Materials Science: Wood and Composites

Quality of Wood

• Checks, shakes, and splits are common defects in wood that affect its integrity.
• A check is a longitudinal crack through the annual grain, while a shake is a separation visible between annual rings.
• Wood stains indicate decay, often appearing as streaks; they can vary from red to white discoloration.
• Decay reduces toughness and may render wood brittle due to fungal activity in damp conditions.
• Proper drying (up to 20%) and varnishing can minimize decay and protect wood from environmental elements.

Plastics in Aircraft Construction

• Plastics are vital in aircraft manufacturing, utilized in forms like thermoset plastic reinforced fiberglass and thermoplastic materials for windows.

Composites Overview

• Composites consist of multiple materials differing in composition, enhancing performance through weight reduction and structural integrity.
• They offer a high strength-to-weight ratio, reduce the number of parts needed, minimize wear, and provide corrosion resistance.
• Disadvantages include high costs and challenging repair processes requiring specialized training and equipment.

Composite Structure Elements

• Reinforcing materials and matrix materials are key components in composite structures.
• Polypropylene is commonly used for airfoil shapes; it is compatible with adhesives and epoxy but dissolves in fuels.
• Balsa wood and laminated hardwoods serve as wood cores, enhancing compressive strength in composite designs.

Matrix Material Functionality

• The matrix holds reinforcing fibers, providing composite strength and transferring external stresses to improve overall structure performance.
• Resin, as an organic polymer, is a common matrix material. Polyester resin is used for non-structural applications, whereas epoxy resin offers better adhesion and chemical resistance.
• Thermosetting resins cannot be reshaped post-curing, playing a critical role in structural applications.
• Vinyl ester resins enhance mechanical properties compared to standard polyester resins.

Resin Systems and Curing Stages

• Curing of thermosetting resins involves three stages: A stage (mixing without reaction), B stage (thickened, tacky state), and C stage (fully cured).
• Prepreg materials are stored at low temperatures to maintain the B stage, preventing further curing until needed.

Working with Resins

• Pot life refers to the workable timeframe of catalyzed resin, while shelf life indicates how long unopened products remain usable.
• A balanced fiber-to-resin ratio (60:40) optimizes strength in composite lay-ups.

Fiber Science

• The effectiveness of reinforcing materials depends on weave pattern, matrix application, fiber strength, and part design.

Manufacturing Processes for Composites

• Heat and pressure application during curing improves fiber saturation and accelerates the curing of the matrix.
• Compression molding employs male and female molds for high-volume production of precision parts.
• Vacuum bagging involves placing assemblies in plastic bags and removing air to apply pressure for repairs.
• Filament winding involves wrapping continuous fiber around a mandrel to create durable structures.
• Wet lay-up is a straightforward method combining fiber reinforcement with a matrix for curing, offering flexibility at the cost of precision.