• creep
• fiber reinforced polymer composites
• pultruded beams
• stability

Fiber Reinforced Polymer (FRP) composites are widely used in the aerospace, naval, and automotive industries. More recently, such materials have also found several applications in civil engineering, e.g. for bridges, small houses, and gridshells. FRPs have considerable advantages mainly related to their high strength and low weight, but also some weaknesses, which limit their application. One weakness is their remarkable creep behavior, mainly due to the viscosity of the polymer matrix and the defects of the reinforcing fibers, which gradually break with time.

A significant amount of experimental researches have been focused on the characterization of the creep behavior of composite materials. Nevertheless, most of the creep models found in the building codes are based on the extrapolation of data from short-term experiments. There are still many uncertainties on how to model the phenomenon as the time scale is of the order of some decades. The prediction of the long-term behavior of viscous materials can be based on short-term experiments by using the Time-Temperature-Stress Superposition Principle (TTSSP). This is an extension of the Time-Temperature Superposition Principle (TTSP). The TTSSP is based on the hypothesis that every creep curve, adapted to Findley’s law, has the same shape at different temperature and stress states. However its validity for Glass Fiber Reinforced Polymer (GFRP) is not evident.

The major objective of the present work is to explore the validity of the TTSSP for Glass Fiber Reinforced Polymer (GFRP) pultruded beams having an epoxy matrix. To this end, 4-point bending tests have been performed in the short term at different temperature and stress levels. Then, according to Findley's law, the TTSSP has been assumed to predict the long-term behavior in secondary (steady-state) creep.

Lastly, a micromechanical model has been developed to explore the possibility of tertiary creep and hence the applicability of the TTSSP.