Issue 43

E. Maiorana et alii, Frattura ed Integrità Strutturale, 43 (2018) 205-217; DOI: 10.3221/IGF-ESIS.43.16 214 Since F p,C = 172 kN, the tightening torque to be applied was found by reading the Voltage, V = 172.000 / 92162 = 1.8663 V; 10 kN correspond to 0.108 V. Three tests were performed, and it was found that although the box of the bolts was closed and correctly stocked, in respect of the data reported in the box regarding the k m , an increase of k i was observed. So for the following slip tests on blasted and painted specimens the tightening torque was 545 Nm, assuming k max = 0.16, maximum value of k i according to the code. An increase in the case of the normal speed tests was observed but in two cases the creep test failed again since relative displacements of 0.02 mm and 0.015 mm were observed after half an hour instead a maximum of 0.002 mm over three hours. The results of the third specimen in the static force test show a slight increase in the slip factor values to 0.47. A last set of specimens, series n.12, steel grade EN 10025-2 [21] S355J2+N, was prepared connecting a central blasted and coated plate, using product n.8, with two cover only blasted plates. Fig. 11 shows an image of the set of specimens. Figure 11 : Blasted and half-coated specimens. For this set, bolts M20 class 10.8 with k m = 0.13 and v k = 0.06 were used. The grease was applied between the screw and the nut. Since the manufacturer declares that the standard production guarantees f ub,min = 1040 N/mm 2 , and EN 1090-2 [2] suggests for the tightening torque method a final torque of 1.1 M s , the final tightening torque was M s = 545 Nm. This result is equal to the previous one using k m = 0.16 but since the grease was applied, it was necessary to respect the manufacturer’s indication. This last procedure to find the tightening torque was discussed with the manufacturer and approved. For specimen number five (V5), the displacement recorded was 0.0400 mm for the upper and 0.0360 mm for the lower limit, thus above the limit of the standard, therefore five tests are not sufficient for the statistic evaluation of the slip factor and an extended creep test procedure should be necessary. Otherwise, apart from the delayed slip of the fifth test, the values obtained by the tests were processed obtaining the mean value of the slip factor  m = 0.338, a standard deviation s  = 0.024, thus a characteristic value  k = 0.289 is achieved. Fig. 12 shows the test results. Figure 12 : Blasted, half-coated with product n.8. Dashed line: mean value  m ; continuous line:  k .