Shear deformation toughening

An electron micrograph of a fracture surface of a CTBN-toughened epoxy resin is shown in Figure 5. This CTBN is particular in that the in situ formed particles are less than 0.5 /zm in diameter. A tensile bar of this system also shows shear deformation which indicate that the small particles have not interfered with the shear deformation characteristic of the unmodified resin. The deformation bands are nearly parallel to the planes of maximum shear stress—i.e., roughly at 45° to the principal... 

Next we looked at the microvoid situation in a bisphenol A modified CTBN-epoxy system. This sample had the highest toughening properties that we developed in the epoxy system because of a two-particle size rubber population that uniquely gives a combination of shear deformation and tensile crazing. Only some of the large particles had microvoid development. Consequently the whitening was much less than when only crazing occurs. The multiple failure sites were still evident. 

We conclude that (a) Shear deformations are dominant in resins toughened with small rubbery particles, (b) Crazing associated with polymer whitening and microvoid development is dominant in resins toughened with large particles, (c) Maximum toughening is obtained under conditions of combined shear and craze deformations. This condition is obtainable when both large and small particles are present. 

Figure 19. Schematic representation of the three different toughening mechanisms in dispersed systems, where the assumed loading direction is vertical (a) induced formation of fibrillated crazes (i.e., with microvoids in them) at the equatorial zones of rubber particles (b) induced formation of homogeneous crazes at cavitated particles and (c) induced formation of shear deformation between cavitated particles.

It appears that the toughening effect near a crack tip occurs as follows after initial deformation, cavitation occurs within or aroimd the soft phase in a zone surrounding the crack tip. This zone is known as the process zone. The newly created surface around these cavities in the process zone allows the latter to grow plastically, thus absorbing some energy. Localized shear deformations, often referred to as shear bands, may also grow from these cavities. If the volume fraction... 

Toughened Polymers with Semicrystalline Matrix. It is well known that toughness of semicrystalline polymers such as PA (polyamide) and PP can be increased similar to the amorphous poljuners by the addition of relatively small amounts of rubber particles such as EPR or EPDM. As in HIPS and ABS, the modifier particles act as stress concentrators, initiating a plastic deformation of matrix strands between the particles as the main energy absorption step. In impact-modified PA and PP at room temperature, plastic deformation takes place through shear deformation (mechanism of multiple shear deformation). 

In rubber-toughened ABS, shear yielding is dominant. Optical microscopy examination by Newman and Strella [126] showed that plastic deformation had occurred in the matrix around the rubber particles. Later studies, notably by Kramer and co-workers, suggested that the rubber particles initiate microshear bands. Donald and Kramer [127] showed that cavitation in the rubber particles initiates shear yielding of the matrix and that shear deformation occurs when the particles are small, and crazing when the particles are large. 

One possible toughening mechanism in such systems could operate by phase transformation, which is well known for ceramic materials [200]. An example is represented by zirconium-containing ceramics [201]. The metastable tetragonal phase of zirconium is incorporated into the ceramic, and under the influence of the stress field ahead of a crack tip, this phase transforms to the stable monoclinic phase. Because the monoclinic phase is less dense than the tetragonal phase, compressive stresses are set up on one of the phases, which superposes on the tensile stress field ahead of the crack tip producing shear deformations, with the effect of increasing the critical fracture energy. 

An efficient toughening mechanism can be set up if the crosslinking is sufficiently low. Soft or rubbery inclusions, the surface-to-surface distance of which is below a critical value, tend to induce shear deformations in the matrix and will, therefore, provide the required conditions... 

The critical strain energy release rates measured in each test are shown in Fig. 15. The fracture toughness measured decreases as the mode II fracture component increased in the tests for this particular material system. This mode mixity dependence of the fracture toughness of adhesively bonded joints apparently is in contrast with the observations of other researchers for other material systems [49-54]. This contradiction can be explained through analyzing the locus of failure. As discussed in Swadener and Liechti [52] and Swadener et al. [53], the locus of failure in their studies was independent of the fracture mode mixity, and the size of the plastic deformation zone at the crack tip increased with the fracture mode mixity. This increased plastic zone was shown to be responsible for a shear-induced toughening mechanism, which consequently, caused the fracture toughness to increase with the mode II components in their studies. In this study, however, as... 

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