Formation shear band

If o o o, then the applied stresses lead to the formation of new glide layers and the shear band has the structure depicted in Fig. 6.16b. During this process, the glide-layer branching due to structure microinhomogeneities in the LRC may occur. However, the preferred orientation of the layers depends on the direction of the forces resulting from shear stresses. 

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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).

In discussing shear deformation, it is convenient to distinguish between the initial elastic and viscoelastic response of the polymer to the applied load and the subsequent time-dependent response. However, the distinction is somewhat arbitrary and is not as fundamental as that between elastic volume response and crazing. Viscoelastic shear deformation continues throughout the period under load. The observed time-dependence of lateral strain reflects both generalized viscoelastic relaxation and shear band formation. Since crazing consists simply of displacement in the tensile stress direction, it makes no contribution to lateral strain therefore —e specifically measures deformation by shear processes. 

Polymer Morphology and Failure Mechanisms. A failed tensile bar of unmodified piperidine-cured epoxy resin shows shear deformation before tensile failure when strained slowly (0.127 cm/sec). We could not produce stable crazes in specimens of unmodified epoxy resins. At all stress levels, temperatures, and conditions of annealing only fracture occurred after shear band formation. The failure to observe crazes in unmodified epoxy resins may be explained by a fast equilibrium condition which exists between crazing on loading and recovery on unloading. 

Fig. 35a. Mechanism of crack I ormalion at intersections of shear bands (i) sequence of shear band formation, jii) chain scission at the intersections (schematic) b Shear crack formation (arrow) in a shear band of type A, leading to a 50 pm wide displacement of type B — shear bands on both sides of the crack...

Because of the difference in form between Eqs. (2) and (3), the mechanisms of deformation and fracture change with the state of stress. For example, polystyrene yields by shear band formation under ccm ression, but crazes and frachues in a brittle matmer under tensile loading. Chants in failure nwchanian with state of stress are e cially important in particulate conqx tes, since the second phase can alter the local state of stress in the surrounding matrix. 

Faflure in multiphase polymers and polymer composites (non-fibrous) is reviewed by Professor Bucknall. Several examples are presented in which the effect of adding a dispersed second phase to a polymer can be either beneficial or deleterious to stress, strain, or work to break. It is shown that two basic modes of local plastic deformation may be operative, namely crazing and shear band formation. 

Shear band formation and evolution, and the change of shear displacement from one location to another, can be analysed numerically. The small and finite displacement on each band, after which the shear displacement is transferred elsewhere, can be explained by a progressive reduction of one or more stress components (and lower mean stress), thus dissipating strain energy. It is not necessary to invoke strain hardening or softening, change of pore pressure or any other intrinsic material weakness in the band. 

The deformed shape resulted from shock induced plastic strain is presented in Fig. 6 by plotting the deformed shape of a slice within the RVE. The deformed shape shows the formation of bands in the region where dislocation microbands are formed. This indicates that dislocation activities under high strain rate loading can be considered as somces for shear band formation. 

The remaining sections employ the deformed lattice and quantum picture of plastic flow to account for shear band formation as a means of achieving fte energy localization and hot spot temperatures necessary for initiation of crystalline explosives by shock or impact. Also briefly examined will be the role of the deformed lattice potential in causing particle size effects and its effect on the plastic deformation and energy dissipation rates. Finally, the dependence of the energy dissipation rate on shear stress will be shown to imply that reaction initiation will be dependent on the shape of the shock wave or impact stimulus. These predictions will be compared with experiment. 

Dislocation Tunneling, Particle Size Effects and Shear Band Formation... 

Figure 10. Shear band formation in a tensile test of the DGEBA/DMHDA system.

Neuhauser, H. (1978) Rate of shear band formation in metallic glasses, Scripta MetalL, 12, 471-474. 

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