Browsing by Author "Radford, Donald W., advisor"

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Open Access
A simulation method and laboratory brake friction dynamometer for tribology studies
(Colorado State University. Libraries, 2009) Nivala, Peter Thompson, author; Radford, Donald W., advisor; Sakurai, Hiroshi, committee member; Heyliger, Paul Roy, 1958-, committee member
Two of the most important parameters of brake system design are the frictional and wear capabilities of the rotor and pad materials. These parameters must meet minimum design requirements in an effort to enhance friction and reduce wear to improve the performance and life of brake system components. The frictional and wear performance of the rotor and pad materials can be assessed through laboratory brake dynamometer testing and evaluation. In the current study, a wear testing simulation and an inertia laboratory brake dynamometer were developed to resolve differences in wear rates of brake materials. Dynamometer testing was conducted to verify the logic of the simulation and the functionality of the dynamometer by measuring wear rates of brake rotor material samples, some of which were subjected to cryogenic heat treatment to modify their wear rates, at varying brake application pressures. Dynamometer testing established that the wear simulation and inertia laboratory brake dynamometer developed during the current study could function together as a suitable tribological experimental apparatus. Specifically, dynamometer testing demonstrated the ability of the experimental apparatus to resolve differences in wear rates of brake materials due to variations in brake application pressure at relatively short test durations; however, dynamometer test results did not show conclusive evidence to suggest an advantage in subjecting the rotor materials used in the current study to cryogenic treatment to lower the rotor or pad wear rates.
Open Access
A thermoplastic matrix continuous fiber reinforced composite impregnation method by direct polymer extrusion
(Colorado State University. Libraries, 2018) Hedin, Kevin M., author; Radford, Donald W., advisor; Ma, Kaka, committee member; Heyliger, Paul, committee member
During component design, continuous fiber reinforced composite material systems are often chosen largely based on their structural efficiency. Their mechanical properties, such as specific strength and specific stiffness, are often cited as significant advantages over the use of other materials. However, composite component production often lacks the capability to provide the local variation necessary to ensure that 1) the reinforcing fibers are best aligned with anticipated loads, and 2) the ideal matrix composition and fiber volume fraction are found throughout the composite part. In practice, these limitations result in composite components that do not demonstrate the maximum possible efficiencies inherent to the fiber-reinforced composite material system. To further increase the flexibility of polymer matrix continuous fiber reinforced composites manufacturing methods, a new thermoplastic impregnation method was developed. This proposed method adds a thermoplastic matrix, which has previously been proven to allow significant variation of local fiber orientation, to the reinforcing fiber just prior to the consolidation of the composite. The increased independence of matrix and fiber addition should allow the local variation of volume and composition of the added matrix, while using less and simpler hardware than previous, similar efforts. In this work, the quality of material deposited from the proposed process is evaluated. The maximum possible quality of the proposed method and also that of a similar process that uses a commercially available material system were determined, primarily using short beam shear (SBS) testing. The material system of both methods consisted of E-glass continuous fiber reinforcement with a PETG matrix. It was found that both manufacturing processes are capable of producing samples with an SBS strength of approximately 53 MPa, and it was concluded that the proposed process has the capability to deposit material of comparable quality to that produced by the baseline method. Subsequent thermal analysis, fiber volume fraction/void content measurement, and metallographic imaging were conducted to investigate the effects of using two different PETG compositions on the SBS strength of composite material produced by the proposed process. It was found that, while using the proposed process, the PETG matrix with a lower glass transition temperature allowed better consolidation of the resulting composite part, ultimately increasing SBS strength. Each process parameter used in the proposed process was evaluated for the practical significance of its effects on SBS strength, which facilitated 1) an understanding of the underlying mechanisms of the process, and 2) a tenable simplification of the process that should reduce operating costs and also demonstrates its robustness via insensitivity to many of the possible process variations. Finally, it was established that the material inputs to the proposed process are relatively inexpensive: Using PETG and continuous E-glass fiber in the proposed process reduces material input cost by at least 52% compared to using commingled PETG and E-glass fibers in the baseline process, on a $/kg basis.
Open Access
Direct digital manufacture of continuous fiber reinforced thermoplastic high aspect ratio composite grid stiffeners and grid stiffener intersections with radically reduced tooling
(Colorado State University. Libraries, 2024) Hogan, Steven J., author; Radford, Donald W., advisor; Heyliger, Paul, committee member; Yourdkhani, Mostafa, committee member
Grid stiffened structures are widely used in the aerospace industry due to their high strength and stiffness to weight ratio and impact damage tolerance. These structures consist of a lattice pattern of stiffening ribs bonded to a thin shell structure, where the stiffening ribs commonly act as the main load bearing members, and the shell acts to cover the ribs and transfer loads through membrane action. These structures offer a variety of beneficial structural properties including high specific strength and stiffness, high impact resistance, high compressive resistance, and high energy absorption. However, the complexity of a grid pattern can lead to excessive manufacturing times, especially for simple constructions such as flat plates. A more promising alternative for manufacturing grid stiffened structures is the use of automated manufacturing methods including ATL, AFP, and filament winding. Because composite grid stiffened structures can be composed entirely of the same composite material, the manufacturing process with these methods can be almost entirely automated, saving time and money. However, the traditional and automated methods of producing composite grid stiffened structures require the fabrication of complex tooling to develop the geometry of stiffening ribs. In addition, all composite grid stiffened structures suffer from the same manufacturing difficulty: for all of the fibers to be continuous through an intersection node, there must be twice as much material at each intersection than in each rib, making intersection compaction extremely difficult. A more recently developed composite manufacturing method is additive manufacturing (AM) in the form of composite 3D printing, which offers a much higher degree of geometric freedom than other autonomous manufacturing methods and does not require tooling. However, composite 3D printing is generally limited to low fiber volume fractions. A manufacturing method with the ability to make high quality, high fiber volume fraction continuous fiber grid stiffened structures without the need for tooling could significantly increase the efficiency and decrease the cost to produce these structures. The current study proposes the use of a novel additive manufacturing method which uses a commingled feedstock and features in situ consolidation to produce grid stiffened structures without the need for tooling. Several stiffener ribs and stiffener rib intersections were produced and tested for composite quality. The fiber volume fraction and void volume fraction through the height and length of printed stiffener ribs and intersections was analyzed to determine if the quality was consistent. A micrograph evaluation was performed on the high aspect ratio stiffener rib and intersection composites to qualitatively evaluate the reinforcement distribution, determine the void locations, and to support the constituent material concentration measurements. The consolidation force was measured during the manufacturing of the samples to better understand the forces experienced during printing and to form a relationship between the consolidation force experienced and the constituent volume fraction of the samples. The results of this study suggest that the application of direct digital manufacture to the placement and consolidation of commingled tow for the fabrication of high aspect ratio grid stiffeners and intersections, without the need for tooling, can readily achieve fiber volume fractions greater than 50% and void fractions as low as 5%. Volume fraction analysis results show that manufactured stiffener ribs and stiffener grid intersections exhibit high fiber volume fractions and low void volume fractions which remain consistent through the height of the samples. Consolidation force measurement results show that a significant decrease in force is experienced between print layers. Microscopic analysis results show that the majority of voids collect at the edges of print layers leading to an increase in void content at the intersection node and potentially masking any quality gradient through the height of samples that may exist. The results of this study show the high potential for the manufacturing of high quality high aspect ratio continuous fiber composite grid stiffener structures through direct digital manufacturing technologies without the need for tooling.
Open Access
Dynamic mechanical analysis for quality evaluation of additively manufactured continuous fiber reinforced thermoplastic matrix composites subject to manufacturing defects
(Colorado State University. Libraries, 2019) Rodriguez, Patrick A., author; Radford, Donald W., advisor; Ma, Kaka, committee member; Heyliger, Paul, committee member
Continuous fiber reinforced polymers (CFRP) have become integral to modern mechanical design as value-added alternatives to metallic, ceramic and neat polymeric engineering materials. Despite the advantages of CFRP, current methods of preparing laminated continuous fiber reinforced polymers are fundamentally limiting in that reinforcement is typically applied only in the plane of the mold or tool. Additionally, key operations inherent to all CFRP processing approaches require a variety of skilled labor as well as costly net-shape, hard tooling. As such, additive manufacturing has risen to the forefront of manufacturing and processing research and development in the CFRP arena. Additive manufacture of continuous fiber reinforced thermoplastics (CFRTP) exhibits the potential to relieve many of the constraints placed on the current design and manufacturing of continuous fiber reinforced structures. At present, the additive manufacture of CFRTP has been demonstrated successfully to varying extents; however, comprehensive dialogue regarding manufacturing defects and quality of the processed continuous fiber reinforced thermoplastics has been missing from the field. Considering the preliminary nature of additive manufacture of CFRTP, exemplary processed composites are typically subject to various manufacturing defects, namely excessive void content in the thermoplastic matrix. Generally, quality evaluation of processed composites in the literature is limited to test methods that are largely influenced by the properties of the continuous fiber reinforcement, and as such, defects in the thermoplastic matrix are usually less-impactful on the results and overlooked. Hardware to facilitate additive manufacturing of CFRTP was developed and continuous fiber reinforced specimens, with high fiber volume fractions (~ 50 %), were successfully processed. Early efforts at evaluating the processed specimens using defect-sensitive Short-Beam Strength (SBS) analysis exhibited limited sensitivity to void content, coupled with destructive, inelastic failure modes. As a path forward, an expanded study of the effects of void content on the processed specimens was conducted by means of Dynamic Mechanical Analysis (DMA). Utilization of DMA allows for thermomechanical (i.e. highly matrix sensitive) evaluation of the composite specimens, specifically in terms of the measured elastic storage modulus (E'), viscous loss modulus (E"), damping factor (tan δ) and the glass transition temperature (Tg) of the processed composite specimens. The results of this work have shown that DMA exhibits increased sensitivity, as compared to SBS, to the presence of void content in the additively manufactured CFRTP specimens. Within the relevant range of void content, non-destructive specimen evaluation by DMA resulted in a measured, frequency dependent, 5.5 – 5.8 % decrease in elastic storage modulus per 1 % increase in void content by volume. Additionally, quality evaluation by DMA realized a marked decrease in the maximum measured loss modulus in the additively manufactured composites, ranging from 7.0 – 8.2 % per 1 % increase in void content by volume. Effects of void content were also measured in both the damping factor and glass transition temperature, where an approximate 1.6 °C drop in Tg was recorded over the relevant range of void content. The results of this work indicate, firstly, that DMA is a superior evaluation method, as compared to SBS, in terms of sensitivity to void content in additively manufactured CFRTP. Additionally, the results of this work provide a clear expansion of the current state of the literature regarding additive manufacture of CFRTP materials in that the effects of prominent manufacturing defects have been assessed with regard to thermomechanical material performance. Furthermore, and finally, the results of this work establish a direct path forward to characterize long-term effects of manufacturing defects, by means of DMA, on the creep-recovery and stress relaxation behavior of the relevant composite material system.
Open Access
Enhancing the deformability of elastic memory sandwich composites with elastic memory/conventional epoxy hybrid facesheets
(Colorado State University. Libraries, 2016) Antonio, Allyson Melia, author; Radford, Donald W., advisor; James, Susan P., committee member; Belfiore, Laurence A., committee member
Shape memory polymer (SMP) composites have the ability to return repeatedly, and with great accuracy, to their cured geometry when heated. By taking advantage of the inherent ability of polymers to exist in a rubbery state at higher temperatures and in a glassy state at lower temperatures, shape memory composites (SMC), which incorporate continuous fiber reinforcement, can accommodate strains up to 5%, compared to the ultimate strain of 1% - 1.5% typical of carbon fibers in aerospace composites. This is possible because the rubbery modulus of an SMP is at least two orders of magnitude lower than its corresponding glassy modulus. The limited lateral stability provided by the matrix in the rubbery state allows the fibers to buckle elastically in sinusoidal waves, called microbuckles, under compressive load. Elastic memory composites (EMC) are a class of SMC that are able to use elastically stored strain energy to exert force, and thereby perform mechanical work. The elastic energy stored by the EMC is not large enough to drive the reverse deformation once cooled to the glassy state, but can be released with applied heat. Because the EMC polymer can processed into low density foam, the combination of EMC polymer foam with continuous fiber EMC to create sandwich panels, is an attractive one. Although microbuckling is the enabling mechanism for strain accommodation, excessive microbuckling results in either matrix failure (delamination) or fiber breakage. Therefore, the conformability EMCs is limited by the amount of flexure that can be achieved without incurring permanent damage. Bending an EMC in the rubbery state induces compressive stress that is amplified by the difference in tensile and compressive moduli. Whereas tension is resisted by the fibers, compression is only supported by the matrix. This causes a shift in the location of the neutral stress plane, from the center of the thickness, far toward the maximum tension surface. Additional bending section thickness from adding foam to the EMC to create a sandwich panel, exponentially raises the maximum stress on the compression surface. This leads to microbuckling failure of EMC sandwich panels at much lower deflections than typical solid laminate EMCs. It is hypothesized that by incorporating permanently bonded conventional carbon epoxy composite plies on the compression surface of the EMC sandwich panel, compression microbuckling failure can be delayed, increasing bending capability. Three base laminate configurations were selected to assess relative deformability in 3-point bending: shape memory only, shape memory with one conventional ply, and shape memory with two conventional plies. To simulate local heating in some specimens, phenolic blocks were bonded to the ends of the foam core and incorporated into the laminate with conventional epoxy, to prevent facesheet shear. The following variables were used to further investigate the viscoelastic behavior of the specimens: bending temperature, deflection rate, and hold time at maximum deflection. Results are presented in the form of force-deflection, rather than stress-strain, as the use of shear end constraint significantly affected the shape of the bent specimens. Global compression facesheet buckling into the foam core caused mid span thinning that was more prominent at higher temperatures. Loading cycle hysteresis due to stress relaxation was recorded at temperatures above and below Tg, being greatest below Tg. It was found, regardless of variables, that the specimens with hybrid elastic memory/conventional epoxy matrix facesheets achieved more than double the deflection of pure EMC.
Open Access
Nanofiber reinforcement of a geopolymer matrix for improved composite materials mechanical performance
(Colorado State University. Libraries, 2015) Rahman, AKM Samsur, author; Radford, Donald W., advisor; Sampath, Walajabad S., committee member; Holland, Troy B., committee member; Heyliger, Paul, committee member
Geopolymers have the potential to cross the process performance gap between polymer matrix and ceramic matrix composites (CMC), enabling high temperature capable composites that are manufactured at relatively low temperatures. Unfortunately, the inherently low toughness of these geopolymers limits the performance of the resulting fiber reinforced geopolymer matrix composites. Toughness improvements in composites can be addressed through the adjustments in the fiber/matrix interfacial strength and through the improvements in the inherent toughness of the constituent materials. This study investigates the potential to improve the inherent toughness of the geopolymer matrix material through the addition of nanofillers, by considering physical dimensions, mechanical properties, reinforcing capability and interfacial bond strength effects. A process optimization study was first undertaken to develop the ability to produce consistent, neat geopolymer samples, a critical precursor to producing nano-filled geopolymer for toughness evaluation. After that, single edge notched bend beam fracture toughness and un-notched beam flexural strength were evaluated for silicon carbide, alumina and carbon nanofillers reinforced geopolymer samples treated at various temperatures in reactive and inert environments. Toughness results of silicon carbide and carbon nanofillers reinforced geopolymers suggested that with the improved baseline properties, high aspect ratio nanofillers with high interfacial bond strength are the most capable in further improving the toughness of geopolymers. Among the high aspect ratio nanofillers i.e. nanofibers, 2vol% silicon carbide whicker (SCW) showed the highest improvement in fracture toughness and flexural strength of ~164% & ~185%, respectively. After heat treatment at 650 °C, SCW reinforcement was found to be effective, with little reduction in the performance, while the performance of alumina nanofiber (ANF) reinforced geopolymer significantly reduced. By means of SEM, EDS and X-ray diffraction techniques, it was found that the longer and stronger SCW is more capable of reinforcing the microstructurally inhomogeneous geopolymer than the smaller diameter, shorter ANF. After heat treatment at 760 ºC, the effectiveness of SCW as reinforcement in both fracture toughness and flexural strength was reduced by ~89% and ~43%, respectively, while, the ANF filled materials performed worse than the neat geopolymer. A strong interaction was suggested between ANF and geopolymer at high temperature by means of chemical reactions and diffusion. SEM & X-ray diffraction results suggested the formation of Al₄C₃ on the SCW surface, which could reduce the interface strength between SCW and geopolymer. Therefore it is suggested that the interface strength should be as high as required for load transfer and crack bridging. Finally, to investigate the potential synergy of a nano-filled matrix material and the fiber/matrix interface toughening mechanism of a continuous fiber composite, composite specimens were produced and tested. Flexural and shear strengths of Nextel 610 continuous fiber reinforced 2vol% SCW filled geopolymer matrix composites were investigated. Specimens were produced with cleaned Nextel fiber and with carbon-coated fibers to investigate the combinations of nano-filled matrix with continuous reinforcement that is well bonded (cleaned fiber) versus poorly bonded (carbon-coated fiber) to the matrix. The results showed that flexural strength of cleaned and coated fiber composites improved by ~35% and ~21% respectively, while shear strength of the similar composite systems improved by ~39.5% and ~24%. The results verified the effectiveness of SCW in toughening not only the neat geopolymer, but also continuous fiber reinforced geopolymer matrix composites.
Open Access
Structural health monitoring in adhesively bonded composite joints
(Colorado State University. Libraries, 2024) Caldwell, Steven, author; Radford, Donald W., advisor; Simske, Steven, committee member; Cale, James, committee member; Adams, Henry, committee member
Composite bonded aircraft structure is a prevalent portion of today's aircraft structural composition. Adequate bond integrity is a critical aspect of the fabrication and operational service life of aircraft structure. Many of these structural bonds are critical for flight safety. Thus, a major concern is related to the assurance of quality in the structural bond. Over the last decade, non-destructive bond evaluation techniques have improved but still cannot detect a structurally weak bond that exhibits full adherend/adhesive contact. Currently, expensive, and time-consuming structural proof testing is required to verify bond integrity. The objective of this work is to investigate the feasibility of bondline integrity monitoring via piezoelectric sensors embedded in the composite joint. Initially, a complex composite joint, the Pi preform, was analytically evaluated for health monitoring viability, with the results showing promising capability. Subsequently, due to experimental complexities, a simple, state-of-the-art composite single lap shear joint is selected for experimentation and analysis to measure and quantify the effect of incorporating a sensor within the bondline to evaluate and expand on the ability of the embedded sensor to monitor and assess the joint integrity. Simple flatwise tension joints are also studied to investigate an orthogonal loading direction through the sensor. The experimental results indicate that the embedded piezoelectric sensors can measure a change in the joint before the integrity degrades and fails on subsequent loadings, resulting in a novel approach for prognostic performance evaluation without detrimentally affecting the performance of the structural joint.
Open Access
The effect of tow shearing on reinforcement positional fidelity in the manufacture of a continuous fiber reinforced thermoplastic matrix composite via pultrusion-like processing of commingled feedstock
(Colorado State University. Libraries, 2017) Warlick, Kent M., author; Radford, Donald W., advisor; Holland, Troy, committee member; Heyliger, Paul, committee member
While the addition of short fiber to 3D printed articles has increased structural performance, ultimate gains will only be realized through the introduction of continuous reinforcement placed along pre-planned load paths. Most additive manufacturing research focusing on the addition of continuous reinforcement has revolved around utilization of a prefrabricated composite filament or a fiber and matrix mixed within a hot end prior to deposition on a printing surface such that conventional extrusion based FDM can be applied. Although stronger 3D printed parts can be made in this manner, high quality homogenous composites are not possible due to fiber dominated regions, matrix dominated regions, and voids present between adjacent filaments. Conventional composite manufacturing processes are much better at creating homogeneous composites; however, the layer by layer approach in which they are made is inhibiting the alignment of reinforcement with loads. Automated Fiber Placement techniques utilize in plane bending deformation of the tow to facilitate tow steering. Due to buckling fibers on the inner radius of curves, manufacturers recommend a minimum curvature for path placement with this technique. A method called continuous tow shearing has shown promise to enable the placement of tows in complex patterns without tow buckling, spreading, and separation inherent in conventional forms of automated reinforcement positioning. The current work employs fused deposition modeling hardware and the continuous tow shearing technique to manufacture high quality fiber reinforced composites with high positional fidelity, varying continuous reinforcement orientations within a layer, and plastic elements incorporated enabling the ultimate gains in structural performance possible. A mechanical system combining concepts of additive manufacturing with fiber placement via filament winding was developed. Paths with and without tension inherent in filament winding were analyzed through microscopy in order to examine best and worst case scenarios. High quality fiber reinforced composite materials, in terms of low void content, high fiber volume fractions and homogeneity in microstructure, were manufactured in both of these scenarios. In order to improve fidelity and quality in fiber path transition regions, a forced air cooling manifold was designed, printed, and implemented into the current system. To better understand the composite performance that results from varying pertinent manufacturing parameters, the effect of feed rate, hot end temperature, forced air cooling, and deposition surface (polypropylene and previously deposited glass polypropylene commingled tow) on interply performance, microstructure, and positional fidelity were analyzed. Interply performance, in terms of average maximum load and average peel strength, was quantified through a t-peel test of the bonding quality between two surfaces. With use of forced air cooling, minor decreases in average peel strength were present due to a reduction in tow deposition temperature which was found to be the variable most indicative of performance. Average maximum load was comparable between the forced air cooled and non-air cooled samples. Microstructure was evaluated through characterization of composite area, void content, and flash percentage. Low void contents mostly between five to seven percent were attained. Further reduction of this void content to two percent is possible through higher processing temperatures; however, reduced composite area, low average peel strength performance, and the presence of smoke during manufacturing implied thermal degradation of the polypropylene matrix occurred in these samples with higher processing temperatures. Positional fidelity was measured through calculations of shear angle, shift width, and error of a predefined path. While positional fidelity variation was low with a polypropylene deposition surface, forced air cooling is necessary to achieve fidelity on top of an already deposited tow surface as evident by the fifty-six percent reduction in error tolerance profile achieved. Lastly, proof of concept articles with unique fiber paths and neat plastic elements incorporated were produced to demonstrate fiber placement along pre-planned load paths and the ability to achieve greater structural efficiency through the use of less material. The results show that high positional fidelity and high quality composites can be produced through the use of the tow shearing technique implemented in the developed mechanical system. The implementation of forced air cooling was critical in achieving fidelity and quality in transition regions. Alignment of continuous reinforcement with pre-planned load paths was demonstrated in the proof of concept article with varying fiber orientations within a layer. Combining fused deposition modeling of plastic with the placement of continuous reinforcement enabled a honeycomb composite to be produced with higher specific properties than traditional composites. Thus, the current system demonstrated a greater capability of achieving ultimate gains in structural performance than previously possible.