Mechanical behavior of polymer composites at cryogenic temperatures

Abstract

High specific strength, stiffness, excellent environmental fatigue resistance and low weight remain the winning alliance that impels fibrous composite materials into new arenas, but other properties are also equally important. Fibrous reinforced plastics (FRPs) offer good vibrational damping and a low coefficient of thermal expansion, characteristics that can be engineered for specialized applications. Commercial composites are used in large markets such as automotive components, boats, consumer goods, and corrosion-resistant industrial parts. Advanced composites, initially developed for military aerospace applications, offer performance superior to that of conventional structural metals and now find applications in satellites, aircraft, and sporting goods and in the energy sector in oil and gas exploration and wind turbine constructions. Cryogenic applications of polymeric fiber composites are mainly in superconductivity, space technology, and handling of liquefied gases. By contrast, because of the heterogeneous nature and anisotropic behavior of FRPs, a structural designer faces challenges in predicting the integrity and durability of FRP laminates during service periods. Polymer composites soften, creep, and distort when heated to high temperatures (>100°C), accompanied by collapse of free volume as the molecular adjustments take place. This can result in buckling and failure of load-bearing composites structures. Severe environmental exposure affects the physical and mechanical properties of polymeric composite materials, resulting in an undesirable degradation and damage. Cryogenic fuel tanks are the most common structural application of FRP at low temperatures. Expose to cryogenic temperatures can cause microcracks as well as delamination in the composites due to thermal residual stresses. These microcracks provide a pathway for the ingress of moisture or corrosive chemicals and are a possible pathway for loss of cryogenic fluids in the tanks. Matrix resins at low temperatures are brittle and do not allow relaxation of residual stresses or stress concentration to take place. At low temperatures, polymers are well below their glass transition temperature and show little viscoelastic behavior. Molecular motion of segments or side groups is still possible, but the degrees of freedom decrease with decreasing temperature. This motion influences the damping behavior of the polymers under cyclic mechanical load. If the temperature-dependent relaxation time of molecular motion is equal to the time of external deformation, maximum power dissipation occurs. Simultaneously, a change in the shear modulus is observed. The goal of this chapter is to extensively study the in-plane mechanical properties of FRP composites at cryogenic temperatures. The composites considered include carbon, glass, and Kevlar fiber-reinforced polymers with different resin matrices.