PSI - Issue 30

O.V. Startsev et al. / Procedia Structural Integrity 30 (2020) 162–166

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M.P.Lebedev et al / Structural Integrity Procedia 00 (2020) 000–000

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1. Introduction Humid air and precipitation are among the major climatic factors affecting PCM. This finding was confirmed in works by Filistovich et al. (2003), Startsev et al. (2004), Startseva et al. (2014). Panin et al. (2014) revealed that PCM adsorbed 0.5-2.5% moisture under natural climatic conditions. Slavin et al. (2018) also revealed that with these relatively low levels of moisture saturation, such physical and chemical transformations as plasticization, structural relaxation, post-cure, destruction, and hydrolysis were activated in PCM polymer matrices. Kablov and Startsev (2017) established that atmospheric moisture, dew, and rain had the greatest impact on the PCM surface layers. Consequently, mechanical properties of the surface layers deteriorated more significantly over time due to climatic effects according to Startsev (2016).

Nomenclature σ t

tensile strength tensile modulus

E t k R

relative retention rate

AE acoustic emission CFRP carbon fiber reinforced polymers DSC differential scanning calorimetry FG fiberglass PC М polymeric composition materials RMS root mean square Q latent heat of ice melting T

absolute temperature of water freezing

V P U

specific ice volume

ice pressure

voltage of acoustic emission It should be noted that the above-mentioned major physical and chemical transformations were caused by molecularly distributed moisture. According to the data of DSC obtained in works of Tsotsis (1989) and Abdelmola and Carlsson (2019), the Raman spectroscopy obtained in works of Bansil et al. (1982) and D'Arrigo et al. (1981), and X-ray diffraction obtained in work et al. Morishige (2010) free water, bound freezing water, and bound non freezing water, which share prevailed, as a rule, were identified in PCM polymer matrices. It was the molecularly distributed moisture that caused plasticization and chemical reactions, which activity intensified with increased temperatures. Therefore, in regions with warm and hot humid climates, PCM ageing, caused by exposure to water, worsened mechanical properties of these materials largely than those after exposure to effects of cold climates. The second part of the work is devoted to substantiation of additional contribution of moisture, absorbed by PCM, into an increase of internal stress levels and micro-cracking in PCM polymer matrices. 2. Role of moisture in PCM under effects of cold climate Impacts of cold climates on PCM containing moisture in capillaries, pores, and microvoids are increased. For example, the proportion of freezing water increased with increasing porosity in porous epoxy polymer studied in Abdelmola and Carlsson (2019) with application of the DSC method. At low porosity water was in a bound state. With a temperature decrease, freezing of free and bound water was expressed by the exothermic peak of DSC with the minimal heat flow at -18 ° С , while with heating, the endothermic peak at 0.6-1.0 ° С linked with ice melting was observed. The peak of heat flux absorption caused by bound ice-free water was observed at -38 ° С . The result confirmed that a temperature decrease to -38 ° С and more was required to transform water into ice in the PCM volume, in accordance with the amount of contained moisture and the pore size.