PSI - Issue 18

Andrzej Katunin et al. / Procedia Structural Integrity 18 (2019) 20–27 Author name / Structural Integrity Procedia 00 (2019) 000–000

26

7

�� � � � �� � ��� �� � ,

(6)

where C , m , and n are constants dependent on the flow type (laminar or turbulent), L is the length of a specimen, Re is the Reynolds number, and Pr is the Prandtl number. According to (6) the heat transfer coefficient  is directly proportional to Re which means that an increase in the air velocity surrounding the specimen will cause an adequate increase in  under certain thermodynamic conditions (see Fig. 6a). The degree of change in the value of  will be determined in this case by the possibilities of the applied system that forces air flow. The second of the analyzed scenarios is based on the mechanism of free convection and the possibility of changing the ambient temperature  0 . In free convection, the heat transfer mechanism is based on the air movement surrounding the specimen under the influence of temperature difference between the specimen and the environment. As with convection forced by the effect of this movement, an increase in the value of the heat transfer coefficient  (see Fig.6b) is proportional to the difference of the mentioned temperatures (Incropera et al., 2007). The currently available climate chambers make it possible to implement such a scenario. 5. Conclusions In this paper the preliminary theoretical analysis and experimental verification of a possibility of minimizing the negative influence of the self-heating effect occurring during fatigue loading of polymeric and polymer-based composite structures on their fatigue life is performed. Based on the presented phenomenological models it was possible to determine three possible scenarios of thermodynamic balance during occurrence of the self-heating effect with an annotation that only two of them are physically realizable. In order to prove the positive influence of surface air cooling of a composite structure subjected to fatigue loading the experimental studies were performed on composite specimens with measurement of the temperature evolution and AE response during the loading. As it was observed, the measurement of both quantities reflected the increase of the fatigue life under the presence of air cooling, and the fatigue life increases further with an increase of blowing velocity. The performed observations allowed to propose two alternative cooling scenarios, where the first one is based on changing the heat transfer coefficient, while the second one is based on changing the ambient temperature. Both scenarios can be realized experimentally to perform further tests on the mentioned phenomena. The results of such tests can be useful during fatigue testing of composite structures, especially during accelerated tests of very high cycle fatigue. Acknowledgements This publication is supported within the framework of the Rector’s grant no. 10/060/RGJ19/0108 in the area of scientific and development research of the Silesian University of Technology, Poland. The authors would like to thank Mateusz Tyczka from the Institute of Fundamentals of Machinery Design, Silesian University of Technology, for 3D-printing the nozzle for an air hose. References Avanzini, A., Gallina D., 2011. Effect of Cyclic Strain on the Mechanical Behavior of a Thermoplastic Polyurethane. Journal of Engineering Materials and Technology 133, 021005. Backe, D., Balle, F., 2016. Ultrasonic Fatigue and Microstructural Characterization of Carbon Fiber Fabric Reinforced Polyphenylene Sulfide in the Very High Cycle Fatigue Regime. Composites Science and Technology 126, 115-121. Incropera, F.P., Dewitt, D.P., Bergman, T.L., Lavine, A.S., 2007. Fundamentals of Heat and Mass Transfer. Wiley, Hoboken, NJ. Kahirdeh, A., Naderi, M., Khonsari, M.M., 2013. On the Role of Cooling on Fatigue Failure of a Woven Glass/Epoxy Laminate. Journal of Composite Materials 47, 1803-1815. Katunin, A., 2010. Analytical Model of the Self-Heating Effect in Polymeric Laminated Rectangular Plates During Bending Harmonic Loading. Eksploatacja i Niezawodnosc – Maintenance and Reliability 48, 91-101. Katunin, A., 2017. Domination of Self-Heating Effect During Fatigue of Polymeric Composites. Procedia Structural Integrity 5, 93-98.