Publication Date

2019-12-09

Availability

Open access

Embargo Period

2019-12-09

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PHD)

Department

Mechanical Engineering (Engineering)

Date of Defense

2019-11-01

First Committee Member

Ryan L. Karkkainen

Second Committee Member

Victoria L. Coverstone

Third Committee Member

Emrah Celik

Fourth Committee Member

Edward A. Dauer

Abstract

Methods of multi-scale analysis through combined physical testing and computational FEM were used to investigate structural battery composites and 3D printed polymers. Multifunctional composite structural batteries are materials capable of storing electrical energy while providing structural rigidity. Two battery cell configurations were considered: a carbon paper based cell with copper and aluminum foils, and a woven carbon fiber-based battery with nickel and iron deposition coatings. Flexural simulations were performed through simulated and physical three-point bend testing. Unidirectional carbon fiber layers in the place of the carbon paper can lead to up to a 233.73 GPa stiffness, significantly greater than the original 13.79 GPa. Honeycomb core can be used to retain carbon paper layers with additional thickness but increased flexural rigidity. Micro-scale electrical current distribution analysis of carbon fiber reinforced composites was performed to investigate improving the overall carbon fiber conductivity. The conductivity of the epoxy matrix conductivity is the most dominant driving force for overall resistance/resistivity of carbon fiber composites. Full-scale panel level simulations assessed the stress concentrations created by changes in panel design can be mitigated with stiff gasket material that reduces sharp panel curvature. Reinforced Cu-Al prototypes were tested in three-point bending; metal foils created a barrier preventing resin flow and left voids and defects resulting in large delaminations. For accurate strain measurements, a custom digital image correlation (DIC) procedure was created using open source software Digital Image Correlation Engine (DICe). A high-resolution camera captures images that, after processing, provide 2D strain fields and a virtual strain gauge that can then be used to generate stress-strain curves and calculate material stiffness. Verification of this technique was performed with 6061 aluminum specimens of known stiffness and resulted in an average error of 2.01% from the literal stiffness value. Multi-scale analysis methods were applied to FDM 3D printed polymers to understand and improve the mechanical behavior. Tensile testing of orthogonal printing orientations showed significant differences in the failure characteristics and tensile strength of printed polymers. Extrusion temperatures, 180°C, 195°C, and 220°C were used to print specimens tested in tension pulling apart the interlayer interfaces to study the effect of temperature on the interface strength. Increasing extrusion temperatures creates a trade-off between achieving adequate interlayer bonding without degradation of the polymer. A custom 3D printed specimen and tensile specimen preparation methodology was created to create a consistent cross-section to create a closely matching computational model. The results of the tensile tests were used to parameterize the interface strength using the models. The PLA interface strength was determined to be 33.75 MPa through FEM modeling of matching geometry, compared to the 43.57 MPa filament strength. Additional models were generated which had reduced or eliminated inter-extrusion gaps to determine the potential mechanical improvements. Eliminating gaps can lead to up to a 16.12% improvement in stiffness and a 19.8% improvement in strength.

Keywords

composites; multi-scale analysis; FEA; structural battery; simulation; 3D printing; DIC

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