Asset Maintenance of Thick Section Fibre-Reinforced Composite Structures

Academic Institution: University of Edinburgh

Industry Partner: Babcock International

Academic Supervisor: Professor Eddie McCarthy

PhD Student: James A Quinn

Summary

Fibre-reinforced polymers (FRPs) are typically used for creating thin, lightweight parts. An example is the carbon-FRP components that are typically found on high-performance cars. For this reason, most of what we understand about FRPs is based on testing and simulating thin samples. However, understanding how FRPs behave when they are thicker presents a challenge.

This thesis tackles the asset maintenance of thick FRP structures (e.g., ships and turbine blades) from 3 perspectives:

  1. Non-destructive Testing (NDT) of thick FRPs

  2. How a specific type of damage—cracks between fibre layers—impacts the strength of thin FRPs

  3. Testing thick FRPs to better understand how they respond to loads

Engineers use various material properties, such as strength, to select the appropriate materials for parts whilst accounting factors like the part’s shape, size and production costs. When we mix two or more materials to make one new material, we call the product a “composite”. Some familiar examples include papier-mâché, concrete, and fibreglass. Fibre Reinforced Polymers (FRPs), are formed by blending lightweight, strong fibres with plastics.

FRPs are typically used for creating thin, lightweight parts. An example is the carbon fibre-reinforced polymer components that are typically found on high-performance cars. For this reason, most of what we understand about FRPs is based on testing and simulating thin samples. However, understanding how FRPs behave when they are thicker presents a challenge. Moreover, determining what classifies an FRP as thick or thin remains a topic of debate: is it the material’s thickness, the number of fibre layers, or the application context?

Certain applications necessitate the use of thick FRPs, like modern minehunter ships. These ships feature thick fibreglass components, some up to 200 mm in thickness. Designing such structures is a complex task, often simplified by making assumptions about how FRPs react to forces. These assumptions work well for thin FRPs but require validation for thick FRPs. This thesis shares experimental data from bending tests on thick FRPs. The data can then be compared to the simplified calculations often used for thin FRP designs, so we can figure out how applicable those assumptions are for thick FRPs

Maintenance of thick FRP structures, such as minehunter ships, is critical. Engineers perform regular inspections using Non-Destructive Testing (NDT) techniques that allow them to examine materials without causing damage. The NDT of thick FRPs is incredibly complicated, because internal damage is often really small. With NDT techniques, the resolution reduces with increased inspection depth. For example, a very thick FRP may be scanned, but very small damage features (e.g., cracks) could be missed, especially if they are located deep in the FRP. One commonly used method is ultrasound inspection, which is evaluated herein. This thesis introduces a novel approach using ground penetrating radar, opening up new possibilities for inspecting thick FRPs.

When damage is discovered during NDT inspections, assessing its severity is crucial. This thesis focuses on a specific type of damage—cracks between fibre layers— and how it impacts the strength of the FRP. Bending experiments were conducted on specimens with varying crack sizes and locations, providing engineers with a tool to estimate the extent of damage on strength, which can be used to assess the significance of such damage in real FRP structures, helping engineers make informed decisions about repairs and reducing unnecessary, environmentally harmful maintenance.

In summary, this research has the potential to extend the service life of assets like ships, improving their overall value by providing engineers with the knowledge and tools necessary to manage FRP structures effectively.

Key Results/Outcomes

  • Presents the novel application of ground penetrating radar for inspection of thick FRP composite structures. The technique proved to be highly successful in finding delamination flaws at depths where ultrasound testing currently lacks capability.

  • Presents a critical analysis of typical in-field advanced ultrasonic testing as used on thick FRPs.

  • A partial factorial investigation exploring the consequence of delamination damage of various sizes, positioned at various locations within FRPs of various thickness, was assessed.

  • The equations used to design and analyse “thick” FRPs make many assumptions about the way the FRP responds to load. However, there is a lack of consensus on how to define “thick” in this context. The definition of the descriptor “thick”, with respect to FRP beams, was challenged experimentally by testing thick FRP beams

Publications

  • Quinn, J.A.; Davidson, J.R.; Bajpai, A; Ó Brádaigh, C.M.; McCarthy, E.D. "Advanced Ultrasonic Inspection of Thick-Section Composite Structures for In-Field Asset Maintenance". Polymers 2023, 15, 3175. https://doi.org/10.3390/polym15153175

  •  Quinn, J.A.; Ward, I.; Robert, C.; Ó Brádaigh, C.M.; McCarthy, E.D. "Criticality of Delamination Flaws in Fibre Reinforced Composites". Twenty-third International Conference on Composite Materials (ICCM23),” in Proceedings of Twenty-Third International Conference on Composite Materials (ICCM23), Belfast, U.K., 2023

  •  Quinn, J.A. "Advanced Ultrasonic Inspection of Thick-Section Fibreglass Composite Structures". In: BINDT Workshop 2023: NDT of composites through life, Poole, UK, 22–23 March 2023; The British Institute of Non-Destructive Testing: Poole, U.K., 2023.

Contact Information

Dr Eddie McCarthy

Senior Lecture in Composites Design and Testing, University of Edinburgh

Ed.McCarthy@ed.ac.uk

Dr James A. Quinn

Research Associate in Design and Testing of Structures, University of Edinburgh

J.Quinn@ed.ac.uk