Off-axis compression behavior and failure mechanisms of needled carbon/quartz fiber reinforced phenolic resin composite based on acoustic emission

The effect of the off-axis angle (note as theta) on compressive properties and failure mechanisms in needled carbon/quartz fiber reinforced phenolic resin (CF-QF/PF) composite has been investigated. For this objective, a series of quasi-static off-axis compression tests were performed, and a more precise and general model for predicting the compressive modulus and strength has been proposed. Further, the acoustic emission (AE) technology, including parameter-based analysis and the Hilbert Huang Transform (HHT) method, was also employed to scrutinize the intricate damage mechanisms. The results show that specimens with theta = 0 degrees exhibit linear and brittle behavior and are the most dangerous scenarios because delamination (accounting for 58% of the total AE accumulative energy) dominates their failure. For specimens with small off-axis angles (0(degrees) < theta <= 45(degrees)), the fiber-matrix interface determines their compressive properties. While for specimens with large off-axis angles(45(degrees) < theta <= 90(degrees)), the enhanced transverse load-bearing capacity of fibers becomes the dominant factor, characterized by significant debonding (33% when theta = 60(degrees)) and fiber breakage (27% when theta = 90 degrees). Finally, these results were verified by optical microscopy (OM) and scanning electron microscopy images (SEM). Highlights Off-axis compression behaviors of needled CF-QF/PF composite are studied. A more precise and general model for predicting off-axis compression properties is proposed. Acoustic emission technology is used to quantify different types of damage. Failure mechanisms are revealed by combining in-situ and offline techniques.