Cavitation and Shear Yielding

Propagation was from left to right. The specimen was stained with osmic acid. Reprinted with permission from J.S. Ullett and R.P. Chartoff, Polymer Engineering and Science, 1995, 35, 13, 1086 1995, John Wiley and Sons Publishers 

Cavitation is followed by the onset of a shear localisation process, which would not have taken place in the net resin under the same conditions. Rubber cavitation and 

All these theoretical predictions are consistent with the results observed by Yee and Person [120, 131], which clearly show shear bands between cavitated particles in rubber-toughened epoxy materials. They also found that the ability of CTBN rubber to toughen epoxy is closely related to CTBN rubber cavitation, which is seen as thick dark circle within the rubber particles under optical microscopy. The need of internal cavitation of rubber particles has been questioned by others [152, 153], but these researchers have considered only the case of uniaxial tension, which have been shown to be quite different than the triaxial stress seen at the crack tip. 

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The stress distribution analysis shows that maximum stress concentration develops in the radial direction at the pole of the particle, and shear yielding is initiated at around 45° on the surface of rigid particles. Debonding occurs at the pole of the particle, and extends to a critical angle [27]. In case of total adhesion, debonding does not occur and there is cavitation in the matrix, at some distance from the particle pole (not at the interface). 

The promotion of energy dissipative processes that delay or entirely suppress fracture processes originating from imperfections in the internal structure or scratches and notches is enhanced by cavitation. In almost all cases, cavitation either makes possible further toughening by activating other mechanisms or itself contributes to the plastic response of the polymer. The most energy dissipative processes, crazing and shear yielding, occur at a reduced stress level. 

On the one hand, it is possible to increase the toughness of polymers by incorporating rubber or polyurethane (HD-TPU) particles. Compliant particles, if well dispersed, may influence the deformation behavior towards a higher energy absorption by activating localized cavitational craze-like and shear yielding mechanisms. Their lower stiffness, however, has a disadvantageous effect on the composite s modulus. 

A proposed mechanism for toughening of mbber-modifted epoxies based on the microstmcture and fracture characteristics (310—312) involves mbber cavitation and matrix shear-yielding. A quantitative expression describes the fracture toughness values over a wide range of temperatures and rates. 

While the surface modification is not effective to suppress cavitation, Yee and coworkers performed an experiment to suppress the cavitation mechanically in a rubber-modified epoxy network. They applied hydrostatic pressure during mechanical testing of rubber toughened epoxies [160]. At pressures above BOSS MPa the rubber particles are unable to cavitate and consequently no massive shear yielding is observed, resulting in poor mechanical properties just like with the unmodified matrix. These experiments proved that cavitation is a necessary condition for effective toughening. 

Fig. 8.1. Toughening mechanisms in rubber-modified polymers (1) shear band formation near rubber particles (2) fracture of rubber particles after cavitation (3) stretching, (4) debonding and (5) tearing of rubber particles (6) transparticle fracture (7) debonding of hard particles (8) crack deflection by hard particles (9) voided/cavitated rubber particles (10) crazing (II) plastic zone at craze tip (12) diffuse shear yielding (13) shear band/craze interaction. After Garg and Mai (1988a).

Furthermore, the model makes it possible to separate the contributions of the three toughening mechanisms as a function of temperature (Fig. 13.6). At high temperatures, the crack-bridging mechanism plays a minor role the void-growth mechanism is very sensitive to temperature and can be completely suppressed at low temperatures. Shear yielding is the main mechanism, except at very high test temperatures where cavitation plays the major role. The contribution of shear yielding depends on the difference between the test temperature and Tg, as discussed in Chapter 12. 

Figure 13.6 Relative contributions (%) of the different toughening mechanisms in epoxy networks versus temperature ( ) rubber bridging ( ) shear yielding and (A) cavitation. (From the results of Huang et a ., 1993b.)... 

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