Structural Health Monitoring in Open-Hole CFRP Laminates Under Fatigue Loading Based on Thermodynamic Entropy Analysis

Abstract

Structural health monitoring of composite laminates has been studied in the past decades. Many empirical and analytical approaches have been proposed in the literature. Recently, thermodynamic entropy-based techniques have gained more attention, due to the potential for removing the dependency of the common approaches on experimental conditions to provide an independent measure of damage. This study applies the thermodynamic entropy concepts and measures the entropy generation due to damage in step-stress tensile fatigue loadings. Several tests are performed on open-hole CFRP laminates, and mechanical and temperature responses are recorded. This allows for the energy dissipation associated with thermal and mechanical mechanisms that contribute to entropy generation to be carefully isolated and measured. Furthermore, the damage associated with residual plastic deformation is measured on the surface using Digital Image Correlation (DIC). Experimental results indicate that although the trajectory of plastic deformation, internal damage evolution, and heat conduction energy dissipation drastically differ in different tests, the final entropy generation values at the time of fracture converge to a critical value, which is known as the entropy endurance limit or fatigue fracture entropy (FFE).