Issue 30

P. Corigliano et alii, Frattura ed Integrità Strutturale, 30 (2014) 304-310; DOI: 10.3221/IGF-ESIS.30.37 305 fully automated hardness scanner with high resolution, and were considered in the FE analysis. The FE model was validated by means of the results obtained using a full-field technique: Digital Image Correlation (DIC) . M ATERIALS AND METHODS he investigated cruciform joint, shown in Fig. 1, is made of S235JR mild steel, which is commonly applied in shipbuilding. Its nominal dimensions are reported in Fig. 2, misalignment is around 1 mm. Full-penetration welding was performed using the MAG process. Tensile tests were carried out on specimens made of the same steel and quasi-static tests in displacement control on welded joints. Figure 1 : Cruciform welded joint. Figure 2 : Specimen geometry. R ESULTS AND DISCUSSION Hardness Measurements he specimen was polished and it was possible to see the three different zones of the material (BM, HAZ, WZ) depicted in Fig. 3. Hardness measurements were performed at the Helmholtz-Zentrum in Geesthacht-Germany, using the Ultrasonic Contact Impedance (UCI) method. The UCI method is based on the natural resonance frequency of a bar, which pushes the Vickers diamond to penetrate into the sample. The measured frequency change depends on the size of the contact surface between the diamond and the sample for a fixed test load, which is related to the hardness of the sample. The aim of the present investigation was to investigate how hardness values influence the global behavior of the weld. The measurements (Fig. 4) showed few zones with different hardness values. Different groups of hardness measurements were identified and they are shown in Fig. 5. The relationship between the ultimate strength  u (in MPa) and HB hardness can be described by a second-order polynomial equation proposed in [9] and given by: 2 0.0012 3.3 u HB HB      (1) This equation is a better approximation, especially for high hardness values, than the commonly used linear relations. The yield strength  y was also found as a function of the Brinell hardness (HB) according to the following equation [9]: 2 0.0039 1.62 u HB HB      (2) T T