Manufacture and Characterization of Fiber Reinforced Epoxy for Application in Cowling Panels of Recreational Aircraft

Abstract

In this study, glass and Kevlar® fibers reinforced epoxy composites were manufactured and characterized using different techniques. The effect of thermal exposure on the flexural properties of the composites was investigated to ascertain its suitability for the intended application in cowling panels of light engine aircraft. Thermogravimetric analysis (TGA) was carried out on both reinforced and unreinforced epoxy resin to evaluate their thermal stability at elevated temperatures. Dynamic mechanical thermal analysis was carried out to evaluate the effects of thermal exposure, applied strain and frequency on the dynamic mechanical response of the composites. The effects of the applied resin hardener and thermal exposure on the flexural strength, flexural modulus and dynamic impact response of the composites were also investigated. The flexural properties were determined using 3-point bending test, while the impact test was carried out using Split Hopkinson Pressure Bar (SHPB). TGA analysis of the reinforced and unreinforced epoxy showed no significant weight loss until the test samples were heated above 250°C in an inert atmosphere. Dynamic Mechanical Thermal Analysis (DMTA) on the composites indicated the glass transition temperature to lie between 80 and 100°C.

The results of the flexural and impact tests showed that the mechanical integrity of both glass and Kevlar® fiber reinforced epoxy composites remained unimpaired by radiative or convective heat exposure for up to 3 h until the exposure temperature exceeded 200°C. This is much higher than the service temperature of cowling panels of light engine recreational aircrafts. When the manufactured fiber reinforced epoxy composites were exposed to temperature above 200°C matrix degradation occurred, which became very significant when the exposure temperature was higher than 250°C. Extensive delamination and matrix cracking occurred when the composites were exposed to the temperature range 250°C - 300°C for 1 h. Fiber-matrix debonding was not observed in the composite except after failure under impact loading. This is evidence of the fact that the epoxy matrix was adequately wetted by both the glass and Kevlar® fibers resulting in the strong fiber/matrix interfacial bonding. While the Kevlar® reinforced epoxy displayed a better damage tolerance under flexural and impact loading, glass fiber reinforced epoxy showed higher strength but lower damage tolerance. Glass fiber reinforced epoxy also showed more resistance to damage under exposure to thermal flux than Kevlar® reinforced epoxy. Under impact loading, the Kevlar® reinforced composite failed by delamination with no fiber rupture, whereas the glass fiber reinforced epoxy failed by matrix cracking, debonding, fiber rupture and fiber pullout.

The results from this research have established the effect of radiative and convective thermal exposure on the mechanical behavior of the fabricated Kevlar® fiber and glass reinforced epoxy composites. The maximum temperature reached on the inner surface of the cowling panels of a typical light engine recreational aircraft due to heat radiations from the engine block has been estimated to be about 65°C. This is lower than the glass transition temperature of the epoxy matrix as obtained from DMTA. The low temperature rise is due to inflow cooling air into the cowling chamber in flight. The results of the current investigations suggest the suitability of composite materials for the intended application. The intensity of thermal exposure, to which the materials will be exposed in such application, may not cause any significant damage to the mechanical integrity of the composite. However, since the difference between the possible exposure temperature and the glass transition temperature is only a little over 20°C, a layer of thermal insulator on the inner surface of the cowling made of fiber reinforced epoxy will be desirable to further sustain the mechanical integrity of the composites when selected for use as choice materials for cowling panels of light engine aircraft.