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On Nonlocal Mechanics Based Ultrasonic Methods for the Detection of Inception of Damage in Composites



Inception of damage quantification during fatigue loading has been a challenge in the field of structural health monitoring (SHM). Interaction between intrinsic damage state and ultrasonic wave signals of composite material is not fully understood. Fiber reinforced composite materials unveils damage precursor in the form of micro crack in matrix and fiber fracture which leads to catastrophic failure of the structure. Here, in this paper tensile-tensile fatigue tests were performed on two different types of ASTM standard composite specimen. For carbon fiber reinforced composite (CFRP) specimens at a regular interval of 10,000 cycles and for glass fiber reinforced composite (GFRP) interrogation of material state during fatigue tests were performed at the interval of 1,000 cycles, until micro cracks are developed. Quantitative Ultrasonic Microscopy is performed during the same intervals. For the Carbon-Carbon composites, the non-local material properties degraded continuously during fatigue loading due to onset of matrix cracking and fiber breakage while for glass fiber reinforced composite specimen material degradation was not detected conclusively. Here an attempt has been made to measure material properties degradation in the form of damage entropy at each pixel point of the selected area by using the scanning acoustic microscopy. The average damage entropy material properties are calculated, after each 10,000 cycles for CFRP and at 1,000 cycles for GFRP. “Damage Entropy” is used as a measure of damage accumulation over number of cycles and are plotted until damage incubates or incepts. Tensile test specimens were instrumented with high frequency-piezoelectric sensors. Interaction of guided lamb wave with local stiffness reduction has also been studied to extract information of damage initiation by intermittently collecting sensor data and analyzing them by using advance signal processing techniques. Optical microscopy was also performed to validate our results of damage inception. Based on the above study, possibility of correlating off-board ultrasonic technique and on-board Lamb wave technique are demonstrated for the detection of precursor to damage in composites.

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