Graduation Semester and Year

2015

Language

English

Document Type

Dissertation

Degree Name

Doctor of Philosophy in Aerospace Engineering

Department

Mechanical and Aerospace Engineering

First Advisor

Andrew Makeev

Abstract

Composite materials have several advantages over metallic materials, with their high specific strength and ability to be elastically tailored to create advantageous structural responses. However, the widespread implementation of composite materials in primary structures is limited by our knowledge of how damage mechanisms interact and spread inside composite parts. To date, characterization has been conducted using one of two methods, surface measurements and subsurface measurements. Surface measurements cannot in general capture the failure of multidirectional composite laminates. Subsurface measurements allow for observations of the internal material structure and damage mechanisms inside the composite which might facilitate greater understanding of the multiple failure modes and their interactions inside composite structure especially for aerospace applications. X-ray computed tomography allows for the investigation of subsurface damage in composite materials by generating a three dimensional representation of the scanned structure. However, as nondestructive measurements using x-rays are based on the density contrast, e.g. small opening of the crack surfaces, the damage might become undetectable if the crack surfaces are closed. Mechanical loading of the composite structure while x-raying can open otherwise hidden damage and reveal the topology essential for understanding failure modes in composite materials and structures.In this work, we have developed a system that allows for mechanical testing of realistic-size specimens including those used in the ASTM standards for measuring mechanical properties of composites, while performing in-situ x-ray computed tomography. We will demonstrate the advantages of this system over no-load CT, and over other, similar systems. The new system improves correlation with structural failure prediction models based on finite element analysis. We then used this new system, along with newly developed fixtures to monitor and characterize delamination growth in Double Cantilever Beam specimens (DCB). It is worth noting that delamination is one of the principal failure modes in composites. Newly developed finite element numerical models that result in improved accuracy in the calculation of the critical strain energy release rates will be presented along with a new method of meshing orthogonal crack front elements for crack fronts that intersect a free surface.Several DCB tests are conducted, tens of CT scans are performed, and hundreds of finite element models are created and processed in order to characterize, validate and investigate crack growth in DCB specimens. As observed by other researches, crack growth in DCB specimens is curved, and using our models, we show that there is significant difference in the values and distribution of Strain Energy Release Rate (SERR) values across the specimen width between the straight (assumed) crack front and the curved (observed) one.

Disciplines

Aerospace Engineering | Engineering | Mechanical Engineering

Comments

Degree granted by The University of Texas at Arlington

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