Issue 47

J. P. Manaia et alii, Frattura ed Integrità Strutturale, 47 (2019) 82-103; DOI: 10.3221/IGF-ESIS.47.08 98 of the fracture surface are the dominant plastic deformation for HDPE. In the central fracture surface of HDPE, two large ellipsoidal voids, surround by oriented fibrils morphology, are visible. The formation of elliptically shaped voids would be either due to plastically deformation inside amorphous phase or a pre-existing defect or even because the combined tension/shear loading (ellipse major axis in the shear direction). The fracture initiated in the centre and propagated towards the surface, where the fracture surface is characterized by crazing and tearing. SEM micrographs of PP in the peripheric-central region show oriented fibrils with the direction of loading (  = 30°), exhibiting high deformation before failure (ductile failure). The inner layer shows a more ductile behaviour than at (  = 90°) and knobs or nodules are observed at the ends that might be caused by thermal softening. The fracture initiated in the outer layer and propagated towards the centre, justifying the presence of fibrous morphology elongations at centre. The centre of the fracture surface is characterized by extensive plastic flow around and inhomogeneous morphology. From PA 6 SEM micrographs, it is clear a brittle failure mode. At central fracture region, all surfaces are covered with a very fine mesh of striation configuration. SEM micrographs show crazing and tearing at the fracture surface end. The striations are a result of successive crack propagation increments. The striations are perpendicular to the shear loading. At a temperature of 50 °C (Fig. 15), HDPE fracture surface becomes more regular, more homogeneous at the centre. Crazing and tearing as well as peeling skin layer are observed. The main fracture feature is the central slope formation, which propagated through the fracture length. Similar geometry was obtained for  = 90°. Although crazing characterizes the ductile behaviour of a material, the central slope formation is considered to represent enhanced ductility (or plastic flow), consistent with decrease in fracture area. HDPE, Temp. of 50 °C PP, Temp. of 50 °C PA 6, Temp. of 50 °C Figure 15 : SEM images of butterfly specimens fractured for combined tensile/shear loading α = 30° at temperature of 50 °C for HDPE, PP and PA 6. In the first column the geometry of specimen and fracture surface location are indicated by the black square. Microfibrils Large Voids Knobs or Nodules Crazing and Tearing Crazing and Tearing Crazing and Tearing Peeling Skin Layer Microfibril Craze Embrittlement Crazing and Tearing Crazing and Tearing