The effectiveness of the effect of microwave radiation and convection heating on the relaxation of internal stresses in cured polymer composite materials

Background and Objectives: The relaxation of internal stresses in pressed carbon and fiberglass plastics under the action of bending loads after modification by heating in a thermal chamber and exposure to a microwave electromagnetic field has been studied. It is shown that for pressed carbon and fiberglass plastics under the influence of bending loads, relaxation processes of internal stresses are characteristic, the intensity of which is determined by the initial state of the material. Materials and Methods: In the experiment, three groups of carbon and fiberglass samples produced by Eurocomplant LLC in Kaluga were used in the form of plane-parallel plates with dimensions of 75x10x5 mm, cut from a panel with dimensions of 500x500x5 mm in the state of delivery. Experiments with the first group of samples were carried out using a special microwave technological installation assembled on the basis of the Zhuk-2–02 radiator (NPP Agroecotech LLC, Obninsk, Kaluga region, Russia) with a horn-type radiator. The time of microwave exposure was recorded upon reaching the set surface temperature determined by the Flir E-40 thermal imager (FLIR, USA). The second group of samples was heated in an artificial light-weather chamber Solarbox 522 model 1500e RH (COFOMEGRA SRL, Italy). The study of internal stress relaxation was carried out according to the three-point bending scheme on a laboratory computer installation with LabVIEW software (IP Mayorov, Orel, Russia). Results: Microwave exposure to carbon and fiberglass contributes to stress relaxation by (5.1–7.2)$\%$ and (6.5–9.8)$\%$, respectively, depending on the magnitude of the external load. After heating in the thermal chamber, stress relaxation was noted by(4.4–6.8)$\%$ and (5.2–9.0)$\%$. For control samples, the degree of relaxation is(4.3–6.5)$\%$ and (4.9–8.55)$\%$, the process stops almost 3 times earlier than for experimental samples. On average, the degree of stress relaxation in samples after exposure to a microwave electromagnetic field is 18.5$\%$ and 12.8$\%$ higher for carbon fiber and fiberglass, respectively, compared with heating in a thermal chamber. Conclusion: Тhis indicates a greater effectiveness of the microwave method of heat treatment in order to stabilize the properties of PCM.