Issue 47

J. P. Manaia et alii, Frattura ed Integrità Strutturale, 47 (2019) 82-103; DOI: 10.3221/IGF-ESIS.47.08 96 Temperature (Fig. 12). For HDPE, the plastic deformation occurs predominantly by a combination of crazing in the fracture surface centre and fibrillation/crazing type of fracture in the surface extremities, which is the last region to fracture. Also, a multiphase layer structure – “peeling skin layer” are shown in peripheric central region-fracture. The fractured skin layer behaves as a “peeling skin layer” and takes a form of high oriented fibrils or as massive craze and tearing [30]. HDPE, Temp. of 50 °C PP, Temp. of 50 °C PA 6, Temp. of 50 °C Figure 13 : SEM images of butterfly specimens fractured for tensile loading 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. Considering that the inherent stress concentration is expected to be high in the middle section, crack nucleates and propagates through wedges. Fracture surface exhibits two main characteristics: ductile fracture associated with crazing at the centre from which fracture initiates; then, a transition area appears showing fibrillation type of fracture, associated with an extensive plastic flow, where crack growth is supposed to be rapid and last region of fracture. In the case of PP, the surface morphology displays an irregular surface with high void and cavitation content, which becomes rather rough and uneven at extremities; similar morphology was already observed on PP flat and cylindrical notched specimens. Brittle failure is the predominant fracture characteristic. A high inhomogeneous multiphase structure is exhibited by PA 6. It is observed that near surface the fracture takes a form of massive craze and tearing-crazing aspect. Dasari et al. [31] defined crazing and tearing, based on their observations made on in iPP-L (long chain high crystallinity polypropylenes) and HDPE fracture surfaces, as the initially crazes formation on the surface, with increase in strain, a crack nucleates and propagates inward through an array of crazes resulting tearing. The core region with high irregular fracture surface is characterized mainly by crazes and brittle failure. SEM micrographs at low magnification shows crazing and tearing in the region away from the centre of the fracture surface. It appears that the fracture initiated near the core of the butterfly specimen, and propagated Peeling Skin Layer Crazing and Tearing Crazing and Tearing Microfibril Cavitation and Voids Large Void Microfibrils Crazing and Tearing Microfibrils Peeling Skin Layer