ORCID Identifier(s)

0000-0002-2086-7061

Graduation Semester and Year

2018

Language

English

Document Type

Dissertation

Degree Name

Doctor of Philosophy in Aerospace Engineering

Department

Mechanical and Aerospace Engineering

First Advisor

Andrew Makeev

Abstract

Efforts to develop strength and life prediction analysis tools for aircraft composite structures have shown a great need to understand the complexity and interaction of failure modes. Many failure models are highly sensitive to transverse nonlinear shear stress-strain and delamination fracture properties, which are notoriously hard to measure. Without proper input parameters, life predictions analyses will not be successful. The objective of this work was to accurately characterize these mechanical properties of carbon fiber reinforced polymer composites through advanced experimental methods. Full-field noncontact deformation measurements using Digital Image Correlation are used to quantify all surface strains components, enabling the design of experiments subject to complex loading. The presented splined-based optimization techniques link measured strains to applied loading, converging to accurate material properties without ad hoc assumptions on the material model. These methods improve on first-order closed-form and iterative numerical solutions due to flexibility and accuracy without the computational expense. Applications of the optimization model on the Short Beam Shear and Thick Adherend shear tests proved the usefulness of these methods. Full three-dimensional characterization of selected carbon fiber-reinforced polymer composites is presented with a discussion of the applicability of simplifying assumptions on the material model. This experimental program showed that the assumption of transverse anisotropy must be verified, as it may be inaccurate depending on the material. Furthermore, large shear strain response of IM7/8552 UD specimens has been measured by asymmetric three-point bending test. Applicability of the common D3518 ±45 degree off-axis tensile test was also deliberated. Shear stress-strain response measured from D3518 specimens disagrees with the Short Beam Shear and Small Plate Twist test results after about 5000 microstrain. This conclusion was shown to be influenced by the specimens’ stacking sequence and microdamage which develops well before damage can be spotted in X-ray Computed Tomography reconstructions. High-magnification deformation measurements are applied to directly measure cohesive laws in precracked Double Cantilever Beam and Thick Adherend shear specimens. Supported by non-destructive in situ crack front measurements by Computed Tomography, the loaded crack tip displacements are directly related to the J-integral. The derived traction-separation law is verified by the excellent agreement of the global response between cohesive model Finite Element Analysis and measurements. This study was the first application of the direct measurement of mode II traction-separation law in Thick Adherend shear specimens.

Keywords

Polymer-matrix composites, Mechanical properties, Digital image correlation, Mechanical testing, Cohesive zone modeling, Delamination

Disciplines

Aerospace Engineering | Engineering | Mechanical Engineering

Comments

Degree granted by The University of Texas at Arlington

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