Graduate Studies


First Advisor

Kevin Cole

Date of this Version

Spring 4-19-2018


Mahlin, M.K., (2018) "Deviation from Isotropy of Quasi-isotropic Advanced Carbon-Carbon-6 (ACC-6)" Master's thesis, University of Nebraska - Lincoln, 2018.


A thesis presented to the Faculty of The Graduate College at the University of Nebraska In Partial Fulfillment of Requirements For the Degree of Master of Science, Major: Mechanical Engineering and Applied Mechanics, Under the Supervision of Professor Kevin Cole. Lincoln, Nebraska: April, 2018

Copyright (c) 2018 Matthew Mahlin

Distribution C: Distribution authorized to U.S. Government Agencies and their U.S. contractors, critical technology, January 26, 2016. Other requests for this document shall be referred to NASA Langley Research Center, Hampton, VA

Note: This version of the thesis has all ITAR data removed for upload to this database.


Quasi-isotropic advanced carbon-carbon-6 (ACC-6) is of interest as hot structures in hypersonic vehicles. This application requires very low margins to achieve high performance, and is sensitive to the accuracy of material property data. Generally, quasi-isotropic laminates are simplified as isotropic with regard to their in-plane properties. However, quasi-isotropic lay-ups exhibit a noticeable reduction of mechanical properties off the fiber aligned axes, at off-axis angles. These off-axis reductions need to be considered when designing high performance hypersonic vehicles built with ACC-6.

To experimentally quantify the angle dependencies of mechanical properties, five different lay-ups of ACC-6 parent material were tested in both tensile and compressive loading. The lay-ups include one Warp-aligned and four quasi-isotropic panels. Macro scale ply properties were measured from the Warp-aligned lay-up in the warp- and fill-aligned directions. The equivalence of warp- and fill-aligned mechanical properties were evaluated. These properties were used to develop an angle dependent tensile strength model and classical lamination theory (CLT) strength analysis. Both models were used to predict angle dependent in-plane properties for the quasi-isotropic lay-ups, and compared with test results. Statistical and regression analyses were performed on test results from the four quasi-isotropic lay-ups.

The first objective of this thesis was to map and quantify the deviation from isotropy of quasi-isotropic lay-ups in tensile strength, tensile modulus, compressive strength, and compressive modulus and Poisson's ratio. To fulfill this objective, testing was performed over a range of test angles, from -5° to 90°. The second objective was to develop a predictive model of quasi-isotropic ACC-6 material properties with respect to test angle. Three sources of deviations from isotropy were investigated including: differences in warp and fill tow properties, influence of fiber-tow alignment for on- and off-axis properties, and processing effects. The third objective was to evaluate current testing practices and inform material property database development. A correction factor to obtain conservative material properties based on the deviation from isotropy is proposed with the intent of reducing testing requirements for the off-axis directions.

Advisor: Kevin Cole