Fatigue Life (Limit) Analysis Through Infrared Thermography on Flax/PLA Composites with Different Reinforcement Configurations  
[PERSON]  
1 Department of Continuum Mechanics and Structural Analysis, Universidad Carlos III de Madrid, 28911 Leganes, Spain; [EMAIL_ADDRESS]. though different properties have been studied in static and dynamic regimes at experimental levels and with mathematical models, information about their properties under fatigue loading conditions is still very limited. Determining fatigue properties is time-consuming and resource-intensive, as the sample must be subjected to repetitive loading up to failure, and the tests must be repeated at different stress levels to build the stress-fatigue life curve [2]. In order to simplify the study and reduce the time, different methods have been developed and implemented, such as nondestructive evaluation (NDE) methods (optical, thermal, electromagnetic, ultrasound, acoustic emission, X-ray, and others) [9; 10]. Especially for the fatigue limit, the surface temperature measured by the thermographic technique is one of the most efficient methods, which was implemented and validated on metals and composites [11; 12; 13; 14; 15]. According to the reported studies, using the thermography technique, the time to complete the entire campaign to determine the fatigue limit may reduce drastically, since it requires around 6000 cycles (20 min considering a loading rate of 5 Hz) to reach stabilization on each sample [16; 17]. During the fatigue damage process (fatigue test under load control), three main stages are presented: the initial stage is usually related to the microstructural reorganization and microcrack initiation, which is characterized by sudden increment in the deformation and temperature; the second stage is related to the crack development on the fiber-matrix interface and in the matrix, where a stable damage progression is presented with a low increment rate of deformation and a stable temperature; the final stage is usually relatively short in the number of cycles, which is characterized with high local increment in the temperature (crack tip area) and large deformation on the sample [14]. In order to estimate the fatigue limit, the second stable stage can be used. Using the superficial temperature and the stress ratio (\(\sigma_{\text{max}}/\sigma_{\text{ut}}\)), [PERSON] consolidated and validated three methods [13]: maximum angle change, minimum curvature radius, and bilinear method. According to the result found by [PERSON], maximum angle change is quite sensitive to the data quality, even failing the estimation if the data error is over 10%; this method is more suitable for metals than composites. On the other hand, the minimum curvature radius method requires a larger amount of data, and even with that, the result is conservative. The last method combines the bilinear and [PERSON]'s methods, having a good correlation with all available data, although it is not conservative in determining the fatigue limit [13]. On the other hand, the natural fiber structure is unique and complex, presenting three main parts: primary cell wall, secondary cell wall, and lumen. The secondary cell wall is the main part responsible for carrying the load; in this wall, cellulose microfibers present a helical shape following the fiber axis, and the lower the microfibril angle, the higher the fiber stiffness. According to the literature, the microfibril angle on flax fiber ranges from 8\({}^{\circ}\) to 11\({}^{\circ}\)[18; 19]; throughout the loading process, especially at low strain rates or cyclic loads, microfibril orientation can be rearranged, reducing the microfibril angle even lower [20]. Furthermore, during the fatigue loading on natural fiber composites, not only does microfiber rearrangement occur, but also the bundle of fibers is rearranged, which makes the behavior even more complex.