Mechanism cavitational craze-like

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. 

The craze front velocity v can be governed by one of two distinct mechanisms of craze matter production. As Argon and Salama have discussed in detail, under the usual levels of service stresses or stresses under which most experiments are carried out, craze matter in single phase homopolymer is produced by the convolution of the free surface of the sohd polymer at the craze tip. This occurs by a fundamental interface instability present in the flow or deformation of all inelastic media when a concave, meniscus-like surface of the medium is being advanced locally by a suction gradient. This is the preferred mechanism of craze advance in homopolymers. In block copolymers with uniform distributions of compliant phases of a very small size, and often weaker interfaces than either of the two phases in bulk, craze advance can also occur by cavitation at such interfaces to produce craze matter as has been discussed by Argon et al. Both of these mechanisms of craze advance lead to very similar dependences of the craze front velocity on apphed stress and temperature that is of the basic form... 

In these types of polymers the micromechanical behavior depends on the interrelation between cavitation and micronecking. Two examples of deformed SBM diblock copolymers with 76% PS and hexagonal-packed poly(butyl methacrylate) (PBMA) cylinders embedded in the PS matrix are shown in Fig. 3.9. Figure 3.9(a) shows craze-like deformation zones running perpendicular to the main strain direction (shown by an arrow). Inside the crazes, both the PBMA cylinders and PS matrix show large plastic deformation. In Fig. 3.9(b) a craze is seen, created by a cellular structure of cavitated PBMA cylinders and plastic deformation of the PS matrix [21], Several types of deformation structures and crazes can be formed in block copolymers initiated by cavitation of one phase and plastic deformation of both phases. However, due to structures on much smaller length scales, there are a number of unique characteristics of the deformation mechanisms [6]. 

The deformation of the polymer within a thin active zone was originally represented by a non-Newtonian fluid [31 ] from which a craze thickening rate is thought to be governed by the pressure gradient between the fibrils and the bulk [31,32], A preliminary finite element analysis of the fibrillation process, which uses a more realistic material constitutive law [36], is not fully consistent with this analysis. In particular, chain scission is more likely to occur at the top of the fibrils where the stress concentrates rather than at the top of the craze void as suggested in [32], A mechanism of local cavitation can also be invoked for cross-tie generation [37]. 

The three mechanisms described above briefly contribute to the overall dilatation of the material under tension, and it is not easy to assign to each process its relative importance in the recorded damage rate A. However, some authors like Keskkula and Schwarz (37) for HIPS, showed from detailed morphological observation that the crazing in the PS matrix is not the only active source of damage, but that the decohesion at the PS/PB interface and cavitation in the PB nodules play a significant role as well. 

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