Shear banding mechanism

Walker, Derivation of the P t Criterion Based on Frey Shear Band Mechanism, BRL Tech Report No. ARBRL-TR-02544, 1984. 

Figure 17.3 A schematic representation of plastic void growth and matrix shear banding mechanisms observed to be involved in the fracture of nanosihca-fiUed epoxy resins. Reprinted from Polymer, 53, Dittanet, P., Pearson, R.A., Effect of silica nanoparticle size on toughening mechanisms of filled epoxy, 1890—1905, Copyright (2012), with permission from...

Figure 10.4 Shear band mechanism with impact modifier.

Two examples of path-dependent micromechanical effects are models of Swegle and Grady [13] for thermal trapping in shear bands and Follansbee and Kocks [14] for path-dependent evolution of the mechanical threshold stress in copper. 

In solid polymers, the energy of impact is dissipated by the formation of crazes or shear bands in the matrix5. Both mechanisms are enhanced by the addition of a second elastomeric phase to the rigid polymer, but not altered in principle. 

The mechanism how a rubber distributed in a network influences the rupture mechanism is not quite well understood yet. It is known that poly(vinyl chloride) forms shear bands when stress is applied and that parts of the rubber which are located in these shear bands may form crazes.13 It might well be that a network structure is efficient for the delocalization of stress energy only in combination with the formation of shear bands. Experimental work is needed to elucidate this further. 

The basic mechanism of toughening is one of void formation and shear band formation (cavitation) when stress is applied. 

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).

As a result of the increase in stress and/or strain, shear bands develop in a large fraction of the sample but, at a certain point, a crack appears and starts to propagate. Several mechanisms for energy absorption, associated with the presence of particles, become active during crack propagation. 

Huang et al. (1993b) proposed a two-dimensional plane strain model, which was successfully used to identify the stress field around the rubbery particles and to simulate the initiation and growth of shear bands between rubbery particles. A model was proposed to quantify the different mechanisms. GIc of the rubber-modified network was written as... 

Mechanical loading —> cavitation in rubber particles —> promotion of shear bands in matrix —> toughness improvement... 

Failure Mechanisms. BPF polycarbonate develops crazes at ascending stresses and fractures in a pseudo-brittle manner similar to polystyrene or PMMA. At room temperature the block polymers develop few separate crazes. As the yield is approached, shear bands grow from the edges. Fracture initiates at an edge from a point where the two shear bands initiated. When a neck forms, the plastic strain in the neck is ca. 80% however fracture occurs shortly after the neck is formed so that the ultimate elongation of the specimen is only 10 or 12%. The shear bands and necks show some stress whitening (Figure 9). 

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