PSI - Issue 30

O.V. Startsev et al. / Procedia Structural Integrity 30 (2020) 162–166 M.P.Lebedev et al / Structural Integrity Procedia 00 (2020) 000–000

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At decreased temperatures, the water concentrated in micro volumes did not crystallize due to a lack of free volume; it turned into a glass-like state, causing an additional increase of levels of interior stresses estimated by the ratio Sokova (2010)

QdT V T dP  

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The stress increased by 1.13MPa with the 1° С temperature decrease. The air temperature decrease to -60 ° С in Yakutsk might cause the increase of interior stresses in PCM with capillary condensed moisture to 68 PMa, which exceeded the interlayer shear strength for some PCM and was an additional reason for micro-crack formation and a decrease of relative retention rate k R . The works of Lopez-Anido et al. (2004), Karbhari (2002), Heshmati et al. (2017) and Jedidi et al. (2015) studied k R of moisture-containing PCM, following cycling in the cooling-heating mode. The results were ambiguous. Thus, researchers Lopez-Anido et al. (2004) identified effects of 20 eight-hour cycles at -18 ° С and effects of a sixteen – hour exposure in hot water on properties of the epoxy adhesive connecting plates of water-saturated FG. The shear strength of the control samples was 16.2 MPa, and after cycling it decreased to 9.2 MPa (by 57%). The effect was explained by an uneven distribution of glue and resulting voids. During freezing, water could have generated crevices due to expansion and these crevices weakened the adhesive strength. Unidirectional 3-layer CFRP based on the VE8117 vinyl ester and a separately cured binder at room temperature and the humidity of 50 % was studied Karbhari (2002). Its samples were aged for 25 days in water at room temperature. The water absorption of the polymer samples was 1.3 %. Then, the dry and water-filled samples were kept at 18 ° С or were subject to thermal cycling with a daily cycle (12 hours at -18 ° С and 12 hours at 20 ° С ). After a 450-day exposure at low temperature, σ t of the dry samples increased by 9 %, and that of the water-filled samples decreased by 14 %. After 450 cycles, σ t decreased by 22%. The effect of low temperatures in CFRP was explained by violation of the adhesive bond of the fiber with the polymer and formation of voids. On the other hand, effects of 125 and 250 cycles (from -20 to +20 ° С ) on the strength of the adhesive bond of CFRP with steel (adhesive-epoxy) were studied Heshmati et al. (2017). The strength of dry samples, as well as the strength of samples aged in distilled and salt water at 45 ° С for up to 90 days, were measured (the water absorption was 1.7-1.4%). Samples soaked in water showed the effects of a decrease in σ t and E t , characteristic of plasticization. Additional “cooling-heating” cycles did not show a significant change in the strength. The thermal cycles from -55 to +130 ° С , characteristic of the flight regimes of a supersonic aircraft, were performed for the IM7/977-2 CFRP for aviation purposes by Jedidi et al. (2006). After 300 cycles, properties of dry and moisturized samples with a 0.8 % water content were studied. No noticeable differences in mechanical properties were identified. Thus, if PCM water saturation does not cause an increase in defectiveness of the polymer matrix and its interface with the fiber, and water is not located as a separate phase in microcapillaries, but it is a molecularly distributed polar plasticizer, then additional cycling in the “cooling-heating” mode does not lead to a significant deterioration of PCM mechanical properties. 3. Micro-cracks at the tip of cracks of moisture-saturated FG according to emission data of acoustic This conclusion about increased defectiveness of PCM containing moisture in macro-defects under effects of low climatic temperatures was confirmed by the following model experiment. The AE method was applied to study effects of low temperatures on properties of the dry and water-saturated fiberglass KAST-V. Dimensions of KAST V samples cut from 2.5 mm thick plates were 130×30 mm. Some of the samples were split from the edge side with formation of a crack located between the composite layers. The samples were dried in a thermostat and then were saturated with water. Then, the samples were cooled to the dry ice temperature, with registration of acoustic emissions applying the method, described by Lependin, Polyakov and Salita (2015). The root mean square voltage of the acoustic emission U was used as the informative parameter. Registration of acoustic emission signals was