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theses

Investigation of the transverse properties in anisotropic Kevlar® fibers are conducted via instrumented indentation methods. The K29, KM2, and K119 fibers are comprised of a skin and core with distinct indentation moduli. Indentations performed at different loads contributed ample data to obtain depth dependent indentation moduli. Indentation moduli are characterized at a depth 10% of fiber diameter to avoid effects from the substrate and additionally at the skin level, 2-13%, 7-13%, and 25-40% of fiber diameter. To account for the curvature of the single fiber, a previously developed modified curved area function was incorporated in comparison to the common flat area function extracted from the Oliver-Pharr method. The indentation moduli derived from the flat area function were undervalued than ones determined from the modified curved area function. The transverse indentation moduli of single Kevlar fibers varied at depths across their diameters. As expected from material composition, the KM2 fiber possessed the largest indentation moduli of 5.28 GPa, whereas the K119 fiber exhibited the lowest at 2.21 GPa.
Two polymers of unique compositions, polydimethylsiloxane (PDMS) and shape memory polymer (SMP), are also examined for their frequency and depth dependent mechanical properties via single and multiple cycle loading. PDMS is a hydrophobic elastomer and exhibits greater elasticity than the hydrophilic SMP, but similarities in material response were distinguished. During single cycle nanoindentation, both planar polymers exhibited smoother loading curves at the lowest frequency as opposed to the higher frequencies. At 3mN load-controlled tests, PDMS and SMP had an average indentation modulus value of 3.94 ± 0.06 MPa and 2.07 ± 0.08 GPa, respectively. Their indentation moduli differed by a factor of 525, supporting the conclusion that PDMS is physically softer than the SMP. As loads and maximum depths increased, the mechanical properties decreased for both materials.
To study periodic response behavior in both polymers, the frequencies for multiple cycle tests were varied at 1, 0.5, and 0.033 Hz for different cycles. During these small-scale fatigue tests on PDMS, 5 and 50 cycle experiments demonstrated a linear trend with a negative slope for indentation moduli, whereas 100 cycle experimental data conformed to power law curves. Contrarily, all cycles and frequencies tested on SMP followed power law curve fitting. As the frequencies decreased, the change in maximum depths increased along with a further depreciation of indentation modulus for both materials. The multiple cycle indentation tests confirmed the consistent trends identified in the single cycle indentations. Overall, the two polymers experienced comparable trends in mechanical properties despite their extensive disparity in chemical composition, indentation modulus, and hardness.

ETD_9723

M.S.

Includes bibliographical references

doi:10.7282/t3-4bm5-we83

ETD graduate

The author owns the copyright to this work.