DOI

10.17077/etd.aqjxf6ie

Document Type

Dissertation

Date of Degree

Spring 2017

Degree Name

PhD (Doctor of Philosophy)

Degree In

Mechanical Engineering

First Advisor

Zhupanska, Olesya I.

Second Advisor

Lu, Jia

First Committee Member

Zhupanska, Olesya I.

Second Committee Member

Lu, Jia

Third Committee Member

Xiao, Shaoping

Fourth Committee Member

Ding, Hongtao

Fifth Committee Member

Bhatti, M. A.

Abstract

In the current thesis, the 4-probe electrical resistance of carbon fiber-reinforced polymer (CFRP) composites is utilized as a metric for sensing low-velocity impact damage. A robust method has been developed for recovering the directionally dependent electrical resistivities using an experimental line-type 4-probe resistance method. Next, the concept of effective conducting thickness was uniquely applied in the development of a brand new point-type 4-probe method for applications with electrically anisotropic materials. An extensive experimental study was completed to characterize the 4-probe electrical resistance of CFRP specimens using both the traditional line-type and new point-type methods. Leveraging the concept of effective conducting thickness, a novel method was developed for building 4-probe electrical finite element (FE) models in COMSOL. The electrical models were validated against experimental resistance measurements and the FE models demonstrated predictive capabilities when applied to CFRP specimens with varying thickness and layup. These new models demonstrated a significant improvement in accuracy compared to previous literature and could provide a framework for future advancements in FE modeling of electrically anisotropic materials. FE models were then developed in ABAQUS for evaluating the influence of prescribed localized damage on the 4-probe resistance. Experimental data was compiled on the impact response of various CFRP laminates, and was used in the development of quasi- static FE models for predicting presence of impact-induced delamination.

The simulation-based delamination predictions were then integrated into the electrical FE models for the purpose of studying the influence of realistic damage patterns on electrical resistance. When the size of the delamination damage was moderate compared to the electrode spacing, the electrical resistance increased by less than 1% due to the delamination damage. However, for a specimen with large delamination extending beyond the electrode locations, the oblique resistance increased by 30%. This result suggests that for damage sensing applications, the spacing of electrodes relative to the size of the delamination is important. Finally CT image data was used to model 3-D void distributions and the electrical response of such specimens were compared to models with no voids. As the void content increased, the electrical resistance increased non-linearly. The relationship between void content and electrical resistance was attributed to a combination of three factors: (i) size and shape, (ii) orientation, and (iii) distribution of voids. As a whole, the current thesis provides a comprehensive framework for developing predictive, resistance-based damage sensing models for CFRP laminates of various layup and thickness.

Public Abstract

In this thesis, the electrical properties of carbon fiber composites are used as a tool for detecting physical damage inside the material. The carbon fiber composites consisted of an epoxy plastic base with carbon fiber reinforcements. The electrical and mechanical properties of these two constituents are quite dissimilar, therefore when combining these constituents into one single composite material; it is often difficult to characterize the properties of this new material. In this work, a robust method has been developed for determining these complex electrical properties using an experimental line-type 4-probe resistance method. Next, the concept of effective conducting thickness was uniquely applied in the development of a brand new point-type 4-probe method, which until now had never been applied to carbon fiber composites. An extensive experimental study was completed to characterize the electrical resistance of these carbon fiber specimens using both the traditional line-type and new point-type methods.

Next, computer models were developed in order to simulate the experiments. These computer simulation models demonstrated predictive capabilities when applied to carbon fiber specimens with varying thickness and fiber orientations. These new models demonstrated a significant improvement in accuracy compared to previous literature and could provide a framework for future advancements in computational modeling of such materials. Additional computer models were created in order to predict damage in carbon fiber specimens subjected to mechanical impact. The impact damage models and electrical models were then combined together to create a model which can detect the presence of mechanical damage using electrical resistance measurements. The computer simulation results showed that as the severity of the damage increased, the electrical resistance of the carbon fiber specimen increased as well. Moreover, as the size of the damage grew vary large, the resistance increased as a non-linear rate. As a whole, the current thesis provides a comprehensive framework for developing predictive, resistance-based damage sensing models for carbon fiber materials with various fiber orientations and thicknesses.

Keywords

Carbon Fiber, Damage Sensing, Electrical Properties, Low Velocity Impact

Pages

xviii, 196 pages

Bibliography

Includes bibliographical references (pages 192-196).

Copyright

Copyright © 2017 Robert James Hart

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