Microscopic damage size in fiber-reinforced polymer-matrix composites: Quantification approach via NDT-measurements

Abstract

The meso-scale morphology of fiber-reinforced polymer-matrix (FRP) composites with fiber plies induces a range of microscopic damage mechanisms when FRP materials are subject to mechanical or thermo-mechanical loading. Both, for improving the damage resistance of FRP composites and for structural design guidelines, understanding the different damage mechanisms and their interaction is important. Acoustic Emission (AE) monitoring of load tests on laboratory-scale FRP specimens yields information on the occurrence of damage as a function of stress level, and typically allows for roughly locating signal sources, and with sophisticated pattern recognition, for identification of different micro- or mesoscopic damage mechanisms. In FRP components and elements empirical criteria for assessing structural integrity, e.g., AE Felicity-ratio, yield quantitative failure predictions. Combining AE information on microscopic damage mechanisms with macroscopic, empirical criteria has not received much attention yet. Identifying which mechanisms, e.g., damage from stress-relaxation or friction of existing crack faces induces the onset of AE signals in FRP composites and thus defines the Felicity-ratio is important for structural integrity characterization. This approach to the Felicity effect on the microscopic scale and its advantages and limitations are presented and discussed.