Issue 47

J. P. Manaia et alii, Frattura ed Integrità Strutturale, 47 (2019) 82-103; DOI: 10.3221/IGF-ESIS.47.08 83 The state of stress, such as the one included by the presence of notches, is one factor that controls the occurrence of extended yielding or brittle failure, even though such polymers behaves as ductile under tensile tests over wide temperature and strain-rate ranges [3]. Many experimental results have shown that the material’s fracture changes under di ff erent loading conditions. Among different damage mechanisms, the stress triaxiality has been recognized as one of the most important fracture controlling factors [4]. Bridgman [5] conducted experiments on a variety of metallic alloys discovering that an increase in stress triaxiality results in a corresponding increase in damage nucleation and growth. The location of high hydrostatic stress is thought to favour craze initiation. Crazing happens due to the nucleation of microvoids in regions of stress concentrations, normal to the maximum principal stress. These voids do not coalesce to form cracks (as in metals) since highly stretched molecular chains, or fibrils, stabilize this process to create crazes, thus craze consists of a web of interpenetrating voids and polymer fibrils [6]. In the current experimental investigation, different stress triaxiality levels are induced by different notch radii in tensile specimens. The triaxial stress state effects on deformation and fracture morphology were examined by means of tests using cylindrical and flat notched specimens with different curvature radii in order to set different triaxial stress triaxialities in the median cross-section, from 0.39 for the cylindrical notched specimen with radius of 30 mm to 0.84 for the flat notched specimens with radius of 5 mm. In addition, combined loading tests were performed with butterfly specimens and Arcan apparatus, resulting stress triaxialities ranging between 0 to 0.58, being those specimens also analysed in terms of deformation behavior as well as fracture surfaces appearance by Scanning Electron Microscopy (SEM). Polymers have a quite significant level of change in physical and mechanical properties over a relatively small change in temperature, which are largely determined by their molecular structure and the resulting bonds. Tijssens et al. [7] showed the importance that temperature plays in crazing of amorphous polymers. Elongation at failure typically increases when the temperature increases and polymer behaves in a much more viscous manner. At relatively low temperatures, the craze damaging mechanisms widens very rapidly. The failure will occur faster, thus behaviour becomes more brittle. At higher temperatures a more spread-out craze zone will develop. Since a craze widens slower as temperature increases, more crazes tend to be initiated. The main purpose of present research is to investigate and describe the morphologies and mechanisms of fracture of HDPE, PP and PA 6 materials tested at crosshead speeds of 200 mm/min, using two complementary experimental approaches: one combining tension/shear loading at three different loading angles (  = 0°, 30° and 90°) at RT and 50 °C, carried out on butterfly specimens and another imposing a triaxial stress state on cylindrical and flat notched specimens with different curvature radii at RT. The mechanisms of fracture are reported and discussed with respect to the different loading conditions with emphasis on the relation between loading angles and temperature on biaxial loading, and notch effects on notched specimens. Also, the deformation response was examined under different stress states, crosshead speeds of 1, 20 and 200 mm/min and two different temperatures (RT and 50 °C), in order to compare the yield load, temperature responses and neck propagation for the three materials. The present paper is organized as follows. In Section 2, we present the investigated material, the different specimen geometries and the mechanical testing protocols. In Section 3, the experimental results are displayed and discussed. Further, specimens fracture morphologies are analysed, by means of SEM micrographs in Section 4. Some concluding remarks are finally given in Section 5. E XPERIMENTAL DETAILS he materials covered in this paper are the HDPE, PP and PA 6, three semi-crystalline thermoplastics. Complex specimen geometries, a designed injection mould made of steel and a biaxial testing apparatus were specifically designed for this research. All the specimens used in this study were manufactured by injection moulding. In order to compare the deformation behaviour (e.g. yielding, necking) under different temperatures and strain rates. The laboratory tests were performed at crosshead speeds of 1, 20 and 200 mm/min at two different temperatures (room temperature and 50ºC). All specimens were loaded using a tensile test machine until fracture. SEM observations of fracture surfaces, were performed only for specimens tested at crosshead speed of 200 mm/min. Materials The materials used in the present studied are the Dow ™ HDPE KT 10000 UE, high density polyethylene resin provided by Dow Chemical Company (Dow); Sabic ® PP PHC27, a semi-crystalline polymer and a multipurpose polypropylene impact copolymer, provided by Saudi Basic Industries Corporation (SABIC) and Promyde ® B30 PMID, a Polyamide 6 impact modified, provided by Nurel Engineering Polymers. T