Assessment of Triboluminescent Materials for Intrinsic Health Monitoring of Composite Damage

Advanced composites offer robust mechanical properties and are widely used for structural applications in the aerospace, marine, defense and transportation industries. However, the inhomogenous nature of composite materials leaves them susceptible to problematic failure; thus the development of a means for detecting failure is imperative. Damage occurs when a load is applied and a distortion of the solid material results in deformation. This process also results in straining of the material. Strain, however, is a physical result of work being performed on a solid material making energy the commonality among all failure mechanisms. This study investigated the feasibility of using Triboluminescent zinc-sulphide manganese (ZnS:Mn) phosphors concentrated in vinyl ester resin for damage monitoring of polymer composites under flexural loading. These particulates react to straining or fracturing by emitting light of varied luminous intensity and detecting the crack initiation presently leading to catastrophic failure(s). Unreinforced vinyl ester resins and fiber-reinforced composite beams incorporated 5 - 50 % wt. concentrations of TL fillers, and were subjected to three-point bend tests. The intent of flexural testing was to observe the transient response of triboluminescence (TL) in both two- and three-phase composite systems throughout the failure cycle of notched beams, while changing the geometric constraints. Results indicate TL crystals emit light at various intensities corresponding to crystal concentration, the notch-length and imminent matrix fracture. The fracturing or deformation energy was estimated by the method of J-integral with varied notch-lengths, where a lower threshold for excitation was found to be approximately <2 J/m^2, far below its critical fracture energy (~ 3 & 7 J/m^2). Consequently, concentrated samples showed nearly 50 % reductions of mechanical moduli due to high loading levels, which subsequently affected the Triboluminescent response. As a result, an optimal 6 % vol. of TL particulates was chosen for further study and exhibited significant signal-to-noise response. Scanning electron microscopy (SEM) revealed particulate inclusions with shearing bands and semblance of particle to resin adhesion, as well as, cases of micro-cracking in reinforced samples. Despite significant parasitic affect to mechanical properties, the luminescent properties of TL occur at rupture for unreinforced composites. The cases of TL concentrated reinforced composites show detection of localized matrix phenomenon which are related to the material response and incurring internal strain-energy prior to any catastrophic failure. This indicates that TL in composite systems has the potential to detect micro-failures (micro-cracks) related to the weak matrix component. The triboluminescent signal was simulated as a rate-dependent model considering the load profile of the composite beam is known.