Date of Award

12-2014

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Mechanical Engineering

Major Professor

Dayakar Penumadu

Committee Members

George Pharr, Gajanan Bhat, John Landes

Abstract

Single carbon fibers are studied using a nano-tensile testing system. This system has unprecedented load and displacement resolution, nN and nm respectively, and the ability to perform dynamic testing for storage and loss modulus during quasi-static tensile extension. Furthermore, improved fiber mounting and alignment procedures coupled with the precision of the nano-tensile testing system assist in unprecedented resolution in single fiber mechanical testing for axial modulus and strength. Hence, using these unique capabilities, the moduli and their statistical distribution of many high performance carbon fibers are reported here. From this, a simplified single parameter model describing the strain dependent modulus is developed. Combining the ability of the nano UTM with nano-machining techniques, the fracture toughness of a single T700 carbon fiber was measured as a function of radius. The mode-I fracture toughness (KIC) for these PAN carbon fibers is 1.73 MPa·m1/2 for notch sizes larger than 300nm, which compares well with limited previous studies. For notch sizes less than 0.3μm, the KIC fracture toughness increases with decreasing notch size, confirming PAN carbon fibers sheath/core microstructure hypothesis. Structural implications regarding nonlinear elastic behavior of single carbon fibers and the radial dependency of the fracture toughness are discussed. Using data obtained from nano tensile testing, a fiber tow model is developed to predict the failure stress of fiber tows. The distribution of elastic modulus decreases the tow failure stress by 15%, but the fiber tow strength is unaffected by the nonlinear stress/strain relationship unless non-uniform loading is considered, which worked in parallel with nonlinearity to further reduce tow strength.

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