On the mechanism of increasing the mechanical characteristics of cured polymer composite materials under the action of a microwave electromagnetic field

Background and Objectives: The aim of the research is to identify the general mechanism of the processes occurring in the microstructure of polymer composite materials when exposed to an ultrahigh-frequency electromagnetic field in the cured state, contributing to an increase in strength characteristics, based on the analysis of the structural features of the matrix and the interfacial layer of cured carbon fiber and the physical foundations of dielectric heating. Materials and Methods: Finite element modeling of the effect of changes in the number of contact interaction surfaces on the elastic-strength properties of an elementary two-dimensional and three-dimensional cell of a polymer composite material and calculations in the Comsol software environment are performed. In the experiments, carbon fiber samples were used in the form of plane-parallel plates with dimensions of 70x(10.0–10.2)x(4.9–5.1) mm. Processing in an ultrahigh-frequency electromagnetic field was carried out on experimental equipment, with a beam-type camera at a frequency of 2450 MHz and an energy flux density of (10–12)x104 , (17–18)x104 and (45–50)x104 MW/cm2 for 30, 60 and 120 s. The characteristics of the microstructure were studied using the MIRA II LMU Tescan scanning electron microscope and the SMM-2000 atomic force microscope. The ultimate strength, modulus of elasticity and elongation under tension were determined on a Zwick/Roell Z100 breaking machine, the ultimate strength at three-point bending and interlayer shear was determined on an upgraded computer laboratory complex with LabWiev software. The kinetics of cracking was evaluated under static loading by acoustic emission method using a computer complex consisting of acoustic emission sensors ZET-601, ZET-7140E, ZET-7191, ZET-7174. Phase changes were evaluated by the DSC method using a differential scanning calorimeter with a high-pressure cell DSC Q20P V24.11. The heating temperature of the samples was determined by thermograms using a FLIR E40 thermal imager and a Testo 830-T1 digital pyrometer. Results: It has been established that in the matrix and the interfacial layer cured polymer composite materials, when interacting with an ultrahigh–frequency electromagnetic field, the energy parameters of which do not reach the values that cause the destruction of the matrix material, changes occur that contribute to an increase in the number of areas of matrix – fiber contact interaction, as a result of which the connectivity of reinforcing components and the uniformity of the redistribution of external loads increases, which significantly increases the mechanical characteristics of the material in the composition of the final product. It is shown that the energy parameters and the exposure time have a significant, and at certain ratios – an extreme effect on the mechanical characteristics of polymer composite materials. It has been established that in rational carbon fiber processing modes, an increase in the limiting stresses of three-point bending is provided by an average of 25–42 percent, interlayer shear – by 14–16 percent, stretching – by 7–8 percent. Conclusion: It has been shown that the general mechanism of structural changes, regardless of the type of filler, contributing to the hardening of cured polymer composite materials, consists in the combined action of thermal and wave processes, as a result of which the matrix under the action of dielectric heating to temperatures 40–70$^\circ$C temporarily passes into a highly elastic state, contributing under the action of wave oscillatory processes stimulated by an electromagnetic field, conformational rotations of macromolecule links and relaxation of residual stresses, an increase in the degree of crystallinity and the formation of an increased number of ordered supramolecular formations during the re-curing process.