Graduation Semester and Year

2013

Language

English

Document Type

Dissertation

Degree Name

Doctor of Philosophy in Aerospace Engineering

Department

Mechanical and Aerospace Engineering

First Advisor

Andrew Makeev

Abstract

Accurate stress-strain constitutive properties are essential for understanding the complex deformation and failure mechanisms for materials with highly anisotropic mechanical properties. Among such materials, glass-fiber- and carbon-fiber-reinforced polymer-matrix composites play a critical role in advanced structural designs. The large number of different methods and specimen types currently required to generate three-dimensional allowables for structural design slows down the material characterization. Also, some of the material constitutive properties are never measured due to the prohibitive cost of the specimens needed. This work shows that simple short-beam shear (SBS) specimens are well-suited for measurement of multiple constitutive properties for composite materials and that can enable a major shift toward accurate material characterization. The material characterization is based on the digital image correlation (DIC) full-field deformation measurement. The full-field-deformation measurement enables additional flexibility for assessment of stress-strain relations, compared to the conventional strain gages. Complex strain distributions, including strong gradients, can be captured. Such flexibility enables simpler test-specimen design and reduces the number of different specimen types required for assessment of stress-strain constitutive behavior. Two key elements show advantage of using DIC in the SBS tests. First, tensile, compressive, and shear stress-strain relations are measured in a single experiment. Second, a counter-intuitive feasibility of closed-form stress and modulus models, normally applicable to long beams, is demonstrated for short-beam specimens. The modulus and stress-strain data are presented for glass/epoxy and carbon/epoxy material systems. The applicability of the developed method to static, fatigue, and impact load rates is also demonstrated. In a practical method to determine stress-strain constitutive relations, the stress approximation must be independent of the deformation measurements, independent of the material properties (geometric stress approximation), and be simple for use in the industry. A remarkable benefit of the full-field deformation measurement is that it lets us observe the physical phenomena of the deformation which enables the derivation of simple and accurate geometric stress approximations. In particular, linear axial through the thickness strain distributions consistently measured in composite short-beam specimens allow a rigorous derivation of extremely simple stress approximations. The observation of linear through the thickness axial strain distributions has become the basis for eliminating the need of using Bernoulli-Euler kinematic assumptions of the rigid cross sections remaining perpendicular to the beam neutral axis throughout the deformation. Such assumptions are not consistent with the deformation mechanisms and therefore are arguable as a rigorous basis for stress approximation. Simple stress approximations are derived in this work based on the observations from the full-field deformation measurements; accuracy of such approximations are verified; and their limitations determined.

Disciplines

Aerospace Engineering | Engineering | Mechanical Engineering

Comments

Degree granted by The University of Texas at Arlington

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