Influence of Temperature on Mechanical Performance of Glass Fiber/epoxy Composite with Continuous and Discontinuous Secondary Carbon Fiber Reinforcement

Abstract

In the current world of structural materials, fiber reinforced polymer (FRP) composites have become a revolutionary material due to their excellent mechanical performance, low density and corrosion resistance. However, these laminated composites usually experience catastrophic failure in nature and poor out-of-plane properties, which raises the possibility that scientifically structured fiber hybridization and modified matrix with short fibers could be used for improved mechanical performance. Carbon fibers have always been of interest to material scientists due to their lack of structural defects and unique mechanical properties. This directs the possibility of incorporating them into widely used glass/epoxy composites through fiber hybridization technic in the form of continuous fibers or modifying the matrix of glass/epoxy composite with waste short carbon fibers to achieve superior mechanical stability. In order to accept these materials for use in different high-end applications, their performance in various in-service environments must be well assured. The present study starts with assessing the three-point flexural and tensile performance of inter-ply glass/carbon/epoxy hybrid composite by altering the hybrid ratio and stacking position. These materials were tested at various elevated temperatures (30 °C, 50 °C, 70 °C, and 110 °C) at 1 mm/min loading speed. In addition, tensile behavior was analyzed at 30 °C and 110 °C temperatures with 0.1, 1, 10 and 100 mm/min loading speeds. The test results of hybrid composites were compared with glass/epoxy (GE) and carbon/epoxy (CE) composites. The stacking position of carbon fibers in GE composite plays a vital role while deformed under a flexural load. Incorporating two stiffer carbon fibers in GE composite, i.e. C2G3 and C1G3C1, performed remarkable increment in flexural strength. All these composites have shown relatively ductile deformation at 110 °C. In tensile test, placing a carbon fiber ply in GE composite (G2C1G2) imparts pseudo-ductility as well as positive hybrid effect in the composite. On the other hand, replacing a glass fiber ply with carbon fiber at both ends (C1G3C1) imparted improved strain to failure and positive hybrid effect in the composites. Further, emphasis was given to the mode-I and II interlaminar fracture toughness (ILFT) of hybrid composites with five layers of carbon fiber (stacking sequence: C5G5, and (G1C1)5) in GE composites at different temperatures (30, 50, and 70 °C) and compared with neat GE and CE composites. The test results exhibited that the stacking position of glass/carbon fibers and test temperature substantially influence the fracture toughness of hybrid composites. At 30 °C, mode I ILFT value of alternative glass and carbon fiber stacking sequence (G1C1)5 of hybrid composite showed 29.38% improvement than CE composite. Further, mode II ILFT of C5G5 and CE composites was 22.29% and 42.13% higher than that of GE composite, respectively. Increasing the test temperature of all composites improved their GIC and GIIC values. The next step towards a comprehensive understanding of the in-service temperature impact on composites is to study the durability of these composites at cryogenic temperature (CT). For doing the same two types of characterizations have been performed, (i) in-situ cryogenic flexural testing of all the composites (i.e., while the sample, as well as the testing fixture were completely submerged in liquid nitrogen bath during the entire testing period), and (ii) ex-situ ageing of the composite samples for various time periods in liquid nitrogen bath followed by flexural testing at ambient temperature. Among all the hybrid composites considered in this study, C2G3 presented the highest enhancement in flexural performance at all testing conditions, which were ∼38.16% at RT, ∼ 29.03% at CT. At CT, all the combinations of hybrid composites showed higher flexural performance than the neat G5 composite. C2G3 achieved the maximum flexural strength, which was 27.82% higher than G5 composite, after 8 h of ageing. The next objective is aimed to utilize the waste carbon fibers, generated from our regular fabrication process in the laboratory, which was cut in the length range of 2 – 5 mm and termed as short carbon fiber (SCF). These SCFs were added to GE composite in varying contents (0.1, 0.3, and 0.5 wt.%) as secondary reinforcement. The flexural and tensile behavior of the SCF modified GE composites were assessed at ambient (30 °C) and elevated (50 °C, 70 °C, and 110 °C) temperatures. The most significant improvement in mechanical performance was achieved by adding only 0.1 wt.% of SCF into GE composite across most of the testing temperatures. At elevated temperatures, all the SCF modified GE composites showed superior mechanical performance over the neat GE composite. The effects of waste SCF reinforcement in GE composite on the overall damage tolerance of the structural composite were examined by both mode I and mode II ILFT in terms of both crack development and propagation. Effect of various SCF content (0.1wt.%, 0.3wt.%, and 0.5wt.%) in GE composites are evaluated experimentally at room as well as at elevated temperatures (30 °C, 50 °C, and 70 °C). Based on the experimental results, GE composite with 0.1 wt.% of SCF at ambient temperature revealed 13.49% and 20.45% increment in mode I and mode II ILFT, respectively, than GE. A positive reinforcement effect is noticed for GIC and GIIC values up to 50 °C. However, due to unfavorable residual interfacial stresses resulting in interfacial debonding and epoxy softening, a negative reinforcement effect is noticed at 70 °C. Flexural tests were performed to assess the integrity and durability of composites at in-situ CT and after ex-situ cryo-ageing in liquid nitrogen for various time intervals (0.25 hrs, 0.5 hrs, 1 hr, 2 hrs, 4 hrs, 8 hrs, and 16 hrs). The composite with 0.1 wt.% of SCFs showed the highest enhancement in flexural performance in all testing conditions, i.e., ∼16% at RT, ∼ 12% at CT, and between 13% - 39% after cryo-ageing over neat GE. Composites with SCFs retained their strength at CT and after cryo-ageing, suggesting that the waste fibers could be economically utilized and preferred over other expensive nanofillers as secondary reinforcements in GE composites for cryogenic applications.