Issue 47

J. P. Manaia et alii, Frattura ed Integrità Strutturale, 47 (2019) 82-103; DOI: 10.3221/IGF-ESIS.47.08 92 voids (“cavities”), and crazing near surface are the dominant modes of deformation. Cavitation might precede crazing due to the formation of microfibrils of oriented chains that span around the faces of the voids. Studies on the development and growth of crazes in thermoplastic materials have indicated that crazing involves alignment, merging, and splitting of microfibrils and nucleation of micro-voids [30]. The difference between crazes and cavities is that the last one do not have an internal structure, thus are unable to transfer stress [19]. The presence of fibrous surface with fibrils pulled out in the fracture surface is an indicator of ductility. It is observed that increasing the stress triaxiality, the fracture surface becomes more brittle and homogeneous, with less formation propensity of longer fibrous surface and more voids content. Dasari et al. [31] observed on their experiments on ethylene - propylene copolymers, wedge shaped micro-cracks parallel to the tensile direction on the surface of the deformed specimen. During tensile straining, the edges multiplied and the plastic flow around the edges led to their separation from the surrounding matrix leading to fibrillation type of fracture. An illustration of this is presented in Fig. 9, column HDPE at central and extremities near surface, due to high radius the material flows more extensively, besides that multiplication of wedges, crazing also occurred. HDPE PP PA 6 Figure 9 : SEM fracture surfaces for flat notched specimens with R=30, for HDPE, PP and PA 6. Note that in the first column, the geometry of specimen and fracture surface location are indicated by the black square. SEM micrographs of PP exhibits rough surface patterns in fracture region, for R=5. The predominantly brittle failure, cavitation and voids, are detected. Cavitation is the formation of voids (cavities) inside amorphous phase during deformation. It is often a precursor of brittle fracture or crazing and a common phenomenon in semi-crystalline polymers [32]. For PP flat notched specimens with R=30, it is observed a mix of rough surface (cavitation and voids) with fibrous surface, in central region-fracture. In the extremities of flat notched specimen R=30, the surface acquires a more inhomogeneous morphology, due to low stress triaxiality which causes the material flowing more extensively. A number of discontinuities or jumps in the fracture process leading to a blocky structure is observed. Considering that the inherent stress concentration is expected to be high in the mid thickness region, a stable crack nucleates and propagates through wedge Fibrillation Crazing Voids Fibrous Morphology Cavitation and Voids Fibrous Morphology Blocky Structure Crazing Blocky Structure Crazing Crazing Cavitation and Voids Crazing