A1 Ductile-Brittle and Crazing Fracture

Topic

The development of an incipient microcrack in the polymer—driven by a combination of hydrostatic pressure and tensile stress—initiates a "craze" rather than an immediate brittle fracture. This craze is a region of the polymer, located just ahead of the incipient microcrack, where the material itself stretches and aligns into mutually parallel microfibrils that bridge the microcrack instead of leaving it completely open. A more intense combination of hydrostatic pressure and tensile stress facilitates the initiation of this craze and allows it to extend from the incipient microcrack into a larger region of the polymer.

The continued stretching of these oriented craze microfibrils—as the microcrack they bridge attempts to open further—dissipates energy within the polymer. Each instance of craze-fibril extension consumes a portion of the energy that would otherwise directly drive microcrack propagation; this dissipated energy provides the polymer's effective toughness derived from crazing. Consequently, if this craze-fibril extension persists for a longer duration before the fibrils finally rupture, the effective toughness resulting from crazing is increased. Thus, this model—characterized by the craze and the effective toughness associated with the crazing process—describes an energy-dissipation mechanism distinct from both the Weibull statistics governing brittle ceramics and the dislocation-driven plasticity characteristic of ductile metals. While a brittle ceramic lacks any comparable dissipation mechanism and fails abruptly according to Weibull statistics, and a ductile metal dissipates energy through dislocation movement across its microstructure, this polymer instead dissipates energy by orienting microfibrils within the highly localized craze region immediately ahead of the microcrack. Because this crazing mechanism differs from both the ceramic and metallic mechanisms, the polymer exhibits a unique ductile-to-brittle fracture signature—invariably preceded by the visible craze—rather than replicating the brittle behavior of the ceramic or the ductile behavior of the metal.

ID:4485