Issue 47

J. P. Manaia et alii, Frattura ed Integrità Strutturale, 47 (2019) 82-103; DOI: 10.3221/IGF-ESIS.47.08 101 C ONCLUSIONS rom the experimental tests, one can conclude that, the yield load in the lowest testing speed occurs at higher displacements, because the polymer macromolecules have the time necessary to induce an orderly alignment. At higher crosshead speeds, increases both the propensity of the material to form a clear and higher yield load at lower displacement values and the material exhibits a more brittle behaviour. In tensile tests of notched specimens, the yield load increases with decreasing the specimen notch radii (increasing the stress triaxiality), while the corresponding displacement decreases. The smaller the notch radius, the higher the stress triaxiality is. It was observed that the specimens gradually started to whiten, near the yield load. As the plastic deformation continued, the stress whitening became clearer and sets the shape changes (dilatation), due to the formation of microvoids. With ongoing deformation, localisation grows to extremes, resulting in void nucleation, void growth, craze formation and catastrophic failure. From the experimental tests, one can conclude that, at RT the drop of the load with the biaxial loading angle attains a maximum value for the loading angle α = 0° (pure shear) and minimum for α = 90° (pure tension). The higher the loading angle, higher is the load required to deform the specimens. The load-displacement curves shift to higher load values with increase of crosshead speed. The plastic stiffness decreases with increasing of crosshead speed for all loading angles. The crosshead speed and loading angle “after yielding” play a key role in the macroscopic deformation behaviour and determine whether the material behaves as brittle or ductile. Decreasing crosshead speed and increasing temperature results in lowering of the load needed to reach a given displacement. Therefore, the yield load shows an explicit dependency on temperature and crosshead speed. Increase of temperature and reduction in crosshead speed lead to a more ductile behaviour. The occurrence of stress whitening in HDPE was found to be dependent upon the temperature and strain rate. The load-displacement curves show that semi-crystalline polymers used at temperatures above g , such as PA 6, becomes more stables. HDPE is chemically the closest in structure to PP, therefore similarities in load-displacement curves, are observed. However, HDPE exhibits better mechanical properties. In flat and cylindrical notched specimens, it is assumed that the fracture toughness of semi-crystalline polymers is controlled by mechanisms such as crazing, void and cavitation formation. One of the major factors controlling the occurrence of yielding or brittle fracture is the state of stress, such as the one included by the presence of notches. HDPE and PP were deformed at temperatures above the glass transition. SEM fracture morphologies of HDPE, PP and PA 6, reveal that the fracture morphologies are highly dependent on stress states. Lower stress triaxiality is a synonymous of ductile fracture. Flat notched specimens, R=5 and R=30, show the formation of an oriented texture of fibrous surface, which increases with decrease of notched radii (low triaxiality). It is observed that rising the stress triaxiality, fracture becomes more brittle and homogeneous, with less propensity for the formation of longer fibrous surface and more voids content. Qualitative SEM observations of HDPE, PP and PA 6 fracture surfaces of flat and cylindrical notched specimens depict inhomogeneous morphology. Mechanisms such as crazing, void and cavitation formation, are observed. In the same surfaces two or more mechanisms, are found. This means that the stress across fracture surface is not equal and is maximum at centre, where the fracture normally begins (large triaxiality). In cylindrical notched specimens, the voids are larger near to the specimen axis, where the stress triaxiality ratio is high, and its distribution density is higher at the specimen centre and decreases toward the specimen border. By comparing the two radii, the voids amount and size decreases as the notch radius increase (lower stress triaxiality ratio). Although the different stress triaxialities imposed by different specimen’s geometries and notches, the fracture surfaces morphologies slight modifies for PP. This feature might be attributed to the internal morphology of polymer. In HDPE cylindrical notched specimens and R=30 at peripherical and central region microfibrils with knobs or nodules with a smooth rounded surface like features, probably formed by the relaxation of hot material, was observed. The microstructure deformation mechanisms of butterfly specimens loaded at α = 90°- tension, room temperature and at temperature of 50 °C, for HDPE, PP and PA 6, show that the fracture morphology exhibits different modes of deformation, mechanisms such as crazing, voids and cavitation formation being observed. Two or more mechanisms are predominant and fracture morphology are inhomogeneous. The inherent stress concentration is expected to be high in the middle section, thus crack nucleates and propagates through wedges. In general, with the increase in temperature, fracture becomes more homogenous, the extent of craze region decreasing and the ductile area/surface increasing. In the case of HDPE, at the central region, the fracture surface is reduced, becoming narrower, suggesting that was the last region to fracture. PA 6 acquires a fracture surface completely different from that obtained with room temperature. With increasing temperature, the fracture surface becomes more ductile with fibril formation at central region. In combined tensile/shear ( α = 30°) it is observed a fracture morphology oriented towards the loading direction, such as in the case of PP, which at RT or at temperature of 50 °C show oriented fibrils with the direction of loading ( α = 30°), F