Shear-deformation bands, fractured tensile

Figure 5. Optical micrographs of fractured tensile specimens showing shear-deformation bands viewed between crossed, polarizing filters to enhance contrast between the birefringent bands and the undeformed matrix (a) bands typical of those observed in 2L, 3LA, 3LA/, 3LE, and 4L materials (b) bands typical of those observed in 3LB and 3LD materials and (c) bands observed in 3LC materials.

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. 

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... 

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. 

In contrast to observations in polystyrene, we do not observe permanent bands our specimens exhibit no residual birefringence upon release from stress. Neither do we observe crazing before failure. However, the specimens do whiten just before failure when viewed edge-on, and this whitening disappears within a few seconds after fracture occurs. We think the oscillations in intensity we observe are likely to be due to incipient shear deformation which disappears after specimen failure. Unpublished results of other workers are reported (see References 11 and 14 in the present Reference 12) to be consistent with the idea that such bands should be difficult to observe in PMMA and in polycarbonate because of their lower draw ratios compared to polystyrene. Studies of an unfilled epoxy polymer (14) in cyclic tensile deformation indicate that shear bands do not remain after removal of stress until a threshold amplitude of deformation is exceeded. 

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