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Vibrational Analysis of Composite Tubes

Nevatia, Gaurav (2025)

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Nevatia, Gaurav
2025
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Julkaisun pysyvä osoite on
https://urn.fi/URN:NBN:fi:amk-2025051913147
Tiivistelmä
This study investigates the dynamic bending modulus of composite tubes (carbon fiber, glass fiber and basalt fiber) using non-destructive, dynamic vibrational analysis according to ASTM E1876 and ISO 6721-3. The primary motivation to choose this method is that static testing methods like 3-point bending of tubes suffers from a change in moment of area during the test. The tubes and information about material properties were provided by L-tec Sports Oy. Experimental procedures involved measuring natural frequencies of composite tubes with varied material compositions, fiber orientations, and reinforcement conditions. Signals are collected by exciting an impulse through knocking the tube placed in a rig and using Audacity software to record the vibration signal. Using Fast Fourier Transform (FFT) and Fourier probing, resonance frequencies were identified, and dynamic modulus values were calculated. The findings were compared with theoretical predictions derived from Finite Element Analysis (FEA) conducted in Inventor Nastran software.
An aluminum tube was used as a reference for validation of this method and proof of concept. The experimental bending frequency (3741.4 Hz) was within 9.88% of the analytical result (4152 Hz) and 6.33% of the FEA Simulated result (3518.6 Hz). The study reveals that the measured modulus generally decreases with increasing winding angles as expected (Gibson, 2011), with anomalies observed at higher angles. Blade usage (a mechanical guide that sits after the resin bath to control how much resin remains on the fibers before they are placed on the mandrel) during lamination improved consistency and reduced variability and also demonstrated modulus upto 2%-4% higher. It can be seen that the observed modulus values frequently exceeded FEA predictions, attributed to factors such as higher-than expected fiber volume fraction, superior fiber-resin bonding, higher material density, elevated elastic moduli (E1 and E2), and residual stresses. Enhanced manufacturing quality, including reduced void content and better fiber alignment, also contributed to the deviations. Resins were ranked based on performance, with CR84/CH84-20 as the one with the highest modulus, followed by 125/237, CR87/CH87-10, and 783/42. The propagated error in bending modulus in the equation used where all the terms except radius where error-free was 14.45%. Standard error of mean (SEM) calculations highlighted increased variability at higher angles, emphasizing the challenges of consistent measurements when the winding angle increases. Some limitations of the methods include the reliance on uniform material properties in FEA modeling, which may oversimplify real-world anisotropic behavior, and the challenges in measuring consistent geometric data for composite tubes due to geometric variations in the shape of the tubes. This research underscores the importance of refined FEA modeling and precise experimental methods to account for real-world complexities, particularly at higher winding angles. It is also proven in this thesis, the Euler-Bernoulli equation coefficients are only valid for thin-walled long tubes. So further studies can be conducted to establish equations and coefficients for various geometries.
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