Thermoset fracture behavior

Practical uses of epoxies include load bearing applications such as structural adhesives and composite matrices. In these applications, their most detrimental feature is a characteristic low resistance to brittle fracture. The desire to improve this property has motivated studies on thermoset fracture behavior for the last two decades. Of particular interest is the relationship between the molecular structure and the failure properties of thermosetting epoxies, the subject of this chapter. 

Under typical test conditions, thermoset fracture behavior is characteristically sensitive to variations in testing rate and temperature Interesting and... 

Fig. 10 a and b. Fracture toughness versus rate (a) and temperature (b) showing typical thermoset fracture behavior. I = initiation A = arrest E = stable crack growth... 

The observation that an increase in temperature or a decrease in rate both result in the same fracture response points toward a viscoelastic influence on thermoset fracture behavior, especially crack initiation. This characteristic behavior of epoxies has been explained qualitatively by consideration of the temperature and strain rate effects on the plasticity of the material at the crack tip . In effect, test conditions which promote the formation of a so-called crack tip plastic zone, or blunt the crack by a ductile process, promote unstable crack propagation. This aspect of unstable fracture is subsequently discussed in more detail. 

The fracture behavior of epoxy thermosets has been of growing interest since the mid-1960 s when investigations by Broutman and McGarry and Mostovoy and Ripling were published. Literature references seem to have peaked in the late 1970 s and early 1980 s when studies on crack blunting mechanisms speculations... 

Although epoxies dominate the thermoset fracture literature, work has been reported on other systems, e.g., polyester resins, phenol-formaldehyde compounds, peroxide cured polystyrene, and highly crosslinked polyurethanes. In general, these materials exhibit fracture behaviors similar to epoxies, and suggest that thermosets, as a class of materials, display characteristic crack growth properties. 

Like the testing variables just described, material variables can influence the fracture behavior of epoxy thermosets. Material variables discussed herein include the types of epoxy resins and amine curatives. 

Zen Zeng, Y.-B., Zhang, M.-Z., Penc, W.-Z., Yu, Q. Microstructure, mechanical properties, and fracture behavior of liquid rubber toughened thermosets. J. Appl. Polym. Sci. 42 (1991) 1905-1910. 

Kan Kanchanomai, C., Rattananon, S., Soni, M. Effects of loading rate on fracture behavior and mechanism of thermoset epoxy resin. Polym. Testing 24 (2005) 886-892. 

T. Helminiak and W. B. Jones, "Influence of Molecular Weight on Polyphenylquinoxaline Thermoset on Fracture Behavior," in Ref. 14. 

Influence of Molecular Weight on Fracture Behavior of Polyphenylquinoxaline Thermosets... 

Polymer-based composites can be divided into thermoset and thermoplastic composites, which due to their different properties show diverse fracture mechanism. Due to tight three-dimensional molecular network structure of the most of thermoset matrixes such as epoxy resins, they exhibit inherent brittle fracture behavior and poor... 

A thermosetting resin produces a polymer that does not soften with further heating and exhibits conchoidal fracture behavior at aU temperatures at which it is stable. Such polymers will undergo thermal decomposition (also called thermolysis) when heated, as their Tg is at a higher temperature than the temperature at which they break down. 

The lengths of the molecular chains dominate large strain behavior and crack propagation in contrast to their minimal influence at small strain levels. Consequently, thermosets are characterized by small deformation zones (Fig. 8.2) and brittle fracture. 

From a practical point of view, the main consequence of physical ageing by structural relaxation is embrittlement (decrease in fracture resistance Chapter 12). For the other aspects of mechanical behavior, ageing has either no effect or a favourable effect (increase of relaxation times, leading to a decrease of creep or relaxation rates). This is the reason why, in most thermoset applications, the knowledge of short-term properties is considered to be sufficient for engineering design, as far as fracture and durability are not concerned. 

Thermosets are polymeric materials which when heated form permanent network structures via the formation of intermolecular crosslinks. Whether the final product has a glass transition temperature, Tg, above or below room temperature, and therefore normally exists as an elastomer or a glass, it is, strictly speaking, a thermo-set. In practice, however, thermosets are identified as highly crosslinked polymers that are glassy and brittle at room temperature. These materials typically exhibit high moduli, near linear elastic stress-strain behavior, and poor resistance to fracture. 

The fact that thermosets are typically brittle and generally exhibit linear elastic stress-strain behavior suggests that linear elastic fracture mechanics (LEFM) and test methods may be applicable. In fact, these approaches have proven very popular, as is evidenced by the successful use of a number of LEFM-based fracture... 

If a material exhibits linear-elastic stress-strain behavior prior to rupture (an ideal behavior approximated by many thermosets), then a simple relationship exists between the material s fracture toughness and its fracture surface energy, J (or G),... 

The properties of thermosetting and thermoplastic resin systems are continually improved to meet increasing performance requirements of end users. One way to enhance material properties is to incorporate nano-modifiers, based on elastomeric silicone particles, which are optionally grafted with other (acrylic) polymers to control dispersibility, viscosity, and other parameters. As an example, epoxy resin formulations have been modified with silicone nanospheres to improve low-stress behavior. Table 1 shows the outstanding fracture toughness improvement of silicone coreshell nanospheres, even at very low particle loading levels. 

Fig. 13.41 The temperature dependence of the critical fracture energy Gic in DGEBA epoxy-resin thermosets, modified either by rubber particles or by debonding glass spheres, either in tests of conventional extension rates or in Izod impact tests, compared with the generally flat behavior of unmodified epoxy resin (Kinloch (1985) courtesy of Springer).

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