PSI - Issue 79

Karolina Głowacka et al. / Procedia Structural Integrity 79 (2026) 155– 160

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decreased to 50% of its initial value, corresponding to a significant loss of mechanical integrity. All parameters were monitored in real time, and the data were recorded for further analysis. As shown by the relationship between force amplitude and the number of cycles (Figure 2), this assumption proved to be valid for the analyzed fiber configuration. In both cases — without pre-tension (Figure 2a) and with 10% pre-tension (Figure 2b) — a 50% decrease in force amplitude corresponded to complete material degradation. a) b)

0 100 200 300 400 500 600 700

0 100 200 300 400 500 600 700

amplitudeof force[N]

amplitudeof force[N]

0

50000 100000

0

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n, number of cycles [-]

n, number of cycles [-]

Figure 2. Representative force amplitude – cycle relationship: (a) reference sample without pre-tension; (b) sample with 10% pre-tension applied

To determine the appropriate fatigue characteristic based on the obtained experimental results, the strain evolution in the material was analyzed. The corresponding schematic representation is shown in Figure 3. During cyclic symmetric bending, the actuator moves in a sinusoidal manner. However, since the specimen is clamped on both sides, the material is predominantly subjected to tensile deformation, and the strain waveform corresponds to the absolute value of a sine function. In this case, the strain range ( ∆ ) is identical to the maximum strain value ( =∆ ). When the specimen was first pre-stretched and then subjected to cyclic bending, the strain waveform was shifted by a constant value corresponding to the additional pre-strain ( ). Consequently, the maximum strain was determined as = ∆ + .

Figure 3. Schematic representation of strain evolution in the actuator and the material during cyclic symmetric bending