Issue 47

J. P. Manaia et alii, Frattura ed Integrità Strutturale, 47 (2019) 82-103; DOI: 10.3221/IGF-ESIS.47.08 89 Tensile test results on cylindrical and flat notched specimens indicate that the presence of notch changes the load- displacement behaviour, despite of the same minimum cross section. The yield load increases with decreasing the specimen notch radii (increasing the stress triaxiality), whereas the corresponding displacement ductility decreases. This behaviour might be attributed to the notch strengthening effect since the notch induces radial and circumferential stresses in addition to the axial stress. These additional stresses could inhibit lamellar rotation toward axial direction; also alter the force acting on the crystalline lamellae, thereby altering their ability to participate in slip processes and delaying the onset of plastic deformation [20]. Beyond the yield load, the specimens gradually started to whiten. As the plastic deformation continued, the stress whitening became clearer and sets the shape changes (dilatation). Therefore, a neck started to form at the same location as onset of stress whitening. This characteristic was clearly noticeable among cylindrical notched specimens. Along with the stress whitening zone, the neck propagated through the notched section of the specimen during the deformation. Plastic dilation in polymers can be assumed to be related to damage. Such damage can be microvoids that grow from local irregularities of the molecular structure. With ongoing deformation, localisation grows to extremes, resulting in void nucleation, craze formation and catastrophic failure [21–23]. Combined tensile/shear loading test Fig. 7 shows the results when a tensile vertical displacement is applied during biaxial loading, for HDPE, PP and PA 6. At RT and for all loading angles, the corresponding load-displacement behaviour shows the expected increase in initial yield force with increase in crosshead speed. It can be observed that the higher the loading angle (from  = 0° to 90°), the higher the load required to deform the specimens, is; whereas the displacement decreases. The simple physical explanation of this phenomenon is that semi-crystalline polymers become more difficult to deform when the molecules get closer to an aligned, stretched conformation [24]. At  = 0°, pure shear, the material behaviour is completely different, the yield displacement is increased and there is large deformation (strain hardening) which means more ductile behaviour. The general shape of all curves, seems to be homothetic, for each biaxial loading angle exhibiting remarkably similar intrinsic behaviour and in the case of PA 6 the load-displacement curves overlap each other, for high plastic strains. This mechanical behaviour might indicate that there is a thermal softening associated with a progressive localization of heat sources at high crosshead speeds as observed by Wattrisse et al. [25]. The initial linear elastic response is followed by a nonlinear evolution, this nonlinearity increasing with applied crosshead speed. At HDPE butterfly specimens with deformation, independently of the loading angle, near fracture point, the crazes, which are bridged by fibrils, open up and fracture occurs upon the failure of these fibrils, without causing cracking and global failure. A careful examination of the fractured specimens for each loading angle, indicates that cracks initiate at the centre of the gauge section in all the cases. However, it is difficult to determine directly from the experiments whether fracture starts from the middle thickness of the cross section or from the surface. Fig. 7 also compares the load-displacement results from biaxial loading at RT with corresponding results in the temperature of 50 °C. A decrease in the yield load or an increase yield load displacement are observed with an increase in temperature and a decrease in the crosshead speed. Similar conclusions on temperature dependence were obtained by Hartmann et al. [26], who performed uniaxial tension tests to the yield load on polypropylene dumbbell specimens as a function of temperature from 22 to 143°C at a strain rate of 2 /min. Therefore, temperature and strain rate have a clear influence on mechanical properties of semi-crystalline polymers. The displacement at failure also increases with an increase of temperature in the case of HDPE, however this tendency is not verified for PP and PA 6. In the case of PP, the displacements at RT and at 50 °C are almost equal and for PA 6 in general are greater at room temperature than at temperature of 50 °C. Stress whitening was observed in the gauge section of butterfly specimens. The occurrence of stress whitening in HDPE was found to be dependent upon the temperature and strain rate. PA 6 mechanical responses at RT are characterized by yield, softening and hardening, as the specimens are inelastically strained (Figs. 6 and 7). At crosshead speed of 200 mm/min, the test requires approximately 1 to 2 s and nearly adiabatic conditions exist due to insufficient time for significant heat transfer to occur. The remarkable softening of the material is observed after yield load due to combined result of strain softening and thermal softening. The softening ends where the load-displacement response is observed to level off even though the temperature in the specimen might continue to rise. Much of the observed effects of the increased crosshead speed on the post yield response of PA 6, was observed by Arruda et al. [27] on polymethylmethacrylate (PMMA). However, at temperature of 50°C the softening behaviour is not observed in Fig. 7, considering the same loading angle. At RT testing the imposed deformations might induce the change of material state, from glassy to glassy-rubbery (glass transition temperature) by increasing its internal temperature. The glass transition marks the onset of extensive molecular motion which is reflected in marked changes in properties, such as volume and stiffness. The material may be more easily deformed and become ductile [28]. On the other hand at temperature of 50°C,