Issue 68

M. C. Chaves et alii, Frattura ed Integrità Strutturale, 68 (2024) 94-108; DOI: 10.3221/IGF-ESIS.68.06

cycle, which correlates with the applied strain level. Similarly, the area under the loading curve represents the maximum potential energy. Fig. 12 illustrates the progressive changes in the loss factor for each tested strain level, thereby establishing a clear relationship between these two forms of energy.

Coefficient of variation (%)

Testing parameters

Elastic modulus (GPa)

Tensile test

8.31 3.34 2.91 7.88

2.82  0.23 3.92  0.13 4.35  0.13 4.29  0.34 4.27  0.07

Fatigue 130% Fatigue 115% Fatigue 95% Fatigue 85%

1.77 Table 5: Comparison of static and dynamic test modulus.

Figure 12: Evolution of the loss factor as a function of the number of normalized cycles.

Similar to the observed behavior in stiffness loss, it is possible to identify two stages. In the first stage, there is a rapid decrease in energy, followed by a second stage of stabilization where degradation is minimal. The loss factor is commonly regarded as a more precise indicator of the degradation process in composite materials due to its higher sensitivity compared to stiffness evolution [25]. The energy dissipated per cycle is generated through internal friction, which produces heat, and micro-plastic strain, which involves crack formation [25–27]. Internal friction increases proportionally with the applied strain level, implying that energy dissipation is higher at higher strain levels. Strain-life curve In the analysis of the strain-life relationship, the stabilization of the loss factor was used as the failure criterion. It was observed that stabilization occurs at different values of normalized cycles depending on the strain percentage: 0.8 N/N f for 85% strain, 0.6 N/N f for 95% strain, 0.4 N/N f for 115%, and 0.2 N/N f for 130% strain. Notably, as the strain percentage increases, the stabilization of the loss factor is achieved more rapidly. The strain-life curve, depicted in Fig. 13, was fitted using the Coffin-Manson model, described previously. SEM analysis The analysis of fracture surfaces was conducted through scanning electron microscopy (SEM). Fig. 14 illustrates the cross sectional surface of a specimen that did not undergo complete fracture during fatigue tests. Regions rich in resin are evident,

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