Advanced Engineering Composites

Overview

• The lecture broadly covers engineering composites, manufacturing processes for polymer matrix composites (PMC), and introduces metal and ceramic matrix composites.
• Specific processes discussed include Molding, Curing, Machining, and Inspection
• The lecture also mentions carbon-carbon composites.

Engineering (Advanced) Composites

• Engineering composites are defined as materials made by combining reinforcing materials with a binder or matrix, without a chemical reaction between them.
• A matrix component (resin) and a reinforcement component (fiber) are present.
• The reinforcement (fiber) bears the loads while the matrix binds and protects the fibers and may also carry auxiliary loads.
• Modern matrix examples include polymers (thermoset, thermoplastic), Carbon/Ceramic (CMCs), and Metal (MMC).
• Modern reinforcements can be organic or synthetic fibers, either short/long (continuous/discontinuous), randomly oriented or aligned.
• Using composites allows for 20-30% weight reduction and property improvements like fatigue and corrosion resistance.

Composite Materials

• Reinforcements in composite materials include organic fibers (Nylon, Polyester), glass fibers, aramid (Kevlar) fibers, carbon fibers, boron fibers, ceramic fibers, silicon carbide fibers, alumina fibers, quartz fibers, and metallic fibers.
• Matrices in composite materials include thermoset polymer matrix (phenolic, epoxy, polyester), thermoplastic polymer matrix (polycarbonate, polyamides), polymer matrix of high characteristics/temperature (PEE, PPS, PEI), metallic matrix (Ni, Ti, Al), ceramic matrix (glass, silice), and carbon matrix.
• Composite matrices can be metallic, polymer, ceramic, or carbon/graphite
• Polymer matrices can be either Thermoset (Epoxy, Phenolic, Polyester) or Thermoplastic (Polyethilene, Polystyrene, Polyamide, Nylon)
• Polymer Matrix Composites (PMC) can sustain working temperatures of up to 260°C
• Metallic Matrix Composites (MMC) can sustain working temperatures of up to 650°C
• Glass Matrix Composites (GMC) can sustain working temperatures of up to 1100°C
• Ceramic Matrix Composites (CMC) can sustain working temperatures of up to 1370°C

Fiber Architectures

• Fiber architectures include Yarns, Rovings, Tapes, Fabrics, Braids, Chopped Fibers, Mats and Pellets

Polymer Matrix Composites (PMC)

• A PMC is composed of a matrix and fibers.
• Reinforcements used in PMC include organic, glass, aramid (Kevlar), and carbon fibers.
• Matrices in PMC include thermoset and thermoplastic polymer matrices.
• PMC advantages are high strength/stiffness, low weight, design flexibility, dimensional stability, high dielectric strength, good corrosion/fatigue resistance, reduced mechanical joints, fiber orientation control, low thermal expansion coefficient, good radar emission absorption, and low electrical conductivity.
• PMC disadvantages are high cost, low strength in the thickness direction, mainly two-dimensional forms, sensitivity to heat/humidity, poor energy absorption, incompatibility with metals (carbon/aluminum), specific strength/strain tests required, expensive inspections, poor lightning impact behavior, limited raw material life, and difficult recycling.
• General Composite Fabrication involves Impregnation, Lay-up, Debulking, and Curing

Manufacturing Processes: Molding

• The molding process needs to be done in a clean room to avoid contaminating the composite material
• Compression Molding: The material is placed between two molds, then closed by pressure. Molds can be at room temperature or hot to cure the resin. Materials can be massive composites or laminates. Massive composites have resin, reinforcing fiber and additive mixtures installed in the mould. Laminates have fiber sheets are places in the mold with resin interspersed
• Contact Molding or Laminate: Including manual molding (hand layup - HLU), spray-up, automatic molding (automatic tape laying ATL, automatic fiber placement AFP) or hot drape forming

Contact Molding: Hand Lay-Up (HLU)

• Hand Lay-Up molding cuts shapes from templates and manually positions plies successively onto the tool
• Materials used are mainly fabric and tape
• Prepreg Lay-Up Processes: Plies previously impregnated with resin where high volumes of fiber are achieved, and structural aeronautical parts are made
• Wet Lay-Up: Uses dry fabrics/tapes, applying resin ply by ply manually resulting in dependence on quality and ability to cure at room temperature with lower cost of raw materials and facilities
• Manual molding process requires flat development, nesting, and plies cutting before laminating
• Composite parts can be "stabilized" with pre-curing with a few plies of adhesive

Contact Molding: Spray-Up

• Spray-up processes involve simultaneous application of resin and chopped fibers by a gun.
• Rollers are sometimes needed to remove porosity.
• Fiber orientations within matrices are random.

Manufacturing Automatic Tape Laying (ATL) and Fiber Placement (AFP)

• Automatic tape laying (ATL) uses a machine to position fiber in tape form utilizing numerical control machines. Employs prepreg unidirectional tapes of various widths and works best on low curvature surfaces.
• Offers advantages of efficient speed, low piece cost, and reduced defects but involves challenges like high initial investment and geometry limitations.
• Automatic fiber placement (AFP) also uses machines, narrower cords or tapes unlike ATL, utilizes multiple feeders, can adapt to complex geometries and is ideal for windings

Hot Drape Forming

• Intermediate process that helps facilitate or eliminate most manual molding. Developed as a complement to ATL with process including applying controlled temperature and vacuum to automatically taped flat laminates and manually molded laminates
• Material-specific temperature cycle parameters are critical because typical parts include c-channel sections with heat sink as main concern in tool design

Liquid Composite Molding (LCM) or Liquid Resin Molding (LRM)

• Multiple processes encompass a family of processes where dry reinforcement impregnated in liquid resin. Examples include RTM, HP-RTM, SQRTM, SCRIMP, VARTM, VAP, and RTI.
• Liquid Composite Molding has various material and processing parameters that are difficult to apply the affect the end results

Resin Transfer Molding (RTM)

• Manufacturing process utilizing dry fiber in closed mold
• Process involves placement of the fiber reinforcement (preform) is placed inside mold. Resin injected and pressure and temperature are applied

Other Liquid Composite Molding (LCM) processes

• Same Qualified Resin Transfer Molding (SQRTM) involves using prepreg preform. Resin is injected for applying hydrostatic press.
• Seemann Composites Resin Infusion Molding Process (SCRIMP) is assisted by vacuum pressure and open tool requiring tight vacuum and flow media integrity
• Vacuum Assisted Resin Transfer Molding (VARTM) used vacuum pressure with characteristices including tooling expense, easy process use of single and double sided tooling and curing capability up to 70%

Filament Winding & Braiding

• Filament winding (FW) combines fiber and resin, manufactures axisymmetric parts such as tubes and tanks, produce parts via rolling fiber on rotary mandrel, and creates very high strength products.
• Filament braiding manufactures composite fabrics, make complex shapes, and requires a mandrel. Controlled by feed rate, produces excellent damage tolereance laminates

Pultrusion

• For long, uniform parts like bars and profiles, pultrusion makes continuous parts by impregnating fibers in liquid polymer then passing through matrix to shape the tool followed by by curing

Manufacturing Processes: Curing

• Curing is necessary to adhere the different plies and involves most of the delicate phases of manufacturing

Curing Main Points

• Curing ensures the consolidation and proper adhesion of plies and is considered the main process
• The main processes include:
• Curing at room temperature
• Curing in oven
• Curing with compression molding
• Curing with fully heated tool
• Curing with vacuum bag
• Curing with autoclave

Curing: Autoclave vs Out-of-Autoclave

• Autoclave: heat and pressure over time in an autoclave that can be classified with or without inert gas or vacuum and external pressure to final consolidation and polymerization
  • Can provide elevated temperature, up to the 700 °F (371 °C approx).
• Out of Autoclave (O0A): thermosets that is combined with oven, self heating tools and is mainly consolidation with means possible

Curing: Room vs Oven

• Room temperature curing is available with polyester since epoxy and phenolics require temperature
• Oven curing may involve contracting tape for parts generated by winding, then introducing part into Oven at elevated temperature

Curing: Heat & Vacuum Bag

• Curing with compression molding introduces compound between two plates that transmit pressure from 20-1000 psi with temperatures usually at 600°F to to achieve flat parts at high production
• Curing with fully heated tool involves tool to providing the heat via resistors into the structure to avoid autoclave
• Curing with vacuum bag is done by vacuum space between composite laminate and a sealed flexible sheet with the goal of ensuring even pressure

Manufacturing Process: Machining

• Involves both conventional machining and shear cutting with unconventional Ultrasonic, Water jet, and Abrasive jet options.
• Water jet cutting: 0.08-0.5mm jet with pressure releasing outlet speeds of up to 900m/s

Manufacturing Process: Inspection

• Used to detect failures (delaminations, porosity, uncured zone) that can include:
• Visual Inspection
• Tap Test
• Ultrasonic Inspection
• X-Ray Inspection
• Other Methods (Tomography, Serography, Thermography etc.)

Metal Matrix Composites (MMC) Characteristics

• They are composite materials made from reinforcing fiber and a metal matrix with reinforcements potentially made from carbon fibers, silicon carbide, graphite, alumina, tungsten while matrices are made from aluminum, magnesium and titanium at low densities
• Reinforcing can be done from continuous fibers, short fibers or particles at temperature strength and high modulus and resistance to fatigue

Manufacturing of MMC by phase

• Processing via liquid state is through molding the matrix material in a liquid state for complex shapes
• Processing via solid state are done by powder metallurgical techniques that are used in tungsten carbide tools
• Processing via two-phase state reaches a semi-solid material from heat and injecting into a mold reinforced

Ceramic Matrix Composites (CMC)

• Composite materials that consist of a reinforcing fibre and a refractory matrix. Carbon fibres silica and silicon carbide are used for their construction
• Characterized by high mechanical strength, resistance to oxidation, fragile behavior ceramic materials. and dependence on thermal properties
• Characterized by semi-solid phase filtration, chemical synthesis and chemical vapor infiltration (CVI)