Gregory Freihofer

Optical stress probe: in-situ stress mapping with Raman and Photo-stimulated luminescence spectroscopy

The optical stress probe system, developed in this work, provides a non-invasive method of monitoring and mapping the optical properties of a material during in situ stress tests. The design and construction of such a system was achieved by coupling a fiber optic probe based spectrometer system with an electromechanical loading system. This novel instrumentation integration enables the quantitative study of Raman or Photo-stimulated luminescence peak shifts with stress, known as piezospectroscopy. It further enables mapping of these spectral shifts over a surface of the specimen under load. To achieve this, a focusing method was developed that optimizes the intensity of specific optical bands of interest with the probe position. Individual software programs for the various systems that make up the instrumentation including the spectrometer, load frame and the XYZ stage were integrated and a single user interface was created. The system was calibrated by replicating published linear correlation between compressive stress and spectral peak position, 2.5cm−1/GPa for polycrystalline alumina.

Portable Piezospectroscopy system: non-contact in-situ stress sensing through high resolution photo-luminescent mapping

Through the piezospectroscopic effect, certain photo-luminescent materials, once excited with a laser, produce spectral emissions which are sensitive to the stress or strain that the material experiences. A system that utilizes the piezospectroscopic effect for non-contact stress detection over a materials surface can capture important information on the evolution of mechanical response under various conditions. Therefore, the components necessary for piezospectroscopic mapping and analysis have now been integrated into a versatile and transportable system that can be used with photo-luminescent materials in any load frame or on a variety of structures. This system combines compact hardware components such as a portable laser source, fiber optics, spectrograph, charge-coupled device (CCD), and an X-Y-Z stage (with focusing capabilities) with a series of data analysis algorithms capable of analyzing and outputting high resolution photo-luminescent (PL) maps on-site. Through a proof of concept experiment using a compressed polycrystalline alumina sample with sharp machined corners, this system successfully captured high resolution PL maps with a step size of 28.86μm/pixel and located high stress concentrations in critical areas, which correlated closely with the results of a finite element model. This work represents an important step in advancing the portability of piezospectroscopy for in-situ and non-contact stress detection. The instrumentation developed here has strong implications for the future of non-destructive evaluation and non-invasive structural health monitoring.

Piezospectroscopic Measurements Capturing the Evolution of Plasma Spray-Coating Stresses with Substrate Loads

Plasma-spray coatings have a unique microstructure composed of various types of microcracks and weakly bonded interfaces which dictate their nonlinear mechanical properties. The intrinsic photo-luminescence (PL) characteristics of alpha-alumina (α-Al2O3) within these coatings offer a diagnostic functionality, enabling these properties to be probed experimentally at the microscale, under substrate loading. The piezospectroscopic (PS) measurements from the coatings are capable of revealing microstructural stress at high spatial resolution. Here, for the first time, the evolution of stresses within air plasma spray (APS) coatings under increasing substrate loads were captured using piezospectroscopy. With mechanical cycling of the substrate, the PS properties revealed anelastic and inelastic behavior and a relaxation of residual tensile stress within the APS coatings. With decreasing substrate thickness, the coating was observed to sustain more stress, as the substrates influence on the mechanical behavior decreased. The findings provide an insight into the microstructural response that can serve as the basis for model validation and subsequently drive the design process for these coatings.

Enhancement in ballistic performance of composite hard armor through carbon nanotubes

The use of carbon nanotubes in composite hard armor is discussed in this study. The processing techniques to make various armor composite panels consisting of Kevlar®29 woven fabric in an epoxy matrix and the subsequent V50 test results for both 44 caliber soft-point rounds and 30 caliber FSP (fragment simulated projectile) threats are presented. A 6.5% improvement in the V50 test results was found for a combination of 1.65 wt% loading of carbon nanotubes and 1.65 wt% loading of milled fibers. The failure mechanism of carbon nanotubes during the ballistic event is discussed through scanning electron microscope images of the panels after the failure. Raman Spectroscopy was also utilized to evaluate the residual strain in the Kevlar®29 fibers post shoot. The Raman Spectroscopy shows a Raman shift of 25 cm−1 for the Kevlar®29 fiber utilized in the composite panel that had an enhancement in the V50 performance by using milled fiber and multi-walled carbon nanotubes. Evaluating both scenarios where an improvement was made and other panels without any improvement allows for understanding of how loading levels and synergistic effects between carbon nanotubes and milled fibers can further enhance ballistic performance.

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

The piezospectroscopic effect, relating a material’s 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.

Optical Stress-sensing Alumina Nanocomposite Coatings for Aerospace Structures

Alpha alumina (α-Al2O3) nanocomposites have a multi-functional stress sensing ability via their photoluminescence (PL) spectral peak shifts defined by Piezospectroscopic (PS) relationships. This has prompted the development of α-Al2O3 nanocomposite coatings to enable real-time stress measurements and damage assessment of structures with the potential for unparalleled spatial resolution and sensitivity. Here, two types of stress-sensing coatings were evaluated, including an atmospheric plasma spray (APS) coating on metallic substrates and an epoxy nanocomposite coating on a composite substrate. The stress sensitivity of the APS coating, represented by the slope of the peak shifts against stress or PS coefficient, varied inversely with the thickness of the substrates between 3.2 and 1.7 cm−1/GPa. These values decreased by more than 50% after tensile cycling for all substrate thicknesses due to microstructural damage in the APS coating indicating the need for repeatability and durability studies. The epoxy nanocomposite coating successfully captured the stress gradients associated with an open hole tension (OHT) composite substrate revealing damage initiation at 77% of failure load, earlier than visual appearance of a surface crack (93%). The findings validate the successful development of quantitative and multiscale spatial resolution stress-sensing coatings, capable of detecting subsurface damage of composite structures, that will take structural testing and integrity monitoring to the next level.

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 Stokes 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 (vG) and D/G intensity ratio.

Ex-situ Raman spectroscopy to optimize the manufacturing process for a structural MWNT nanocomposite

The G and D doublet, an intrinsic Raman feature of multi-walled nanotubes (MWNTs), was monitored in an ex-situ fashion for all intermediate stages of a manufacturing process of a MWNT/polymer nanocomposite. The G peak position and D/G ratios were monitored to characterize the changes in load transfer and disorder, respectively, of the MWNTs. Differences in Raman characteristics desired during the manufacturing process for different structural applications are discussed. Techniques are presented that could optimize any manufacturing parameters for pressurized filtration and resin infusion using the G position and D/G integrated intensity ratio. Similar Raman control techniques could be applied to any intermediate phase of a manufacturing process to develop enhanced novel carbon nanotube composites for structural applications.

Optical Spectra of Carbon Nanotubes for Stress Measurements of Advanced Aerospace Structures

SDM 2010 student paper competition 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 singlewalled carbon nanotubes. Laser excitation wavelengths were varied and eects 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.

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 stress- and 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.