PSI - Issue 47

R. Nobile et al. / Procedia Structural Integrity 47 (2023) 176–184 R. Nobile et al. / Structural Integrity Procedia 00 (2019) 000 – 000

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With the aim of comparing the results relating to the stiffness degradation, obtained by processing fatigue data, with the thermal results, a cumulative damage parameter D k (N) (Nobile et al. (2022)) was defined, expressed as: D K (N) = [1 − KK(N) 0 (N 0 ) ]∗100 (2) where K(N) and K 0 (N 0 ) represent the stiffnesses relative to a generic instant of the fatigue life (N) and to the initial reference state (N 0 ) respectively. 3. Results and discussion 3.1. Electrical resistance monitoring results The effect of the change in resistance as the temperature changes was experimentally determined to avoid errors in the data processed (Nobile et al. (2021)). The contribution of the thermal resistance variation due to the temperature variation was evaluated through a linear relationship using Eq. (3):

∆R th =R th −R 0 =A∆T

(3)

where ΔR th is the resistance variation with respect to the initial value, ΔT = (T -T 0 ) is the temperature difference from the initial value and A = - 0.2782 mΩ °C -1 is the temperature coefficient of the resistance determined by heating the specimen by a 1000 W halogen lamp and measuring the change in resistance as shown in Fig. 4a. A linear decrease of the resistance with temperature appears evident from the graph in Fig. 4b expressed by Eq. (3).

(a) (b) Fig. 4. (a) Experimental setup for evaluate temperature effect; (b) ERC vs. temperature in the heating phase for experimental evaluation of the resistance temperature coefficient (A). The resistance variation measured experimentally during the tests (R exp ), including the thermal term and that due to damage, is shown in the graph of Fig. 5a. From the trend of the graph (Fig. 5a), it is generally observed an increase of the strength values from 5 to 30 % of the fatigue life immediately after a small initial reduction for all the specimens tested. Subsequently, starting from 60-70 % of the fatigue life, the electrical resistance shows a further rapid increase in the delamination propagation phase before the sudden failure of the specimen. For all tested specimens, the temperature variation graph shown in Fig. 5b, also monitored in real time by a T-type thermocouple positioned on the specimens during all the fatigue tests, shows a rapid linear increase in the initial load phases, followed by a second phase of linear increase with less slope. Finally, starting from approximately 80 % of the fatigue life, a rapid increase in temperature is observed before final failure. The behavior of the specimen A5 is an