PSI - Issue 65

P.B. Severov / Procedia Structural Integrity 65 (2024) 215–224 P.B. Severov / Structural Integrity Procedia 00 (2024) 000–000

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also recorded in the sections where s min is constant, which probably confirms the presence of processes associated with the behavior of partially destroyed material in the damage zones. The sections s max and s min are excluded from further analysis. The loading of the specimen was cyclical, with an increase in the range of displacement of the active gripper. Each cycle consists of two sections: an increasing displacement (s ↑ ) and a decreasing displacement (s ↓ ). For example, the loading cycle 2 lc has sections s ↑ from 4 to 5 and s ↓ from 6 to 7 (see Fig. 1).

Fig. 1. Cycles 1 lc - 9 lc . Specimen loading diagram

Fig. 2. Cycles 1 lc - 9 lc . The total stress-strain diagram (experimental data)

4. Research results

During the planning stage of the experiment, prior to its implementation, a task was set to identify mechanical hysteresis in a specimen cut from a unidirectional CFRP plate along its fibers. A specimen of maximum stiffness with unidirectional fibers laying in the direction of the tensile force of the testing machine was specially selected. With a positive outcome from this basic experiment, studying mechanical hysteresis on less stiff specimens with other types of fibers laying should not present any significant challenges. To confirm the presence of mechanical hysteresis in the specimen under study, after completing the experimental part of the work, it was sufficient to determine the slopes of the stress-strain diagram at the vertices of the cycles when changing the direction of displacement of the cylinder plunger. As a result, it was found that for each loading cycle near the left vertex, the slope of the stress-strain diagram in the section of increasing displacement is greater than the slope in the sections of decreasing displacement. Conversely, near the right vertex, the slope in the sections of decreasing displacement is greater by Severov (2023). This confirmed the presence of a hysteresis loops in the specimen of maximum stiffness and the possibility of their experimental detection. The polynomial approximation of the experimental data ε xi and σ xi at the sections of increasing displacement (upper branches of hysteresis loops) and at the sections of decreasing displacement (lower branches of hysteresis loops) allowed one to study the properties of hysteresis loops in the domain of their definition from ε xmin to ε xmax under quasi-static repeated loading during damages accumulation. Hysteresis loops for loading cycles 1 lc to 9 lc are shown in Fig. 3 as graphs of sixth-degree polynomial equations for the nine upper σ x ↑ branches and eight lower σ x ↓ branches.

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