ORCID Identifier(s)

0000-0002-3118-1989

Graduation Semester and Year

2020

Language

English

Document Type

Dissertation

Degree Name

Doctor of Philosophy in Aerospace Engineering

Department

Mechanical and Aerospace Engineering

First Advisor

Andrew Makeev

Abstract

Advanced polymer matrix composites are widely used in designing high-performance and lightweight spacecraft, rotorcraft, and automotive structures, due to its superior performance such as specific stiffness, specific strength, and toughness, which are appealing in reducing the weight of these structures. One critical factor that limits the efficient design of structures composed of fiber-reinforced composites (FRP) is the vulnerability of the material under compression loading since compressive strength of FRP material is typically a lot less compared to the tensile strength. Moreover, composite structure may undergo compression loading either by accidentally or by deliberately, large margins of safety are usually imposed in the composite design process, causing in heavyweight and inefficient design. Therefore, the advantage of high elasticity and strength of FRP cannot be fully utilized. Understanding, modeling and finally predicting the compressive response of composite material is a long-standing as it is one of the most complex and widely studied yet unresolved issues. Additionally, the multiplicity of damage modes, their interaction, and complex failure mechanism limits the accurate failure prediction and life assessment of composite structures under compression loading. The uncertainty due to the complexity of failure mechanism, impose risk in a composite design process that cannot be improved by lengthy and expensive testing. Validated analysis techniques capturing the proper mechanism of compressive damage and interaction from initiation to ultimate strength and fatigue failure are essential to provide accurate life assessment of the composite structure. Not to mention, it may assist to identify rooms for improvement to the material system. Fiber micro-buckling or fiber kinking is reported as a primary failure mode of compression failure for FRP composite. Unlike the tensile properties which are generally dictated by reinforcing fibers, under compression, the surrounding matrix and the fiber-matrix interface play a significant role, since they provide the lateral support and restraint to fiber rotation due to compressive load. In this regard, fiber-kinking theory (FKT) is considered as the primary failure mechanism of the composite under compression loading. The objective of this research is to develop a finite element-based methodology to predict fiber-compression fatigue failure which is a well-recognized weakness of carbon-offer reinforced composites. In order to achieve proper fatigue failure prediction in compression loading, preliminary studies have been performed to see if the implemented fiber kinking model can predict the compression failure in quasi-static loading. Therefore, two different carbon/epoxy materials were tested according to the ASTM standard, and the CT scan images were compared with the computational prediction. It was found that including fiber-kinking theory in the computational model help captures the fundamental failure mechanisms such as compression strength, crack location-propagation, and large fiber rotation after failure initiation. The study also shows that accurate prediction of fiber rotation angle which drives the compression failure plays a significant role in capturing the proper material behavior under compression. Based on these outcomes fatigue tests were performed in a short beam shear specimen for two different carbon/epoxy material systems. Through these tests, the key parameter contributing to the change in the material mechanical property during fatigue is identified as the accumulation of permanent shear deformation. Therefore, the fiber-kinking theory is extended and modified to include this change of mechanical shear property during fatigue loading. Additionally, the proposed fatigue model is implemented in the material constitutive model at the fiber misalignment frame using ABAQUS/Explicit FE formulation. Finally, ASTM standard test method D6484 Open Hole Compression is performed, and model predictions are correlated with the test results. The proposed fatigue model based on continuum damage mechanics offers a strong promise in resolving the weakness of the currently available meso-scale fatigue model to predict fatigue compression failure. Furthermore, the proposed model has the ability to predict compressive strength and fatigue behavior in the multidirectional laminate which not only increases the confidence in the material allowable but also has a strong potential to tremendously reduce qualification testing and expand the material composition and layup design space.

Keywords

Fiber Kinking, FKT, CFRP, CDM, Composite, Constitutive Model. Compression Failure, Damage

Disciplines

Aerospace Engineering | Engineering | Mechanical Engineering

Comments

Degree granted by The University of Texas at Arlington

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