Resin Removal Repair of Composites

  • Jack J Lindley-Start

Student thesis: Doctoral ThesisEngineering Doctorate (EngD)

Abstract

The increased use of composite materials in the aerospace industry for safety critical
structures presents the industry with a challenge: how should these structures be repaired
when they become damaged in service? Currently, the only certified repair scheme is the
bolted patch repair which degrades the performance of the repaired structure. Commonly
proposed alternative techniques, such as the bonded patch repair, create new structures rather
than repairing the existing structures, which may present an insurmountable barrier to
certification of the repair scheme. A promising alternative repair approach is the resin
removal repair.
This thesis aims to investigate the hypothesis that resin removal repairs are able to restore
sufficient flexural and tensile mechanical performance to be used in industry. The scope of
this thesis was limited to investigating resin removal repair schemes on: thermoset resin
systems, uni-directional carbon fibre laminates, and resin dominated damage (such as
delaminations and matrix cracks) that resulted in minimal fibre distortion.
This thesis investigated this hypothesis by conducting the following experiments:
1. Thermogravimetric analysis (TGA) was used to investigate the abilities of ThermalOxidative Resin Removal (TORR), Thermal Activation of Semiconductors (TASC),
and Inert Gas Blanketed Resin Removal (IGBRR) to remove the resin of a composite
whilst retaining its fibres at the milligramme scale.
2. A bespoke experimental arrangement, microscopy, and X-Ray Computed Tomography
(XCT) scanning were used to assess the effectiveness of the TORR and IGBRR
approaches at the laminate scale.
3. The applicability of a resin injection and a resin infusion approach was investigated
using XCT scanning, gravimetry, and microscopy.
4. Tensile and four-point bend tests were conducted to establish the tensile and flexural
strength and stiffness restoration abilities of the resin removal repair.
This research established that, with the resin removal repair investigated, after repair, tensile
strength was reduced by 60 % and flexural strength was reduced by 70 % compared with
virgin laminate samples, and tensile and flexural stiffnesses were approximately 85 % of the
values for the virgin laminate. This indicated that additional development would be required
before sufficient mechanical performance restoration was achieved for resin removal repairs
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to be employed in industry. The key challenges to a successful repair were identified as the
presence of char on the fibres after the resin removal stage, residual and thermal stresses
leading to damage within the laminate caused by the repeated heating cycles (initial part
curing at 180 oC, two resin removal processes at approximately 600 oC, and reinstated resin
curing at 180 oC), and the removal of the fibre’s sizing.
Therefore, future research to develop resin removal repairs further may choose to focus on:
1. Methods for reducing the creation of char, such as inducing oxygen through the
laminate’s thickness, a two stage resin removal process involving an initial pyrolysis
stage in an inert gas environment followed by a char oxidation stage in air, or a bulk
heating approach such as microwave heating.
2. Methods for managing the thermal and residual stresses, for example pre heating the
laminate to higher temperatures than the service temperatures quoted by the suppliers
(120 oC for the resin system investigated in this research) of the resin.
3. Methods for reinstatement the fiber’s sizing or otherwise preparing the resin removed
fibres for bonding to the repair resin, such as plasma treatment.
Date of Award2 Dec 2021
Original languageEnglish
Awarding Institution
  • University of Bristol
SupervisorLuiz Kawashatia (Supervisor) & Hammerton Ian (Supervisor)

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