PSI - Issue 47

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

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R. Nobile et al. / Structural Integrity Procedia 00 (2019) 000 – 000

phenomena: for example, the change in the electrical resistance of a material subjected to fatigue load has been proposed to evaluate fatigue damage (Xia et al. (2003)); alternatively, ultrasound propagation velocity variation (Omari et al. (2013), Dattoma et al. (2019), Nobile et al. (2022)) or the Thermoelastic Stress Analysis (TSA) technique has also been used to monitor fatigue damage in composites (Dulieu-Barton et al. (2011)). In particular, the Electrical Resistance Change has been recognized by several studies as a sensitive parameter to crack initiation phase and in general to the fatigue damage process. In composite structures, the fatigue damage phenomenon is more pronounced in areas of stress concentration such as the connection of components by rivets (Skorupa et al. (2017), Grilo et al. (2013). For the study of these structures, samples with different geometric configurations of the rivet hole are used. The electrical resistance measurement has recently become an active non-destructive evaluation method to detect internal damage of Carbon Fiber Reinforced Polymers (CFRP) laminate, since carbon fiber is an excellent electrical conductor and has been used as a strain sensor for many years (Coron et al. (1969), Park et al. (2006), Irving et al. (1998), Seo et al. (1999), De Baere et al. (2010)). In the present paper, Electrical Resistance measurements were performed in real-time to detect damage on a batch of CFRP specimens without interrupting the test. Furthermore, the temperature of the specimens was also monitored to subtract its effect from the acquired raw data. From the processed resistance signal, a progressively rapid increase in values from about 5-30 % of the fatigue life was observed due to initial damage evolution. Subsequently, starting from about 60-70 % of the fatigue life, the resistance undergoes a further rapid increase corresponding to delamination. The latter behavior is consistent with the stiffness degradation. Thermal data, expressed through appropriate damage parameters, also showed an interesting gradual increase in the values due to fatigue damage evolution in excellent agreement with the stiffness degradation.

Nomenclature ERC

Electrical Resistance Change

A resistance temperature coefficient ΔR exp experimental resistance variation ΔR th thermal resistance variation D diss dissipative contrast parameter 2. Materials and methods 2.1. CFRP specimens and experimental tests

CFRP composite specimens, considered for fatigue damage monitoring by ERC and Infrared Thermography IRT measurements are manufactured using the Liquid Resin Infusion (LRI) process, a promising technology for producing large, thick, or complex structural parts. The tested specimens correspond to an open-hole configuration and consist of 16 layers with stacking sequence of [+45, -45,0,90] 2S . The geometry and specimen’s sizes are shown in Fig. 1 a-b.

(a) (b) Fig. 1. Specimen’s geometry for ERC method (dimensions in mm); (b) 3D CAD view of tested samples.