Precursor/incubation of multi-scale damage state quantification in composite materials: using hybrid microcontinuum field theory and high-frequency ultrasonics.

A systematic framework for incubation of damage- state quantification in composites is almost absent in the current practice. Identification and quantification of the material state at its early stage has become significantly important in the field of structural health monitoring. Interaction between the intrinsic material state and ultrasonic wave signals, e.g., nonlinear ultrasonic, higher harmonic generation, etc., in metals are quite well known and well documented in the literature. However, it is extremely challenging to quantify the precursor to damage state in composite materials. Thus, in this paper, a comparatively simple but efficient novel approach is proposed to quantify the "incubation of damage" state using scanning acoustic microscopy. The proposed approach exploits the hybrid microcontinuum field theory to quantify the intrinsic (multi-scale) damage state. Defying the conventional route of bottom-up multi-scale modeling methods, a hybrid top-down approach is presented, which is then correlated to the ultrasonic signature obtained from the materials. A parameter to quantify incubation of damage at meso-scale has been identified in this paper. The intrinsic length-scale-dependent parameter called "damage entropy" closely resembles the material state resulting from fatigue, extreme environments, operational hazards or spatio-temporal variability, etc. The proposed quantification process involves a fusion between micromorphic physics and high-frequency ultrasonics in an unconventional way. The proposed approach is validated through an experimental study conducted on glass-fiber reinforced polymer composites which are mechanically fatigued. Specimens were characterized under a scanning acoustic microscope at 50 and 100 MHz. The imaging data and the sensor signals are characterized to quantify the incubation of damage state by the new parameter damage entropy.