Influence of the recovery duration on the cyclic creep response of carbon fibre reinforced polymer composites
Due to the viscoelastic nature of polymeric matrix, polymer composites exhibit time-dependent degradation in modulus and strength. The former is exhibited as time-dependent increase in strain under constant loading, known as creep. Upon unloading, this viscoelastic strain will recover completely if enough time is allowed. During cyclic loading the time for recovery may not be enough resulting in partial recovery. The effect of this partial recovery on subsequent creep and recovery has hardly been studied in the past and hence is the focus of this thesis. Numerous parameters are known to influence the creep response of viscoelastic materials, namely: the magnitude and rate of application of the load, temperature, creep duration. However, the influence of recovery duration, particularly in the case of cyclic creep loading, is more limited. Hence, the objective of this thesis was to investigate the effect that the recovery duration had on the cyclic creep response of a unidirectional carbon fibre reinforced polymer composite. Utilizing creep durations of 20s and 40s, and recovery durations varying from 1s to 80s, manufactured composite samples were subjected to cyclic creep loading conditions at two stress levels (8 / 0.1 MPa, 12.5 / 0.5 MPa) and four temperatures (room temperature, 80 oC, 120 oC, 180 oC). The maximum recorded strain during creep and recovery durations increased with the number of cycles for a given set of loading conditions and their magnitude decreased with increase in recovery duration for a given cycle. The unrecovered strain and the unrecovered stored energy at the end of each loading cycle increased with the number of cycles but decreased with recovery duration for a given cycle. The test temperature did not alter this effect of creep recovery duration; however, the magnitude of strains (creep, recovered, and unrecovered) increased with increase in test temperature. Similar effect was observed with applied peak load. Based on these results, it can be concluded that the recovery duration has significant effect on the creep and recovery response of carbon fibre reinforced epoxy composites during cycling loading.
CFRP, Creep, viscoelastic, Carbon Fibre Reinforced Polymer, Carbon Fiber Reinforced Polymer, Cyclic Creep