Theses and Dissertations at Montana State University (MSU)
Permanent URI for this collectionhttps://scholarworks.montana.edu/handle/1/733
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Item Development and characterization of a split Hopkinson pressure bar for testing high shock accelerometers(Montana State University - Bozeman, College of Engineering, 2022) Berg, Charlotte Katherine; Chairperson, Graduate Committee: David A. MillerExtreme environments pose significant challenges in the aerospace and defense fields. Prior to equipment deployment, dynamic testing is often conducted to identify potential failure points from high shock. Accelerometers are vital sensors for characterizing system response to transient inputs, such as impact and vibration, faced by atmospheric reentry vehicles or explosive protection equipment. However, accurate data collection is often inhibited by sensor damage or nonlinear response to dynamic inputs. A controlled experimental system capable of recreating extreme conditions is needed to assess sensor limitations and design options prior to full-scale structural testing. Conventionally built for high strain-rate material testing, a Split Hopkinson Pressure Bar uses colinear elastic bar impact to produce high energy, short-duration stress waves. Material samples are sandwiched between elastic bars and dynamic properties are derived from stress waves reflected by or transmitted through the sample. In this work, a modified Hopkinson Pressure Bar was developed to assess accelerometer response to high-energy, dynamic inputs. The modified system consists of a gas gun that propels a short steel striker into a longer steel incident bar, producing a nondispersive stress wave. A single-axis Endevco 7270 piezoresistive accelerometer is mounted on a fly-away structure aligned with the incident bar. The stress wave that reaches the accelerometer creates a shock impulse of similar magnitude and frequency as an explosive test. The Hopkinson Pressure Bar was characterized with a range of input configurations and produces acceleration pulses with amplitudes up to 100,000 g and durations of 0.2 ms. Strain signals were compared to accelerometer output in two mounting configurations over a range of shock levels. There was good agreement between strain-derived acceleration and peak accelerometer accelerations. Experimental system capabilities and limitations are presented alongside current challenges and directions for future research. The setup developed for this research increases sensor and material testing capabilities under extreme environmental conditions.Item Developing bio-inspired methodologies for encoding angular position from strain(Montana State University - Bozeman, College of Engineering, 2020) Lange, Christopher William; Chairperson, Graduate Committee: Mark JankauskiAs mechanical systems rely more on closed-loop control, the sensors which supply feedback information are essential. Additionally, in systems where sensor function is critical, sensor redundancy is important to retain functionality if one or more sensors fail. Redundancy can be achieved through multiple high-fidelity sensors which measure the same type of information, such as gyroscopes or accelerometers. However, multiple high-fidelity sensors can increase cost significantly. This thesis explores the potential to replace or augment the functionality of angular position sensors using strain measurements. Strain gauges are already used in system health monitoring systems. By utilizing these already implemented sensors to measure angular position, we can remove the additional cost of redundant angular position sensors. However, for complex systems, the mapping between strain and angular position is unclear. By incorporating reduced order, physics-based models into machine learning techniques, we can efficiently transform high-order strain data into angular position. To demonstrate the potential of using alternative sensing methods, we developed a reduced order model of a parametrically excited flexible pendulum. Inspiration for this simplified system comes from insect halteres, which are small sensory organs evolved from insect hind wings which provide rapid information about body rotation. The parametrically excited flexible pendulum allows a single axis of rotation and single direction of flexibility to be paired, and their relationship studied. By varying parameters within the model such as pendulum length and modulus as well as parametric excitation amplitude and frequency, the Gaussian process regression learning can be optimized to reduce training time and increase untrained prediction accuracy. Inputs of strain and parametric excitation position along with their respective first and second derivatives are then analyzed to determine which inputs are interrelated and therefore un-necessary, thus reducing the input required. This provides the essential first steps towards using machine learning to implement multiple sensor, deformation based, multi axial angular position sensing in complex systems.Item Developing a load acquisition system for a multiaxial test frame(Montana State University - Bozeman, College of Engineering, 2019) Carpenter, Aaron James; Chairperson, Graduate Committee: David A. MillerMaterial testing has traditionally been completed by using a uniaxial load frame which isolates a single stress component. Engineers however, design components for applications in a multi-axial world to withstand stress in multiple directions. The In-Plane Loader (IPL) at Montana State University expands the realm of material testing to three degrees of freedom within a two-dimensional plane. Applications of the IPL include composite material testing and experimental validation of constitutive models in multiple axes. The multi-axial test frame has been in place at MSU for several years. One of the primary challenges associated with the IPL is its ability to accurately measure multi-axial load components. The purpose of this work was to develop and validate an updated multi-axial load acquisition system for the IPL. The procedure included design, manufacture, implementation, and validation of the system. Validating the system in multiple axes required isolating single stress components along each of the planar axes. Tension tests were completed to isolate the vertical component, and shear tests were completed to isolate the horizontal component. Each of the results were compared to results of standardized test procedures designed to isolate their respective stress components. Digital image correlation was implemented as a non-contact method of measuring displacement for the testing procedures. The data collected in this study provides confidence in the ability to measure multi-axial loading in combination with digital image correlation to expand the capabilities of multi-axial testing. The system provides the ability to study load dependent failure of materials as well as displacement dependent failure. The information presented provides an understanding of challenges associated with multi-axial testing which hopes to assist in the development of future multi-axial test frames.Item Validation of high strain rate, multiaxial loads using an in-plane loader, digital image correlation, and FEA(Montana State University - Bozeman, College of Engineering, 2018) Stroili, Christopher; Chairperson, Graduate Committee: David A. MillerMontana State University's In-Plane Loader (IPL) is a machine designed to test for mechanical properties at multi-axial states of stress and strain by in-plane translation and rotation. Historically the machine has been used to characterize composite lay-ups, where applying multi-axial loads can better describe anisotropic materials. The IPL testing machine uses Digital Image Correlation (DIC) software and a stereoscopic camera system to measure strains on the surface of the test coupon by tracking a stochastic pattern applied to the gage section. The focus of this work was to test the capabilities beyond quasi-static composites testing, specifically looking to explore the feasibility of testing plastics and metals at strain rates from 10 0 to 10 3 s -1. This work explored the speed and loading capabilities of the IPL and determined a suitable coupon geometry which balances gage section area with material strength. 304 Stainless Steel was tested both on the IPL and in uniaxial tension. Experimental tensile test data was fit to a Johnson Cook strain rate sensitive constitutive model. This constitutive equation was then used with an implicit dynamic finite element analysis (FEA) model. To study the validity of high rate testing of steel in the IPL, strain from the DIC experimental data was compared with the FEA results. While the strains predicted by the FEA model varied from experimental results, a better understanding of the IPL capabilities has been achieved. Moving forward, a series of recommendations have been made so that high strain rate multi-axial testing of metals can be implemented with more robust constitutive models.Item A photoelastic investigation of three-dimensional contact stresses(Montana State University - Bozeman, College of Engineering, 1968) Schafer, Douglas CraigItem Normal and shear stresses between a rigid sphere and an elastic half-space(Montana State University - Bozeman, College of Engineering, 1976) Alzheimer, James MartinItem Effects of externally applied tensile stresses on the moisture diffusion characteristics of epoxy glass composites(Montana State University - Bozeman, College of Engineering, 2013) Stoffels, Mark Thomas; Chairperson, Graduate Committee: David A. MillerMarine Hydrokinetic (MHK) Power involves using the power of moving water to create clean, renewable energy. The primary structure of MHK energy devices are most commonly constructed using epoxy glass composite materials. Unstressed epoxy glass composites absorb moisture when subjected to a humid environment; this moisture absorption leads to degradation of mechanical properties of the composite. This phenomenon is relatively well documented and understood. However, under most operating conditions the structure will be under some combination of externally applied stresses. The objective of this study is to characterize the effects of externally applied stresses on the moisture diffusion parameters of epoxy glass laminates, and how these changes ultimately influence mechanical properties. A model is proposed which relates externally applied tensile stresses to changes in absorption capacity as well as diffusion rate. The model postulates that changes seen in the diffusion process are the result of stress-dependent changes in the free volume of the epoxy resin. The free volume changes of the resin are calculated through laminate plate theory; the free volume change becomes a function of fiber angle as well as a host of elastic properties of the constituents. Consequently, according to the proposed model, changes in diffusion parameters are dependent upon the magnitude of applied stress, the loading angle, as well as elastic properties of the constituents. In order to experimentally test the proposed model, a series of epoxy glass laminate samples were manufactured at varying fiber angles and submerged in a moist environment while subjected to varying levels of tensile loading. Weight gain measurements we recorded throughout the diffusion process until full saturation was achieved. The experimental values exhibited excellent agreement with the novel theoretical model.