Acoustic emission as a tool in monitoring fatigue damage accumulation in fiber reinforced metal laminate

Fiber reinforced metal laminates (FRML) are currently used as aerospace material. The common one is Glare® (registered trademark of Alcoa) which is one of the materials used in the fuselage of the commercial aircraft Airbus A380. Due to its structure, Glare® 1A has been reported to possess good overall mechanical, as well as fatigue properties. Its laminates consisting of unidirectional glass fibers have been found to suppress crack initiation and propagation due to "bridging" mechanisms. Within this context, several modeling approaches have been suggested to describe damage evolution in FRML with limited success, however, in correlating experimental data with model predictions. To create a reliable damage model, the results of a thorough experimental program are presented in this paper, which provide the basis to create constitutive laws that the model would use to predict progressive damage. Specifically, quasi static tensile tests followed by fatigue experiments were conducted on FRML Glare®1A using Acoustic Emission (AE) on line monitoring. Fatigue loading on flat uniform specimens was applied, for several stress ratios (Δσ_app/σ_y) in the range of 0.6-0.8 and with fixed applied number of cycles (10⁵). The degradation in the fatigue resistance was tracked in terms of both modulus and other mechanical properties. Trends in the measured mechanical properties were compared to AE data. Mechanical and AE results were compared with relevant micromechanical models for laminated materials. Complementary characterization using optical and SEM microscopy was performed for detection of microcracking and interfacial debonding, as well as for fracture mode classification. The present study indicates that the damage process in Glare® 1A involves seven different damage mechanisms. Furthermore, the normalized modulus is suggested as a macromechanical parameter that can quantify damage degradation during fatigue loading, while being also sensitive to the applied stress . Additionally, this normalized modulus was found to agree with AE cumulative counts curves, showing the same exponential decay as predicted by a relevant model.