth and identifying pertinent failure criteria. 

As it is known [58, 59], the main deficiency of epoxy polymers, their tendency to brittle fracture, is considered. It was considered for a long time that brittleness is an inalienable property of crosslinked systems, however, a number of papers has appeared recently that show that epoxy polymers can be deformed in tensile tests, displaying macroscopic yielding [60]. The authors [52] showed the possibility of plastic strains in impact test conditions and explained the factors influencing the type of fracture of epoxy polymers. 

Figure 6.18 The diagram of load-time (P - t) for plastic fracture of epoxy polymer SCE-DADPS - 1.2, plotted by averaging the results of five experimental diagrams...

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. 

Most of the recent advances in the understanding of the fracture behaviour of epoxy polymers has been through the application of fracture mechanics 2) and the present Chapter is therefore concerned with the study of the mechanisms and mechanics of crack growth in crosslinked epoxies using fracture mechanics. 

We have used this model to investigate stress distributions in and around the rubber particle, or around a void, in a matrix of epoxy polymer. This chapter describes the modeling of stress concentrations in rubber-toughened epoxy and gives a simple model for predicting the fracture energy, Gc, of such a material. 

Although it is not the intention to provide a comprehensive thesis on the origins of LEFM, it is useful to review the test method that is so often used to characterize the fracture toughness of epoxy polymers. The most common geometry to characterize the fracture toughness of epoxy polymers is the single edge notched, three-point bend test... 

Figure 17.1 Scanning electron micrographs of the fracture surface of epoxy polymer containing 9.6 vol% nanosilica (voids with nanoparticles are circled in the central image).

Zamanian, M., Mortezaei, M., Salehnia, B., Jam, J.E., 2013. Fracture toughness of epoxy polymer modified with nanosilica particles Particle size effect. Engineering Eracture Mechanics 97, 193—206. 

Figure 5.38 The optical microphotograph of floccules on the fracture surface of epoxy polymer SCE-DADPS. Enlargement is 125x [138]...

Since in the experiment the constant value of was used then according to Equation 6.25 the value of is only the function of the plasticity of epoxy polymers, characterised by the value, and their degree of defectness, characterised by parameter Since in the experiment the samples without a notch were used then parameter characterises the critical structural defect (CSD) [66, 67]. Electron microphotographs (enlargement 300x) of the fracture surfaces, an example of which is adduced in Figure 6.21, were used for estimation of the value of... 

Figure 6.21 An electron microphotograph of fracture surface in impact tests of a sample of epoxy polymer SCE-DADPS-1.2. Enlargement 300x [52]...

A number of amorphous thermoplastics are presently employed as matrices in long fiber composites, including polyethersulfone (PES), polysulfone (PSU), and polyetherimide (PEI). AH offer superior resistance to impact loading and higher interlaminar fracture toughnesses than do most epoxies. However, the amorphous nature of such polymers results in a lower solvent resistance, clearly a limitation if composites based on such polymers are to be used in aggressive environments. 

Kinloch, A. J. Mechanics and Mechanics of Fracture of Thermosetting Epoxy Polymers. Vol. 72, pp. 45-68. 

Mechanics and Mechanism of Fracture of Thermosetting Epoxy Polymers... 

In the case of linear-elastic-fracture-mechanics, and nearly all epoxy polymers obey the requirements for LEFM to be employed, a simple relationship exists between KIc and G,c... 

The results shown in Table 1 clearly reveal that the fracture energies of even unmodified, simple epoxy polymers, i.e. about 100 to 300 J/m2, are at last one hundred times the energy required to break solely covalent bonds, i.e. lessthan 1 J/m2. This demonstrates that other energy absorbing processes, such as plastic deformation, must take place at the crack tip. 

Turning to the multiphase thermosetting epoxy polymers (Table 2), then for the rubber-modified materials the greater fracture resistance arises from a greater extent of energy dissipating deformations occurring in the material in the vicinity of the crack tip 1 -8-35.38). The deformation processes are ... 

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