PSI - Issue 18

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

21

2

1. Introduction Polymeric composites subjected to cyclic loading (fatigue, vibration) reveal generation of heat due to mechanical energy dissipation resulted from the viscoelastic nature of polymers being used as a matrix of composites. This phenomenon, called the self-heating effect, can be dangerous for the structure, since at certain circumstances, e.g. when the critical self-heating temperature is exceeded, it might dominate the process, which leads to the accelerated degradation and structural failure (Katunin, 2017; Katunin, 2019). Such an accelerated degradation is unfavorable in several practical problems, where the self-heating effect appears. In particular, the heating-up of a tested structure is not likely during fatigue testing of composite structures (e.g. the tension/compression tests of the thermoplastic polyurethane cylindrical hollow specimens performed by Avanzini and Gallina (2011) under the self-heating temperature control), especially in very high-cycle fatigue tests, when the excitation frequencies (which have a direct connection with the amount of a dissipated heat) are in the ultrasonic range (see e.g. studies of Backe and Balle (2016)). Due to the mentioned problems with self-heating it is often controlled during fatigue testing, primarily by air cooling. The first studies of air cooling of the polymeric structures subjected to self-heating were performed by Ratner and Korobov (1965) and Oldyrev (1967). They investigated the influence of air cooling on structural durability and proved that the surface cooling may significantly decrease the self-heating temperature, and thus, extend its structural life. The recent studies on structural characterization of polymeric composites subjected to the self-heating and air cooling were performed by Kahirdeh et al. (2013) which covered experimental measurements of the self-heating temperature and acoustic emission as well as developed an empirical model for quantitative evaluation of degradation degree of the tested structures under such conditions. Lahuerta et al. (2014) investigated the influence of air cooling of thick polymeric laminates subjected to fatigue loading with occurring self-heating effect. In both cases the extension of structural residual life of polymeric composites subjected to self-heating by air cooling was confirmed. In the following study, the kinetics of the self-heating effect in the presence of air cooling was investigated, which allowed to distinguish characteristic scenarios of self-heating temperature evolution. Moreover, the influence of the high-speed airflow onto the thermodynamic balance was analyzed. The obtained theoretical results were then confirmed experimentally by testing composite specimens with various airflow speeds by means of thermography and acoustic emission. The preliminary experimental results obtained within this study allowed to conclude about effectiveness of application of the air cooling approach for extension of structural residual life of polymers and polymeric composites. 2. Modelling of the self-heating with air cooling The thermodynamics of the self-heating process of a cyclically loaded composite structure can be described by the non-stationary heat transfer equation (Katunin, 2010): � � � � �� � � � �� � � � � � �� � � � �, (1) where  ( t ) denotes the temperature distribution over the composite structure at time t ,  (  ) is the density and c (  ) ,  (  ) are the thermal capacity and conductivity, respectively. The term Q d ( t ) in (1) represents dissipation energy and is defined as (Katunin, 2010): � � � � � � � � �� � �� � � � �� � ��⁄� , (2) where  denotes angular frequency of loading;  ,  are the stress and strain amplitudes, respectively. The generation of energy Q d ( t ) is usually observed as the self-heating phenomenon, and its occurrence depends on different factors