Issue 43

E. Maiorana et alii, Frattura ed Integrità Strutturale, 43 (2018) 205-217; DOI: 10.3221/IGF-ESIS.43.16 209 Heistermann et al. [11] studied the slip resistance in lap joints with long open slotted holes while Annan and Chiza [12] presented a work about the characterization of slip resistance of high strength bolted connections with zinc-based metallized faying surfaces and Annan and Chiza [13] the slip resistance of metalized-galvanized faying surfaces in steel bridge construction. Latour et al. [14] made an experimental analysis on friction materials for supplemental damping devices while Pavlović et al. [15] presented friction connection vs. ring flange connection in steel towers for wind converters. Ferrante Cavallaro et al. [16] presented the experimental behavior of innovative thermal spray coating materials for FREEDAM joints while Li et al. [17] the slipping coefficient study of frictional high strength bolt joint. Through Finite Element Analysis and experimental study, in Huang et al. [18] the mechanical behavior including slip vs. load ratio, load transfer factors, stress state, and friction stress distribution of this type of joints was studied in detail. Both FEA results and experimental ones show that the loads resisted by bolts in the edge rows are, as expected, larger than the ones by bolts in the middle rows. A report of the Federal Highway Administration [19] has shown that ambiguities within the test method might increase the variability of reported friction coefficients. The report outlines that: - variability of slip coefficients attained for the same coatings were noted by coating manufacturers despite no change in formulation. The most common approach is to use a multilayer paint system with a zinc-rich primer; - labs following the same RCSC [3] procedure were sometimes reporting very different slip coefficients for identical coatings; - the major finding was the manner in which each lab measured slip displacement which contributed to the greatest variability in frictional coefficient results. So, the aim and the main contribution of this work is not only to collect and evaluate the slip factor for different surfaces treatments, through an extensive product comparison and testing but also compare the European and American method for the friction coefficient determination. E XPERIMENTAL TEST METHODS FOR THE DETERMINATION OF THE SLIP FACTOR or the European Code, the procedure for the determination of the characteristic value  k of the slip factor was found testing a series of five specimens as descripted in Annex G of the EN 1090- 2 [2] “Slip test”. For each series, firstly four models are tested applying an incremental tensile load with a velocity of about 0.4 kN/s, to obtain a test duration between 10 and 15 min; in a second stage, the 5 th test was performed to evaluate long-term effects. In the first four tests (short-time tests), the slip loads F Si are recorded when a slip of 0.15 mm occurs. The 5 th model (long- term test) is loaded with 90% of the mean slip loads reached in the previous four tests, during 3 h to assess the behavior under sustained loads. If the difference between the slip measured at the end of 5 min and 3 h after the load application does not exceed 2 μm, the test is valid and the slip load shall be determined as for the previous four tests. If this condition is not verified, a minimum of three extended creep tests should be performed. The validity of the 5 th test still depends on an additional condition: the standard deviation S Fs of the slip loads obtained in the five tests, i.e. ten values, cannot exceed 8%. The slip factor is calculated with Eqn. (5): 2.05 k m μ μ μ s   (5) For the American Standard, the procedure for the determination of the mean value  m of the slip factor derives directly from a series of results found testing five specimens as described in Appendix A of the RCSC [3]. It is important to note that for RCSC [3], testing setup to determine the slip factor is different respect European standard and the single value µ i per specimen is 2 si i p,C F μ F  (6) F