PSI - Issue 75

Sixin Liu et al. / Procedia Structural Integrity 75 (2025) 200–204 Sixin Liu/ Structural Integrity Procedia (2025)

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Bonding was performed using a surface-prepared TA4610 adhesive, with a curing time of 24 hours under mild compression to ensure a defect-free interface. Adhesive bonding of polymers to metallic substrates in fatigue-critical environments is a well-established technique [7]. Fatigue testing was conducted on an Instron E3000 servo-hydraulic testing system under displacement-controlled loading at a frequency of 5 Hz. The loading waveform was sinusoidal, with a peak tensile load determined to reflect the mid-range fatigue threshold of the aluminium base. The results showed consistent sensor fracture at 29,800 – 31,500 cycles, a variance of less than ±5% from the simulated failure point. Visual inspection post-testing confirmed the presence of through-thickness cracks originating from the notch root, and supplementary monitoring using a high-speed camera captured crack propagation over the final 1,000 cycles. These findings corroborate the finite element model and validate the sensor’s utility as a life -fraction indicator. 5. Conclusion and Future Work This study presents a novel approach to structural fatigue monitoring through the development of passive, visually readable polymer-based sensors. By leveraging geometric stress amplification in polypropylene via tailored V-notch configurations, the sensor can be designed to fail at targeted intervals of the host structure’s fatigue life. Finite element analysis and physical testing confirm the predictability and repeatability of sensor failure, demonstrating its potential as a low-cost, power-free alternative to traditional fatigue monitoring systems. Future research will explore several directions to enhance the functionality and robustness of the sensor. These include: • Multi-layered composite architectures to delay delamination and improve environmental resistance; • Integration of piezoelectric or conductive fillers (e.g., carbon nanotubes, graphene) for hybrid sensing and electric signal generation [8]; • Deployment in field-scale components across aerospace, marine, and civil infrastructure domains; • Long-term durability studies under variable humidity, UV exposure, and temperature fluctuations to evaluate performance in operational environments [9]. Ultimately, the proposed sensor technology offers a promising pathway toward intuitive, scalable, and proactive fatigue life management, particularly for structures where conventional sensing solutions are infeasible or economically prohibitive. References [1] S. Suresh, *Fatigue of Materials*, 2nd ed. Cambridge: Cambridge University Press, 1998. [2] D. Montalvão, N. M. M. Maia, and A. M. R. Ribeiro, "A review of vibration-based structural health monitoring with special emphasis on composite materials," *Shock Vib. Dig.*, vol. 38, no. 4, pp. 295–324, 2006. [3] Y. Huang and J. Zhang, "Viscoelastic behavior of isotactic polypropylene: modeling and experiments," *Polymer*, vol. 45, no. 24, pp. 8451– 8460, 2004. [4] S. Khan, H. Kamarulhaili, and M. A. Sheikh, "Stress concentration factors for V-notched polymer specimens," *Int. J. Fatigue*,vol. 101, pp. 367–375, 2017. [5] R. I. Stephens, A. Fatemi, R. R. Stephens, and H. O. Fuchs, *Metal Fatigue in Engineering*, 2nd ed. New York: Wiley, 2000. [6] H. Tsuji and A. Mizuno, "Fatigue properties of polypropylene and its blends," *Polym. Test.*, vol. 19, no. 8, pp. 949–959, 2000. [7] A. J. Kinloch, *Adhesion and Adhesives: Science and Technology*, London: Chapman and Hall, 1987. [8] N. Hu, H. Fukunaga, C. Yan, and Y. Li, "Prediction of electrical conductivity of polymer–carbon nanotube composites," *Sens. Actuators A Phys.*, vol. 163, no. 1, pp. 410–416, 2010. [9] C. R. Farrar and K. Worden, *Structural Health Monitoring: A Machine Learning Perspective*, Chichester: Wiley, 2012.