Date of Award

2020

Degree Type

Dissertation

Degree Name

Doctor of Philosophy in Mechanical Engineering and Applied Mechanics

Department

Mechanical, Industrial and Systems Engineering

First Advisor

Arun Shukla

Abstract

Soft materials have been used extensively as energy absorbers in different industries such as marine, protective equipment, automotive, aerospace and transportation due to their light weight, low impedance, low mechanical stiffness and low strength. Foams, rubbers, polymers, hydrogels, and biological tissues are some examples of soft materials. Low density closed cell foams are widely used in marine and transportation industry as energy absorbers due to their light weight and low impedance. Mechanical responses of these materials are very sensitive to loading medium, rate and direction. Fundamental investigation into the mechanical response of these materials is paramount before they can be incorporated into the design of future structural applications that would be exposed to low and high loading rates. For this purpose, a comprehensive study was conducted to investigate the underwater mechanical response of low density (35 – 100 kg/m3) closed cell polymer foams under hydrostatic and underwater shock loadings. Moreover, shear thickening fluids have received significant attention for various applications such as traction control, smart structures and body armors. AMCS hydrogel as a shear thickening fluid is widely used in energy mitigation application due to its load or temperature induced gelation. Structural shock mitigation of AMCS hydrogel under different loading rates was investigated, and its mitigation performance was compared with water.

The underwater constitutive behavior of PVC foams with varying densities was investigated experimentally. The experiments were conducted in an optically clear acrylic tube, which allowed for visualization of the specimen and the application of 3D Digital Image Correlation. A series of calibration experiments was conducted to investigate the applicability of the Digital Image Correlation technique for measuring the deformation of underwater objects inside of a curved acrylic tube of considerable thickness. The results of the calibration experiments demonstrated that a submerged object located in the middle of the acrylic tube appears magnified in the radial direction. This apparent magnification was taken into account during the analysis of the deformation for all underwater experiments. The hydrostatic loading was achieved by fitting the acrylic tube with a nylon piston and compressing the piston with an Intron testing machine. Hydrostatic load of up to 5 MPa was achieved during quasi-static compression of the piston. The load applied by the Instron machine was coupled with the Digital Image Correlation data to analyze the constitutive behavior of the PVC foams. The hydraulic crush pressure, bulk modulus, and energy stored up to densification strain were determined for each foam density.

A dynamic loading facility was developed to investigate the underwater shock response of PVC foams of varying densities. The shock loading facility consists of a water filled hollow cylindrical structure, with one end fully closed and the other end fitted with a nylon piston. A rigid striker was used to impact the piston, which creates an underwater shockwave. The facility was comprised of four separate sections, where the middle section is an optically clear acrylic window, and the other three sections are aluminum. The optically clear acrylic window was utilized for the employment of three-dimensional Digital Image Correlation in conjunction with high-speed photography (90,000–100,000 frames per second) to obtain full-field deformation data of the foams during shock loading. Pressure data was recorded using piezoelectric pressure sensors at different locations along the underwater shock tube. Peak pressures in the range of 1–10 MPa with exponential decays were generated by changing the striker velocity. Furthermore, quasi-static hydrostatic response of pre shocked foams was evaluated using a previously developed underwater loading facility. Strain rate of 103 s−1 was obtained in foam specimens during the experiments. Findings showed substantial delay between the underwater shock loading and material response. Polyvinyl chloride foams recovered 80–90% of their original shape after underwater shock loading and also retained much of their energy absorption capacity.

Shock mitigation performance of aqueous methylcellulose hydrogel and water for structural applications was investigated through two dynamic loading instruments: Instrumented bar and shock tube. While aqueous methylcellulose solutions have previously been found to attenuate impact-induced forces passing through them by a unique liquid-to-solid phase transition, this is the first time studied as shock mitigators to structural elements. The results obtained with aqueous methylcellulose as mitigator were compared with an equivalent experiment conducted with water as damping medium. The liquid was loaded into a specially designed hollow aluminum box, built to allow transmission of dynamic stress waves to a thin back plate. Determination of the liquid’s attenuation performance was based on the 3D Digital Image Correlation technique with high-speed photography to obtain the full-field real-time deformation data of the back-face plate throughout the dynamic loading event. It was found that upon high rate loading with the instrumented bar, the aqueous methylcellulose solution decreases the maximum out of plane displacement resulting from the dynamic loading by as much as 40% compared to water, and significantly damps the structural vibrations of the back-face plate. On the other hand, upon relatively low rate loading with shock tubes, water and aqueous methylcellulose solutions provide the same magnitude of out of plane displacement, however, the damping ratio (Logarithmic Decrement) of the structure through aqueous methylcellulose solutions is 45 % greater than through water. The findings were analyzed and rationalized in terms of imparted mechanical power.

Cyclic hydrostatic compressive loading response of closed cell ethylene vinyl acetate (EVA), polyethylene (PE), and poly vinyl chloride (PVC – Divinycell H45) foams with an advertised density of 45 kg/m3 was investigated through a specially designed facility at 10-3 s-1. 3D Digital Image Correlation (DIC) technique was used with high resolution photography to obtain the full-field deformation data during the cyclic hydrostatic loading event. Additionally, underwater multiple-nonsimultaneous shock loading response of same foams was investigated with another specially designed facility at 103 s-1. 3D DIC technique was also used with high speed photography (90000-100000 frames per second) to obtain the full-field deformation data during the shock loading event. Monotonic hydrostatic compressive loading experiments showed that the H45 foam can absorb by 60% greater energy than EVA45 and PE45 foams. However, cyclic hydrostatic loading experiments showed that such higher energy absorption capability of H45 foam is not preservable as being dissimilar to EVA45 and PE45 foams. Cumulative energy absorbed throughout the hydrostatic cyclic loading for EVA45 foam is 50 % and 540% greater than PE45 and H45 foams, respectively. Results from the underwater multiple-nonsimultaneous shock loading experiments showed that the strain rate H45 foam experiences is considerably less than those of EVA45 and PE45 foams experience under similar underwater shock loadings. Moreover, results also showed that decreasing the applied impulse by 50% does not decrease the maximum volumetric strains EVA45 and PE45 foams undergo noticeably while it significantly decreases the maximum volumetric strain H45 foam undergoes.

Available for download on Tuesday, May 03, 2022

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