J. P. Manaia et alii, Frattura ed Integrità Strutturale, 47 (2019) 82-103; DOI: 10.3221/IGF-ESIS.47.08 82 Elastoplastic and fracture behaviour of semi-crystalline polymers under multiaxial stress states João P. Manaia, Francisco A. Pires, Abílio M. P. de Jesus Department of Mechanical Engineering, Faculty of Engineering, University of Porto, Rua Dr. Roberto Frias, s/n 4200-465, Porto, Portugal. joaomanaia@netcabo.pt, fpires@fe.up.pt, ajesus@fe.up.pt ABSTRACT. The deformation behaviour and fracture mechanisms of high- density polyethylene (HDPE), polypropylene (PP) and polyamide 6 (PA 6) are investigated experimentally under different stress states and at different crosshead speeds of 1, 20 and 200 mm/min. Fracture surface morphologies were investigated in a series of specimens tested at 200 mm/min under combined tension/shear loading at three different loading angles (  = 0°, 30° and 90°) at room temperature (RT) and 50 °C. In addition, the effects of notch profile radii (stress triaxiality) on HDPE, PP and PA 6 fracture behaviour have been studied at RT, using flat and cylindrical notched specimens. Specimens’ geometries were carefully designed to achieve various loading conditions and allowing to explore initial stress triaxialities ranged from 0 in pure shear loading (  = 0°) to a maximum of 0.84 for flat notched specimens with radius of 5 mm. The yield load shows an explicit dependency on temperature and crosshead speed. The fracture surfaces analysed reveals damage mechanisms such as crazing, void and cavitation formation. Two or more mechanisms are predominant, which means that the stresses along fracture process are not uniform. KEYWORDS. Semi-crystalline polymers; Fracture; SEM; Stress triaxiality; Multiaxial loading; Butterfly specimen. Citation: Manaia, J. P., Pires, F. A., Jesus, M. P. de J., Fracture morphologies of semi crystalline polymers under different stress states, Frattura ed Integrità Strutturale, 47 (2019) 82-103. Received: 03.11.2018 Accepted: 20.11.2018 Published: 01.01.2019 Copyright: © 2019 This is an open access article under the terms of the CC-BY 4.0, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. INTRODUCTION ne of the most relevant subjects of applied polymers science is the understanding of the deformation mechanisms and the fracture properties of semi-crystalline polymers. Mechanical studies on semi-crystalline polymers show that properties such as tensile strength and elongation at break are associated with the deformation and rearrangement of crystalline and amorphous phases. The imposed loadings draw crystalline lamellae and amorphous phase into viscoelastic and plastic deformation. The deformation of semi-crystalline polymers is complex, multistage and are strongly dependent on the underlying structure as well as external parameters such as applied stress, strain rate, temperature and pressure and they are often accompanied by volume change during their viscoplastic deformations [1,2]. O