PSI - Issue 72

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

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(a)

(b)

Fig. 5. Normalized frequency ratio versus normalized number of cycles of thermoplastic coupons for (a) f = 160.4 kHz and (b) f = 3.5 kHz.

Fig. 6. Experimental and empirical residual frequency ( f = 3.5 kHz) versus normalized number of cycles of thermoplastic coupons.

5.2. Estimation of residual strength and fatigue life from residual frequency

Fig. 7(a) presents the experimental and numerical residual force as a function of the experimental and numerical DI, derived from Equations (1) and (4), respectively. Using linear trendlines, the residual force of the coupons can be predicted either experimentally using the DI obtained from C-scan images (Equation (1)) or numerically, based on the DI from the fatigue delamination model (Equation (4)). These two processes operate independently, meaning the model’s predictive capability is not compromised by reliance on experimental input. Fig. 7(a) does not incorporate random vibration analysis in its predictions. However, this aspect is addressed in Fig. 7(b), where residual force is plotted against normalized frequency. By utilizing either measured or predicted frequencies, the residual force can be effectively estimated. The significant deviation between the experimental and numerical variation of residual force degradation with respect to DI and normalized frequency is primarily due to two factors: (a) Limitations of the numerical model in accounting for the complexities of the testing environment, (b) Simplified damage representation, as the model considers only delamination, while other damage mechanisms present in the experimental setup are not included.