Abstract
Advanced composites which offer robust mechanical properties are being increasingly used for structural applications in the aerospace, marine, defense and transportation industries. However, the anisotropic nature of composite materials leaves it susceptible to problematic failure; the development of means for detecting failure is imperative. As design and functionality requirements of engineering structures such as spacecraft, aircraft, naval vessels, buildings, dams, bridges and ground-based vehicles become more complex; structural health monitoring (SHM) and damage assessment is becoming more rigorous. Though structures involved have regular costly inspections, the damage associated with composites in SHM systems can lead to catastrophic and expensive failures. Industry and research have no single technique used on its own to provide reliable results. Integrating several nondestructive evaluation (NDE) techniques could provide a solution for real-time health monitoring. Such studies, utilizing acoustic emission (AE), A-scans, C-scans, and laser shearography have reported considerable success. Nevertheless, damage detection has to be reliable and cost effective.
The answer may lie with the development of SHM systems by the use of triboluminescent crystals, as well as optical fibers embedded in the composite matrix. These crystals react to straining or fracture by emitting light of varied luminous intensity. Thus, a fiber-reinforced plastic (FRP) laminate doped with Triboluminescent (TL) or Mechanoluminescent (ML) crystals, acting as health sensors to its host material, will give an indication of crack initiation well ahead of catastrophic failure(s). The development of an in-situ health monitoring system for safety critical structures is a viable route through ‘Triboluminescence’. Assessing the viability of a proposed structural sensor system requires cross-linking between key areas in science and engineering.
Initial testing has shown that light can propagate through doped resins alone, as well as doped FRP laminates. The luminous intensities relation to impact velocity adds credence to a monitoring system that can characterize impact activity. However, Triboluminescent crystals have high material density. In response, a two-dimensional rotational mold was built to counteract massive settling under normal vacuum molding processes. Micro-structural evaluations using scanning electron microscopy (SEM) and EDAX imaging have aided in demystifying particulate dispersion of TL fillers through use of image processing.
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