Repair scheme for the damaged hull

Damage Assessment

The damaged sandwich panel consists of two high strength GRP skin and skins separated by a low density, lower strength foam core.

Such structures can be subjected to three damage scenarios. The damage can be limited to one skin (Type A), to one skin and the core (Type B), or to both skins and the core (Type C). Type A damage generally involves matrix cracking, fiber breakage and delaminations in the skin. The damage may or may not extend through the full thickness of the skin. Type A damage can also include debonding of the skin from the core. Type B damage involves Type A damage to one skin combined with crushing or shear cracking of the core. Type C damage involves the same damage mechanisms as Type B except both skins are affected. Type C damage can fully penetrate the sandwich structure.

Assuming the hole approximately 150mm in diameter is a though hole and damaged both skins and foam core, we have Type C damage. So we need to repair this damage according to Type C repair scenario. Since access can be gained from both sides, The repair of Type C damage could be done.

Repair Type Decision

Repair approaches are divided into non-patch, usually fo minor defects, and patch, usually for major defects. Patch repairs are also divided into

Bonded External Patch Repairs

External patches can be employed to repair laminates and skins of honeycomb panels of thickness up to about 2mm composites. The main problem with external patches is that, as in a single-lap joint, there is an eccentric load path that results in bending in the patch and peeling stresses in the adhesive and composite.

Bonded Scarf or Flush Repairs

Scarf repairs are employed to repair thicker section material than is possible with external repairs and also where the repair must show minimum protrusion above the original surface. The main advantages of scarf joints are the uniform shear-stress distribution in the adhesive, and, due to lack of eccentricity in the load path, the low peel stress.

Bolted External Patch Repairs

Bolted patches are generally employed for field repair of thick laminates where the shear stress requirements exceed the capability of adhesives for external patch repairs and where the complexity of repair implementation and material removal requirements. Moisture problems also limit field applications of bonded repairs in thick composites. Bolted repairs are well suited to the repair of battle damage since they are easy to apply by unskilled personnel and require minimum facilities.

Both bonded and bolted repair methods could be applied to repair marine sandwich structures. Bonded repairs were considered to be lighter in weight and distribute the load more evenly over a wider area. However, they require careful surface preparation, are difficult to inspect and are more difficult to perform correctly. Bolted repairs are more easily carried out and require minimal surface preparation. However, they add bulk and weight and require holes to be drilled through the structure which can introduce further damage and create stress concentrations. Additionally, bolted repairs are more difficult to implement on sandwich structures and need to be water-tight.

Since the yacht has been rammed well above the water line, it could continue its operation with small leakages. So repair could be done in facilities. Also since it has been rammed there is no stress cracking due to design considerations. Same section could be performed; shear stress requirements do not exceed the capability of adhesives. According to these considerations, bonded scarf repair would be more applicable.

Permanent Composite Structure Repair

For all bonded repairs, removal of all damaged material starting at the centre of the damaged region and surface preparation is of vital importance.

Damage to sandwich structures often involves damage to the core material. The damaged core can be filled either with a foaming adhesive, a laminate or a new core section bonded in place. The latter method is usually adopted as it best restores the properties of the sandwich structure.

The approved general guidelines for laminate and sandwich repairs should be followed. These repair operations should be carried out in a workshop areas to ensure high quality repairs.

The repair of Type C damage to GRP/foam sandwich structures requires the replacement of both skins and the core.

The repair procedure for Type C damage is shown in Figure and described following;

• Remove damaged material.

1. Remove the damaged skins, working from the centre of the damaged region on both sides outwards until sound material is encountered.
2. Remove the exposed damaged core.

• Taper sand

1. Prepare the foam core
2. Sand the edges of both skins to a taper per layer providing a scarf angle

• Install the replacement foam

1. Use a paste adhesive designed to bond foam. However, instead of using vacuum, the adhesive was injected into the bondline using a caulking gun.
2. Use a backing plate where required.
3. Use the appropriate grade of foam.
4. Use the minimum amount of adhesive.

• Replace the skins

1. Use the same number of layers as the original skin.
2. Apply one extra layer of GRP.

• Replace the skins on other side

1. Use the same number of layers as the original skin.
2. Apply one extra layer of GRP.

Quality Check / NDT

For comprehensive inspection of repaired part, Non Destructive Test (NDT) can ve used. The inspector should examine the quality of the repaired area and particular attention should be given to the interface between the original part and the repaired area.

Usual inspection methods use some form of ultrasonic test equipment where the reflection of the ultrasonic sound waves detect and identify any damaged areas or faults.