Issue 41

S.M.J. Razavi et alii, Frattura ed Integrità Strutturale, 41 (2017) 432-439; DOI: 10.3221/IGF-ESIS.41.54 435 R ESULTS FROM FATIGUE TESTS he fatigue tests were performed by using a servo-hydraulic MTS810 test system with a load cell capacity of 250 kN. All tensile stress-controlled fatigue tests were carried out over a range frequency varying from 5 to 10 Hz depending on the level of the applied load. A constant value of the load ratio, R=0, was employed in all tests. After the tests the specimens were examined and the fracture surfaces were analysed to get information about the crack initiation and propagation. Failure always happened, as expected, in correspondence of the first bolt of the connection, as visible in Fig. 4. In particular Fig. 4a shows a lateral view and Fig. 4b an upper view of the specimen. Fig. 5 shows two broken parts of a specimen. In particular Fig. 5a shows the holed plate and Fig. 5b the fracture surface in proximity of the bolted connection. The representative failures shown in the figure always started from the net section in correspondence of the hole. From there the crack propagated through the material until the net section was so much weakened that finally a static crack caused the final failure. Multiple initiation points are well visible with a regular propagation until the final failure. This is in agreement with [22] which shows that in members with drilled holes a surface crack at the wall of hole was found always predominant. According to the fracture surfaces it can be observed that the crack has not a constant configuration in thickness. Thus, it is verified that the preloading applied to bolts influence crack initiation phase. Fig. 6 shows an example of the zinc layer from a SEM image. Fig. 7 summarizes the results from fatigue tests of hot-dip galvanized bolted connections subjected to a nominal load ratio R=0. The stress range over load cycles N is plotted in a log-log diagram. The unbroken specimens (run-out samples), over three million cycles, were not considered in the statistical analysis. They are marked with an arrow in Fig. 7. The mean curve corresponding to a probability of survival, Ps, equal to 50% is reported in the figure as well as the scatter band defined by lines with Ps 10%-90%. The dashed line refers, instead to a Ps of 97.7% for a direct comparison with the Eurocode detail (EN 1993-1-9). The typical expression for the S-N curve in EC3 is reported below: 6 2 10 k k R R c N       (1) The inverse slope k value of the S–N curve and the scatter index T referred to Ps 10%-90% are also shown. The complete list of data related to hot-dip galvanized bolted connections is summarised in Tab. 2 properly marking the run-out specimens. The results from the statistical re-analysis is provided in Tab. 3. From the re-analyses of the data it is clear that considering a Ps of 97.7% at two million cycles Δ σ is equal to 100 MPa which is slightly lower than the corresponding classified category Δ σ c=112 MPa derived from EC3 for the considered uncoated bearing-type connection. In contrast to [22], the inverse slope of the curve, k, is very close to that suggested by EC3. The data from hot dip galvanized specimens are plotted together with data from uncoated specimens characterized by the same geometry and tested in this research program. It is possible to observe that all data fall inside a narrow scatterband and that the reference value at two million of cycles and corresponding to a Ps of 97.7% remains almost the same (101 MPa) with almost no significant differences between galvanized and not-galvanized specimens (see Fig. 8). This result is in agreement with that reported in [22] where it was shown that the use of preloaded high strength bolts gave a remarkable positive influence on the achieved fatigue life and that the detrimental effect of hot dip galvanizing can be easily neutralized. The advantage of this method is the easiness of handling with the maximum of efficiency of the bolted connection under fatigue loading. In this optic the accurate procedure described in section 3 for specimens preparation and assembly is surely necessary to guarantee a good repeatability of the connections in the different specimens. This procedure permits to allow beneficial compressive stresses in the neighbouring of the holes which are advantageous for the fatigue behaviour. In fact, as described in [23], the difference between galvanized and non-galvanized simple plates weakened by a central hole is much higher than that reported in the present paper for bolted connections. In the research conducted by Berto et al. [23] a non-negligible deviation approximately equal to 30% has been found between coated and uncoated specimens with an insignificant reduction of the fatigue life due to influence of galvanization process. In that case two well different scatter bands were given without the possibility of providing a unified band for coated and uncoated specimens which is instead possible in the present investigation dealing with bolted connections. Some creep analyses are also planned [24]. T