Issue 47

J. P. Manaia et alii, Frattura ed Integrità Strutturale, 47 (2019) 82-103; DOI: 10.3221/IGF-ESIS.47.08 91 As a general rule, however, semi-crystalline polymers are used at temperatures between glass transition temperature and a practical softening temperature which lies above g and below ௠ , these are the cases of HDPE and PP. In Figs. 5 to 7, although the different stress states, loading angle conditions, the general shape of all curves for each material and test, seems to exhibit remarkably similar intrinsic behaviours, when compared with increase crosshead speed. At higher crosshead speed, increases the propensity of material to soften, however it is not so pronounced as in PA 6. It seems that in semi-crystalline polymers used at temperatures above g becomes more stables. Relatively to the molecular structure, HDPE has a long linear chain (lower degree of branching), therefore has stronger intermolecular forces. HDPE is chemically the closest in structure to PP, therefore similarities in load-displacement curves, are observed. However HDPE exhibits better mechanical properties [29]. T EST R ESULTS - F RACTURE S URFACE MORPHOLOGIES Analysis of flat notched specimens fracture morphologies under tension at stress triaxialities between 0.62 and 0.84 ll flat notched specimens made of HDPE, PP and PA 6 experienced full section fracture under tension at crosshead speed of 200 mm/min and at RT. Fracture surfaces of representative specimens are displayed in Figs. 8 and 9. HDPE PP PA 6 Figure 8 : SEM fracture surfaces for flat 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 are indicated by the black square over the cross-section. Qualitative SEM observations of HDPE fracture surfaces at several magnifications for flat notched specimens, R=5 and R=30, show the formation of an oriented texture of fibrous surface at the centre with a number of longer fibrils. At the centre, due to the higher stress triaxiality, occurs the initial fracture, while at the near surface, the fracture surface becomes more inhomogeneous, because that is the last region to fracture. It is believed that cavitation at centre, due to numerous A Crazing Fibrous Morphology Crazing Voids Cavitation and Voids Blocky Structure Crazing Crazing Voids Crazing Cavitation and Voids