Acoustic Emission at the Crack Tip during Cooling of a Moisture-Saturated Composite
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AL CHEMISTRY
Acoustic Emission at the Crack Tip during Cooling of a Moisture-Saturated Composite O. V. Startseva,*, V. V. Polyakovb, D. S. Salitab, and Corresponding Member of the RAS M. P. Lebedeva,c Received May 7, 2020; revised June 24, 2020; accepted August 4, 2020
Abstract—The effect of low temperature on the properties of moisture-saturated glass-fiber-reinforced plastic KAST-B was investigated by studying acoustic emission. During cooling of dried and moisture-saturated specimens with surface defects, the rms acoustic emission and the acoustic emission count rate remain within the background noise. It was determined for the first time that, if water is localized in a crack in the glassfiber-reinforced plastic, then its cooling leads to an intense acoustic emission. It was assumed that this emission is caused by local damages of the polymer matrix because of the increase in the internal stresses as water turns to ice. Keywords: composite, moisture saturation, acoustic emission, internal stresses, crack, micro damages DOI: 10.1134/S0012501620370010
Modern polymer composite materials (PCMs) based on epoxy matrices and glass-, organic-, and carbon-fiber reinforcements are used to produce various aircraft components operating under the most severe climatic conditions, including extremely cold climate areas [1].
mers exposed to 10 years in the Alaskan climate was impaired to a greater extent than that after exposure to the humid subtropical climate. In a similar example [4], the compressive and structural strengths of eight brands of PCMs after 1–5 years of exposure to open climatic conditions of Yakutsk decreased more significantly (by 10–15% and more) than those after exposure to the conditions of warmer area (Batumi, Sochi, Gelendzhik, and Moscow).
The development of new PCMs with improved mechanical properties includes solving the problem of increasing their lifetime under severe operating conditions. However, for a number of carbon-, glass-, and other fiber reinforced polymers, it is still unclear why their strength undergoes a suddenly fast decrease under cold weather conditions, which compares to or even exceeds the decrease in the mechanical performance under tropical conditions.
It is commonly believed [4] that the main cause of the aging of PCMs in a cold climate is the damaging effect of internal stresses induced by the nonuniform thermal expansion of reinforcing fibers and polymer matrices with decreasing temperature. These internal stresses during seasonal and daily thermal cycles cause the formation of microcracks, their coalescence, and the generation of macro damages in the bulk of the binder or at the interface with the fibers. For this reason, the strength of PCMs decreases [5]. However, in some cases, the mechanical performance of PCMs does not deteriorate or vary insignificantly even after thermal cycling at high thermal cycle amplitudes. For example, unidirectional carbon-, glass-, and basaltfiber-reinforced polymers based on the Tyfo® S Epoxy two-component epoxy matrix (United States) wer
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