Brian Shonkwiler

An Improved Short-Beam Method for Measuring Multiple Constitutive Properties for Composites

The short beam shear (SBS) test method combined with digital image correlation (DIC) deformation measurement is used in this work to study constitutive properties of the IM7/8552 carbon/epoxy composite material system. Dependence of the shear stress approximation on the SBS specimen span-to-thickness (s/t) ratio has been investigated. A stress model developed in a previous study is expanded to account for this dependency. In addition, the effect of nonzero transverse normal stresses on a simple closed-form approximation for tensile and compressive axial moduli has been assessed. Results from the expanded closed-form solution verify that transverse normal stress has little influence on the measurement of the axial moduli. Also, the SBS test setup has been refined as a result of the sensitivity of the measured axial strain distribution to the specimen alignment in the test fixture. The test results obtained in this work are compared with previously published results to demonstrate the ability of a single unidirectional SBS test to capture multiple constitutive properties including axial tensile and compressive moduli, the nonlinear shear stress–strain response in the plane of loading, and to show that measured material properties do not depend on the test specimen size.

DIC Data-Driven Methods Improving Confidence in Material Qualification of Composites

This work presents initial results of research activities focused on utilizing the Digital Image Correlation technique for measurement of the complex deformation of composite materials towards the development of consolidated common material qualification processes. These common processes will enable reduced material qualification test matrices potentially accommodating substitution of resin types and other modifications of the material systems. In particular, data-driven methods based on full-field DIC strain measurements and finite element analysis developed at the University of Texas Arlington Advanced Materials and Structures Lab, are verified on carbon-fiber reinforced/untoughened and toughened polymer-matrix composite material systems. The DIC data-driven methods allowed verification of simplifying assumptions, increasing our confidence in material allowables. The methods also allowed identifying standing challenges in lamina characterization, including inconsistencies of certain ASTM standard test fixtures and analysis methods, preventing accurate measurement of lamina characteristics key to laminate analysis.