Non-Contact Laser Ultrasonic Inspection for Cryogenically Introduced Interlayer Defects in Epoxy-Based Composites
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Traditional transducer-based techniques for Non-Destructive Evaluation (NDE) are limited by fixed frequency-bandwidth for generation and sensing, and thus provide unsatisfactory resolution for certain types of material defects. Thermo-Acousto-Photonic NDE (TAP-NDE) is a proven alternative that is non-invasive and non-contact, and suited for real-time applications. This paper focuses on employing TAP-NDE to examine the presence of microcracks and fissures in multi-layered composites. Tests were performed on layered composite panels of specific epoxyresin composition and constant thickness to identify localized delaminations formed by subjecting the specimen to cryogenic cycling. Interrogation of the undamaged specimen using laser-generated broadband surface waves revealed a standard reference knowledge base, as seen in the instantaneous frequency-time domain. Tests were repeated after each specimen was subjected to a set number of cycles of liquid nitrogen cycling, which caused damages at the micron scale in the bulk material. Analyses showed changes in the time of wave arrival and absence of prominent high frequency components. Wave velocity and dispersion characteristics of the cycled specimen were altered. Thus, the specimen, on cryogenic cycling, was found to undergo a decrease in stiffness, which is speculatively the result of micro-voids, fissures or delaminations between layers. Hence, when combining with the basic notion of instantaneous frequency, TAP-NDE acts as an effective broadband generation and sensing technique, demonstrating feasibility and greater versatility for inspecting layered composites as against contemporary narrowband techniques.
