Issue 47

J. P. Manaia et alii, Frattura ed Integrità Strutturale, 47 (2019) 82-103; DOI: 10.3221/IGF-ESIS.47.08 94 This is in accordance with the experience from the present study. The HDPE and PA 6 cylindrical notched specimens, in particular the cylindrical geometry with R=30 were too ductile to complete fracture, while the specimens with R=5 were able to fully fracture the cross section in a brittle manner. The effect of stress triaxiality on fracture surface of PP is minimal however with increasing of notch radius seems to lead to a more ductile behaviour. SEM micrographs of cylindrical notched specimens with notch radius R=5 of HDPE showed two fracture morphologies: brittle fracture, where crack growth is supposed to be rapid and mix brittle/ductile fracture with some microfibrils content near surface, due to crazing fracture mechanism. The central fracture surface is less uneven than the fracture close to the material surface because the stress concentration is higher in the centre, probably from where the cracks nucleates and propagates. For R=30 a uniform fibrillar structure over the cross section is visible, indicating that the fracture was ductile in nature, exhibiting enhanced ductility. The uniform fibrillar structure might be indicating that the surface layer had failed after void nucleation and multiple formation of microfibrils. At peripherical and central region, microfibrils with knobs or nodules with a smooth rounded surface like features probably formed by the relaxation of hot material, is observed. Similar features were found in research work performed by Brough et al. [33] on scanning electron micrographs of HDPE fracture surfaces. It is observed a large peeling skin for notched specimens with R=5. The stress triaxiality rules the fracture morphology, which becomes more ductile with large fibril formation at low stress triaxiality (notch, R=30), on the other hand becomes more brittle with increasing the stress triaxiality (notch, R=5). HDPE PP PA 6 Figure 11 : SEM fracture surfaces for cylindrical notched specimens with R=30, for HDPE, PP and PA 6. Note that in the first column the geometry of specimen and fracture surface observation location are indicated by the black square. The fracture surface of PP for notch radius R=5 exhibits a uniform fracture aspect, predominantly with rough surface, dominated by brittle fracture (cavitation and voids). Absence of fibrils formation is verified. In the case of the notched specimen with R=30, there is a fibrillar structure, pulled out from the background surface, at the central fracture surface, which is consequence of low stress triaxiality. The fracture becomes more ductile. Similar to fracture morphology observation on PP flat notched specimens, also with cylindrical notched specimens, the effect of stress triaxiality slightly Knobs or Nodules Microfibrils Fibrous Morphology Fibrous Morphology Striations Voids Voids Cavitation and Voids Microfibrils with Knobs