midterm
A structure with an "Interlaminar Shear" design would relate to Matrix Dominated properties.
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The sources do not appear to define when a structure would be built with an "In-Plane Shear" design.
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Flexural modulus refers to stiffness.
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Tg refers to glass transition temperature.
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A main concern with Tg is that the wet Tg of the matrix resin dictates the maximum service temperature of a structure without degrading.
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Matrix Dominated designs have the following properties:
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Compressive strength
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Off-axis bending stiffness
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Interlaminar shear strength
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Service temperature
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Fiber Dominated designs have the following properties:
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Tensile strength
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Flexural modulus
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Unidirectional: This is a material where the fibers run in one direction, giving it the highest strength-to-weight ratio. It requires more plies to achieve quasi-isotropic, balanced, and symmetric properties.
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Quasi-isotropic: A quasi-isotropic composite has either discontinuous (chopped) fiber in a random orientation, or has continuous (long) fibers oriented in directions such that equal strength is developed all around the laminate when loaded in edgewise tension or compression. Generally, a laminate is considered to be quasi-isotropic when laid up with an equal number of plies at each of 0°, 90°, +45°, and -45° angles.
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Sandwich Core: "Sandwich" structures rely on relatively thin skins bonded to a lightweight but usually thick core material. Compared to solid laminates, they offer a much higher bending stiffness-to-weight ratio.
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Warp Clock is an internationally recognized system for controlling fiber orientation.
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Counter Clockwise (CCW) is shown from the manufacturing point of view, with the fiber orientation viewed from the inside of the panel looking toward the tool surface.
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Clockwise (CW) is shown from the engineering point of view from the outside of the structure/tool surface looking in. This perspective is most often used in the repair of composites.
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The orientation of plies matters because laminate performance is dependent on fiber orientation.
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Symmetry means plies are symmetric about the middle of the laminate when viewed in cross-section. Symmetry helps with dimensional stability through processing.
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Balance refers to a laminate with an equal number of plus and minus angled plies. Balance helps to avoid twisting under applied loads.
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Nesting is the placement of harness-satin woven cloth plies so that the dominant fiber direction at the surface of one ply aligns in the same direction as the interfacial surface of the adjacent ply.
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Stacking is the placement of plies in the layup without regard for the position of the warp face.
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Plain weave and twill weave fabrics are typically stacked.
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Three risks to service life are:
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Fuels and Oils
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Ultraviolet light
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Acids/Alkalies
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A conductive path for Lightening Strike Protection (LSP) can be provided by:
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Bonding aluminum foil to the structure as the outermost ply
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Bonding aluminum or copper mesh to the structure either as the outermost ply or embedded within a syntactic surfacing film
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Incorporating strands of conductive material into the laminate
PPE is not optional when working with composites [1, 2].
The same type of gloves will not always be appropriate, so it is important to ensure that the equipment is well-stocked [3]. The material selected for protective gear is determined by the hazards involved [4, 5].
There are four routes of exposure [6]:
Absorption can happen through the eyes, hands, and skin [5, 7]. To protect the skin, clothing, and possibly a Tyvek suit or apron, should be worn [4]. Safety glasses and face shields protect the eyes [3]. Gloves protect the hands [3].
Inhalation is breathing a substance into the lungs [6]. Respirators can be used for protection [4, 8].
Injection introduces foreign material into the bloodstream or tissue [6]. Attention to the task, avoiding rushing, and having proper first aid supplies are key to preventing injection [8, 9].
Ingestion is swallowing chemicals, which can occur if chemicals are left on hands, clothing, or beards, or through contaminated food, drinks, or cigarettes [10]. To prevent this, there should be no food in the lab, gloves should be removed before drinking or eating, and hands should be washed thoroughly and often [8].
Adhesive bonding is a process that transfers loads between substrates and has the ability to transfer loads more efficiently [11, 12]. Long term durability requires proper surface preparation, the right adhesive, uniform bondline thickness, uniform clamping, and proper cure [11]. Fastening involves loads transferred at the fastened joint [11].
Co-curing is curing the laminate while simultaneously bonding it to some other uncured structure, or to a core material such as honeycomb, foam, or end-grain balsa, and involves curing in one process [13, 14]. Co-bonding is curing together of two or more elements where at least one has already been fully cured and one is uncured, and requires meticulous surface preparation [14, 15]. Secondary Bonding involves two or more pre-cured or already cured composite parts joined together by adhesive bonding [13, 14]. The only chemical or thermal reaction is the curing of the adhesive joining the parts and requires precise tooling to control location of mating parts [13].
Surface preparation for bonding to a fiber reinforced plastic surface depends upon chemical attachment and attraction and sharing of electrons at the interface Covalent bond [16, 17]. The process to prep metal for bonding includes chemical pretreatments to remove the oxidized layer of metal and etching a mechanical micro-surface into the exposed metal-oxide layer [16, 17]. To ensure the surface free energy value is high enough for bond that coalent bond to actually maintain itself, the material must be bonded, bagged, and under vacuum literally within a half hour from completion of the etching process [18, 19].
For mechanical fastening of thick composites, use large diameter fasteners with high bearing strength and an interference fit [20, 21]. Large-head, expanded sleeved fasteners and stainless steel sleeves are also used which expand diametrically inside the hole as the titanium fastener body is drawn through it [20, 21]. Bonding of thin composites is preferred over mechanical fastening [22, 23].
For drilling carbon and glass fiber reinforced composites, carbide drills with a positive or extreme rake angle, fine chisel edge, and gradual taper up to final diameter should be used [24-26]. Diamond coated carbide drills are also highly effective for carbon fiber and fiberglass because small diamond bits are deposited on the cutting edge and embedded in a metal matrix material [24, 26, 27]. For drilling aramid fiber reinforced composites, a brad –point twist drill (self-centering, modified, two-flute twist drill with a sharp center point surrounded by two sharp peripheral cutting edges) should be used [26, 27].
Shear Design Applications:
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Interlaminar Shear: Structures experiencing bending loads. The matrix provides resistance to interlaminar shear, with forces parallel to the laminate layers.
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In-plane Shear: Structures experiencing torsional loads. The matrix resists fiber buckling under compression, a major factor in a composite's compressive strength.
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Resin Properties:
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Thermoset Resins:
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Used for highly loaded structures.
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Exhibit high strength.
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Joined by cross-linking.
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Thermoplastic Resins:
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Suitable for areas needing toughness.
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Offer impact resistance.
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Enable high-volume, fast processing.
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Flexural Modulus and Glass Transition Temperature (Tg):
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Flexural Modulus: Indicates stiffness, showing how easily a part bends.
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Tg (Glass Transition Temperature): The temperature at which a resin or fiber transitions from a rigid to a rubbery state due to increased molecular mobility.
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Main Concern with Tg: Laminate distortion can occur if a structure isn't supported above this temperature. Achieving a specific cure temperature for a limited time may not produce the desired ultimate Tg.
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Matrix Properties:
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Polyester: Inexpensive with good environmental resistance but lower strength and higher shrinkage than epoxies.
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Vinyl Ester: Better environmental resistance and strength than polyester, with less shrinkage than polyester but more than epoxy; higher cost than polyester.
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Epoxy: Superior adhesion, high elongation factors, and good environmental resistance; more expensive than polyester and vinyl ester.
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Phenolic: Brittle with poor adhesive characteristics but excellent static dissipation and good chemical/flame resistance.
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Polyurethane: High toughness, fracture resistance, higher elongation properties, and good adhesive properties; lower cost compared to most epoxies.
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Benzoxazine: Similar flame resistance and low smoke toxicity to phenolic; long-term room temperature storage and adaptable to closed molding processes.
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Cyanate Ester: Excellent balance of mechanical performance and toughness; sensitive to moisture uptake prior to processing.
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Bismaleimide (BMI): Better thermal stability than epoxies, brittle, and good hot/wet properties; more expensive than epoxies.
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Polyimide: Can be thermoset or thermoplastic, requires high-temperature cure, difficult to process, but good flame resistance and low smoke toxicity.
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Stages of Thermoset Matrix Cure Cycle:
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A-Stage: First reaction stage; low viscosity, can be diluted with solvents.
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B-Stage: Resin is viscous and tacky but not flowing, facilitating storage and handling.
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C-Stage: Final reaction stage; resin is cross-linked, insoluble, and infusible.
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Fiber and Matrix Dominated Design:
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The sources do not contain information about properties of a Matrix Dominated design or a Fiber Dominated design.
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Prepreg Out-Time:
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You need to know the amount of time prepreg is out of frozen storage and exposed to the air. Moisture contamination can occur if the prepreg bag is opened, allowing condensation to form on the material.
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Definitions:
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Flow: When the resin is at its lowest viscosity and maintains this viscosity for a period of time.
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Gel: The transition phase from a liquid to a solid state.
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Optimal Compaction Point: The later part of the flow phase where resin viscosity is starting to rise, making it the best time to apply pressure.
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Vitrification: When the resin becomes a glassy solid at a given temperature.
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Cure: The extent to which a resin achieves its full intended properties and characteristics.