One of the most severe damage modes in modern wind turbines is the failure of the adhesive joints in the trailing edge of the large composite blades. The geometrical shape of the blade and current manufacturing techniques make the trailing edge of the wind turbine blade more sensitive to damage. Failure to timely detect this damage type may result in catastrophic failures, expensive system downtime, and high repair costs. A novel sensing system called the In-situ Triboluminescent Optical Fiber (ITOF) sensor has been proposed for monitoring the initiation and propagation of disbonds in composite adhesive joints. The ITOF sensor combines the triboluminescent property of ZnS:Mn with the many desirable features of optical fiber to provide in-situ and distributed damage sensing in large composite structures like the wind blades. Unlike other sensor systems, the ITOF sensor does not require a power source at the sensing location or for transmitting damage-induced signals to the hub of the wind turbine. Composite parts will be fabricated and the ITOF integrated within the bondline to provide in-situ and real time damage sensing. Samples of the fabricated composite parts with integrated ITOF will be subjected to tensile and flexural loads, and the response from the integrated sensors will be monitored and analyzed to characterize the performance of the ITOF sensor as a debonding damage monitoring system. In addition, C-scan and optical microscopy will be employed to gain greater insights into the damage propagation behavior and the signals received from the ITOF sensors.
Structural health monitoring of civil infrastructure systems like concrete bridges and dams has become critical because
of the aging and overloading of these CIS. Most of the available SHM methods are not in-situ and can be very
expensive. The triboluminescence multifunctional cementitious composites (TMCC) have in-built crack detection
mechanism that can enable bridge engineers to monitor and detect abnormal crack formation in concrete structures so
that timely corrective action can be taken to prevent costly or catastrophic failures. This article reports the fabrication
process and test result of the flexural characterization of the TMCC. Accelerated durability test indicated that the 0.5
ZnS:Mn/Epoxy weight fraction ITOF sensor configuration to be more desirable in terms of durability. The alkaline
environment at the highest temperature investigated (45 °C) resulted in significant reduction in the mean glass transition
and storage moduli of the tested ITOF thin films. Further work is ongoing to correlate the TL response of the TMCC
with damage, particularly crack opening.
The human nervous system (HNS) provides one of the most advanced examples of how to monitor the structural state of
a complex system. In attempts to mimic the HNS, a key component has been the development of the sensory receptor.
This paper reports on the development of a triboluminescence (TL)-based sensory receptor that converts mechanical
energy from fatigue or impact loads and cracks propagation, into optical signals. This sensor system has potential for
wireless, in-situ and distributed sensing (WID). The approach differs from existing fiber optic methods in that it does not
require any external light source to function. The optical signal is generated through mechanical excitation of the highly
triboluminescent ZnS:Mn. It is then transmitted through optical fibers to photomultiplier tubes (PMT) for detecting,
quantifying and locating (with further analysis), intrinsic damage in critical engineering structures like concrete bridges.
The TL sensory receptor consists of a sensitized portion of a polymer optical fiber (POF) coated with epoxy containing
ZnS:Mn crystals. The sensory receptors were then incorporated into cementitious and polymer samples. Results from
preliminary investigation showed that the TL sensory receptor gives repeatable responses under multiple impact loads.
The triboluminescent intensity of the signal is directly related to the magnitude of the impact load. Results from dynamic
mechanical analysis show a reduction in the Tg of the ITOF coating (TSR) with higher concentration of the
triboluminescent (ZnS:Mn) crystals for the epoxy system used. There was however significant enhancement of the
modulus with increase in the TL crystals. High-performance epoxy system with the principles of particulate composites
would be applied in subsequent work to optimize the properties and performance of the TL sensor system.
Triboluminescence (TL) is a mechanical and luminescent phenomena enabling damage sensing capabilities in
materials. Depending on material compound, various excitation mechanisms result in emissions stimulated by
rubbing or fracture, and give an indication of internal stress. Design of Experiments helped ascertain experimental
knowledge of the multiphase composite system containing ZnS:Mn phosphors (0 - 40%) and vinyl ester resin
(VER). This statistical approach proffered an empirical model used to validate triboluminescent production. Data
shows concentration compiled with impact energy has a significant effect on the luminous intensity. Light intensity
was measured by a photomultiplier tube and a photo-voltaic detector. The signal intensity range was determined for
each. The photovoltaic detector acts as a low-light sensor in the range of 0.61 - 0.116 A for impacts less than 0.4 J.
Microscopy revealed plates with reasonable dispersion and view of micro-structural inclusions. DMA indicates the
inclusion of ZnS:Mn produces a moderate change in Young's modulus and thermo-kinetic properties.