Mechanical performance of stiffened CFRP panel cured via magnetic particle-induced in-situ heating

Carbon fiber reinforced polymer (CFRP) composites have been widely used in the manufacture of aerospace structures due to their excellent mechanical properties and material characteristics. However, as a trend of large-scale and complexity for CFRP structures, the conventional oven or autoclave molding process gradually exposes problems such as limited part size, long molding cycle, low energy utilization, and uneven curing. To solve the above problems, a new method of curing CFRP composite structures based on electromagnetic induction in-situ heating with magnetic particles of Mn-Zn ferrite as susceptor was proposed, and in-depth investigation of the feasibility of molding complex stacking sequenced composite structures using this method was conducted.

Surface modification and physical mixing methods were used to disperse 13 wt% of spherical Mn-Zn ferrite with a diameter of about 5-10 μm in epoxy resin, and carbon fiber composites reinforced panels with two cross-sectional shapes of T-type and I-type with complex lay-up angle design were fabricated by hand-lay-up process. The heating characteristics, mechanical properties, damage characteristics, and porosity of the two types of reinforced panels under the electromagnetic induction heating process were investigated by experimental methods, and compared with the oven curing method. The electromagnetic induction heating method mainly consists of a high-frequency induction heater connected to a circular copper coil and a vacuum pump. The specimen and the mold were wrapped in a vacuum bag and placed in the coil. The high-frequency alternating current generated by the high-frequency induction heating machine produces an alternating magnetic field in the coil, which in situ heats up the magnetic particles in the specimen and realizes in situ curing of the specimen. To avoid induced eddy currents in the specimen, which interferes the uniform and smooth heating of the specimen, the specimen is placed horizontally in the coil so that the magnetic induction lines are parallel to the specimen surface. At the same time, the specimen vacuum bag will be led out of the conduit connected to the vacuum pump to realize the whole heating process of uninterrupted vacuum, to achieve the role of exhaust and compaction.

The electromagnetic induction method allows in-situ, uniform, temperature-controlled heating and curing for both two CFRP panels, independent of the lay-up angle. The bending stiffness of the T-shaped CFRP plate obtained by this new method was 558333 GPa*mm, which was 5.2% higher than that obtained by the oven molding under the same curing time. The maximum load bearing capacity was 5268.48N, which was 11.2% higher than that of the oven molding. The flexural load of the I-type plate obtained by induction heating curing was 4736N, which was 3.3% higher than that obtained by the oven molding under the same curing time. The porosity of the T-type and I-type CFRP panels obtained by induction heating curing was 5.71% and 6.69%, respectively, which was 0.4% and 0.3% higher than that of the control panels obtained by the oven heating method.Conclusions: The proposed electromagnetic induction in-situ curing method can realize curing and molding of CFRP stiffened panels to which the layup is complex. Although the method leads to a certain increase in porosity, but compared to the traditional oven curing method, the mechanical properties of the composite material fabricated under this new method is significantly increased. The electromagnetic heating curing method using the Mn-Zn ferrite magnetic particles as well as inductive heating equipment is cost-effective, portable, and convenient for the rapid setup and molding arrangement. Current research shows that the maximum heating temperature of the magnetic particles can reach 120~150℃, which is suitable for molding most thermosetting resins. The two key technologies that need to be broken through at a later stage are: 1) high dispersion of magnetic particles and interfacial bonding technology; 2) pore suppression technology. If the above two problems can be solved, based on the in-situ heating of magnetic particles, the method can well solve the problem of temperature gradient and curing residual stress caused by uneven heating of large components, which is very suitable for the manufacturing and molding of large or thick CFRP structures.