Issue 47

J. P. Manaia et alii, Frattura ed Integrità Strutturale, 47 (2019) 82-103; DOI: 10.3221/IGF-ESIS.47.08 93 and craze containing regions that grow inward from the surface leading to a blocky structure [30]. Brittle and crazing, are the two modes of fracture, observed for both notch radii. The effect of stress triaxiality slightly modifies the fracture surface and influences the void growth and size. It is observed from SEM micrographs of PA 6, clearly different fracture surfaces, when compared with HDPE or PP. The fracture surface for specimen with R=5 is smooth at the centre with some voids content and slight rough at the extremities, the last region to fracture, whereas the surface for the specimen with R=30 is rough, with a very fine mesh and absence of fibrils formation. Some block structures are observed at near surface. Both fracture morphologies are an indicator of brittleness. Crazing and brittle fracture are the dominant modes of failure. Even though in PA 6 with specimens with R=5 the brittle fracture is smooth, occasionally spherical voids were observed, probably due to high stress triaxiality which induces the void formation [12]. Also, crazing mode of fracture occurred at some distance away from the centre. Even with the same specimen geometries, the fracture surface shows different morphologies. The fracture surface is slightly rough for PA 6, specimens with R=30, more heterogeneous for PP and shows a filament like structure for HDPE. Analysis of Cylindrical Notched Specimens Fracture Morphologies under Tension at Stress Triaxialities between 0.39 and 0.64 To explore the underlying fracture mechanisms under tension at crosshead speed of 200 mm/min and at RT, SEM images were taken for the fracture surfaces of cylindrical notched specimens made of HDPE, PP and PA 6. Fracture surfaces of representative specimens with two different notch radii are displayed in Figs. 10 and 11. HDPE PP PA 6 Figure 10 : SEM fracture surfaces for cylindrical notched specimens with R=5, for HDPE, PP and PA 6. Note that in the first column the geometry of specimen and fracture surface location observation are indicated by the black square. Laiarinandrasana et al. [12] discussed the effect of temperature, crosshead speed and stress triaxiality on the fracture of Polyvinylidene Fluoride (PVDF), a semi-crystalline polymer. To this end, tensile tests were performed on cylindrical notched specimens. It was reported more ductile fracture behaviour for specimens with larger notch radii (lower triaxialities), while increasing the stress triaxiality ratio (decreasing the notch radius) or increasing strain rate seems to favour the brittle fracture. Brittle Ductil Crazing Cavitation and Voids Voids Voids and Striations Striations and Crazing Cavitation and Voids Peeling Skin Layer