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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monitoring. Of particular concern is the presence of internal or hidden damage caused by low-velocity impacts, making it challenging to detect such damage during in-service inspection. To ensure the vibrational reliability and prevent dangerous dynamic stress states in the presence of local damage their influence on the modal characteristics of structural elements have been studied. The dynamics of composite structures with local inhomogeneities remain an area of interest for scientists, engineers, and designers of new technology. Previous studies conducted by Saravanos and Hopkins (1996), Li and Crocker (2006), and Derkach et al. (2022) have focused on determining the influence of local damages, such as open cracks, single delaminations, and their location along the length of cantilever beams, on the modal characteristics. As shown by Grady and Meyn (1989), Pérez et al. (2014), impact damage of composite structural elements is characterized by complex fractures, involving a combination of layer cracking and delamination. Detecting such damages based on changes in modal characteristics is extremely difficult since they are insensitive to these forms of damage. Consequently, more sophisticated methods of vibration diagnostics are needed to address the issue. The characteristics of non-ideal vibrational resistance in composite materials and structural elements, along with the associated dissipation of vibration energy, hold promise for addressing this issue, as considered by Kyriazoglou and Guild (2005), Hammami et al. (2015), Cao et al. (2017), and Kabannyk (2023). In light of the above, this study aims to establish the influence of local damages on the damping capacity of composite materials and structural elements made of carbon fiber-reinforced plastic (CFRP). Additionally, the study aims to investigate the energy dissipation mechanisms during cyclic deformation of locally-damaged specimens. Solving these problems is important as it will contribute to the development of scientific principles for vibration diagnostics of local inhomogeneities in systems, thereby ensuring functional efficiency and increasing the reliability of highly loaded composite elements in modern mechanical engineering. While previous works have examined the influence of damage on the viscous energy dissipation in the material, the effect of such damage on nonlinear or amplitude-dependent energy dissipation has almost not been considered. However, the presence of delaminations is known to increase dry friction on the delamination interfaces, potentially leading to higher levels of amplitude dependent energy losses. Given the current trends in the designing of structural elements using composite materials, further development of these methods is necessary. Therefore, the goal of this study is to determine the effect of predetermined local damage caused by quasi-static punching using a spherical indenter on the changes in natural frequency and the characteristics of amplitude dependent energy dissipation in CFRP composite beam specimens. To achieve the research goal and further develop vibration diagnostic methods for composite structural element damage, we developed a research methodology. It involves predetermined damaging cantilever specimens and subsequently analyzing hysteresis losses during vibrations. The acoustic emission (AE) technique has been applied to predetermined damage to the woven CFRP beam specimen. 2. Application of AE technique for predetermined damage of CFRP The expressing the numerical value of the integral damage indicator of a structural element is a complex issue. It is even more difficult to express integral damage numerically in the case of local damage. The presence of local damage in a cantilever beam is indicated by a reduction in bending stiffness and a decrease in natural frequency. The extent of this reduction depends on the stiffness decrease of the structural element under investigation. Considering this, and the practical objective of detecting minor damage, dissipative properties can be regarded as a more susceptible vibrational indicator for mild to moderate local damage. To quantify the damping capacity of a woven CFRP sample in relation to local damage, we use the continuum damage indicator D . It is set 0 for an undamaged sample and at 1 for a fully damaged local area of the sample, which created during the quasi-static perforation test (Fig. 1a). The AE signals are used to determine the extent of local damage. The functional diagram of signal registration system in the case of predetermined local damage is shown in Fig. 1(b). The contact force during quasi-static punching is determined by measuring the longitudinal deformation of the indenter with two strain gauges (SG) collocated on the cut-out areas, which allows excluding the bending influence on the signal. The signal from the diagonal of the SG bridge enters the HX711 integrated analog-to-digital (A/D) converter with a built-in differential amplifier (DA).