In recent years, theoretical understanding regarding toughening mechanisms has been
advanced for both rubber and thermoplastic toughened epoxy resins. The relationships
between microstructure and fracture behavior of toughened thermoset were illustrated
quantitatively. But some researchers thought there still exists .a great deal of controversy (Hedrick et al., 1990). or an .argument over details (Huang, et al., 1993). on the nature of the toughening mechanisms in modified epoxy resin networks. Several reviews (Garg and Mai, 1988, Huang et al.1993, Pearson, 1993) have given detailed descriptions of the existing toughening mechanisms proposed to explain the improved toughness for rubber and thermoplastic toughened epoxy resins, respectively. Some important aspects of these toughening mechanisms are summarized here.

1. Crack-Pinning Mechanism (Lange et al., 1970,1971). This theory stated that as the crack begins to propagate through the resin, the crack front bows out between the filler particles but remains pinned at the particles. A schematic diagram of the crack-pinning mechanism is shown in Fig. 1.1 (Pearson, 1993). This mechanism is based on small particles as toughening agents. Because this mechanism operates mainly with inorganic fillers (Evans, 1972, Rose, 1987) that resist fracture during failure of the epoxy matrix resin, it is generally less important in ductile matrix materials.
2. Rubber Tear-Energy Toughening Theory (Kunz-Douglass et al., 1980, Sayer et al.,1983). Based on the early toughening mechanisms of toughened thermoplastics and the idea that the particles stretch across the crack opening behind the crack tip and hinder the advance of the crack, the researchers suggested that the energy absorbed in fracture is the sum of the energy required to fracture the matrix and to break the rubber particles.
3. Microcracking Mechanism (Evans et al., 1985, Oritz, 1987). A schematic diagram of the crack-pinning mechanism is shown in Fig. 1.2 (Pearson, 1993). Microcracks due to rubber particles cause tensile yielding and, thus, a large tensile deformation. Voids result when the microcracks open, and these voids permit large strains. Debonding or microcracking effectively lowers the modulus in the frontal process zone around the crack tip, and thus effectively reduces the stress intensity there. But this theory couldn.t explain many phenomena, such as stress-whitening, the large amount of plastic deformations, higher fracture toughness at a higher temperature (Kinloch, 1983), and the fact that nonreactive rigid thermoplastic particles also may toughen some systems.
4. Localized Shear Yielding (or Shear Banding) Mechanism (Kinloch et al., 1983，Pearson et al., 1986). A schematic diagram of the crack-pinning mechanism is shown in Fig. 1.3 (Pearson, 1993). Kinloch et al. suggested that the rubber-tear mechanism only makes a secondary contribution to toughening, but it does not represent the major toughening mechanism. They proposed a mechanism that involves dilatational deformation of the matrix, and cavitation of the rubber particles in response to the triaxial stresses near the crack tip, combined with shear yielding between the holes formed by the cavitated rubber particles. The stress-whitening was attributed to light scattering by these holes, and the major energy absorption mechanism was suggested to be the plastic deformation of the matrix. Plastic deformation blunts the crack tip, which reduces the local stress concentration and allows the material to support higher loads before failure occurs.
5. Particle Bridging (Rigid Particles) Mechanism (Sigl, et. al 1988). A schematic diagram of the particle-bridging mechanism is shown in Fig.1.4. In this toughening echanism, the authors proposed that a rigid or ductile particle plays two roles: (1) It acts as a bridging particle that applies compressive traction in the crack wake. (2) The ductile particle deforms plastically in the material surrounding the crack tip, which provides additional crack shielding. Sigl et al. also pointed out that the shielding resulted from yielded particles is negligible, and that the particle bridging provides most of the improvement in toughness. In contrast to the crack-pinning mechanism, the particlebridging mechanism favors large particles and emphasizes the energy-to-rupture needed of the ductile phase.
6. Crack-Path Deflection Mechanism (Faber and Evans 1984). A schematic diagram of the crack-pinning mechanism is shown in Fig. 1.5 (Pearson, 1993). The crack-path deflection may explain the increase in toughness by a stress intensity approach. There are both mode I and mode II characters of the crack opening. Most materials are more resistant to mode II crack opening. The deflection of the crack path decreases the mode I crack opening, but increases the mode II crack opening. Therefore, the materials exhibit a higher apparent toughness. This mechanism does not need to consider the particle size of modifiers, but it is thought that uneven spacing provides better results than does uniform spacing.
7. Massive Shear-Banding Mechanism (Argon1989). Argon proposed that additional crack-tip shielding in rubber-modified epoxy resins occurs due to the reduction in yield stress by the stress concentration of the compliant rubber particles that facilitate shear yielding. Kim and Brown (1987) and Pearson (1990) provided evidence to show that shear-yielding also exists in thermoplastic particles modified epoxy system.
Even though there are various toughening mechanisms proposed by different researchers, it seems that a single theory can not explain every experimental result and phenomenon of toughening. One reason may be the discrepancy of the raw materials chosen by different researchers, because the initial properties of raw materials have significant influence on the final fracture properties of epoxy materials. Another reason lies in the fact that the fracture itself is a complex phenomenon, and a single theory cannot represent 18 every detail. So the toughening mechanism of epoxy resins may be a combination of the above two or several more mechanisms. Huang et. al. combined the schematic diagrams of several toughening mechanisms and gave a comprehensive schematic diagram to describe the different toughening mechanisms involved in the fracture of rubbertoughened epoxy polymers, as shown in Fig. 1.6.



Fig. 1.1 A schematic diagram of the crack-pinning mechanism (Pearson, 1993)

Fig. 1.2 A schematic diagram of the microcracking mechanism (Pearson, 1993)

Fig. 1.3 A schematic diagram of the shear-yielding mechanism (Argon, 1989)

Fig. 1.4 A schematic diagram of the particle-bridging mechanism (Pearson, 1993)

Fig. 1.5 A schematic diagram of the crack-deflection mechanism (Pearson, 1993)

Fig. 1.6 Different toughening mechanisms of rubber modified epoxy polymers
(The diameter of the plastic, or precess zone is 2ry)