Abstract
Considering that structural composites are typically composed of off-axis plies, i.e. quasi-isotropic stacking sequence, their strength and stiffness are time-dependent due to the viscoelastic character of polymer matrices. This work consists of determining creep, recovery, and stress relaxation of carbon fiber-reinforced polymer (CFRP) composites. Long-term experimental analyses are conducted via dynamic mechanical analysis under several temperatures and stress levels. From the experimental observations, the changes in the relaxation mechanisms are predicted using Fancey's latch model. The rate of relaxation at different temperatures is also covered. Since at certain strain levels the viscoelastic behavior cannot be properly determined, the stress-relaxation is determined using the time-temperature superposition (TTS) principle, considering nine temperatures at three strain levels in order to cover the three main regions of the composite system (glassy, glass transition and rubbery regions). The models and experiments herein presented can be extended to any polymeric system.
Highlights
Lightweight components along with suitable mechanical performance make fiber-reinforced polymeric composites the best candidate for marine, aeronautical and aerospace structures [1,2,3,4]
This work consists of determining creep, recovery, and stress relaxation of carbon fiber-reinforced polymer (CFRP) composites
According to Sreekala et al [15], the knowledge of stress relaxation behavior under different strain levels allows predicting the dimensional stability of load-bearing structures and the retention of force for bolts fastened to composites
Summary
Lightweight components along with suitable mechanical performance make fiber-reinforced polymeric composites the best candidate for marine, aeronautical and aerospace structures [1,2,3,4]. Structural composite materials need to attend short- and long-term design requirements. Among short-term requirements, some mechanical properties of interest are flexural [5], tensile [6,7], compression [8], interlaminar shear strength [5,9] and impact [9]. Concerning long-term tests, two widely accepted test methods are stress relaxation and creep [10,11,12,13,14]. According to Sreekala et al [15], the knowledge of stress relaxation behavior under different strain levels allows predicting the dimensional stability of load-bearing structures and the retention of force (by modulus) for bolts fastened to composites. In case the unrecovered strain is too large after removing the applied stress on the material, it might have decreased dimensional stability and even lead to structural failure. Depending on loading stress and temperature, total recovery can be achieved [16,17,18]
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