Identification of Internal Damage in CFRP Cross-Ply Laminates Subjected to Cyclic Loadings by Thermal Measurement

This paper presents the identification of internal damage in CFRP cross-ply laminates subjected to arbitrary cyclic loadings based on thermal measurements via entropy. Long-term durability of carbon fiber reinforced plastic (CFRP) is one of the most important issues for composite materials in the field. When a polymer material is subjected to some load, the material entropy increases with deformation, even at the stage where void nucleation is not yet found. This can be verified by molecular dynamics simulation. Subsequently, a void of a few angstroms in size appears with an increase in loadings, and some voids are united and become larger voids. Consequently, the polymer results in ultimate failure. Using molecular simulation, damage (void content) and entropy are linked. Next, by representative volume element (RVE) and some micromechanical models, the ply property, including time- and load-dependent degradation, is determined for each direction. The determined ply properties are utilized in mesoscale analysis, which is ply-level homogenization, i.e., CFRP laminate analysis. For this stage, the degradation of ply properties is quantified by load history. Thus, we can predict the residual strength and lifetime based on nanoscopic phenomena for CFRP laminates without any experimental results, through entropy value. For the entire damage behavior described above, thermal measurements are implemented to identify the entropy value. The possibility for quantitative detection of invisible internal damage is presented in this study.