Issue 43

P. Zampieri et alii, Frattura ed Integrità Strutturale, 43 (2018) 90-96; DOI: 10.3221/IGF-ESIS.43.06 92 A CCELERATED CORROSION PROCESS Material and geometric properties of the specimens he structural detail under investigation is a slip resistant bolted joint made with high strength preloaded bolts (with geometry carried out from Berto et al 2016) [3-4]. In particular, two plates of 10mm thickness, 40mm width and 229.5mm length are longitudinally joined by means of two cover plates and n.6 M12 class 10.9 bolts tightened with a torque of 91Nm (Fig. 1). The plates are made of S355 structural steel with drilled holes. All the plates were treated with a SA3+ sandblasting process that guarantees a white metal cleaning and consequently a quite uniform friction coefficient equal to 0.5. In regard the friction, between the bolt shank, the nut and the bolt washers, the manufacturer’s technical data sheet was considered, which provides a friction coefficient value equal to 0.128. (a) (b) Figure 2 : Equipment used for the corrosion process: (a) phase of samples immersion; (b) phase of samples exposure on air. Accelerated corrosion treatment The accelerated corrosion process was applied on n. 10 specimens. The procedure was set up by referring to ISO 11130 norm. The process consists in submitting samples to a sequence of immersion and drying cycles, without interruption for the duration of 672h (corresponding to 4 weeks). Every cycle is made up of two minutes long immersion step in a distilled water solution with 5% NaCl followed by a sixty minutes long drying step made with dry air at a temperature of about 35°C. The equipment used to carry out the corrosion cycles consists of the following parts:  Two tanks for the specimens immersion named V1 and V2: samples are completely immersed in saline solution in V1 tank; V2 tank allows to empty the V1 volume and to let samples on air at the controlled temperature of 35°C for the drying phase;  Two immersion pumps named P1 and P2: P1 pump, located inside V1 tank, allows to empty the tank to expose specimens to the air during the drying phase; the P2 pump, located inside the V2 tank, allows to move the solution from V2 to V1 and to immerse the specimens;  Two electromagnetic sensors called S1 and S2: both inside V1 tank. The S1 sensor is positioned at a higher level than that of the samples; it has the function of detecting the maximum level of the solution in the V1 tank and therefore sending the pumping interruption signal to P2 pump (which was transferring the solution from V2 to V1 tank). The S2 sensor, positioned at a lower level than that of the samples, sends to P1 pump the pumping signal (it allows to transfer the solution from V1 to V2 tank).  An electric panel to power to the pumps and sensors, and a simple control system (made by a couple of timers) having the function of managing the tanks filling and emptying steps;  A third tank V3 placed above the V1 and used as an hermetic closure; an heating device positioned on the top of the V3 tank has the purpose to keep the air temperature at a constant value of 35°C. The outline of the equipment is shown in Fig. 2. Prior to initiating the accelerated corrosion process, samples were weighed with an electronic scale to be able to estimate the mass drop caused by the oxidative phenomenon. Every 168 hours of T