PSI - Issue 59

Valerii Kobzar et al. / Procedia Structural Integrity 59 (2024) 344–351 Valerii Kobzar and Oleh Derkach / Structural Integrity Procedia 00 (2023) 000 – 000

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Fig. 3. AE pulses N and local damage parameter D for three indented specimens under local quasi-static punching of the woven CFRP material.

It should be noted that the dependency N = f ( D ) may vary for different materials, sample sizes, AE sensor, and its location. Therefore, in order to ensure consistency and comparability, it is important to fix the location of the AE sensor, the position of the sample, and the threshold for the entire series of experiments. This allows for accurate and meaningful comparisons of the AE activity and its relationship to the level of local damage ( D ) in the specimens. 3. Modal and damping analysis techniques The block diagram (Fig. 4) illustrates the experimental setup consisting of a mechanical system and a vibration excitation system for the cantilever beam specimens. The prismatic sample 1, which includes an extended in the root section, is clamped within a massive steel plate 2 weighing 150 kg. The specimen is also clamped with a fixed force. To minimize energy losses, the plate and sample assembly are suspended on the steel strings 3, which have a diameter of about 2 mm and a length of approximately 3 m, which help to reduce the transmission of vibrations to the foundation. Resonant flexural vibrations of cantilever specimens are excited using an electromagnet (EM) powered by an amplifier (PA). To excite vibrations, thin plates made of ferromagnetic material 4 are attached to the free end of cantilever beam specimen. The experimental setup employed for exciting resonant vibrations is the self-oscillation technique, which utilizes a mechanical system with positive feedback. The EM operates within the range of small deformations to minimize its influence on the mechanical system. The weak coupling between the mechanical system and the EM allows to occur vibration resonance of the specimen. The amplitude of vibrations is automatically maintained at the specified amplitude by regulation the EM current. The target amplitude is set using a software, and once the amplitude stabilizes to the target value, the vibration frequency is measured. Various damping characteristics are used to describe energy dissipation in materials, with the logarithmic decrement ( δ ) of vibrations being a primary one. As shown by Boginich et al. (2023) it is determined through the free vibration method by observing the rate at which amplitudes of vibrations decrease. This characteristic, represented as δ = f ( ε ), is amplitude-dependent and correlates with cyclic strain amplitude ( ε ). It is shown, that the value of the logarithmic decrement of vibrations significantly depends on the amplitude of cyclic strain. Hence, when conducting experimental studies, it is crucial to measure deformations in the most stressed part of the sample. In our case, dynamic deformations were obtained using a SG placed closed to the root section of the specimen.