Hybrid Carbon Fiber Alumina Nanocomposite for Non-Contact Stress Sensing Via Piezospectroscopy

Carbon ber composites have become popular in aerospace structures and applications due to their light weight, high strength, and high performance. Recently, scientists have begun investigating hybrid composites that include fibers and particulate fillers, since they allow for advanced tailoring of mechanical properties, such as improved fatigue life. This project investigated a hybrid carbon ber reinforced polymer (HCFRP) that includes carbon fiber and additional alumina nanoparticle fillers, which act as embedded nano stress-sensors. Utilizing the piezospectroscopic e ect, the photo-luminescent spectral signal of the embedded nanoparticles has been monitored as it changes with stress, enabling non-contact stress detection of the material. The HCRFPs stress-sensitive properties have been investigated in-situ using a laser source and a tensile mechanical testing system. Hybrid composites with varying mass contents of alumina nanoparticles have been studied in order to determine the e ect of particle content on the overall stress sensing properties of the material. Additionally, high resolution photo-luminescent maps were conducted of the surfaces of each sample in order to determine the particulate dispersion of samples with varying alumina content. The dispersion maps also served as a method of quantifying particulate sedimentation, and can aid in the improvement of the manufacturing process. The results showed that the emitted photo-luminescent spectrum can indeed be captured from the embedded alumina nanoparticles, and exhibits a systematic trend in photo-luminescent peak shift with respect to stress. The stress maps showed a linear increase in peak shift up to a certain critical stress, and matched closely with the DIC strain results. Therefore, the non-contact stress sensing results shown in this work have strong implications for the future of structural health monitoring and nondestructive evaluation (NDE) of aerospace structures

Fiber Length and Orientation in Long Carbon Fiber Thermoplastic Composites

Carbon fiber composites have become popular in aerospace applications because of their lightweight yet strong material properties. The injection molding process can be used to produce discontinuous fiber composites using less time and resources than traditional methods, thereby broadening carbon fiber composites’ applications in different industries. Utilization of longer fibers offers more load carrying capability and superior strength properties for injected molded composites. Since the fiber length and the orientation distribution in Long Fiber Thermoplastics (LFTs) directly affects LFT composites’ material properties, there is a need to study the microstructure of LFTs and characterize fiber length and orientation distributions. Therefore, this work aims to experimentally measure fiber length and orientation in pre-manufactured carbon fiber LFT composites in order to validate computer simulations of the injection molding process, and to therefore better predict mechanical properties. Fiber orientation distribution was measured by the optimization of several grinding and polishing steps followed by microscopic imaging of a sample’s cross-section. On the other hand, fiber length distribution was measured through the development of epoxy burn-off, down-selection, and fiber separation procedures, followed by microscopic imaging and manual fiber length measurements. By specifically optimizing these procedures for the analysis of carbon fiber LFTs, a detailed method has been developed to analyze the fiber length and orientation distributions and quantify any bias in the characterization techniques. Using the methods developed in this work, computer simulations can be validated and microstructure properties can be analyzed, allowing for better material strength predictions and industry implementation of LFTs

Characterization And Modeling Of Discontinuous Fiber Composites

Composite materials, which are light and strong, are of great interest to engineers in the aerospace industry. Specifically in this work, a discontinuous short fiber reinforced polymer composite whose matrix is Polypropylene and fibers are Electric-glass oriented in different directions was studied. The performance of this material is highly dependent on its microstructure, and therefore the objective of this research is to non-destructively characterize the microstructure of the composite material. This includes characterization of its fiber orientation and length, fiber volume fraction, and void volume fraction. To do this, X-ray micro-computed tomography has been used, providing two dimensional cross-sectional images that stack to form a three-dimensional image of the microstructure. Advanced image-processing methods have been used to determine the fiber volume fraction, the void volume fraction, and the fiber length distributions. Characterization of the microstructure will help predict its mechanical properties and establish a general framework for characterizing and predicting the strength of composite materials. Through the advanced characterization and strength prediction methods discussed in this work, engineers will eventually be able to quickly and non-destructively evaluate materials and thereby reduce large scale testing in aerospace applications

Investigating Damage in Discontinuous Fiber Composites Through Coupled In-Situ X-Ray Tomography Experiments and Simulations

Composite materials have become widely used in engineering applications, in order to reduce the overall weight of structures while retaining their required strength. Due to their light weight, relatively high stiffness properties, and formability into complex shapes, discontinuous fiber composites are advantageous for producing small and medium size components. However, qualifying their mechanical properties can be expensive, and therefore there is a need to improve predictive capabilities to help reduce the overall cost of large scale testing. To address this challenge, a composite material consisting of discontinuous glass fibers in a polypropylene matrix is studied at the microstructural level through coupled experiments and simulations, in order to uncover the mechanisms that cause microvoids to initiate and progress, as well as certain fiber breakage events to occur, during macroscopic tension. Specifically, this work coupled in-situ X-ray micro computed tomography (µ-CT) experiments with a finite element simulation of the exact microstructure to enable a 3D study that tracked damage initiation and propagation, and computed the local stresses and strains in the microstructure. In order to have a comprehensive 3D understanding of the evolution of the microstructure, high fidelity characterization procedures were developed and applied to the µ-CT images in order to understand the exact morphology of the microstructure. To aid in this process, ModLayer - an interactive image processing tool - was created as a MATLAB® executable, and the 3D microstructural feature detection techniques were compared to traditional destructive optical microscopy techniques. For damage initiation, this work showed how high hydrostatic stresses in the matrix can be used as a metric to explain and predict the exact locations of microvoid nucleation within the composite’s microstructure. From a damage propagation standpoint, matrix cracking - a mechanism that has been notably difficult to predict because of its apparent stochastic nature - was studied during damage propagation. The analysis revealed the role of shear stress in fiber mediated flat matrix cracking, and the role of hydrostatic stress in fiber-avoidance conoidal matrix cracking. Overall, a sub-fiber simulation and an in-situ experimental analysis provided the microstructural physical phenomena that govern certain damage initiation and progression mechanisms, further enabling the strength and failure predictions of short fiber thermoplastic composites

Design of Low Cost Carbon Fiber Composites via Examining the Micromechanical Stress Distributions in A42 Bean-Shaped versus T650 Circular Fibers

Recent advancements have led to new polyacrylonitrile carbon fiber precursors which reduce production costs, yet lead to bean-shaped cross-sections. While these bean-shaped fibers have comparable stiffness and ultimate strength values to typical carbon fibers, their unique morphology results in varying in-plane orientations and different microstructural stress distributions under loading, which are not well understood and can limit failure strength under complex loading scenarios. Therefore, this work used finite element simulations to compare longitudinal stress distributions in A42 (bean-shaped) and T650 (circular) carbon fiber composite microstructures. Specifically, a microscopy image of an A42/P6300 microstructure was processed to instantiate a 3D model, while a Monte Carlo approach (which accounts for size and in-plane orientation distributions) was used to create statistically equivalent A42/P6300 and T650/P6300 microstructures. First, the results showed that the measured in-plane orientations of the A42 carbon fibers for the analyzed specimen had an orderly distribution with peaks at |ϕ|=0∘,180∘. Additionally, the results showed that under 1.5% elongation, the A42/P6300 microstructure reached simulated failure at approximately 2108 MPa, while the T650/P6300 microstructure did not reach failure. A single fiber model showed that this was due to the curvature of A42 fibers which was 3.18 μm−1 higher at the inner corner, yielding a matrix stress that was 7 MPa higher compared to the T650/P6300 microstructure. Overall, this analysis is valuable to engineers designing new components using lower cost carbon fiber composites, based on the micromechanical stress distributions and unique packing abilities resulting from the A42 fiber morphologies

Characterizing Mechanical Properties Of Hybrid Alumina Carbon Fiber Composites With Piezospectroscopy

Carbon fiber composites have become popular in aerospace structures and applications due to their light weight, high strength, and high performance. Recently, scientists have begun investigating hybrid composites that include fibers and particulate fillers, since they allow for advanced tailoring of mechanical properties, such as improved fatigue life. This project investigated a hybrid carbon fiber reinforced polymer (HCFRP) that includes carbon fiber and additional alumina nanoparticle fillers, which act as embedded nano stresssensors. Utilizing the piezospectroscopic effect, the photo-luminescent (PL) spectral signal of the embedded nanoparticles has been monitored as it changes with stress, enabling noncontact stress detection of the material. The HCRFPs stress-sensitive properties have been investigated in-situ using a laser source and a tensile mechanical testing system. Hybrid composites with varying mass contents of alumina nanoparticles have been studied in order to determine the e↵ect of particle content on the overall stress sensing properties of the material. Additionally, high resolution photo-luminescent maps were collected from the surfaces of each specimen in order to determine the particulate dispersion of specimens with varying alumina content. The dispersion maps also served as a method of quantifying particulate sedimentation, and can aid in the improvement of the manufacturing process. The results showed that the emitted photo-luminescent spectrum can indeed be captured from the embedded alumina nanoparticles, and exhibits a systematic trend in photo-luminescent peak shift with respect to stress, up to a certain critical stress. Therefore, the non-contact stress sensing results shown in this work have strong implications for the development of multi-functional hybrid composites to support structural health monitoring and nondestructive evaluation (NDE) of aerospace structures

Particle Size Effect On Load Transfer In Single Particle Composite Samples Via X-Ray Difiraction

Particulate composites are widely used in many aerospace applications including protective coatings, adhesives, or structures, and their mechanical properties and behavior have gained increasing significance. The addition of modifiers such as alumina generally leads to improved mechanical properties. In this work, samples with an isolated alumina particle embedded in an epoxy matrix were created to replicate the ideal assumptions for many particulate mechanics models. The effect of particle size on load transfer is determined here using a unique X-Ray Difiraction experimental set-up at the Canadian Light Source. At the Very Sensitive Elemental and Structural Probe Employing Radiation from a Synchrotron (VESPERS) beamline, a custom miniature mechanical load frame was used to apply compressive loads to each sample. At three different compressive loads, the alumina within each sample was exposed to a hard X-ray beam which created a difiraction pattern that was collected by a 2-D detector. A trend of increasing load transfer with increasing particle size was observed during the analysis of the difiraction rings. Results from this work provide experimental insight into the effect of particle size on load transfer in single particle composites and can serve to experimentally validate the theoretical load transfer models that currently exist

Experimental Piezospectroscopic Measurements To Study Load Transfer In A Single Alumina Particle Embedded Within An Epoxy Matrix

The addition of alumina into an epoxy matrix enables the non-invasive study of the load transfer between the particle (alumina) and the matrix material (epoxy). This ability stems from the photoluminescence (PL) characteristics of alumina, whose peak position is dependent on the stress the alumina sustains, also known as piezospectroscopy (PS). In this work, samples with an isolated particle embedded in a matrix were created to replicate the ideal assumptions for many particulate mechanical models. Photo stimulated luminescent spectroscopy was used to collect the spectral emission from the mechanically loaded samples. Results from this work will provide experimental insight into the load transfer properties of particulate composites that have gained significance for their improved mechanical properties. Findings from this experimental data can serve to experimentally validate the theoretical and numerical load transfer models that currently exist. Copyright 2014. Used by the Society of the Advancement of Material and Process Engineering with permission

Quantifying Alumina Nanoparticle Dispersion In Hybrid Carbon Fiber Composites Using Photoluminescent Spectroscopy

Composites modified with nanoparticles are of interest to many researchers due to the large surface-area-to-volume ratio of nano-scale fillers. One challenge with nanoscale materials that has received significant attention is the dispersion of nanoparticles in a matrix material. A random distribution of particles often ensures good material properties, especially as it relates to the thermal and mechanical performance of composites. Typical methods to quantify particle dispersion in a matrix material include optical, scanning electron, and transmission electron microscopy. These utilize images and a variety of analysis methods to describe particle dispersion. This work describes how photoluminescent spectroscopy can serve as an additional technique capable of quickly and comprehensively quantifying particle dispersion of photoluminescent particles in a hybrid composite. High resolution 2D photoluminescent maps were conducted on the front and back surfaces of a hybrid carbon fiber reinforced polymer containing varying contents of alumina nanoparticles. The photoluminescent maps were analyzed for the intensity of the alumina R1 fluorescence peak, and therefore yielded alumina particle dispersion based on changes in intensity from the embedded nanoparticles. A method for quantifying particle sedimentation is also proposed that compares the photoluminescent data of the front and back surfaces of each hybrid composite and assigns a single numerical value to the degree of sedimentation in each specimen. The methods described in this work have the potential to aid in the manufacturing processes of hybrid composites by providing on-site quality control options, capable of quickly and noninvasively providing feedback on nanoparticle dispersion and sedimentation