Date of Award

1-30-2024

Publication Type

Dissertation

Degree Name

Ph.D.

Department

Mechanical, Automotive, and Materials Engineering

Keywords

Anisotropy, Deformation mechanism, High strain rate, Mechanical characterization, Modeling;Polymeric foams

Supervisor

William Altenhof

Rights

info:eu-repo/semantics/openAccess

Creative Commons License

Creative Commons Attribution 4.0 International License
This work is licensed under a Creative Commons Attribution 4.0 International License.

Abstract

The main objective of this study was to advance the experimental, numerical, phenomenological and analytical methods of assessing the dynamic compressive response of polymeric foams, especially at an intermediate strain rate range of 50 s-1 to 600 s-1. Different experimental apparatuses and techniques, including the universal compression/tension testing machine (strain rates up to 0.5 s-1), custom-build droptower (a strain rate range of 50 s-1 to 200 s-1) and pneumatic testing apparatus (a strain rate range of 200 s-1 to 600 s-1) were utilized in the macro-mechanical characterization. The microstructure of the investigated polymeric foams was studied by means of Scanning Electron Microscopy (SEM) and Energy Dispersive x-ray Spectroscope (EDS). Numerical, analytical and phenomenological models were modified and calibrated to characterize and predict polymeric foams’ anisotropic rate-sensitive mechanical response. In the first phase of the study, the compressive response of polyether sulfone (PES) foam was investigated under both quasi-static and elevated strain rates. Anisotropic behavior was assessed by testing three orthogonal loading directions, revealing distinct deformation mechanisms. Elevated strain rates, ranging from 50 s-1 to 200 s-1, showcased a substantial rate dependency. The compressive response was simulated using Finite Element Analysis (FEA), achieving an impressive average validation metric of 97%. Localized deformation in the foam rise direction was identified, with specialized equations developed to quantify this phenomenon. The observed variation in the deformation mechanism and rate sensitivity of PES foams when loading in different material directions granted the need for further investigation on the influence of the specimen shape and profile on the mechanical characterization of polymeric foams. In the second phase, rigid Polyvinyl Chloride (PVC) foams were investigated for the effects of the specimen size and density variation on deformation mechanisms and mechanical properties. Different through-thickness direction density variation patterns, varying from 2.6% to 26.3%, governed localized deformation and post-yield stress drop-off behavior. Plateau stress exhibited sensitivity to foam density, while specimen thickness influenced loading elastic modulus. Intermittent unloading-reloading cyclic testing revealed that the thickness effect on the apparent unloading elastic modulus was negligible despite the thickness’s significant effect on the loading modulus. The limitations of Split Hopkinson Pressure Bar (SHPB) apparatuses in dynamic testing limited the exploration of specimen size influence in the dynamic mechanical response of polymeric foams. Consequently, most research on the influence of specimen size on the mechanical response was limited to the quasi-static loading regime. The specimen size effect was reported to be negligible in the quasi-static regime. A droptower testing machine equipped with a 45.45 kg dropping entity and a novel energy dissipation system was utilized to test specimens with different profile sizes and shapes dynamically. The findings from this work, for the first time, revealed that, unlike the quasi-static regime, dynamic behavior was sensitive to specimen profile, prompting the use of impact velocity rather than engineering strain rate in reporting rate sensitivity. Equations were developed to quantify the localized deformation of polymeric foams based on the specimen’s instantaneous dimensions during dynamic compression testing, revealing the significant effect of the specimen size on the correlation between the engineering and localized strain rates. Thus, the specimen thickness and profile size should be consistent in the quasi-static and dynamic experiments to achieve an accurate mechanical characterization, which has rarely been achieved in previous studies. In the study’s third phase, a pneumatic apparatus was utilized to characterize PVC foams under various strain rates, up to 600 s-1, corresponding to 15 m/s impacting velocity, and loading directions to address prior technical limitations. PVC foams with six different nominal densities were subjected to strain rates ranging from 0.005 s-1 to 600 s-1. Specimens possessing a consistent profile size and shape were utilized in the quasi-static and dynamic tests. A modified Nagy model coupled with a nonlinear Avalle relationship accurately represented stress/strain responses. It was found that conducting tests at a single dynamic strain rate (e.g. 400 s-1) effectively captures the rate sensitivity of PVC foam within the 200 s-1 to 600 s-1 strain rate range. Additionally, the influence of relative density on the foam’s rate sensitivity was quantified. In the final phase of the study, the characteristics of PVC foam obtained from the previous phases of this study were utilized to design, test and validate the foam section of a novel energy dissipation system involving AA6061 extrusions and a PVC foam. This hybrid energy dissipation system demonstrates enhanced mechanical performance, surpassing traditional axial crushing modes regarding energy absorption effectiveness.

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