Browsing by Author "Yourdkhani, Mostafa, committee member"

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Open Access
Additive manufacture of dissolvable tooling for autoclave processing of fiber reinforced polymer composites
(Colorado State University. Libraries, 2022) Morris, Isaac, author; Radford, Donald, advisor; Yourdkhani, Mostafa, committee member; Heyliger, Paul, committee member
Autoclave processing of advanced fiber reinforced polymer composites (AFRPC) uses applied heat and pressure to yield high quality composite components. Geometrically accurate and thermally stable molds or tools are used to maintain the part form until the part cures and rigidizes. For high-volume production runs, molds may be made from materials such as metals, ceramics, or AFRPCs. However, tooling made from these materials can be costly to manufacture and are not suitable for low volume production runs. This is especially true for complex geometries in trapped tooling situations where the cured composite shape prevents tool separation. In this situation, composite manufacturers rely on sacrificial washout tooling materials that are machined or cast to shape to create the tool. However, these sacrificial materials still come with significant challenges. For example, the surfaces of these tools are often porous and require sealing, and their washout can result in corrosive waste that makes disposal challenging. Additionally, these tools are brittle and monolithic in nature, making them fragile to handle and slow to heat up during cure. An alternative may be to use high temperature, dissolvable thermoplastic materials in melt extrusion additive manufacturing to create complex washout tooling. However, there is a lack of information regarding the types of soluble materials and the structural configurations that make this type of tooling successful in autoclave use. To begin to address this, samples made from several materials, and one insoluble model material, were processed in stepwise fashion at increasing autoclave processing temperatures to evaluate the impacts of material and structure on autoclave robustness. Then, mid-sized composite specimens were produced on 3D-printed tooling that evaluated the interaction between the composite and the tool, including surface quality and deformation. Finally, a trapped tooling geometry was used to manufacture several composites at processing conditions of 157°C at 414kPa, well above the use temperature of the tested materials. These trials focused on reducing deformation by adjusting the tool wall thickness and vacuum bagging configuration. It was shown that 3D-printed dissolvable tooling can be used as an alternative to traditional washout tooling for autoclave processing. The materials Stratasys ST-130 and Infinite Material Solutions AquaSys 180 were used to manufacture tools that were processed at autoclave conditions of 121°C at 345kPa with minimal deformation. Surface quality was also found to be acceptable without machining or sealing, eliminating this step from the production of traditional washout tools. Finally, a modified tool design and vacuum bagging technique were demonstrated that significantly reduced the deformation of tooling at processing temperatures that significantly exceed the use temperature of the material.
Open Access
Development of a high-voltage laser triggered switch facility including initial optical and electrical diagnostics
(Colorado State University. Libraries, 2019) Rose, Charles E., author; Yalin, Azer P., advisor; Menoni, Carmen S., committee member; Yourdkhani, Mostafa, committee member
Pulsed power programs have been part of the United States strategic plan to address the nation's energy and defense needs since the 1960s. With escalating energy demand, one of the greatest challenges of our time is to develop clean and reliable energy sources with controlled fusion being an exciting and favorable candidate. Developing this technology has been an arduous and taxing effort with a breakthrough (supposedly) coming just around the corner for decades. Arguably, one of the leading testbeds for fusion research is Sandia National Laboratories (SNL) Z machine which is part of SNL's pulsed power program. The Z machine can create fusion-like conditions and allows the global research community to investigate pathways forward to a viable fusion reactor. Integral to developing future pulsed power technology and the next Z-pinch style machines, high voltage spark gap switches are an active research area and the focus of this thesis.
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
Direct digital manufacturing of uniform thickness continuous fiber grid stiffened composites through tow spreading via roller based deposition
(Colorado State University. Libraries, 2024) Ratkai, Harry, author; Radford, Donald, advisor; Yourdkhani, Mostafa, committee member; Heyliger, Paul, committee member
Grid stiffened structures are an effective method for lightweighting designs. While continuous fiber composites are attractive materials for creating grid stiffened structures, there are two major impediments to the wider acceptance of such structures: the high capital costs for manufacturing and the material buildup at the crossover points. The high capital costs not only come from the complex tooling but also from the need to cure the parts after deposition. The material buildup at the crossover points is not only geometrically undesirable but can reduce the mechanical performance of the part. Many options to overcome this additional thickness have been implemented, but the majority cut the continuous fiber at the crossover, further reducing the performance. Previous work at Colorado State University has demonstrated that crossovers can be manufactured using a nozzle-based gantry printer and continuous glass fiber/PET commingled tow with a minimal thickness buildup at the crossover, all with radically reduced tooling, without compromising the structural performance. Unfortunately, the direct digital manufacturing system used did not utilize a cut and refeed system for the commingled tow; thus requiring the part to be made using continuous pathing or for a person to manually stop, cut and restart the tow at the end/beginning of each discrete path. These shortfalls of the nozzle-based printer make this technology, in its current form, impractical for adoption by industry. This work details the development of a robotic end effector for a new manufacturing method utilizing a heated roller for deposition and a programmable cut and refeed system. Initially, a comparison of the two methods of deposition, nozzle and roller, was done; both systems made crossover samples where part thickness and void and fiber volume fractions were measured. Next, an optimization of process parameters was performed on the beam and crossover sections, separately, for the roller-based end effector. Both the beams and crossovers were evaluated using thickness measurements, void and fiber volume fraction measurements and microscope imaging. Finally, a molding shoe was attached to the end effector to determine the effectiveness of molding the beam side walls, in-situ. It was demonstrated that the roller-based system can manufacture grid stiffened parts with less thickness deviations and fewer voids then the nozzle-based system. Additionally, optimized processing parameters were found for beams at three different deposition speeds, 450mm/min, 600mm/min and 750mm/min. Under the best conditions. The system is capable of direct digital manufacture of continuous fiber reinforced composite grids with under 2% void content. By slowing the deposition speed and increasing the consolidation force at the crossover points, the system is able to spread and thin the tow, thus, minimizing the thickness buildup at the crossover points. Using the understanding developed in determining optimized parameter two additional demonstrations of the capabilities of the system were completed: a preliminary example of full molding of the grid cross-section and the manufacture of curvilinear grids via in-plane steering. Combined, the outcomes demonstrate that a roller-based system with cut and refeed can produce grid stiffened structures with discrete fiber paths, that have crossovers of uniform thickness, at higher deposition rates than previous nozzle-based technology.
Open Access
Mechanistic visco-elastic modeling of shear deformation and failure in internally-reinforced geosynthetic clay liners
(Colorado State University. Libraries, 2022) Baukus, Aaron, author; Bareither, Christopher A., advisor; Scalia, Joseph, committee member; Yourdkhani, Mostafa, committee member
Analysis and prediction of the shear behavior of a non-heat-treated needle punched geosynthetic clay liner (NHT NP GCL) have been conducted using a mechanistic model. A three-element Kelvin-chain model was employed to simulate the incremental loading of a rapid loading shear test. A performance analysis initially was conducted to evaluate variation in model parameters with respect to differences in physical properties of GCLs (i.e., peel strength) and experimental conditions (i.e., normal stress, temperature, creep shear stress). The optimized model parameters demonstrated sensitivity to the variation in internal and external factors and yielded empirical relationships that were carried forward to test model applicability for predicting time-to-failure for an internally-reinforced GCL. These data trends in combination with creep-test data were used to calibrate the creep deformation model. Time-to-failure predictions performed with the calibrated creep deformation model resulted in a percent error < 9%. A modified model-calibration procedure was developed to extend model applicability to stress conditions common in practice. The modified calibration procedure was used to predict NHT NP GCL creep deformation in a hypothetical landfill cover system. The time required for the projected deformation to surpass 3 mm exceeded one million years for all stress conditions evaluated, which suggested that the NHT NP GCL will not experience creep failure in the low-stress cover scenarios evaluated.
Open Access
On the integration of materials characterization into the product development lifecycle
(Colorado State University. Libraries, 2024) Dare, Matthew S., author; Simske, Steve, advisor; Yourdkhani, Mostafa, committee member; Herber, Daniel, committee member; Radford, Donald W., committee member
The document is broken down into four sections whereby a more complete integration of materials characterization into the product development lifecycle, when compared to traditional approaches, is researched and considered. The driving purpose behind this research is to demonstrate that an application of systems engineering principles to the characterization sciences mechanism within materials engineering and development will produce a more efficient and comprehensive understanding of complex material systems. This will allow for the mitigation of risk, enhancement of relevant data, and planning of characterization procedures proactively. The first section proposes a methodology for Characterization Systems Engineering (CSE) as an aid in the development life cycle of complex, material systems by combining activities traditionally associated with materials characterization, quality engineering, and systems engineering into an effective hybrid approach. The proposed benefits of CSE include shortened product development phases, faster and more complete problem solving throughout the full system life cycle, and a more adequate mechanism for integrating and accommodating novel materials into already complex systems. CSE also provides a platform for the organization and prioritization of comprehensive testing and targeted test planning strategies. Opportunities to further develop and apply the methodology are discussed. The second section focuses on the need for and design of a characterizability system attribute to assist in the development of systems that involve material components. While materials characterization efforts are typically treated as an afterthought during project planning, the argument is made here that leveraging the data generated via complete characterization efforts can enhance manufacturability, seed research efforts and intellectual property for next-generation projects, and generate more realistic and representative models. A characterizability metric is evaluated against a test scenario, within the domain of electromagnetic interference shielding, to demonstrate the utility and distinction of this system attribute. Follow-on research steps to improve the depth of the attribute application are proposed. In the third section, a test and evaluation planning protocol is developed with the specific intention of increasing the effectiveness of materials characterization within the system development lifecycle. Materials characterization is frequently not accounted for in the test planning phases of system developments, and a more proactive approach to streamlined verification and validation activities can be applied. By applying test engineering methods to materials characterization, systems engineers can produce more complete datasets and more adequately execute testing cycles. A process workflow is introduced to manage the complexity inherent to material systems development and their associated characterization sciences objectives. An example using queuing theory is used to demonstrate the potential efficacy of the technique. Topics for further test and evaluation planning for materials engineering applications are discussed. In the fourth section, a workflow is proposed to more appropriately address the risk generated by materials characterization activities within the development of complex material systems when compared to conventional engineering approaches. Quality engineering, risk mitigation efforts, and emergency response protocols are discussed with the intention of reshaping post-development phase activities to address in-service material failures. While root cause investigations are a critical component to stewardship of the full system lifecycle during a product's development, deployment and operation, a more tailored and proactive response to system defects and failures is required to meet the increasingly stringent technical performance requirements associated with modern, material-intensive systems. The analysis includes a Bayesian approach to risk assessment of materials characterization efforts through which uncertainty regarding scheduling and cost can be quantified.
Open Access
Soft and shape morphing robots driven by twisted-and-coiled actuators
(Colorado State University. Libraries, 2022) Sun, Jiefeng, author; Zhao, Jianguo, advisor; Maciejewski, Anthony, committee member; Gao, Xinfeng, committee member; Yourdkhani, Mostafa, committee member
Soft robots are a new type of robot with deformable bodies and muscle-like actuation, which are fundamentally different from traditional robots with rigid links and motor-based actuators. Owing to their elasticity, soft robots outperform rigid ones in safety, maneuverability, and adaptability. With their advantages, many soft robots have been developed for manipulation and locomotion in recent years. To enable their unique capabilities, soft robots require a key component—the actuator. Many different actuators have been used, including the conventional pneumatic-driven and cable-driven methods, as well as several novel approaches proposed recently such as combustion, dielectric elastomers, chemical reactions, liquid–vapor transition, liquid crystal elastomer, and shape memory alloy. Besides existing actuation approaches, another promising actuator for soft robots is the twisted-and-coiled actuator (TCA). Compared with existing actuation methods, TCAs exhibit several unique characteristics: like large energy density and being directly actuated by electricity with a small voltage. All of these characteristics will potentially enable small-scale and untethered soft robots that in general are difficult to be accomplished by pneumatic and tendon-driven methods. Further, unlike shape memory alloys, TCAs are intrinsically soft, making it possible to embed them in any shape inside a soft body to generate versatile motion. To better actuate soft robots with TCAs, we introduce a novel fabrication technique of contraction TCAs that will have uniform initial gaps between neighboring coils. In this case, they can contract larger than 48% without a preload, termed free stroke. We also characterize such a TCA and compare it with self-coiled TCAs. Besides the free stroke property, the TCA can also be directly used as a sensor that provides its displacement information. To better design, optimize, and control TCAs for various applications, we developed a physics-based model based on TCAs' physical parameters as opposed to system identification methods, since such physics-based models are expected to be a general model for different types of TCAs (self-coiled, free-stroke, conical) We demonstrate soft robots with programmable motions by placing TCAs in different shapes inside a soft body. Specifically, we embed TCAs in a curved U shape, a helical shape, and straight shapes in parallel to enable three different motions: two-dimensional bending, twisting, and three-dimensional bending. We also combine the three motions to demonstrate a completely soft robotic arm that mimics a human forearm. A model is also developed to simulate the TCA-driven soft robots. The framework can model 1) the complicated routes of multiple TCAs in a soft body and 2) the coupling effect between the soft body and the TCAs during their actuation process. When not actuated, a TCA in the soft body is an antagonistic elastic element that restrains the magnitude of the motion and increases the stiffness of the robot. By stacking several modules together, we simulate the sequential motion of a soft robotics arm with three-dimensional bending, twisting, and grasping motion. The presented modeling and simulation approach will facilitate the design, optimization, and control of soft robots driven by TCAs or other types of artificial muscles. Finally, we design shape morphing robots that can morph the shape of their bodies to adapt to a different environment. These robots can be built with shape-morphing modules. A shape-morphing module has a variable stiffness element that allows it to switch between soft and rigid states. While it is in a soft state, it can morph to different configurations driven by TCAs. We demonstrate robots built with these modules can morph to different shapes that facilitate grasping and locomotion.
Open Access
Synthesis of monolayer MoS₂ via chemical vapor deposition
(Colorado State University. Libraries, 2020) Varra, Travis, author; Sambur, Justin, advisor; Prieto, Amy, committee member; Yourdkhani, Mostafa, committee member
Two-dimensional materials, specifically transition metal dichalcogenides (TMDs), have emerged as ideal candidates for lightweight and flexible optoelectronic applications. Unlike bulk solids, single layer TMDs exhibit a direct bandgap that makes next-generation device applications possible. This work describes the synthesis of single layer MoS2 via chemical vapor deposition (CVD). This method involves thermal vaporization of MoO3 and S precursors in a tube furnace. The influence of reaction conditions (e.g., temperature, pressure, reaction holding time, carrier gas flow rate, and precursor separation distance) on MoS2 sample morphology was quantified using optical microscopy. Isolated equilateral triangles with 11 μm-long edge lengths were reproducibly grown on Si/SiO2 substrates. The layer thickness was determined using Raman and photoluminescence spectroscopy.