Date of Award

1-1-2012

Document Type

Dissertation

Degree Name

Ph.D.

Department

Mechanical Engineering

First Advisor

Maciej Kumosa

Keywords

acoustic emission, atmospheric aging, fatigue, finite element analysis, polymer matrix composite

Abstract

Mechanical failure modes associated with elevated temperature exposure of the load bearing unidirectional hybrid composite (based upon glass and carbon fibers reinforcing a high temperature epoxy matrix) of a next generation transmission line design were investigated in this research. In particular, the flexural performance (in both static and fatigue loading) of the composite which had been exposed to elevated temperatures for prolonged periods of time was studied. To this end, a fatigue test was developed in an attempt to simulate the multi-axial loading conditions present on transmission lines. This test was used to evaluate the fatigue behavior of unaged specimens, as well as the evolution of fatigue performance of aged specimens. Furthermore, a four point loading configuration was used to assess the effect that aging had on the static flexure strength of the hybrid composite.

It was found that the magnitude in static and fatigue material property reduction increased with aging time, and was dictated by which aging mechanism was dominant. Microstructural changes revealed that the modest reduction in mechanical properties at intermediate aging times was predominantly attributed to thermal oxidation, while for longer aging times physical aging was the primary cause for the substantial reduction.

Finite element models were developed to quantify a newly discovered stiffening phenomenon observed in the composite subjected to fatigue. A mesh morphing scheme was developed to account for the scatter in the stiffening phenomenon due to the inherent geometric variability of the carbon fiber composite. In addition, 3-dimensional viscoelastic representative volume element finite element models were developed to understand the damage mechanisms of the hybrid composite system.

Finally, the utility of waveform based broadband acoustic emission was explored to identify the types of damage which were occurring within the hybrid composite, as well as which material a particular mechanism originated from. To demonstrate the method as a potential in-service nondestructive evaluation technique, explicit dynamic finite element models simulating the 3 most common composite failure mechanisms (i.e. matrix cracking, fiber/matrix delamination, and fiber fracture) were developed. Based upon the spectral content of the simulated signals, a damage classification scheme based upon the method of Partial Powers was developed. The efficacy of the methodology was validated using waveforms captured during the quasi-static flexure testing of aged hybrid composite rods.

Provenance

Recieved from ProQuest

Rights holder

Brian Michael Burks

File size

373 p.

File format

application/pdf

Language

en

Discipline

Materials Science, Mechanical engineering

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