This thesis works on the topic of fiber-reinforced plastics and discusses the measurement of strain with embedded sensors. Embedding sensors into a structure fundamentally poses challenges arising from the differences in mechanical properties of sensor and structure. This thesis works on the research area of Self-Sensing, where these challenges are overcome by using carbon fibers for both load-carrying and strain-sensing functions. Starting with a literature review, this thesis proposes three research hypotheses, which are targeted to describe the Self-Sensing properties of unidirectional carbon fiber reinforced plastics (CFRPs) for strain measurements. These hypotheses assume, that the electric anisotropy of the material results in a complex voltage distribution within a Self-Sensing specimen. In order to discuss this point further, a two-dimensional piezoresistivity model based on the Laplace equation is introduced. The developed model newly allows to quantify the electric potential changes in specimens with arbitrary geometrical dimensions and electric anisotropy.
Furthermore, this thesis discusses a set of experimental results on the piezoresistive properties of unidirectional CFRP made with the pultrusion process. Overall, the results of the experiments indicate that the most repeatable results are obtained for specimens with electric contacts at their cut-end. This approach allows to manufacture Self-Strain-Sensing rods with a gauge factor of approximately 1.9 that can be used in a multifunctional manner for both load-carrying and strain-sensing purposes. Furthermore, a novel measurement setup is developed, which allows to acquire the electric potential distribution on the surface of electrical conductors with very high spacial resolution. This experimental setup newly reveals that the current flow in specimens can be more complex than assumed in a two-dimensional model.
Author(s): Patrick Scholle
Series: Mechanics and Adaptronics
Publisher: Springer
Year: 2023
Language: English
Pages: 203
City: Cham
A Two-Dimensional Piezoresistivity Model for Anisotropic Materials and its Application in Self-Sensing of Carbon Fiber Reinforced Plastics
Acknowledgements
Contents
Symbols and Abbreviations
General Notation
Latin Symbols
Greek Symbols
Abbreviations
1 Introduction
1.1 Introduction to Self-Sensing Carbon Fiber Reinforced Structures
1.2 Thesis Outline and Contents
References
2 State of the Art of Embedded Strain Sensors for Fiber Reinforced Plastics
2.1 General Overview of Strain Sensing Technologies
2.1.1 Resistive Strain Gauges
2.2 Embedding of Strain Sensors into Fiber Reinforced Plastics
2.2.1 Examples of Sensor Connections in Research and Industry
2.2.2 The Impact of Integrated Sensors on the Mechanical Strength of Fiber Reinforced Structures
2.3 Summary and Outlook
References
3 Electrical Homogeneity and Fiber Waviness: Predominant Factors for Self-Strain-Sensing Carbon Fiber Structures—A Literature Study
3.1 Basic Experimental Techniques and Definitions
3.1.1 Electrical Test Setups
3.1.2 Contacting Carbon Fiber Laminates
3.2 Piezoresistivity
3.3 The Resistance Change of Single Carbon Fibers Due to Mechanical Strain
3.3.1 Examinations of Single Bare Carbon Fibers Under Tensile Load
3.3.2 Examinations of Single Carbon Fibers Embedded into Polymer
3.4 Embedding Carbon Fiber Rovings into a Polymer for Strain Sensing
3.5 Using Large Carbon Fiber Reinforced Plastic Structures as Strain Sensors
3.5.1 Longitudinal Resistance Change with Longitudinal Strain
3.5.2 Transverse and Through-Thickness Resistance Change Due to Longitudinal Strain
3.6 Formulation of the Research Hypotheses
References
4 Concerning the Influence of Current Inhomogeneity on Self-Strain-Sensing Properties of Carbon Fiber Reinforced Plastics
4.1 General Electrical Properties of Carbon Fiber Materials
4.1.1 Longitudinal Resistivity
4.1.2 Transverse Resistivity
4.2 A Two-Dimensional Piezoresistivity Theory for Strain Sensors …
4.2.1 Two-Dimensional Laplace Equation for Anisotropic Mediums
4.2.2 Development of a Two-Dimensional Piezoresistivity Model
4.2.3 Model Verification with Finite Element Studies
4.3 Experimental Evaluation of the Gauge Factor of Single Carbon Fiber Filaments
4.3.1 Methods and Materials
4.3.2 Experimental Results
4.3.3 Discussion of the Single Fiber Experiments
4.4 Manufacturing Reliable Electrical Contacts to Cured Carbon Fiber Reinforced Plastics
4.4.1 Multi-fiber Strain Sensors: A Maximally Reduced Model
4.4.2 Surface Preparation Methods and Materials
4.4.3 Surface Contacting Methods Based on Filled Adhesives
4.4.4 Surface Contacting Methods Based on Electrodeposition
4.4.5 Comparison of Contact Resistances for Various Contacting Methods
4.4.6 Results and Discussion
4.4.7 Dynamic Evaluation of Contact Resistances During Initial Strain Cycles
4.4.8 Results and Discussion
4.4.9 Conclusion
4.5 Evaluation of the Developed Conduction Model Without External Load
4.5.1 Methods and Materials
4.5.2 Results and Discussion
4.6 Piezoresistivity of Pultruded CFRP Rods with Homogeneous Current …
4.6.1 Methods and Materials
4.6.2 Results
4.6.3 Discussion
4.7 The Combined Effect of Longitudinal and Transverse Piezoresistivity …
4.7.1 Methods and Materials
4.7.2 Results
4.7.3 Discussion
4.8 Main Findings, Summary, and Outlook
References
5 Direct Measurement of the Potential Distribution on Conducting Surfaces
5.1 Concerning the Retroactivity of Electrical Contacts on Carbon Fiber Reinforced Plastics
5.2 Methods and Materials
5.3 Initial Experimental Investigation of Machine Parameters
5.3.1 Stability of Potential Measurements over Time
5.3.2 Overall Repeatability of the Measurements
5.3.3 Example Measurement of the Out-of-Plane Current Homogeneity in Thick Conductors
5.4 Experimental Determination of In-Plane Current Homogeneity …
5.4.1 Potential Distribution in Isotropic Conductors
5.4.2 Potential Distribution in a Pultruded Specimen with Silver Paint Surface Contacts
5.4.3 Potential Distribution in a Pultruded Specimen with Electroplated Surface Contacts
5.4.4 Potential Distribution in a Pultruded Specimen with Electroplated End Contacts
5.4.5 Potential Distribution in a Prepreg Specimen with Silver Paint Contacts
5.4.6 Discussion
5.5 Main Findings, Summary, and Outlook
References
6 Implementation Aspects of Self-Strain-Sensing Carbon Fiber Rods
6.1 Using Embedded Self-Strain-Sensing Carbon Fiber Rods for Strengthening and Strain Sensing of GFRP
6.1.1 Manufacturing Aspects of Integrated Carbon Fiber Sensors: Methods and Materials
6.1.2 Influence of the GFRP Laminate on the Sensor Characteristics of the CFRP Sensor Under Uniaxial Tension
6.1.3 Influence of the Integrated CFRP Rod on the Tensile Properties of GFRP Under Uniaxial Strain
6.1.4 Summary
6.2 Design of an Automated Process for Localized Electrodeposition onto Carbon Fiber Rovings
6.2.1 Literature Review on Galvanic Deposition on Carbon Fibers and Their Reinforced Plastics
6.2.2 Process Design
6.2.3 Application as a Carbon Fiber Roving Strain Sensor
6.2.4 Summary
6.3 The Influence of Temperature on the Electrical Resistance of Carbon Fibers
6.3.1 Methods and Materials
6.3.2 Results and Discussion
References
7 Summary, Conclusions, and Outlook
7.1 Summary
7.2 Concept Challenges
7.3 Application Scenarios
7.4 Outlook
References
Appendix A Derivations of Other Boundary Conditions and Models for the Laplace Equation
A.1 Γ1 for Different α
A.2 Finite Size Contacts
A.3 Piezoresistivity Model for Voltage Contacts on the Opposing Side
Appendix B Surface-Contact Specimens Dimensions
Appendix C Full Measurement Results for Gauge Factor Studies
C.1 End-Contact Specimen
C.2 Surface-Contact Specimen
C.3 Embedded Specimens
