Graduation Semester and Year

2019

Language

English

Document Type

Dissertation

Degree Name

Doctor of Philosophy in Mechanical Engineering

Department

Mechanical and Aerospace Engineering

First Advisor

Andrew Makeev

Second Advisor

Daniel P. Cole

Third Advisor

Ashfaq Adnan

Abstract

There has been a strong demand in using high-modulus (HM) carbon-fiber composites potentially enabling lightweight aircraft structures with significant weight savings. However, extremely low fiber-direction compressive strength has been a well-recognized weakness of the HM composites, preventing their implementation in aircraft platforms. Fiber-direction compressive strength of HM and intermediate-modulus (IM) carbon fiber-reinforced polymers (CFRP’s) is presumably governed by microstructural stability. However, strong decrease in fiber-direction compressive strength of HM carbon-fiber composites, compared to their intermediate modulus (IM) counterparts, contradicts predictions from available microstructural buckling theories. A recent work demonstrated that significant improvement in the fiber-direction compressive strength of HM carbon-fiber composites can be achieved by hybridizing IM and HM carbon fibers in HM CFRP toughened with nano-silica. In particular, a new HM material solution achieving fiber-direction compressive strength of IM legacy composites but with more than 30% higher axial modulus has been developed. On the other hand, enabling effective design of the new high-performing composite material requires accurate physics-based models to capture fiber-direction compressive strength behavior of HM CFRPs. Such models would provide the most effective way to explore diverse material design options with optimum microstructural configurations. The absence of accurate models has been attributed to complexity and potential multiplicity of the governing failure modes and their interaction. Therefore, observing the fiber-direction compressive failure presumably governed by microstructural stability has been a major prerequisite for developing a rigorous modeling strategy. Kink-band formation in HM CFRPs was observed for the first time. This observation confirms that fiber-direction compressive strength is indeed governed by microstructural stability, which can be used as a basis for accurate model development. Furthermore, some of the major challenges in developing models capturing the microstructural stability include meeting their input data requirements. In order to generate such data and observe the physics phenomenon at a scale small enough for the important features to be captured, in-situ scanning electron microscopy (SEM) based experiments are introduced to measure the essential microstructural material properties, e.g. fiber-to-matrix interface shear strength behavior within the actual composite ply. This work looks into weak fiber-matrix interface as a potential mechanism driving the fiber-direction compressive strength decrease of the HM CFRP’s. The experiments show approximately 30% decrease in the average values of the fiber-matrix interface shear strength for the HM carbon fibers compared to the IM carbon fibers in the new material system. And such a strong reduction corresponds to a 22% lower fiber-direction compressive strength of the HM CFRP. In addition, the fiber-matrix interface shear strength reduction correlates with decreasing fiber-direction compression strength predicted by a microstructural buckling model which properly accounts for the fiber-matrix interface shear strength behavior. The results support the idea of integrating off-the-shelf IM carbon fibers with stronger fiber-matrix interface and higher shear modulus into a HM carbon-fiber composite to improve fiber-direction compressive strength of the HM composite material system.

Keywords

High-Modulus (HM) Carbon Fiber, Polymer Matrix Composite (PMC), Composite Materials, Carbon Fiber Reinforced Polymer (CFRP), Compressive Strength, Fiber Hybridization, Fiber-matrix interface shear strength, In-situ scanning electron microscopy (SEM)

Disciplines

Aerospace Engineering | Engineering | Mechanical Engineering

Comments

Degree granted by The University of Texas at Arlington

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