Accurate Three-dimensional Characterization Of The Nonlinear Material Constitutive Properties For Laminated Composite Materials
Cline, Julia Elaine
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Accurate and efficient full three-dimensional characterization of mechanical properties of composite materials, including stress-strain curves and strength characteristics, is essential for understanding complex deformation and failure mechanisms of composites and optimizing material qualification efforts. Non-contact full-field deformation measurement techniques such as digital image correlation (DIC) allowing for assessment of all surface strain components on the entire specimen surface, enable simpler experimental setups and material specimen designs for more efficient and accurate material characterization. This work presents a new method enabling simultaneous assessment of the shear stress-strain curves in all three principal material planes. The method uses a small rectangular plate torsion specimen. The method is relying on digital image correlation to capture deformation including the out-of-plane strain components; and on finite element model (FEM)-based stress calculation. Material properties measured using short-beam shear tests are used as initial approximations of the material model in the FEM stress analysis. Iterative FEM updating methods are used to assess the shear nonlinear stress-strain relationships of composite materials. A stress convergence method is developed to assess the interlaminar shear stress-strain curves by updating material properties until the stress state at the maximum point converges. Results for the interlaminar stress-strain curves are presented. A full-field optimization method is developed to assess the nonlinear shear behavior in all principal material planes simultaneously based on the minimization of the error between FEM-predicted strains and DIC-measured strains. The use of the full-field data significantly reduces the number of FEM iterations compared to the stress convergence method and allows for the investigation of nonlinear material coupling among shear and tensile modes in the large strain regime. The short-beam shear method is expanded to construct an initial approximation based on full-field strain measurements. Results include the in-plane and two interlaminar shear stress-strain curves simultaneously captured for IM7-carbon/8552-epoxy material system. Dependency of the in-plane and interlaminar shear response on axial and transverse stresses in the nonlinear regime is discussed.