PSI - Issue 41

R. Nobile et al. / Procedia Structural Integrity 41 (2022) 421–429 Riccardo Nobile et al. / Structural Integrity Procedia 00 (2019) 000 – 000

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proposed different techniques to monitor it in a Structural Health Monitoring (SHM) logic: for example the variation of the ultrasound propagation velocity was used to determine the fatigue damage (Omari et al. (2013); Dattoma et al., (2019)), or the change of signals originated by embedded or bonded sensors such as piezoelectric UT sensors, fiber optic, acoustic, was also adopted (Vipperman, (1999); Foedinger et al., (1999); Chung, (2001)); alternatively, non-contact full-field methods have been proposed, as Thermoelastic Stress Analysis TSA (Middleton et al., (2019), based on the thermoelastic effect that correlates the stress distribution to the temperature changes detected in the inspected component or material subjected to cyclic loads (Vassilopoulos et al., (2011); Palumbo et al., (2021)). The Electrical Resistance Change (ERC) is a method that has received limited attention in terms of structural monitoring especially for its use on metals. This method does not involve any embedding or gluing; therefore it does not present the problems described in the case of the sensors. Moreover, the method allows monitoring the entire structure, while the use of sensors tends to allow monitoring only selected locations. The method has been recognized by numerous studies to detect damage on CFRP composites and by the authors as a parameter sensitive to the nucleation and propagation phase of the crack and in general to the fatigue damage process (Park et al., (2006); Nobile and Saponaro, (2021)). This method does not require expensive instrumentation and does not cause degradation of the test material. In the present paper Electrical Resistance method was used for real-time monitoring of damage on notched specimens AISI 316L stainless steel subjected to fatigue. The temperature of the specimens was also on-line monitored during the test using three T thermocouples to eliminate its effect on the electro-mechanical response. From the results, it was observed, that resistance decreases in the initial stages of loading and subsequently, starting from about 20-40% of the fatigue life, shows a rapid increase to be associated to an irreversible change in the material due to the fatigue damage close to the notch tip. Before final failure, from about 80-90% of the fatigue life, the resistance increases rapidly in the propagation phases of the crack coherently with the stiffness reduction. The applied ERC technique proved to be valid for studying the evolution of damage and for predicting and evaluating the fatigue life of metals effectively.

Nomenclature ERC

Electrical Resistance Change ΔR exp experimental resistance variation ΔR th thermal resistance variation A resistance temperature coefficient

2. Material and experimental procedure The specimens used in this work have a U-shaped notch in the gage length. Fig. 1 shows the geometry and the nominal dimensions of the specimens used for the fatigue tests. The monotonic properties of the 316L stainless steel (Kelly (2006)) at room temperature are summarized in Table 1.

(a)

(b)

Fig. 1. (a) Geometry of the specimens AISI 316L used for fatigue tests (dimensions in mm); (b) fatigue tested specimens.