Repair Methods for Composites 250

ambient temperature

The nominal temperature of the air that surrounds devices and systems. When performing repairs on composite parts, work in an area where the ambient temperature is below 90°F (32°C).

balanced laminate

A laminate in which the individual plies occur in +/- pairs at various angles except for 0° and 90°. The pairs must be the same thickness but they are not required to be in any particular order.

balsa wood

A type of wood that is lightweight and springy. Balsa wood may be used in the core of a sandwich panel.

bolt

A threaded cylindrical device used for fastening parts. Bolts are used in bolted repairs of composites.

bolted repair

A composite repair method in which the damaged area is covered with a titanium plate that is secured to the part with fasteners and screws. Bolted repairs are typically done on thick laminates designed to bear heavier loads.

bonded repair

A composite repair method in which the damaged area is covered with some type of patch. The three basic types of bonded repair are patch repair, scarf repair, and stepped repair.

buckling

The bulging, bending, bowing, or kinking of an object as a result of compressive stress. Combining or stacking individual plies together to form a laminate helps to prevent buckling.

catalyst

A substance that facilitates chemical reactions in another substance. Catalysts are used to promote hardening in resins and gel coatings.

composite

A material made by combining a binding resin with small filaments of solid material. Composites have the strength of metal, the light weight of plastic, and the rigidity of ceramics.

core material

The middle of a sandwich panel composite. Sandwich cores can be made of foam, balsa wood, or honeycomb.

crossply laminate

A laminate in which the ply orientations are oriented at right angles to each other, with ply orientations limited to 0° and 90° only. It is usually best to arrange stacking sequences with fibers oriented in different directions.

delamination

The separation of the layers in a laminate. Delamination can occur locally, in a small area of the laminate, or it can affect the entire laminate.

face plate

The outer skin of a sandwich panel. Also known as face sheet.

face sheet

The outer skin of a sandwich panel. Also known as face plate.

fiberglass

Reinforcement material made from extremely fine strands of glass. Fiberglass is the most commonly used reinforcement material in composites.

fill repair

A relatively simple composite repair method in which resin and filler is applied to the damaged area to fill in nicks, scratches, and dents. Fill repairs are used for minor, non-structural damage on laminates, or on the outer skin of a sandwich panel.

film adhesive

A thin, transparent sheet that bonds to a surface. Film adhesive is used in bonded repairs of composites.

finishing

Any machining process that surfaces, conditions, edges, cleans, etc. a part or component. Usually the last step after a workpiece is constructed.

foam

The resulting material after mixing a plastic resin with substances that cause the formation of tiny air bubbles in the resin. Foam may be used in the core of a sandwich panel.

gel coat

The protective outer coating of a composite material. Gel coat requires the addition of a catalyst before applying so that the coating will harden.

hexagonal

Six-sided. Honeycomb cells in a sandwich core may be hexagonal.

honeycomb

A type of sandwich core in which empty cells are separated from each other by a solid wall. Honeycomb cells can be square or hexagonal.

injection repair

A composite repair method used primarily for delamination. Resin is injected into the delaminated region and pressure is applied to the part.

interlaminar

Located between individual layers of the laminate.

interlaminar shear

The displacement of plies within a laminate. Combining or stacking individual plies together to form a laminate helps to prevent interlaminar shear.

intralaminar

Located within a ply of the laminate.

lamina

An individual layer of the laminate. Also known as a ply.

laminate

Two or more layers of fibrous material that have been bonded together for strength. One of the most common reasons for repairing a composite part is damage to the laminate.

matrix

The material that binds together the reinforcing fibers of a composite. The matrix is usually a viscous material that hardens to give shape to the composite part and protect the fibers from damage.

midplane

The middle layer of the laminate.

negligible damage

Cosmetic damage that is so slight, it does not affect the structural integrity of the composite material and poses no risk of part failure. Negligible damage can be corrected with a simple procedure, such as buffing, or it can be left as is.

nonrepairable damage

Damage that is so extensive, severe, or expensive that the part must be discarded and replaced with another. Damage may also be deemed nonrepairable simply because the repair work is too costly to justify keeping the part.

nut

A fastening device containing a hole and internal threads that mate with a screw or bolt.

parallel laminate

A laminate in which all of the layers are oriented so that the fibers are parallel to the direction of the greatest tension. In composite materials, reinforcement fibers provide the greatest strength when they are oriented in the direction of the load.

patch

Another name for the titanium plate used in a bolted repair.

patch repair

A type of bonded repair in which replacement material is inserted to fill the damaged area and covered with a series of successively larger repair plies.

pilot hole

An undersized hole that holds a screw more securely when the screw is driven into the hole. Pilot holes are used in bolted repairs of composites.

ply

An individual layer of the laminate. Also known as a lamina.

pot life

The period of time after mixing during which a resin remains suitable for use. When repairing composites, keep in mind that the pot life of the resin is limited.

primary bond

A bond that forms between atoms and that involves the exchanging or sharing of electrons. In composites, the primary bond is formed when the composite is originally cured.

reinforcement material

The part of the composite that provides strength, stiffness, and the ability to carry a load. In manufacturing, fibers are the most commonly used reinforcement.

repairable damage

Surface damage and structural damage that can be reversed with repairs, and the part restored to acceptable quality and durability levels. Always consult with someone experienced in composite inspection to determine whether the part can or should be repaired.

resin

A substance made from either synthetic or natural polymers and used for composite matrices. In essence, a resin is a polymer that has not been processed into its final form.

sandwich panel

A composite structure in which two stiff outer skins are bonded to a relatively thick but lightweight core. Sandwich panels are preferred in applications requiring a structure that is both highly resistant to bending and extremely lightweight.

scarf repair

A type of bonded repair in which the damaged area is sanded to produce a tapering effect. Film adhesive is laid over the damaged area and covered with a series of successively larger repair plies.

scarfing

Sanding an area of a part to produce a tapering effect. Scarfing is a common way to create a large but visually attractive repair on a composite.

secondary bond

A bond that involves attraction between molecules, but no transfer or sharing of electrons. Secondary bonds are not as strong as primary bonds.

stacking sequence

The arrangement of ply orientations within a laminate. The properties of the laminate vary depending on the stacking sequence.

stepped repair

A type of bonded repair in which the damaged area is sanded to resemble a series of steps. Film adhesive is laid over the damaged area and covered with a series of successively larger repair plies.

symmetrical laminate

A laminate in which the layers below the midplane form a mirror image of the stacking sequence of the layers above the midplane. Symmetrical laminates may have an odd number of layers or an even number of layers.

visual inspection

A visual assessment of surface defects on the composite. Visual inspection is one of the most commonly used non-destructive inspection methods for composites.