EVALUATION BY TEST OF TIGHTENING TORQUE FOR PRELOADED STAIN-LESS STEEL BOLTING ASSEMBLIES

Preloaded stain-less steel bolting assemblies are currently used in minor and secondary structures such as ventilated facades, but the national and international regulations condition their use on the execution of specific tests to determine the sliding resistance. This approach is justified by the dif-ferent viscoplastic behaviour compared to carbon steel, which can lead to a greater loss of preload over time and by the galling, that is a formation of streaks during tightening with damage of the bolt. This study investigates the use of bolted joints in stainless steel and hypothesizes, based on labora-tory tests, valid formulations in the project for the definition of the preload and the consequent sliding resistance of the joint. Using non-lubricated stainless-steel bolts with geometries according to UNI EN ISO 4014, 4017 standards, tests were carried out on the suitability for preloading, on the possible presence of galling and on the sliding resistance of joints. The preload suitability tests were carried out following the current regulations for preloaded carbon steel bolting, UNI EN 14399 2 and 3, and the achievable preloads were found to be lower than those defined for common steel. The most important conclusion is that it is possible to use these preloaded bolts, with a design preload equal to 50% of that defined in the case of carbon steel. On the other hand, for the factor k, laboratory tests have shown that the values for these bolts are included in a range between 0.25-0.40, higher than carbon steel preloaded bolting. Since the interval is quite wide, further laboratory tests are therefore necessary to define the k factor of these joints made by stainless steel, to avoid over-tightening or in the other hand, having a too low preload. The study constitutes further investigation with laboratory tests on the application of the correct tightening torque without damaging the bolt, compared to the previous publication dealt with in 2019 Bologna’s conference, which was limited to sliding tests of the joint.