Nanocomposite Coating Mechanics via Piezospectroscopy

Coatings utilizing the piezospectroscopic (PS) effect of alpha alumina could enable on the fly stress sensing for structural health monitoring applications. While the PS effect has been historically utilized in several applications, here by distributing the photo-luminescent material in nanoparticle form within a matrix, a stress sensing coating is created. Parallel to developing PS coatings for stress sensing, the multi-scale mechanics associated with the observed PS response of nanocomposites and their coatings has been applied to give material property measurements, providing an understanding of particle reinforced composite behavior. Understanding the nanoparticle-coating-substrate mechanics is essential to interpreting the spectral shifts for stress sensing of structures. In the past, methods to experimentally measure the mechanics of these embedded nano inclusions have been limited, and much of the design of these composites depend on computational modeling and bulk response from mechanical testing. The PS properties of Chromium doped alumina allow for embedded inclusion mechanics to be revisited with unique experimental setups that probe the particles state of stress under applied load to the composite. These experimental investigations of particle mechanics will be compared to the Eshelby theory and its derivative theories in addition to the nanocomposite coating mechanics. This work discovers that simple nanoparticle load transfer theories are adequate for predicting PS properties in an intermediate volume fraction range. With fundamentals of PS nanocomposites established, the approach was applied to selected experiments to prove its validity. In general it was observed that the elastic modulus values calculated from the PS response were similar to that observed from macroscale strain measurements such as a strain gage. When simple damage models were applied to monitor the elastic modulus, it was observed that the rate of decay for the elastic modulus was much higher for the PS measurements than for the strain gage. A novel experiment including high resolution PS maps with secondary strain maps from digital image correlation is reviewed on an open hole tension, composite coupon. The two complementary measurements allow for a unique PS response for every location around the hole with a spatial resolution of 400 microns. Progression of intermediate damage mechanisms was observed before digital image correlation indicated them. Using the PS nanocomposite model, elastic modulus values were calculated. Introducing an elastic degradation model with some plastic deformation allows for estimation of material properties during the progression of failure. This work is part of a continuing effort to understand the mechanics of a stress sensing PS coating. The mechanics were then applied to various experimental data that provided elastic property calculations with high resolution. The significance is in the experimental capture of stress transfer in particulate composites. These findings pave the way for the development of high resolution stress-sensing coatings

Enhancing Cnt-composites With Raman Spectroscopy

Carbon Nanotubes (CNTs) have been the subject of intense research for their potential to improve a variety of material properties when developed as nano-composites. This research aims to address the challenges that limit the ability to transfer the outstanding nano-scale properties of CNTs to bulk nano-composites through Raman characterization. These studies relate the vibrational modes to microstructural characterization of CNT composites including stress, interface behavior, and defects. The formulation of a new fitting procedure using the pseudo-Voigt function is presented and shown to minimize the uncertainty of characteristics within the Raman G and D doublet. Methods for optimization of manufacturing processes using the Raman characterization are presented for selected applications in a polymer multiwalled nanotube (MWNT) composite and laser-sintered ceramic-MWNT composite. In the first application, the evolution of the MWNT microstructure throughout a functionalization and processing of the polymerMWNT composite was monitored using the G peak position and D/G intensity ratio. Processing parameters for laser sintering of the ceramic-MWNT composites were optimized by obtaining maximum downshift in stress sensitive G-band peak position, while keeping disorder sensitive D/G integrated intensity ratio to a minimum. Advanced Raman techniques, utilizing multiple wavelengths, were used to show that higher excitation energies are less sensitive to double resonance Raman effects. This reduces their influence and allows the microstructural strain in CNT composites to be probed more accurately. iii The use of these techniques could be applied to optimize any processing parameters in the manufacturing of CNT composites to achieve enhanced properties

Stress and structural damage sensing piezospectroscopic coatings validated with digital image correlation

The piezospectroscopic effect, relating a material\u27s stress state and spectral signature, has recently demonstrated tailorable sensitivity when the photo-luminescent alpha alumina is distributed in nanoparticulate form within a matrix. Here, the stress-sensing behavior of an alumina-epoxy nanoparticle coating, applied to a composite substrate in an open hole tension configuration, is validated with the biaxial strain field concurrently determined through digital image correlation. The coating achieved early detection of composite failure initiation at 77% failure load, and subsequently tracked stress distribution in the immediate vicinity of the crack as it progressed, demonstrating non-invasive stress and damage detection with multi-scale spatial resolution

Characterization And Performance Of Stress- And Damage-Sensing Smart Coatings

Mechanical enhancement of polymers with high modulus reinforcements, such as ceramic particles, has facilitated the development of structural composites with applications in the aerospace industry where strength to efficiency ratio is of significance. These modifiers have untapped multifunctional sensing capabilities that can be enabled by deploying these particles innovatively in polymer composites and as coatings. This chapter highlights some of the recent and novel findings in the development of piezospectroscopic particle-reinforced polymers as smart stressand damage-sensing coatings. The sections in this chapter describe the piezospectroscopic effect for alumina-based particulate composites, show the derivation of multiscale mechanics to quantify substrate stresses with piezospectroscopy, and demonstrate their performance in stress and damage sensing applied to a composite material. The noninvasive instrumentation is outlined and discussed for current and future applications in the industry ranging from manufacturing quality control to in-service damage inspections

Damage Mapping Of Composites With Piezospectroscopic Coatings

In composite structures there are many different types of damage with varying effects on mechanical performance. This work explores a revolutionary non-destructive evaluation (NDE) technique to diagnose composite damage with Piezospectroscopic (PS) coatings. Several PS phenomena are shown here which reveal intrinsic damage patterns of a composite coupon. A unique integration of piezospectroscopy and digital image correlation (DIC) enables determination of local mechanical properties. With applied multi-scale damage mechanics, it is shown that this integration produces a degrading elastic modulus map with in-situ mechanical loading. Using a phenomenological approach, the PS coating\u27s properties are studied as a comprehensive damage indicator

Damage Mapping With A Degrading Elastic Modulus Using Piezospectroscopic Coatings

The development of piezospectroscopic (PS) composites has enabled the creation of a non-destructive evaluation (NDE) technique which integrates piezospectroscopy, digital image correlation (DIC) and analytical multi-scale mechanics to map the elastic modulus of a coated material. The measured elastic modulus was represented as a normal distribution with a mean value (32.2 GPa) which is within 8% of the conventionally recorded modulus (35 GPa). Damage mechanics are applied to map elastic degradation in situ mechanical loading with an average uncertainty (∼ 10 GPa) that was sufficient in observing subsurface, progressive damage patterns which are unique to the coated material

Measuring Tensile Stresses In Cnf/Polymer Composites Using Raman Spectroscopy

This research focuses on developing relationships between the mechanical properties and the optical spectra of carbon nanofiber (CNF) reinforced polymer composites. Stress distribution in the sample at discrete force increments is obtained and the effects on the G band of the Raman spectra of CNF are observed. A linear shift of carbon nanofiber Raman frequencies is created by strain applied to their molecular structure. Results of loading experiments combined with optical spectroscopy shows the stress dependency of these bands which can be used in the future for design, analysis and non-destructive structural stress testing of CNF/polymer composites. Copyright © 2011 by the American Institute of Aeronautics and Astronautics, Inc

Investigation Of Temperature Dependent Multi-Walled Nanotube G And D Doublet Using Pseudo-Voigt Functions

A pseudo-Voigt (PV) function is used as a representation of the Stoke\u27s phonon frequency distributions for a multi-walled nanotube (MWNT) composite G and D doublet. Variable peak assignments with the PV function have been shown to enhance the resolution of these bands commonly used for characterization of carbon nanotube (CNT) composites. The peak assignment study was applied to an in-situ temperature experiment where the addition of new sub-bands in the G and D doublet was determined to reduce the uncertainty of the Raman characteristics. Fitting the spectrum with five pseudo-Voigt bands was concluded to give the most consistent results, producing the lowest uncertainty values for G-peak position (mG) and D/G intensity ratio. © 2013 Society for Applied Spectroscopy

Optical Spectra Of Carbon Nanotubes For Stress Measurements Of Advanced Aerospace Structures

The potential of carbon nanotubes being utilized for structural applications presents a need to develop non-invasive measurement capabilities for tailoring of reinforcements and to establish the structural integrity of such materials. This research is focused on developing relationships between mechanical properties and the optical spectra of single-walled carbon nanotubes. Laser excitation wavelengths were varied and effects on the G bands of the Raman spectra are presented here. Results of an in-situ thermal experiment on bulk carbon nanotube samples show the temperature dependency of these bands, which are commonly used to quantify the elastic modulus for CNT composites. Copyright © 2010 by the American Institute of Aeronautics and Astronautics, Inc

Multiscale Mechanics To Determine Nanocomposite Elastic Properties With Piezospectroscopy

The piezospectroscopic (PS) properties of chromium-doped alumina allow for embedded inclusion mechanics to be revisited with unique experimental setups that probe the particles\u27 state of stress when the composite is under applied load. These experimental investigations of particle mechanics will be compared to the Eshelby theory and a derivative theory. This work discovers that simple nanoparticle load transfer theories are adequate for predicting PS properties in the low to intermediate volume fraction range (≤20%). By applying the multiscale mechanics to a PS response, the inverse problem was demonstrated to reveal the elastic modulus of the composite. The implications for this technique are damage monitoring through observation of reduced mechanical properties in addition to a method to assist with engineering nanomaterials