PSI - Issue 68

P.N.B. Reis et al. / Procedia Structural Integrity 68 (2025) 1301–1304 P.N.B. Reis et al. / Structural Integrity Procedia 00 (2025) 000–000

1302

2

are crucial. Furthermore, these structural elements are also frequently exposed to complex fatigue load histories that are marked by variations in the frequency, amplitude, stress ratio, and shape of cyclic stresses. Additionally, it cannot be ignored that the viscoelastic behavior of polymer matrices significantly affects the fatigue life of the composite, particularly for composites whose fibers are not aligned with the load direction (Reis et al., 2009). Many studies have been carried out to evaluate the fatigue response and, more recently, the traditional approach based on fatigue residual life and static residual strength has been replaced by considerations based on damage mechanisms that occur during fatigue life or thermodynamics and acoustic phenomena (Miyano et al., 1999. Bourchak et al., 2007; Himmel, 2002; Loutas et al., 2017). In this context, it has been common to use methods based on S-N curves, a methodology that is very time-consuming and expensive. To avoid these weaknesses, new methodologies have emerged, including those based on self-heating temperature. Efficient, faster, and less expensive, they evaluate the temperature during each load level due to the viscoelastic nature of the polymeric matrix and determine, for example, the fatigue limit due to the sudden change in the relationship between the loading stress and temperature accumulation (Palumbo et al., 2016; Huang et al., 2019; Katunin et al., 2022). In this context, the critical self-heating temperature appears as an indicator of fatigue limit, because the appearance of damage in the composites leads to new friction surfaces and, consequently, to new sources of heating that lead to increased self-heating temperature (Katunin, 2019). Therefore, because the literature is not abundant in terms of studies involving the fatigue response of hybrid composites (Swolfs, 2019), this study intends to analyse the critical self heating temperature as an indicator of a fatigue limit of hybrid laminates based on natural fibers. In addition to natural fibers being renewable, biodegradable, and available worldwide, they are lightweight, abundant, non-abrasive, and inexpensive. For example, they have a specific weight about half that of glass fibers and a tensile modulus very similar to that of aramid fibers (Reis, et al., 2007). In this context, it is not surprising that natural fibers are increasingly attracting interest from the industrial community, and their mechanical characterization is necessary as reinforcing elements in composites laminated or hybridized with other types of fibers. 2. Materials and experimental procedure Composite laminates were produced with different fibers and involving a Sicomin SR GreenPoxy 56 resin with a SD Surf Clear hardener. This resin is characterized by containing 56% of its molecular structure from plant origin. To study the hybridization effect on fatigue response, carbon and Kevlar fibers in the form of woven bidirectional fabric (taffeta with 196 g/m 2 and 170 g/m 2 , respectively) were used. Configurations [3F], [1C + 2F + 1C], and [1K + 2F + 1K] were produced, where the “numbers” are the number of layers and the “letters” corresponding to flax (F), carbon (C), and Kevlar (K) fibers, respectively. The laminates were manufactured manually and using a vacuum system. For this purpose, the composite laminates were prepared and placed inside a vacuum bag, and the system subjected to a 2.5 kN load in a press for 24 h to ensure a constant fiber volume fraction and uniform laminate thickness. Post-curing in an oven at 40 ° C for 24 hours completed the manufacturing stage of the composite laminates. The samples used in this study were obtained from those composite plates and subsequently tested for fatigue using the custom fatigue test rig available at the Faculty of Mechanical Engineering of the Silesian University of Technology. This test rig and equipment used for this study is described in detail in Katunin and Wachla (2020). Nine samples were tested for each unique combination of loading properties. The fatigue tests were carried out at constant displacement, at 30 Hz, and at room temperature. 3. Results and discussion Fig. 1 shows a typical curve for the temperature resulting from self-heating, which is characterized by three typical phases, the last of which shows a sudden increase due to competition between the different damage mechanisms. In this context, the point associated with the beginning of this phase represents the critical self-heating temperature, the value of which can be easily obtained using the double-exponential model proposed by Katunin (2012). Table 1 presents the average critical self-heating temperature values. In addition, acoustic emission has also proved to be a very useful tool for detecting and propagating the different damage mechanisms that arise in composite materials. The literature even relates the response of this technique to the self-heating temperature, the effectiveness of which we intend to evaluate here for the hybrid composites under study.