Understanding key terms related to the manufacturing, supply, machining, and fabrication of thermoset composites is essential for today’s professionals. This master list combines Atlas Fibre’s original glossary terms with expanded coverage of machining, fabrication, and foundational composite terminology.
Abrasive Tool Wear: The accelerated dulling of a cutting edge caused by the hard glass or carbon reinforcing fibers within a composite, which act like sandpaper against the tool. Because thermoset composites are far more abrasive than metals or plastics, tool wear directly drives surface finish, dimensional accuracy, and tooling cost.
Anisotropy: The property of being directionally dependent, implying different properties in different directions. In thermoset composites, anisotropy is often due to the orientation of the reinforcing fibers, resulting in varying mechanical properties in different directions.
Autoclave Curing: A high-pressure, high-temperature curing process used to fabricate high-performance thermoset composites. Ensures excellent consolidation and void-free parts, commonly used in aerospace and high-end automotive industries.
B-Stage: An intermediate, partially cured state of a thermoset resin in which it is solid but not fully cross-linked, allowing further processing before final cure. Prepregs are supplied in the B-stage so they can be laid up and then consolidated under heat and pressure.
Chemical Compatibility: The ability of these materials to resist corrosion, degradation, or deterioration when in contact with various chemicals. Read more about chemical compatibility.
Chemical Resistance: The ability of a material to withstand exposure to various chemicals without degrading or losing its properties. Thermoset composites often exhibit high chemical resistance, making them suitable for harsh chemical environments.
Climb vs. Conventional Milling: Two milling strategies defined by the cutter’s rotation relative to the direction of feed – climb milling rotates with the feed, conventional against it. Choosing the correct strategy for a given laminate helps control delamination, fiber pull-out, and edge quality.
Co-curing: The simultaneous curing of different components (such as skins and cores in a sandwich structure) in a single operation. Enhances the bond between the components and can streamline manufacturing.
Coefficient of Thermal Expansion (CTE): The rate at which a material expands with an increase in temperature. A low CTE in thermoset composites ensures better dimensional stability under thermal cycling.
Creep Resistance: The ability of a material to resist deformation under sustained load over time. High creep resistance means the material can maintain its shape and structural integrity even when subjected to long-term stresses.
Cross-Linking: The chemical reaction in which polymer chains form permanent bonds during curing, converting a liquid or pliable thermoset resin into a rigid, infusible solid. Cross-linking is what gives thermoset composites their heat resistance and dimensional stability and makes the cure irreversible.
Cure: The process of hardening a thermoset resin through heat, pressure, or the addition of curing agents. Curing transforms the resin from a liquid or soft state into a rigid, solid state.
Degree of Cure: The extent to which a thermoset resin has undergone the curing process. Often expressed as a percentage and affects the mechanical and thermal properties of the composite.
Delamination: The separation of a laminate’s reinforcing layers (plies), often triggered during drilling, routing, or edge trimming when cutting forces pull the plies apart. It is one of the most common machining defects in thermoset composites and can compromise structural integrity, making tool selection and feed control critical.
Depth of Cut: The thickness of material removed in a single pass of the tool. On thin or brittle laminates, shallower passes typically reduce delamination and edge chipping while improving dimensional control.
Dielectric Strength: The maximum electric field a material can withstand before it breaks down and conducts, typically expressed in volts per mil. High dielectric strength is a defining advantage of composites such as G10 and FR-4 in electrical insulation applications.
Dimensional Stability: The ability of a material to maintain its dimensions and shape when exposed to varying environmental conditions such as temperature and humidity. Dimensional stability ensures minimal expansion or contraction, preserving the performance and fit of the component.
Dust Extraction: The capture and containment of the fine particulate generated when machining composites, which can be a respiratory hazard and is abrasive to equipment. Effective dust extraction is essential for operator safety, air quality, and consistent machining performance.
Edge Chipping: Small fractures or breakouts along a cut edge or hole exit, caused by the brittle nature of the cured resin. Controlling chipping is essential where clean hole exits or tight edge tolerances are required.
Fatigue Resistance: The ability of a material to withstand repeated loading and unloading cycles without significant degradation in performance. High fatigue resistance ensures long-term durability under cyclic stresses.
Feed Rate: The speed at which the cutting tool advances through the workpiece, usually expressed in inches per minute or per tooth. The correct feed rate balances productivity against heat buildup and defects such as delamination and fiber pull-out.
Fiber Pull-Out (Fraying): A surface defect in which reinforcing fibers are torn or dragged out of the resin matrix rather than cleanly sheared, leaving rough, fuzzy edges. Minimizing pull-out with sharp tooling and correct parameters is a key indicator of machined edge quality.
Fiber Volume Fraction: The ratio of the volume of reinforcing fibers to the total volume of the composite material. Affects the overall mechanical properties of the composite, such as strength and stiffness.
Fixturing (Workholding): The clamps, jigs, or vacuum systems that hold a workpiece securely and accurately during machining. Proper fixturing prevents the movement and vibration that would otherwise cause chatter, out-of-tolerance dimensions, or delamination.
Flexural Strength: The ability of a material to resist deformation under load in bending. A measure of the tensile and compressive strength of a material when subjected to a bending force.
Fracture Toughness: The ability of a material containing a crack to resist fracture. Critical for thermoset composites in applications where structural integrity and damage tolerance are important.
Glass Transition Temperature (Tg): The temperature at which a thermoset polymer transitions from a hard, glassy state to a soft, rubbery state. The glass transition temperature, or Tg, marks the upper limit of the material’s operational temperature range.
Hardness: The measure of a material’s resistance to deformation, indentation, or scratching. Indicative of the surface durability and wear resistance of the material.
Hygrothermal Aging: The degradation of material properties due to the combined effects of moisture absorption and temperature variations. Essential for predicting the long-term performance of thermoset composites in humid and variable-temperature environments.
Impact Strength: The ability of a material to absorb energy and resist impact without fracturing. High impact strength is essential for applications where the material may be subjected to sudden forces or shocks.
Interlaminar Shear Strength (ILSS): The measure of the shear strength between the layers (laminae) of a composite material. High ILSS is important for the structural integrity of laminated thermoset composites.
Isothermal Curing: The process of curing a thermoset resin at a constant temperature. Helps in controlling the curing kinetics and achieving uniform properties throughout the composite part.
Laminate & Ply (Lamina): A laminate is a material built up from multiple stacked layers, each individual layer being a ply or lamina, bonded together by the cured resin. The number, material, and orientation of the plies determine the strength, stiffness, and thickness of the finished composite.
Matrix (Resin Matrix): The continuous resin phase that binds the reinforcing fibers together, transfers load between them, and protects them from the environment. In thermoset composites the matrix is a cross-linked resin such as epoxy, phenolic, or silicone.
Microcracking: The formation of tiny cracks within the resin matrix of a composite material. Can occur due to thermal cycling, mechanical loading, or environmental exposure, affecting the composite’s mechanical properties and durability.
NEMA Grade: A standardized classification (defined under NEMA LI-1) that specifies the reinforcement and resin system of an industrial laminate along with its properties — for example G10 and FR-4 (glass-epoxy), G11 (high-temperature glass-epoxy), G7 (glass-silicone), and G3 (glass-phenolic). NEMA grades give engineers a common reference for selecting a laminate to match electrical, thermal, and mechanical requirements.
Operating Temperature: The maximum ambient service temperature that a material can endure without experiencing significant degradation or loss of mechanical properties. Read more about selecting the right material for operating temperature.
PCD (Polycrystalline Diamond) Tooling: Cutting tools tipped or coated with synthetic diamond, used because the hard, abrasive fibers in composites rapidly dull conventional tooling. PCD holds a sharp edge far longer – often five to eight times the life of carbide when cutting glass-epoxy – producing cleaner cuts and reducing delamination.
Post-Curing: The process of heating a thermoset composite part after the initial curing to further enhance its properties. Improves the material’s mechanical strength, thermal stability, and chemical resistance.
Prepreg: Fibers (such as carbon, glass, or aramid) that are pre-impregnated with a thermoset resin matrix in a partially cured state. Prepreg is used in composite manufacturing to ensure consistent resin content and distribution.
Reinforcement: The fiber phase – such as glass, carbon, or aramid, in woven, chopped, or continuous form – that carries the majority of the load and gives a composite its strength and stiffness. The type, form, and orientation of the reinforcement largely determine the composite’s mechanical performance.
Residual Stress: Stresses that remain in a thermoset composite material after the curing process due to differential shrinkage, thermal expansion, or chemical reactions. Can impact the mechanical performance and dimensional stability of the composite.
Resin Transfer Molding (RTM): A manufacturing process for fabricating thermoset composites where resin is injected into a mold containing dry fiber preforms. Allows for high-quality, complex-shaped composite parts.
Spindle Speed / Surface Feet per Minute (SFM): The rotational speed of the tool or workpiece (RPM) and the resulting speed of the cutting edge relative to the material (SFM). Optimizing speed for composites limits frictional heat, which can soften the resin matrix and degrade the cut. Learn more about feeds and speeds.
Surface Finish (Ra): A measure of machined-surface roughness, commonly reported as the arithmetic average roughness (Ra) in microinches or micrometers. Achieving a fine finish on composites depends on sharp tooling, correct speeds and feeds, and controlled fiber shearing.
Swiss-Type Machining (Swiss Turning): A lathe process in which the workpiece is guided through a bushing and cut close to its point of support, enabling very slender, high-precision parts. Swiss machining or swiss turning is well suited to small, tight-tolerance composite components.
Tensile Strength: The maximum stress that a material can withstand while being stretched or pulled before breaking. Tensile strength typically indicates the material’s ability to bear loads and resist tension.
Thermal Conductivity: The property of a material that indicates its ability to conduct heat. Lower thermal conductivity is often desirable for applications requiring thermal insulation. Search by Mil-Spec to find the thermal conductivity of composite materials or view the material comparison chart.
Thermo-oxidative Stability: The resistance of a thermoset composite to degradation at elevated temperatures in the presence of oxygen. Crucial for applications requiring long-term thermal exposure and oxidative environments.
Thermoset (vs. Thermoplastic): A polymer that undergoes an irreversible chemical cure to form a rigid, cross-linked structure that cannot be re-melted, in contrast to a thermoplastic, which softens and re-solidifies with heat. This permanence gives thermoset composites their superior heat resistance, chemical resistance, and structural stability.
Tolerance: The allowable deviation from a specified dimension on a finished part. Thermoset composites can be held to tight tolerances, though their anisotropy and thermal behavior must be accounted for to do so consistently.
Toughening Agents: Additives used in thermoset resins to improve their toughness and resistance to crack propagation. Common toughening agents include rubber particles, thermoplastic phases, and core-shell particles.
Viscoelasticity: The property of materials that exhibit both viscous and elastic characteristics when undergoing deformation. Important in understanding the response to stress, particularly under dynamic loading conditions.
Viscosity: The measure of a fluid’s resistance to flow. In the context of thermoset resins, viscosity is crucial during the processing and molding stages, affecting the ease of impregnation of fibers and the quality of the final composite part.
Wet-out: The process of fully impregnating reinforcing fibers with a resin to eliminate voids and achieve good fiber-matrix adhesion. Proper wet-out is essential for maximizing the mechanical properties of the composite.
Young’s Modulus: Also known as the modulus of elasticity, it is a measure of the stiffness of a material. Defined as the ratio of stress (force per unit area) to strain (proportional deformation) in a material in the linear elasticity regime of the stress-strain curve.
