Browsing by Author "Heyliger, Paul, committee member"

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Open Access
A Haversian bone model of fracture healing in a simulated microgravity environment
(Colorado State University. Libraries, 2015) Gadomski, Benjamin C., author; Puttlitz, Christian M., advisor; Browning, Raymond, committee member; Donahue, Tammy, committee member; Heyliger, Paul, committee member
Ground-based models of weightlessness and microgravity have provided valuable insights into how dynamic physiological systems adapt or react to reduced loading. Almost all of these models have used rodent hindlimb unloading as the means to simulate microgravity on isolated physiological systems. Unfortunately, results derived from rodent studies are significantly limited when one tries to translate them to the human condition due to significant anatomical and physiological differences between the two species. Therefore, it is clear that a novel animal model of ground-based weightlessness that is directly translatable to the human condition must be developed in order for substantial progress to be made in the knowledge of how microgravity affects fracture healing. In light of this, four specific aims are proposed: (1) develop a ground-based, ovine model of skeletal unloading in order to simulate weightlessness, (2) interrogate the effects of the simulated microgravity environment on bone fracture healing using this large animal model, (3) develop a computational model of weightbearing in ovine bone under different experimental conditions in order to characterize the loads experienced by the fracture site, and (4) develop countermeasures that enhance bone fracture healing in the presence of simulated microgravity. Successful completion of this project will substantially elevate the understanding of how fracture site loading affects the subsequent healing cascade in the presence of microgravity and will form the foundation for designing future rehabilitation protocols to facilitate bone healing during long-duration spaceflight.
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
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
Additive manufacturing of an intervertebral disc repair patch to treat spinal herniation
(Colorado State University. Libraries, 2021) Page, Mitchell Ian, author; Puttlitz, Christian, advisor; Heyliger, Paul, committee member; Susan, James, committee member; Kirk, McGilvray, committee member
Chronic low back pain is ubiquitous throughout society. The consequences of this disease are extensive and lead to physical, mental, and financial suffering in the affected population. Herniation of the intervertebral disc (IVD) is the primary cause of chronic low back pain due to the essential mechanical role of the IVD in the spinal column. Degenerative changes to the IVD tissues, in particular the annulus fibrosus (AF), lead to a pronounced vulnerability to herniation. Although numerous treatments for intervertebral disc herniation currently exist, these treatments are typically palliative and prone to hernia recurrence. Accordingly, there is a distinct need for an IVD hernia therapy that can provide long-term pain relief and recovery of spinal function. One novel strategy to repair the intervertebral disc is to tissue-engineer a construct that facilitates regeneration of the healthy and functional IVD tissue. Advances in additive manufacturing technology offer the fabrication of complex tissue-engineered structures that augment biological content and biocompatible materials. Therefore, this work sought to engineer an additive manufactured repair patch for IVD herniation towards an improved treatment for chronic low back pain. Specifically, the aims of this work were to leverage experimental and computational methods to: (1) to characterize the mechanics of additive manufactured angle-ply scaffolds, (2) evaluate the tissue response of cell-laden scaffolds cultured with dynamic biaxial mechanical stimuli, and (3) to design and implement an annulus fibrosus repair patch. The mechanics of additive manufactured scaffolds for AF repair were experimentally characterized in a physiologically-relevant, biaxial loading modality. To assess sensitivity of the scaffold mechanics to additive manufacturing parameters, a broad scope of scaffold designs were evaluated with a parameterized finite element model. A custom incubator was developed, cell-laden scaffolds were cultured with a prescribed, multi-axial mechanical loading protocol, and ECM production within the scaffold was evaluated. A finite element model was developed to aid in understanding the relationship between global scaffold loading and the local, inhomogeneous cellular micromechanical environment within the scaffold. The developed TE material was translated into an implant and was implemented in a large animal model. The efficacy of the AF repair strategy was also evaluated in finite element model of the human lumbar spine. This work formed a multi-scale approach to consolidate biological and mechanical efficacy of a novel AF repair strategy. Ultimately, this approach may facilitate regeneration of the AF and represent a revolutionary treatment for chronic low back pain.
Open Access
An analysis of domain decomposition methods using deal.II
(Colorado State University. Libraries, 2021) Rigsby, Christina, author; Tavener, Simon, advisor; Bangerth, Wolfgang, committee member; Heyliger, Paul, committee member; Liu, Jiangguo, committee member
Iterative solvers have attracted significant attention since the mid-20th century as the computational problems of interest have grown to a size beyond which direct methods are viable. Projection methods, and the two classical iterative schemes, Jacobi and Gauss-Seidel, provide a framework in which many other methods may be understood. Parallel methods or Jacobi-like methods are particularly attractive as Moore's Law and computer architectures transition towards multiple cores on a chip. We implement and explore two such methods, the multiplicative and restricted additive Schwarz algorithms for overlapping domain decomposition. We implement these in deal.II software, which is written in C++ and uses the finite element method. Finally, we point out areas for potential improvement in the implementation and present a possible extension of this work to an agent-based modeling prototype currently being developed by the Air Force Research Laboratory's Autonomy Capability Team (ACT3).
Open Access
Assessment of the effects of ligamentous injury in the human cervical spine
(Colorado State University. Libraries, 2012) Leahy, Patrick Devin, author; Puttlitz, Christian, advisor; Heyliger, Paul, committee member; Sakurai, Hiroshi, committee member; Santoni, Brandon, committee member
Ligamentous support is critical to constraining motion of the cervical spine. Injuries to the ligamentous structure can allow hypermobility of the spine, which may cause deleterious pressures to be applied to the enveloped neural tissues. These injuries are a common result of head trauma and automobile accidents, particularly those involving whiplash-provoking impacts. The injuries are typically relegated to the facet capsule (FC) and anterior longitudinal (ALL) ligaments following cervical hyperextension trauma, or the flaval (LF) and interspinous (ISL) ligaments following hyperflexion. Impacts sustained with the head turned typically injure the alar ligament. The biomechanical sequelae resulting from each of these specific injuries are currently ill-defined, confounding the treatment process. Furthermore, clinical diagnosis of ligamentous injuries is often accomplished by measuring the range of motion (ROM) of the vertebrae, where current methods have difficulty differentiating between each type of ligamentous injury. Pursuant to enhancing treatment and diagnosis of ligamentous injuries, a finite element (FE) model of the intact human full-cervical (C0-C7) spine was generated from computed tomography (CT) scans of cadaveric human spines. The model enables the quantification of ROM, stresses, and strains, and can be modified to reflect ligamentous injury. In order to validate the model, six human, cadaveric, full-cervical spines were tested under pure ±1.5 Nm moment loadings in the axial rotation, lateral bending, flexion, and extension directions. ROM for each vertebra, facet contact pressures, and cortical strains were experimentally measured. To evaluate injured ligament mechanical properties, a novel methodology was developed where seven alar, fourteen ALL, and twelve LF cadaveric bone-ligament-bone preparations were subjected to a partial-injury inducing, high-speed (50 mm/s) tensile loading. Post-injury stiffnesses and toe region lengths were compared to the pre-injury state for these specimens. These experimental data were incorporated into the FE model to analyze the kinematic and kinetic effects of partial ligamentous injury. For comparison, the model was also adapted to reflect fully injured (transected) ligaments. Injuries simulated at the C5-C6 level included: 1) partial FC injury, 2) full FC injury, 3) partial FC and ALL injury, 4) full FC and ALL injury, 5) partial LF and full ISL jury, 6) full LF and ISL injury, 7) partial FC, ALL, LF, and full ISL injury, and 8) full FC, ALL, LF, and ISL injury. The model was also modified to replicate injury to the right alar ligament. Five cadaveric cervical spines were tested under pure moment conditions with scalpel-sectioning of these ligaments for validation of the full-injury models. Comparisons between the intact and various injury cases were made to determine the biomechanical alterations experienced by the cervical spine due to the specific ligamentous injuries. Variances in ROM and potential impingement on the neural tissues were focused upon. The overarching goals of the study were to identify a unique kinematic response for each specific ligamentous injury to allow for more accurate clinical diagnosis, and to enhance the understanding of the post-injury biomechanical sequelae.
Open Access
Bio-inspired design for engineering applications: empirical and finite element studies of biomechanically adapted porous bone architectures
(Colorado State University. Libraries, 2020) Aguirre, Trevor Gabriel, author; Donahue, Seth W., advisor; Ma, Kaka, committee member; Heyliger, Paul, committee member; Simske, Steven, committee member
Trabecular bone is a porous, lightweight material structure found in the bones of mammals, birds, and reptiles. Trabecular bone continually remodels itself to maintain lightweight, mechanical competence, and to repair accumulated damage. The remodeling process can adjust trabecular bone architecture to meet the changing mechanical demands of a bone due to changes in physical activity such as running, walking, etc. It has previously been suggested that bone adapted to extreme mechanical environments, with unique trabecular architectures, could have implications for various bioinspired engineering applications. The present study investigated porous bone architecture for two examples of extreme mechanical loading. Dinosaurs were exceptionally large animals whose body mass placed massive gravitational loads on their skeleton. Previous studies investigated dinosaurian bone strength and biomechanics, but the relationships between dinosaurian trabecular bone architecture and mechanical behavior has not been studied. In this study, trabecular bone samples from the distal femur and proximal tibia of dinosaurs ranging in body mass from 23-8,000 kg were investigated. The trabecular architecture was quantified from micro-computed tomography scans and allometric scaling relationships were used to determine how the trabecular bone architectural indices changed with body mass. Trabecular bone mechanical behavior was investigated by finite element modeling. It was found that dinosaurian trabecular bone volume fraction is positively correlated with body mass like what is observed for extant mammalian species, while trabecular spacing, number, and connectivity density in dinosaurs is negatively correlated with body mass, exhibiting opposite behavior from extant mammals. Furthermore, it was found that trabecular bone apparent modulus is positively correlated with body mass in dinosaurian species, while no correlation was observed for mammalian species. Additionally, trabecular bone tensile and compressive principal strains were not correlated with body mass in mammalian or dinosaurian species. Trabecular bone apparent modulus was positively correlated with trabecular spacing in mammals and positively correlated with connectivity density in dinosaurs, but these differential architectural effects on trabecular bone apparent modulus limit average trabecular bone tissue strains to below 3,000 microstrain for estimated high levels of physiological loading in both mammals and dinosaurs. Rocky Mountain bighorn sheep rams (Ovis canadensis canadensis) routinely conduct intraspecific combat where high energy cranial impacts are experienced. Previous studies have estimated cranial impact forces up to 3,400 N and yet the rams observationally experience no long-term damage. Prior finite element studies of bighorn sheep ramming have shown that the horn reduces brain cavity translational accelerations and the bony horncore stores 3x more strain energy than the horn during impact. These previous findings have yet to be applied to applications where impact force reduction is needed, such as helmets and athletic footwear. In this study, the velar architecture was mimicked and tested to determine suitability as novel material architecture for running shoe midsoles. It was found that velar bone mimics reduce impact force (p < 0.001) and higher energy storage during impact (p < 0.001) and compression (p < 0.001) as compared to traditional midsole architectures. Furthermore, a quadratic relationship (p < 0.001) was discovered between impact force and stiffness in the velar bone mimics. These findings have implications for the design of novel material architectures with optimal stiffness for minimizing impact force.
Embargo
Computational methods for the analysis of cell migration and motility
(Colorado State University. Libraries, 2024) Havenhill, Eric Colton, author; Ghosh, Soham, advisor; Heyliger, Paul, committee member; McGilvray, Kirk, committee member; Zhao, Jianguo, committee member
Collective cell migration (CCM) is necessary for many biological processes, such as in the formation or regeneration of tissue, fibroblast movement in wound healing, and the movement of macrophages and neutrophils in the body's immune response, to name a few. CCM is commonly modeled with PDEs, however these equations usually model the population density, rather than the displacement field describing the movement of any arbitrary cell. One unknown aspect of this movement is the various methods that cells use to facilitate communication to each other. Chemical communication plays a substantial role in directed cell movement, however, other mechanical methods, such as the propagation of stresses through a shared substrate to neighboring cells and cell behavior in a crowded environment, also play an important role which is less understood. The quantification of the kinematic and dynamic characteristics in CCM would present several novel advancements in understanding the collective cell behavior. First, the dynamic mode decomposition (DMD) framework is utilized. DMD allows for the recovery of a dynamic system, in the form of an ODE or PDE, by sampling the states of a system. In the context of the cell migration, the displacements of fibroblasts during a scratch-wound assay are obtained, which result in a governing PDE through the DMD process. This PDE is used in conjunction with modern optimal control theory to develop a 2D and 3D trajectory for the migration of controllable cells to a target. On an individual level, with the hybrid use of modern static structural optimization and simple non-linear control, a cell's cytoskeleton during migration can be studied, providing for the quantification of the traction force exerted on the substrate. The results of this analysis are compared with stress and structural optimization models in ANSYS and FEBio, which uses the finite element method, so that a reasonable range of these stresses during CCM can be provided. To further study the individual mechanics of cell migration, the proposed hybrid model is extended to a fully dynamic model which predicts the cytoskeletal stress fiber formations over time that require the minimal amount of material with the use of optimal control theory. The results of this research could provide useful applications in many real-world situations, from the generating of a trajectory for microrobots during drug delivery to the study of the collective migration of organisms including cells.
Open Access
Constitutive modeling of the biaxial mechanics of brain white matter
(Colorado State University. Libraries, 2016) Labus, Kevin M., author; Puttlitz, Christian M., advisor; Donahue, Seth, committee member; Heyliger, Paul, committee member; James, Susan, committee member
It is important to characterize the mechanical behavior of brain tissue to aid in the computational models used for simulated neurosurgery. Due to its anisotropy, it is of particular interest to develop constitutive models of white matter based on experimental data in order to define the material properties in computational models. White matter has been shown to exhibit anisotropic, hyperelastic, and viscoelastic properties. The majority of studies have focused on the shear or compressive properties, while few have tested the tensile properties of the brain. Brain tissue has not previously been tested in a multi-axial loading state, even though in vivo brain tissue is in a constant multi-axial stress state due to fluid pressure, and data from uniaxial experiments do not sufficiently describe multi-axial stresses. The main objective of this project was to characterize the biaxial tensile behavior of brain white matter via experimentation and constitutive modeling. A biaxial experiment was developed specifically for testing brain tissue. Experiments were performed at a quasi-static loading rate, and an Ogden anisotropic hyperelastic model was derived to fit the data. A structural analysis was performed on biaxially tested specimens to relate the structure to the mechanical behavior. The axonal orientation and distribution were measured via histology, and the axon area fraction was measured via transmission electron microscopy. The measured structural parameters were incorporated into the constitutive model. A probabilistic analysis was performed to compare the uncertainty in the stress predictions between models with and without structural parameters. Finally, dynamic biaxial experiments were performed to characterize the anisotropic viscoelastic properties of white matter. Biaxial stress-relaxation experiments were conducted to determine the appropriate form of a viscoelastic model. It was found that the data were accurately modeled by a quasi-linear viscoelastic formulation with an isotropic reduced relaxation tensor and an instantaneous elastic stress defined by an anisotropic Ogden model. Model fits to the stress-relaxation experiments were able to accurately predict the results of dynamic cyclic experiments. The resulting constitutive models from this project build upon previous models of brain white matter mechanics to include biaxial interactions and structural relations, thus improving computational model predictions.
Open Access
Design, fabrication, and characterization of 3D printed ceramic scaffolds for bone regeneration
(Colorado State University. Libraries, 2024) Baumer, Vail Olin, author; Prawel, David, advisor; McGilvray, Kirk, committee member; Heyliger, Paul, committee member
Synthetic bone tissue scaffolds are a promising alternative to current clinical techniques for treating critically large bone defects. Scaffolds provide a three-dimensional (3D) environment that mimics the properties of bone to accelerate bone regeneration. Optimal scaffolds should match the mechanical properties of the implantation site, feature a highly porous network of interconnected channels to facilitate mass transport, and exhibit surface properties for the attachment, proliferation, and differentiation of bone cell lineages. 3D printing has enabled the manufacture of complex scaffold topologies that meet these requirements in a variety of biomaterials which has led to rapidly expanding research. Structural innovations such as triply periodic minimal surfaces (TPMS) are enabling the production of scaffolds that are stiffer and stronger than traditional rectilinear topologies. TPMS are proving to be ideal candidates for bone tissue engineering (BTE) due to their relatively high mechanical energy absorption and robustness, interconnected internal porous structure, scalable unit cell topology, and smooth internal surfaces with relatively high surface area per volume. Among the material options, calcium phosphate-based ceramics, such as hydroxyapatite and tricalcium phosphate, are popular for BTE due to their high levels of bioactivity (osteoconductivity, osteoinductivity and osteointegration), compositional similarities to human bone mineral, non-immunogenicity, tunable degradation rates, and promising drug delivery capabilities. Despite the potential for TPMS ceramic scaffolds in BTE, few studies have explored beyond the popular Gyroid topology. Of the many TPMS options, the Fischer Koch S (FKS) has been simulated to be stronger, be more isotropic, have higher surface area, and absorb more energy than Gyroid at high porosities. In this report, we present a method for photocasting any TPMS in hydroxyapatite which is used to 3D print the first FKS ceramic scaffold. Results indicated that the resolution and accuracy of the process is suitable for BTE, and the custom software for producing the scaffolds was made available to the open-source community. Then, FKS and Gyroid scaffolds were designed to match the properties of trabecular bone using this method for use in critical bone defect repair. The scaffolds were printed and characterized using compressive and flow-based testing to reveal that, while both designs could mimic the low end of natural bone performance, the FKS were 32% stronger and only 11% less permeable than Gyroid. These findings emphasized the need for further characterization of these scaffolds beyond mechanical analysis and into studies of cell growth. To accomplish this, a custom multi-channel perfusion bioreactor was designed to culture cells on these scaffolds to investigate differences in cell behavior with higher efficiency than current designs. The design, capable of culturing many samples simultaneously, was validated using computational fluid dynamics and cell growth assays to demonstrate osteogenic effects and repeatability. In this work, novel TPMS scaffolds were fabricated from hydroxyapatite with sufficient accuracy and quality for large defects, testing of these scaffolds matched trabecular bone performance and suggested that FKS may be superior to Gyroid, and lastly, a four-channel bioreactor system was designed and validated to enable researchers to further characterize scaffolds for BTE.
Open Access
Determination of seismic performance factors for cross laminated timber shear wall system based on FEMA P695 methodology
(Colorado State University. Libraries, 2018) Amini, Mohammad Omar, author; van de Lindt, John W., advisor; Mahmoud, Hussam, committee member; Heyliger, Paul, committee member; Senior, Bolivar, committee member
Cross Laminated Timber (CLT) was initially introduced in Europe and has recently gained popularity in North America where it is seen as a sustainable alternative to steel and concrete in midrise construction. Although most CLT structures to date have been constructed in low seismic regions, recent tests have indicated that CLT based lateral force resisting systems can successfully be utilized in regions of higher seismicity. Despite the many advantages that CLT offers, the lack of a design code and systematic design procedure is one of many challenges inhibiting widespread adoption of CLT in the US. The purpose of this study was to investigate the seismic behavior of CLT based shear wall systems and determine seismic performance factors, namely, the response modification factor (R-factor), the system overstrength factor (Ω), and the deflection amplification factor (Cd), using the FEMA P695 procedure. The methodology is an iterative process that includes establishing design requirements, developing archetypes, performing a series of tests, developing and validating nonlinear models, nonlinear static and dynamic analysis, and evaluating performance; all in conjunction with a peer panel to provide input. Nine index buildings that include, single-family dwellings, multi-family dwellings, and commercial (including mixed-use) mid-rise buildings were developed. Archetypes were then extracted from these index buildings. Testing performed at the component and subassembly levels include connector tests and isolated shear wall tests. A subsequent full-scale shake table test was performed for system level demonstration. A critical aspect of this study is use of generic connectors whose properties are already addressed by a design specification to facilitate building code recognition. Test-based performance for these generic connectors is reported as part of this study to facilitate evaluation of proprietary alternatives for seismic equivalence. Connector tests were performed on angle brackets, used for attachment of the wall to the supporting element, and inter-panel connectors. These tests showed connector thickness to be important in achieving the desired ductile behavior with lesser thickness (12 gauge) being the more favorable. Quasi-static cyclic tests were conducted for a portfolio of CLT shear walls to systematically investigate the effects of various parameters. CLT demonstrated rigid behavior with energy dissipation concentrated in the connectors. Boundary constraints and gravity loading were both found to have a beneficial effect on the wall performance, i.e. higher strength and deformation capacity. Specific gravity also had a significant effect on wall behavior while CLT thickness was less influential. Higher aspect ratio panels (4:1) demonstrated lower stiffness and substantially larger deformation capacity compared to moderate aspect ratio panels (2:1). However, based on the test results there is likely a lower bound for aspect ratio (at 2:1) where it ceases to benefit deformation capacity of the wall. Multi-panel configuration comprised of high aspect ratio panels connected through vertical joint demonstrated considerably larger deformation capacity. Shake table tests showed the proposed system's potential to meet life-safety code requirements and its applicability in US seismic regions. A CLT shear wall design method was developed and refined based on the test results. Phenomenological models were used in modeling CLT shear walls. The archetypes were designed based on the proposed design method and were numerically evaluated by assessing their performance using nonlinear static and dynamic analyses. Based on the rigorous process, an R factor of 3 is proposed for the CLT shear wall systems and an R factor of 4 is proposed for the cases with high aspect ratio panels only. Results from the study will be proposed for implementation in the seismic design codes and standards in the US.
Open Access
Development of novel mechanical diagnostic techniques for early prediction of bone fracture healing outcome
(Colorado State University. Libraries, 2021) Wolynski, Jakob G., author; McGilvray, Kirk, advisor; Puttlitz, Christian, advisor; Heyliger, Paul, committee member; James, Susan, committee member; Wang, Zhijie, committee member
To view the abstract, please see the full text of the document.
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
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
Dynamic structural analysis of ramming in bighorn sheep
(Colorado State University. Libraries, 2015) Drake, Aaron Michael, author; Haut Donahue, Tammy L., advisor; Donahue, Seth W., advisor; Stansloski, Mitchell, committee member; Heyliger, Paul, committee member
Concussions are the most common traumatic brain injury and are caused by impulsive loads applied to the skull, resulting in relative motion of the brain within the brain cavity. Despite wearing helmets, athletes involved in full contact sports, such as football, are highly susceptible to concussive injuries. Short term symptoms of concussions include nausea, headache and confusion and there is evidence of more serious, long term effects from repeated concussions. Furthermore, the physical mechanisms of concussions are not well understood, making them difficult to diagnose and treat clinically. Male bighorn sheep sustain massive impact loads to the head during ramming, which is done as a means of determining hierarchy and gaining mating privileges. These large animals thrust themselves, horns first, at one another and collide violently, repeating this ritual for up to several hours until the subdominant male succumbs. After a collision, the animals are stunned momentarily but otherwise appear to suffer no ill effects, based on behavioral observations. This simple fact provided the motivation to examine the dynamic structural behavior of bighorn sheep horns and skulls. For reference, the average translational brain cavity accelerations observed during finite element model impact were found to be 111g (1091 m/s²) and impacts thought to be damaging to human brains occur at around 100g. A dynamic finite element impact model was produced using the geometry, obtained from a CT scan, of a mature male bighorn sheep’s skull and horns. Quantitative and qualitative results of the simulation were examined to determine mechanisms of energy dissipation and stress distribution during an idealized impact event. Video analysis of particularly forceful ramming sequences of wild bighorn sheep was carried out to estimate the dynamics involved with ramming. In order to investigate the relative contributions of the horn curl as well as the internal foamy bone architecture, three separate finite element models were produced. One model had one half of the horn length removed, another had the internal foam-like bone removed and these models were compared to the fully intact model to determine the structural contributions of these features during impact. Removing one half of the horn curl had the effect of increasing the peak brain cavity translational acceleration by 49%. Eliminating the internal foamy bone architecture resulted in a dramatic 442% increase in brain cavity rotational accelerations. The dynamic (vibrational) response of bighorn sheep horns and skulls was investigated using two, related methods: finite element modal analysis and experimental modal analysis. The finite element modal analysis revealed five dominant natural frequencies with values ranging from 118 to 309 Hz. Experimental modal analysis revealed several natural frequencies between 100 and 300 Hz, however, differentiating specific modes was difficult. For both vibrational analyses the dominant vibrational mode shape was side-to-side oscillations of the horn tip. This study hopes to promote and guide further research on the mechanisms of brain trauma prevention in bighorn sheep, with an emphasis on the structural and material characteristics of the horn and skull, to increase our understanding of, and ways to prevent traumatic brain injuries in humans.
Open Access
Effect of bone geometry on stress distribution patterns in the equine metacarpophalangeal joint
(Colorado State University. Libraries, 2012) Easton, Katrina L., author; Kawcak, Chris, advisor; McIlwraith, Wayne, committee member; Puttlitz, Christian, committee member; James, Susan, committee member; Shelburne, Kevin, committee member; Heyliger, Paul, committee member
Catastrophic injury of the equine metacarpophalangeal joint is of major concern for both the equine practitioner and the American public. It is one of the major reasons for retirement and sometimes euthanasia of Thoroughbred racehorses. The most common type of catastrophic injury is fracture of the proximal sesamoid bones and lateral condyle of the third metacarpal bone. Many times these injuries are so disastrous that there is no possibility of fixing them. Even in the injuries that are able to be fixed, complications arising from the fracture such as support limb laminitis may ultimately lead to the demise of the horse. Therefore, prevention of these types of injuries is key. In order to decrease the incidence of injury, it is important to understand the risk factors and pathogenesis of disease that leads to them. This project was established to create a finite element model of the equine metacarpophalangeal joint in order to investigate possible risk factors, namely bone geometry, and its effect on the stress distribution pattern in the joint. The first part of the study involved in vitro experiments in order to provide a comprehensive dataset of ligament, tendon, and bone strain and pressure distribution in the joint with which to validate the finite element model. Eight forelimbs from eight different horses were tested on an MTS machine to a load equivalent to that found at the gallop. Beyond providing data for validation, the study was the first to the author's knowledge to measure surface contact pressure between the distal condyles of the third metacarpal bone and the proximal sesamoid bones. A pressure distribution pattern that could lead to an area of high tension in the area of the parasagittal groove was found. This result could help explain the high incidence of lateral condylar fractures that initiate in this location. The second part of this study focused on the development and validation of a finite element model of the metacarpophalangeal joint. A model was created based on computed tomography (CT) data. It included the third metacarpal bone, the proximal phalanx, the proximal sesamoid bones, the suspensory ligament, medial and lateral collateral ligaments, medial and lateral collateral sesamoidean ligaments, medial and lateral oblique sesamoidean ligaments, and the straight sesamoidean ligament. The mesh resolution was varied to create three models to allow for convergence. The converged model was then validated using data from the previous part of the study as well as data from the literature. The result was a finite element model containing 121,533 nodes, 112,633 hexahedral elements, and 10 non-linear springs. The final section of this study used the converged and validated finite element model to study the effect of varying bone geometry. The model was morphed based on CT data from three horses: control, lateral condylar fracture, and contralateral limb to lateral condylar fracture. There was an area similar between all three groups of increased stress in the palmar aspect of the parasagittal grooves where fractures are thought to initiate. Other results showed distinct differences in the stress distribution pattern between the three groups. Further investigation into these differences may help increase the understanding of a horse's predisposition to injury. In conclusion, this study has shown that joint geometry plays a role in the stress distribution patterns found in the equine metacarpophalangeal joint. The differences in these patterns between the three groups may help explain the increased risk of a catastrophic injury for some horses. Further studies are warranted to better define the parameters leading to these changes.
Open Access
Effect of mixed-mode loading on fatigue and fracture assessment of a steel twin box-girder bridge
(Colorado State University. Libraries, 2019) Irfaee, Mazin M., author; Mahmoud, Hussam, advisor; Heyliger, Paul, committee member; Atadero, Rebecca, committee member; Stright, Lisa, committee member
Steel twin box-girders are considered an attractive option for the construction of bridges due to their basic design, simple form, and ease of creation. Despite their advantages, they are considered fracture critical and as such there is an additional mandate for these bridges to inspected more in depth. This causes their inspection cost to be approximately two to five times greater than that of bridges with non-fracture critical members. The required additional inspection in the U.S. is mainly driven by rare historical events of bridge collapse for bridges that were not steel twin box girders. In addition, the mandated additional inspection does not reflect the inherent level of redundancy in most bridges. Therefore, it is important to quantify the potential for fracture and the level of redundancy in steel two-girder bridges in general, and twin box girders in particular, to minimize their inspection cost. Recognizing the inherently large scatter in fatigue performance, evaluating crack propagation and potential for fracture should, however, be performed in a probabilistic manner using detailed models that represent accurate behavior of the bridge. In this study, a detailed numerical finite element model of steel twin tub-girder bridge is developed and crack growth analysis, potential for fracture of its main tubs, and its overall redundancy is evaluated. The crack growth analysis is performed using multi-mode linear elastic fracture mechanics while accounting for uncertainties in the random variables associated with crack propagation and fracture. The results of the crack growth analysis are utilized to develop fragility functions that specify inspection intervals versus probability of failure where failure is characterized by dynamic crack growth. The analysis conducted to quantify the potential for fracture show distinct possible failure modes that vary from brittle fracture to ductile fracture. The extreme loading case shows that the bridge overall is not at risk of collapse. It is important to note that this conclusion cannot be generalized for all tub girder bridges since the level of redundancy is expected to vary between bridges depending on many factors such as girders geometries, plate thickness, fabrication, among others. However, the presented approach and the corresponding results provide a systematic way by which fracture critical bridges can be evaluated.
Open Access
Evaluating the bond durability of FRP-concrete systems subjected to environmental exposures
(Colorado State University. Libraries, 2012) Mata Carrillo, Oscar Rafael, author; Atadero, Rebecca, advisor; Heyliger, Paul, committee member; Glick, Scott, committee member
The poor current condition of transportation infrastructure in the U.S. is well documented. With traffic volumes on the rise, as well as limited funding available to maintain and rehabilitate aging bridges, cost effective means of improving the performance and durability of these structures must be employed. Fiber Reinforced Polymers (FRPs) offer one potential solution. Their use has been progressively growing in the field of civil engineering as the material's high strength to weight ratio, non-corrosive nature, and ability to conform to existing geometry make it appealing in the reinforcement of existing reinforced concrete structures. In most applications of FRP to strengthen an existing structure, the FRP-concrete bond is essential. Bond is needed for proper transfer of stresses among interfaces. From a durability standpoint, the long-term bond performance is also a major concern. As a result, a long-term durability study was conducted in the laboratory to evaluate the behavior of the bond between the FRP and concrete. Small concrete specimens were prepared, reinforced with FRP material, and subjected to various environmental scenarios such as wet-dry cycles, freeze-thaw cycles, and constant immersion in water, as well as deicing agents. Direct tension pull-off tests and three-point flexural tests were conducted on these specimens to determine any degradation in bond strength over time. Finally, the pull-off test method was evaluated by means of previous research studies and recommendations about preparation procedures were made.
Open Access
Hydrological assessment of field-scale GeoWaste and waste rock test piles
(Colorado State University. Libraries, 2020) Hassanzadeh Gorakhki, Mohammad Reza, author; Bareither, Christopher, advisor; Shackelford, Charles, committee member; Scalia, Joseph, committee member; Heyliger, Paul, committee member; Butters, Greg, committee member
Mine waste rock and mine tailings are generated in substantial quantities an d must be managed to protect human health and the environment. Challenges in mine waste management facilities include geotechnical stability, environmental contamination, water management, and post operation (long term) closure. Waste rock and tailings co-disposal is a management technique that can address many of the aforementioned challenges. GeoWaste is a mixture of fast-filtered tailings and waste rock blended to isolate waste rock particles within a tailings-dominated matrix. A field-scale experiment that included a waste rock pile and GeoWaste pile was conducted at a mine in Central America to evaluate if GeoWaste suppresses sulfide oxidation and production of metal-rich acid rock drainage relative to waste rock. The objectives of this study were to (i) evaluate hydrologic performance of the piles, (ii) conduct in situ infiltration tests on the piles, (iii) determine field-scale hydraulic parameters for GeoWaste and waste rock, and (iv) develop numerical models to predict water content and oxygen concentrations within the piles. Water content, temperature, electrical conductivity, and oxygen concentration within the piles were monitored for 26 months. Sealed double ring infiltrometer tests were conducted at the end of the pile experiment and test pile subsequently were excavated to assess the spatial distribution in geotechnical characteristics. Inverse modeling was completed in HYDRUS-2D based on infiltration data to determine hydraulic conductivity and moisture retention parameters for the test piles. Field- and laboratory-scale hydraulic parameters were used in HYDRUS-1D and HYDRUS-2D to develop seepage models to predict moisture movement during the 26-month pile experiment. Oxygen concentration was predicted for the GeoWaste pile in HYDRUS-1D via the solute transport module, Fick's 2nd law, the oxygen consumption rate, oxygen diffusion in gas and water phases, and Henry's constant.