PSI - Issue 72

Niki Tsivouraki et al. / Procedia Structural Integrity 72 (2025) 141–148

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1. Introduction In recent decades, the composite content by weight in aircraft has increased significantly, now reaching up to 50%. While thermosets remain the predominant choice for aircraft structures, thermoplastics are gaining attention due to their unique advantages, particularly in terms of dynamic behaviour, recyclability, and weldability, Pantelakis et al. (2020). Despite their growing adoption in the aerospace industry, further research into the fatigue behaviour of thermoplastics is essential to ensure safe operation and smooth aircraft performance. Structural integrity and service life are critical aspects, requiring evaluation from multiple perspectives. A comprehensive understanding of an aircraft’s condition and performance can be achieved by conducting experiments, developing numerical models, and leveraging existing material property datasets while continuously expanding them, Fekete et al. (2016). With the adoption of the damage tolerance design philosophy in composite aircraft structures, developing methodologies to assess residual fatigue life and strength has become essential. Among the various proposed approaches, random vibration-based methods are gaining prominence due to their natural applicability to continuously vibrating components, such as aircraft wings. The implementation of these approaches relies on modal-based metrics, which can be categorized into three main subcategories. The first and most common approach examines the behavior of natural frequencies (NFs) and damping ratios (DRs) as the number of fatigue cycles increases, Adams et al. (1975), Bedewi et al. (1997), Abo-Elkhier (2014) and Han et al. (2022). The second involves analysing the behavior of natural frequencies using theoretical mathematical models to more accurately capture experimental modal responses, Moon et al. (2003) and Wu et al. (2020). More recently, a third approach has emerged, combining NFs, support vector machine (SVM) algorithms, and neural networks to assess residual fatigue properties, Liang et al. (2024). The first two subcategories integrate modal tests with empirical models to address the problem, while the third combines and compares multiple methods to evaluate their effectiveness. The objective of this study is to develop a comprehensive methodology for estimating the residual fatigue life and strength of thermoplastic coupons by exploiting their random vibration characteristics. The methodology is developed both experimentally and numerically. Additionally, the study aims to investigate the correlation between the evolution of progressive fatigue damage and the modal characteristics of thermoplastic coupons. 2. Experimental 2.1. Materials and coupons The coupons and the tabs that were used in the present study were made from thermoplastic TC 1225 LM PAEK prepreg plies with a fiber volume fraction of 66%. The 0.14 mm-thick plies are stacked in a quasi-isotropic stacking sequence with a lay-up of [-45/0/45/90] 2s . The coupon dimensions were in accordance with ASTM standards <, Test Method for Tensile Properties of Polymer Matrix Composite Materials and Test Method for Tension-Tension Fatigue of Polymer Matrix Composite Materials, measuring 250 mm x 25 mm x 2.24 mm and the bonded tabs measuring 50 mm x 25 mm x 2.24 mm. 2.2. Experimental procedure Fig. 1 illustrates the experimental procedure for assessing residual fatigue life and strength. A total of 28 coupons were tested during the experiments. Initial tensile and fatigue tests were performed for material characterization. Additionally, interrupted fatigue tests were conducted, during which the coupons were subjected to random vibration tests and Ultrasonic C-Scan immersion tests every 10,000 cycles. From these tests, the modal characteristics — including natural frequencies (NFs), damping ratios (DRs), spectral distance, and power spectral density (PSD) Welch spectrums — were determined, along with the damage index. Following the fatigue loading, static tensile tests were carried out to evaluate the residual strength of the coupons. By combining all these results, an experimental assessment of residual fatigue life and strength was achieved. The equipment used for these experiments is described in detail in [12].