Tensile Testing of Pinned Hybrid CFRP/Titanium Joints and Damage Monitoring with Electrical Resistance Measurements

  • Dengg, Andreas (Institute of Structural Lightweight Design)
  • Kralovec, Christoph (Institute of Structural Lightweight Design)
  • Schagerl, Martin (Institute of Structural Lightweight Design)

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In civil aircraft design and manufacturing, the variety of used materials is constantly increasing. To exploit the full potential of the materials, reliable new joining technologies are required. This is especially true for the combination of carbon fiber reinforced polymers (CFRP) and titanium parts. In the recent past, metallic pin arrangements are proposed and investigated, which protrude into the CFRP to improve the strength and damage tolerance properties of conventional, bonded joints. A possible way to monitor such “pinned hybrid joints” is structural health monitoring (SHM). With the help of direct current electrical resistance measurements (DC ERM), damage-related, irreversible electrical resistance change of the joint is evaluated. In a recent study at the author’s research laboratory, the irreversible electrical resistance change of a pinned hybrid CFRP/titanium single-lap shear (SLS) joint was demonstrated with the help of a self-developed resistance model, as joint damage initiated and progressed under quasi-static, low-cycle tension-tension loading until specimen failure. In contrast to similar reported experiments with DC ERM, the method proposed by the authors does not require any conductivity-enhancing additives (e.g., carbon nanotubes) to the isolating CFRP matrix material. Here, the metallic pins enable the desired electrical properties of the joint with their improved contact between the metallic adherend and the fibers of the CFRP adherend. To monitor the joint’s structural degradation due to the determined damage mode of cracks at the overlap ends, the longitudinal stiffness of the joint can be extracted during loading by the optical method of digital image correlation (DIC). Furthermore, a finite-element (FE) model of the specimen was created and validated with the obtained results. In the present investigation, the DC-ERM measurement recent results of the pinned hybrid CFRP/titanium SLS joint are re-evaluated for damage monitoring by means of an improved method. The new method aims to increase the robustness of the DC ERM-based damage evaluation approach through simpler data processing, a dimensionless damage indicator, and improved inferences about the condition of the specimen during mechanical loading. Differences to the previous methodology and the potential advantages of the novel method are outlined and discussed. Further, an improved instrumentation setup for future tests under high cycle fatigue loading is presented and dis