Date of Award

2018

Document Type

Open Access Thesis

Department

Mechanical Engineering

Sub-Department

College of Engineering and Computing

First Advisor

Addis Kidane

Abstract

In this study, the effect of layer stacking on the energy absorption characteristics of density-graded cellular polymers subjected to high velocity impact is investigated experimentally. The focus in the present work is to characterize the constitutive response and deformation mechanisms of these Functionally Graded Foam Materials (FGFM) using full-field measurements. Uniform foam Quasi-static and dynamic testing of uniform foam specimens are completed for each nominal density used in the layered foam samples. The low strain-rate experiments are performed in order to determine the mechanical properties of foam specimens. The high-rate experiments for homogenous foam specimens were completed to further characterize the constitutive response and also to provide baseline data for comparison to the response of FGM specimens. High rate loading experiments are performed using Split Hopkinson Pressure Bar (SHPB) combined with ultra-high-speed imaging to measure in-situ deformations and observe the formation and propagation of elastic and inelastic stress waves during impact. The FGM specimens were fabricated in-house by bonding different bulk density polymeric foam layers in different stacking arrangements. The effect of the orientation of the discrete layers on the dynamic response is quantified using high speed imaging with digital image correlation (DIC). The challenges with dynamic equilibrium due to low mechanical impedance of soft materials are carefully considered. The effects of inertia and material compressibility are included in analysis. The approach uses DIC to gather the full-field data which is used to measure the acceleration and density, later used to estimate the stress gradients developed in the material. The temporal and spatial distributions of the inertia stress are superimposed with the boundary stress measured from the transmitted signal to satisfy the equation of dynamic stress equilibrium. The local and global responses are both examined in order to assess the overall performance of each gradient sequence. The average stress-strain curves obtained are then used to find the total energy absorbed during loading. Since the desired final goal is to be able to optimize this graded cushioning structure for any specific situation, the “best” arrangement of the FGM system is defined to be the layered system that has the highest energy absorption based on the model being used to characterize response. Recommendations for the extension of this work will be made at the end.

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