Mechanical Properties of Cryogenically Cycled Thermoplastic Composites under Long-Term Moisture Exposure

Fiber-reinforced thermoplastic composites (FRTC) are widely utilized in various engineering applications, including space, defence, transportation, and energy because of their high strength, durability, and lightweight properties. However, one of the primary challenges in using thermoplastic composites in aerospace is their mechanical and thermal performances at cryogenic conditions, less than -150 C. To address these moisture concerns, this study investigated the effects of cryogenic aging on the mechanical and surface properties of three carbon fiber-reinforced composite specimens with different resin systems from TORAY Composite Materials America, Inc.: A (T1100G/TC1225), B (T1100G/3960c, 70 gsm areal weight), and C (T1100/3960c, 192 gsm areal weight). The specimens were subjected to cryogenic aging by immersion in liquid nitrogen (-196 °C) for durations of 120 and 240 minutes. In addition to this, moisture ingression tests were performed on samples submerged in water for 13 weeks and subject to characterization studies for understanding how cryogenic conditioning would impact dry and wetted samples mechanically. Gravimetric analysis, water contact angle measurements, Fourier-transform infrared (FTIR) spectroscopy, and three-point bending tests were conducted to assess the impact of cryogenic aging on the composites. The results revealed that the PAEK-based specimen A exhibited superior cryogenic resistance, with lower moisture absorption, higher hydrophobicity, and better retention of flexural properties compared to the epoxy-based specimens B and C. Cryogenic aging led to micro-cracking of the resin matrix and degradation of the matrix-fiber interface, resulting in a decrease in flexural strength and modulus, as well as an increase in brittleness, particularly in the epoxy-based composites. The findings highlight the importance of matrix system selection for composite materials intended for cryogenic applications and provide valuable insights into the mechanisms of cryogenic aging in carbon fiber-reinforced composites.