Issue 47

J. P. Manaia et alii, Frattura ed Integrità Strutturale, 47 (2019) 82-103; DOI: 10.3221/IGF-ESIS.47.08 84 Differential Scanning Calorimetry (DSC) tests were performed on 8 to 10 mg of solid granulates of HDPE, PP and PA 6 at a heating rate of 10 °C /min, using a TA Instruments DSC Q20. The reproducibility of the measurements was verified by a second run. The crystal weight fraction (  cw ) is calculated as the ratio of the measured melting enthalpy (  f ) and the theoretical melting enthalpy of a perfect crystalline polymer (  0 f ) taken equal to 293 J/g, 209 J/g and 188 J/g, respectively for HDPE, PP and PA 6 [8,9]:     0 f cw f (1) Crystallinity weight fractions of approximately 55.1%, 40.4% and 49.1%, respectively for HDPE, PP and PA 6 were found. Also, the melting temperatures    m were confirmed to be 139.6 °C, 170.7 °C and 227.4 °C, respectively. The DSC tests also allowed to disclosure the glass transition temperature    g of PA 6, which was 54± 2 °C. HDPE and PP glass transition temperatures were not confirmed since they show negative values and our DSC tests only covered positive temperatures. However according to literature [9], HDPE and PP materials show glass transition temperatures the order of -100 and -20ºC, respectively. Specimens’ geometries Triaxial stress states were reproduced by means of tests using cylindrical and flat notched specimens with different curvature radii in order to set different triaxial stress states in the median cross-section, from 0.39 for the cylindrical notched specimen with a radius of 30 mm to 0.84 for the flat notched specimen with a radius of 5 mm. In addition, a butterfly specimen was designed for biaxial tensile/shear loading, using an Arcan apparatus. The biaxial testing allows exploring initial stress triaxialities ranged between 0 and 0.58. Geometries and dimensions of the cylindrical notched specimens for uniaxial tensile experiments, are described in Fig. 1. The geometric parameters were chosen in order to set different stress states in the gauge section. The minimal cross section diameter is equal to 5 mm, but the length of the non-uniformly reduced section is assigned with two different notch radii: R=30 mm and R=5 mm. Both ends of cylindrical notched specimens were machined with M 14 threads for mounting in the testing machine. Figure 1 : Cylindrical notched specimens: (1) radius of 30 mm and (2) radius of 5 mm. (Dimensions in millimeters). The main reasons to include notches in the specimen geometry is to confine the plastic deformation and the onset of fracture processes into the notched region [10]. A formula for the stress triaxiality was first derived by Bridgman, who analysed the stress distribution in cylindrical metal specimens with different notches. Bridgman’s formula involves the relationship between the smallest cross section 0 a and the notch radius, R . At the centre of the median cross-section the stress triaxiality ratio,  , is maximum and is defined as the ratio of the hydrostatic stress  h and the von Mises equivalent stress  eq , which according the Bridgman formula results in [4,11]: (1) (2)