PSI - Issue 5

Andrzej Katunin et al. / Procedia Structural Integrity 5 (2017) 93–98 Andrzej Katunin et al. / Structural Integrity Procedia 00 (2017) 000 – 000

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determine a difference between the measured temperature and the approximation model (this difference indicated a beginning of macrocrack development, and thus a critical value of a self-heating temperature) and Raman spectroscopy analysis (Katunin et al. (2012)) that allowed for evaluation of residual cross-linking processes during heat generation in polymeric composite structures. Recent studies of the author’s research group (Katunin et al. (2017)) have been focused on a multiphysical approach to evaluation of the criticality of self-heating. In this study, tested specimens were cyclically loaded until reaching a specific self-heating temperature value on their surfaces in a range of 40÷100°C with a step of 5°C, and further, parameters obtained directly from fatigue tests (acceleration of vibrations, loading force, surface temperature, acoustic emission) as well as parameters obtained from microscopic observations and tensile tests (residual elastic modulus, ultimate tensile strength, maximal force at failure) were used for evaluation of a critical self-heating temperature value. The obtained results showed that the self-heating effect initiates cracking of a composite matrix at 65-70°C, which is a much lower temperature value in comparison to the glass-transition temperature of the same composite (Katunin and Gnatowski (2012)), which is in a range of 124-157°C, depending on loading parameters. The above-described studies, however, were performed in a non-stationary self-heating regime, i.e. the self-heating dominated the fatigue process. Therefore, the presented results seem to be underdrawn and do not provide a full picture of the criticality of the self-heating phenomenon. Following this, it is essential to perform a series of experimental studies in a regime of the stationary self-heating, i.e. stabilized at certain temperature values. Performing such studies allows for determination of both influence of particular self-heating temperature values on intensity of the structural degradation as well as determination of the critical self-heating temperature by comparison of a number of loading cycles to failure between particular cases. Such an analysis allows for full qualitative and quantitative description of influence of the self-heating effect on fatigue of polymeric composites and developing a relation between self-heating temperature values and the structural lifetime of polymeric composites. The specimens used for fatigue tests were manufactured from a 14-layered unidirectional glass/epoxy composite and supplied by Izo-Erg S.A. (Gliwice, Poland). A description of manufacturing process as well as basic mechanical and dynamic properties of these specimens can be found in Katunin and Gnatowski (2012). The composite sheet of a thickness of 2.5 mm was cut to specific dimensions of specimens: width of 10 mm and length of 100 mm. An effective length, i.e. the length between specimen holders which participated in loading, of each specimen equaled 40 mm. The specimens were loaded with a constant frequency of 30 Hz. The tests were performed on a laboratory test rig, which is presented on a scheme (Fig.2a) and a photography (Fig.2b). 2. Specimens and testing procedure

Fig. 2. Experimental test rig for performing fatigue tests: (a) scheme, (b) photography.

A tested specimen 5 was clamped in a specimen holder 4 and excited by the TIRA ® TV-51120 electrodynamic shaker 1 through a stinger 3 with a specimen holder 6 connected to a force sensor PCB Piezotronics ® 208C03 7 at the