PSI - Issue 79

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

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test setup, which provided a support span of 120 mm in the basic configuration. The specimens were divided into two groups: • without pre-tension (reference condition), • with pre-tension of approximately 10%, applied prior to the fatigue loading cycles. 2.2. Experimental method Fatigue tests were carried out using a cyclic three-point bending method in a symmetric setup. Due to the low stiffness and high flexibility of the specimens, it was necessary to fix them rigidly at both ends of the test rig. As a result, despite the nominal bending configuration, the dominant deformation mode was uniaxial tension rather than pure bending. A dedicated test fixture was designed for this purpose, and its detailed construction is described in [14]. A photograph of the test stand is shown in Figure 3.

Figure 1. Photograph of the test fixture used for fatigue testing of rubber composite belts

The tests were displacement-controlled. The maximum displacement amplitude was limited by the testing machine’s range and the geometry of the double-sided bending fixture. To obtain a representative fatigue characteristic, various displacement amplitudes were applied — from 13 mm to 28 mm — resulting in different strain levels. The experiments were conducted at frequencies ranging from 0.5 Hz to 3.5 Hz. For the pre-tensioned specimens, an initial elongation was introduced prior to cyclic loading. This was achieved by increasing the distance between the supports, thereby applying a defined pre-strain (~10%). After setting the pre tension, the fatigue test procedure followed the same protocol as for the reference specimens. 3. Results In polymeric materials, and particularly in rubber composites, cyclic loading at higher frequencies is often associated with self-heating due to internal energy dissipation. This phenomenon typically accelerates fatigue damage and leads to a reduction in service life. However, the experimental results did not reveal a clear dependence of fatigue life on loading frequency. The primary distinction between low- and high-frequency loading was instead observed in the shape of the hysteresis loops on the force – displacement diagrams, indicating changes in energy dissipation mechanisms rather than a direct reduction in durability. 3.1. Failure criterion The preliminary tests showed that the investigated samples did not fail abruptly. The analysis of the relationship between force amplitude and the number of loading cycles, under a constant displacement amplitude, revealed a gradual, nearly linear decrease in force. As material degradation progressed due to the propagation of microcracks, the curve became increasingly irregular. Nevertheless, no distinct point of fracture was observed that would indicate complete failure of the specimen. Therefore, the failure criterion was defined as the point at which the force amplitude