Date of Award
2024
Degree Type
Dissertation
Degree Name
Doctor of Philosophy in Mechanical Engineering and Applied Mechanics
Department
Mechanical, Industrial and Systems Engineering
First Advisor
Carl-Ernst Rousseau
Second Advisor
Arun Shukla
Abstract
Chapter 1: This study experimentally analyses the deformation and energy retention behavior often closed-cell PVC foam disks constituting a one-dimensional granular chain subjected to low-velocity impacts. A pressure-driven impactor loads disk chains of two densities, each at 1 and 10 m/s. The deformations of the disks are captured using high-speed cameras, and the chain’s input and output forces are recorded during the impact. Energy retained by the chain and the disks is calculated using the deformations of the disks, input, output, and contact forces. The results show that the energy retained by the chain of H130 foam is 3 times greater than that of the H35 foam at 1 m/s and is about twice as high at 10 m/s. Normalized with respect to input energy, the relative energy retained by the chains of H35 foam is more than that of the H130 foam for the same impact velocity and is higher at lower impact velocities for both foams. In addition, the energy the disk chain retains decays along the chain, and the decay rate is higher for lower impact speeds, implying that the disk chain should be longer for higher impact speeds to mitigate the energy fully.
Chapter 2: A series of experiments is conducted to study energy transfer in a chain of foam disks as a function of temperature. These experiments use two different densities of closed-cell Polyvinyl Chloride foams as an array. A puncture testing machine impacts the chain at a velocity of 1 m/s at five different temperatures. Piezoelectric sensors capture the output and input forces of the chain during the impact, while high-speed cameras record the disk deformations. The chain’s energy retention is computed using disk deformations and loading forces. Results indicate that high-density foam chains exhibit significantly higher force and energy retention than low-density counterparts. Notably, the foam’s stiffness decreases with rising temperature, more prominently in higher-density foam. However, the proportion of energy retained by the chains remains relatively consistent across different temperatures. Additionally, both foam types show a pattern of exponential decay in energy retention along the chain. These insights offer implications for designing and optimizing energy-absorbing foam systems, paving the way for enhanced energy mitigation in low-velocity impact applications.
Chapter 3: This study investigated stress wave propagation through 1D granular chains of H130and H250 foam disks, with average impact velocities ranging from 17.57 to 37.98 m/s for H130 foam and from 17.93 to 38.08 m/s for H250 foam. Analysis focused solely on the incident stress wave, excluding the reflected wave. The mid-planes of the disks were chosen for analysis due to their uniaxial force components along the chain's length. The results show that the stress wave velocity is faster in the H250 foam chain due to its higher density and stiffness. Wave velocity increases with impact velocity but decreases as it travels along the chain, with a more pronounced reduction in the H130 foam compared to the H250 foam. The peak normal forces in the H250 foam chain disks are approximately three times greater than those observed in the H130 foam chain disks at comparable impact velocities. The peak normal forces in both foam chains decrease rapidly with increasing impact velocity, especially over the first few disks. As the wave propagates further from the impact source, the rate of attenuation slows, with more gradual force reduction in the H250 foam due to its higher density and stiffness. Energy propagation through the mid-planes of the disks decreases as the wave moves along the chain, with faster attenuation at higher impact velocities. Plastic dissipation at the contact points contributes to this energy loss, which becomes more significant at higher impact velocities. These findings confirm that the waves propagating through the foam chains are elasto-plastic in nature.
Chapter 4: Closed-cell polyvinyl chloride (PVC) foam offers several benefits, including its lightweight nature, high stiffness and specific strength, moisture resistance, and low thermal conductivity. These features make it an excellent core material for composite sandwich structures. This study aims to develop a deep neural network (DNN) model to accurately predict both stress and instantaneous modulus up to the densification phase based on density, strain, strain rate, and anisotropy ratio for all commercially available closed-cell H series PVC foams. To achieve this, the DNN model has been trained on stress-strain data from various studies published in the existing literature, encompassing different strain rates, densities, and anisotropy ratios of commercially available closed-cell H series PVC foams. The developed DNN model demonstrates exceptional accuracy in predicting stress and effectively captures the general trend of instantaneous modulus despite inherent noise in the actual data. This is evidenced by R2 scores of 0.99 for stress and 0.77 for instantaneous modulus. These results underscore the model’s robustness, reliability, and potential as a powerful tool for accurately modeling the mechanical properties of closed-cell H series PVC foams.
Recommended Citation
Madhirala, Vikranth Kumar Reddy, "1D GRANULAR CHAINS OF CLOSED-CELL PVC FOAMS UNDER LOW AND MEDIUM VELOCITY IMPACTS" (2024). Open Access Dissertations. Paper 4443.
https://digitalcommons.uri.edu/oa_diss/4443