Issue 47

J. P. Manaia et alii, Frattura ed Integrità Strutturale, 47 (2019) 82-103; DOI: 10.3221/IGF-ESIS.47.08 99 With temperature increase the fracture surface of PP exhibits an irregular aspect with microfibrils formation. The area fraction of fibrous morphology is increased with increase of temperature, with respected to RT. Large amount of deformation all over the area with a number of longer fibrils and large voids, also some evidence of thermal softening, are observed at fracture extremities. PA 6 SEM micrographs shows microfibrillar failure in the mid-thickness regions of the fracture surface versus craze embrittlement (brittle mode of failure due to high crosshead speed and temperature). The fracture surface becomes rougher and more irregular, with increasing the temperature. At extremities, the fracture surface shows high inhomogeneous surface. At this region, the fracture surface is characterized by crazing and tearing. Generally, in a tensile test, one expects the highest hydrostatic stress to be in the centre of the specimen and this could be the reason for the brittle fracture feature in the centre and more ductile in the longitudinal extremes. SEM images of the fracture surfaces of the butterfly specimen loaded under pure shear (  = 0°, stress triaxiality: 0) at room temperature and at temperature of 50 °C, for HDPE, PP and PA 6, are depicted in Figs. 16 and 17, respectively. At room temperature, HDPE and PP show a fracture surface predominantly of ductile type with crazing and tearing, and fibril formation associated with the shear loading. Pure shear leaves a relatively smooth fracture surface and orientated geometry. HDPE, Room Temperature PP, Room Temperature PA 6, Room Temperature Figure 16 : SEM images of butterfly specimens fractured for shear loading at room temperature for HDPE, PP and PA 6. In the first column the geometry of specimen and fracture surface location are indicated by the black square. For PP, the fracture surface generated is particularly “clean” with a smooth surface in a form of waved structures, with fibril formation, along the shear stress direction, whereas in the case of HDPE a regular surface with almost geometric triangular patterns at the specimen centre is observed. Inside the fracture surface it is observed the fibrils formation and alignment. The symmetry suggests that the fracture starts near surface and propagates inwards through the crazed surface on both sides Crazing and Tearing Crazing and Tearing Crazing and Tearing Crazing and Tearing Crazing and Tearing Microfibril