Issue 47

J. P. Manaia et alii, Frattura ed Integrità Strutturale, 47 (2019) 82-103; DOI: 10.3221/IGF-ESIS.47.08 87 apparatus consists of two pairs of plane semi-circular loading steel plates and the butterfly specimen placed in between. The array of pin holes locations on the outer edge of the loading plates provide a range of biaxial loading angles  . The biaxial loading angle can be varied in steps of 15° from 0° to 90°. Two removable clamps positioned the butterfly specimen in the symmetry plane of the arcan apparatus. Two screws at each side provided sufficient clamping pressure. The four pins, two in opposite side, prevents any rotation or misalignment either vertical or horizontal during the biaxial loading. Therefore, the out-of-plane displacement is avoided, as suggested Mohr and Doyoyo [16]. Testing machines The mechanical tests at RT were carried out by a servohydraulic MTS 810 universal testing machine, with a 10KN load cell, while the mechanical tests at 50 °C were performed inside the Instron environmental chamber (temperature rating from - 150 °C to +350 °C), with the biaxial apparatus attached to the Instron 3367 loading machine. The tests were performed after the specimen stabilizing in the chamber for 20 minutes at the required temperature (50 °C) in order to obtain sufficiently constant cross-sectional temperature (assure thermal equilibrium). In this work, RT means that the air temperature was 21 °C (deviation ± 2 °C) and relative humidity of 55% (deviation ± 10 %), which was the same atmosphere in which all specimens were conditioned. During the tests, a MTS 634.25 axial extensometer with a gauge length of 50 mm, was used to measure the elongation of both cylindrical and flat notched specimens, whereas for biaxial tests the displacements were continuously measured by internal actuator LVDT. Both displacement and force were recorded at an appropriate frequency, depending on crosshead speed. A batch of five specimens (5 repetitions) for each crosshead speed (1, 20 and 200 mm/min), specimen geometry and loading angles (  = 0°,  = 30° and  = 90°) were tested at RT, while at 50 °C a batch of three specimens at two crosshead speeds (1 and 200 mm/min) and at the same loading angles as in RT, were performed. SEM observations were performed by the field-emission scanning electron microscopy, instrument FEI Quanta 400 FEG ESEM, with a nominal resolution of <2 nm. The secondary electron (SE) mode allowed a detailed examination of the semi- crystalline structure. The intensity of the digitized micrographs is defined on a 256 greyscale[17]. The specimens were fixed with double coated carbon tape which, is also used for dissipation of the electron beam charge and heat. All samples were also coated with Platinum before scanning with SEM using Jeol IFC 1600 coating machine. Tab. 1, depicts the total number of SEM observations. Specimens Geometry HDPE PP PA 6 Flat notched Radius=5 RT=1 RT=1 RT=1 Radius=30 RT=1 RT=1 RT=1 Cylindrical notched Radius=5 RT=1 RT=1 RT=1 Radius=30 RT=1 RT=1 RT=1 Combined tensile/shear loading α = 0° RT=1 50 °C=1 RT=1 50 °C=1 RT=1 50 °C=1 α = 30° RT=1 50 °C=1 RT=1 50 °C=1 RT=1 50 °C=1 α = 90° RT=1 50 °C=1 RT=1 50 °C=1 RT=1 50 °C=1 Table 1 : Total number of specimens used in SEM observations (crosshead speed of 200 mm/min). T EST RESULTS – DEFORMATION BEHAVIOUR he mechanical response of HDPE, PP and PA 6, were examined under different stress states until fracture. In order to investigate a wider range of stress states and strain rates, tensile tests were performed on flat notched and cylindrical notched specimens with different notch radii at different crosshead speeds of 1, 20 and 200 mm/min. In addition, biaxial loading tests at three loading angles, were performed at room temperature and 50 °C. ISO 527-1 defines the yield stress as the first stress at which an increase in strain occurs without an increase in stress [18]. As a convention in this paper, and hereinafter, from the load displacement curves the yield load is considered as the first maximum force at which an increase in displacement occurs without an increase in load, when noticeable. Tensile flat notched and cylindrical notched tests Despite belonging to semi-crystalline polymers with crystallinity percentage from DSC scans of 55.1%, 40.4% and 49.1%, respectively for HDPE, PP and PA 6, their mechanical behaviour are somewhat different. The load-displacement curves of T