Degradation Monitoring of Fatigued Notched Carbon Fiber Composites using DIC and Modal Techniques

An experimental study has been conducted on notched quasi-isotropic carbon fiber laminates subjected to both constant and variable amplitude tension-tension fatigue loading in order to investigate the effect of fatigue damage development on the residual properties. Quasi-static tests were initially performed on notched composites using digital image correlation (DIC) to determine the strain profiles at selected transverse sections of the outer ply of the laminates. These results are used as baseline for subsequent comparison with strain profiles obtained and tracked using DIC for specimens fatigued at different fatigue lifetime ratios. The results showed significant effect of fatigue damage development on strain redistribution even at early stages of fatigue and the capability of DIC technique to predict the damage initiation and its location. X-ray computed tomography (CT) was used to determine and understand the sequence of damage development throughout the fatigue lifetime of notched composites and attribute the observed damage mechanisms with the occurred change of strain profiles. Additional quasi-static tests were also performed on notched composites fatigued at different fatigue lifetime ratios in order to examine the effect of fatigue damage on the residual strength. The results showed a progressive increase of strength with increasing number of cycles till specified threshold after which the strength tends to decrease. Moreover, experimental modal analysis were also carried out to assess the effect of fatigue damage development on the residual frequency responses (RFRs) as well as the mode shapes. The results showed the importance of specific higher modes in detecting early fatigue damage.