Bulkhead Reinforcement: Unidirectional Fibreglass and Edge Capping

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Restoring structural integrity and finishing a cut composite panel.

Author: Shayne

Key Message: Cutting into a cored panel necessitates a two-stage process to restore its structural integrity: first, reintroducing in-plane stiffness and strength along the cut edge with a strategic reinforcement (like unidirectional glass); and second, re-establishing the critical load path between the skins and sealing the core with a structural edge cap.

Introduction


During the interior refit of Paikea, we cut through the existing bulkheads to make larger doorways in the hulls. However, we cannot just leave a foam and e-glass panel edge cut and the foam exposed. In order to keep the strength in the bulkhead we need to ensure we reintroduce the strength and stiffness by reinforcing the cut out and adding an edge capping to tie the two skins on each side of the bulkhead together. The goal was to create a strong, lightweight reinforcement that would resist flexing around a bunk opening. This report details the decision to use unidirectional fibreglass and the method developed to laminate it into a narrow channel and then place an edge capping to tie the 2 sides together and improve strength.

1. The Problem: Exposed Core and a Weakened Structure


The act of cutting through the sandwich panel severs the continuous fibreglass skins, creating a weak point along the edge. The exposed core is vulnerable to moisture ingress and impact damage, and the panel loses a significant degree of its inherent stiffness. Simply filling the edge with putty is structurally inadequate.

2. Laminate Design and Material Selection


The primary requirement was to restore axial stiffness along the length of the bulkhead edge.

  • Material: Unidirectional E-glass was selected as it places the maximum number of fibres in the direction of the required load path, offering an efficient stiffness-to-cost ratio for this application.
  • Laminate Schedule: The design specified a channel 20mm wide by 5mm deep, filled with 10 layers of 600gsm uni, creating a robust, stiff spine along the cut edge.

3. The Wet-Out and Placement Challenge


The primary difficulty was impregnating and placing the dry, springy unidirectional tape into a narrow, vertical channel.

  • Pre-wetting: The uni was pre-impregnated with epoxy on a separate surface.
  • Handling Difficulty: The wetted fibre bundle became difficult to handle and roll to the required diameter without the woven “W fibre” found in bi-axis fabrics to hold it together.
  • Solution: The laminate was carefully rolled and worked into the channel in stages, a time-consuming process that leveraged the 40-minute pot life of the epoxy system.

4. A Practical Consolidation Method


Without a workshop and all the precise tools available, an effective consolidation method was needed to compact the fibres and remove excess resin.

  • Technique: Highly adhesive masking tape was applied tightly over the channel opening, acting as a temporary flexible dam holding the fiber in place while it cured.
  • Result: This method applied sufficient pressure to consolidate the laminate effectively, resulting in a well-compacted part with minimal voids after cure.

5. The Essential Final Step: Structural Edge Capping

With the unidirectional reinforcement cured, the final step is to apply a structural edge cap.

  • Primary Purpose: Re-establishing the Load Path. The most critical function of the edge cap is to structurally tie the two outer skins of the bulkhead together. In an uncut panel, loads are distributed across the continuous skins. Cutting the panel severs this path. The edge cap acts as a bridge, allowing stresses and loads to transfer from one skin to the other, restoring the panel’s monolithic structural behaviour.
  • Secondary Purpose: Sealing and Durability. The edge cap also seals the exposed core from moisture ingress and provides a durable, finished edge that protects against impact and prevents delamination from starting at this vulnerable point.

Conclusion


This two-stage process of adding unidirectional reinforcement and a structural edge cap is a fundamental and correct method for finishing any cut composite panel. It ensures the structural integrity of the bulkhead is not only maintained but enhanced, while providing a durable, long-lasting finish.

Want More?

See more of our cutting out bulkheads in the posts below. If you are interested in a deep dive into composites, then consider joining our Youngbarnacles Members Hub, we provide detailed discussions on our projects that share real world tips on how to use cutting edge materials in your own projects. You can check out our Members Forum Discussion on Edge Capping there too.

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