Engineering glossary for product design, manufacture and simulation
Plain definitions of the terms that come up as a product moves from specification to production, from tolerance stack-up and draft angles to the FEA, CFD and thermal analysis behind the design.
- Additive manufacturing
- Building a part layer by layer directly from a CAD model, in polymer or metal, by processes such as fused filament fabrication, selective laser sintering, stereolithography and metal powder bed fusion. It needs no tooling, which suits prototypes, complex internal features and low volumes; it is also called 3D printing.
Prototype & Test - Anisotropic material
- A material whose stiffness, strength or thermal conductivity depends on direction. Fibre composites, glass-filled mouldings, wood and printed circuit boards are common examples, and they need direction-dependent properties in a simulation.
- Bend allowance
- In sheet metal, the length of material along the neutral axis that a bend consumes, used to work out the flat blank size. For a bend angle θ in radians, inside radius R and thickness t, it is θ(R + Kt), where the K-factor, typically about 0.3 to 0.5, locates the neutral axis as a fraction of the thickness from the inside face.
- Bill of materials (BOM)
- A structured list of every part, sub-assembly, fastener and raw material needed to build a product, with quantities, part numbers and materials. It is issued with the manufacturing drawings and drives purchasing and assembly.
Engineering Pack - Boundary condition
- A known value or constraint applied at the edge of a model, such as a fixed support, an applied pressure, an inlet velocity or a surface temperature. Boundary conditions represent how the modelled region interacts with everything outside it.
Engineering simulation - Boundary layer
- The thin region of fluid next to a surface in which the velocity changes from zero at the wall to the free-stream value. Its thickness and state, laminar or turbulent, control wall friction and heat transfer.
CFD and fluid dynamics - Buckling
- A sudden sideways deflection of a slender member or thin wall under compression or shear, which can occur at a stress well below the material’s strength. Linear buckling analysis estimates the critical load of an idealised structure; non-linear analysis follows the behaviour as buckling develops.
Non-linear analysis - Cavitation
- The formation of vapour bubbles in a liquid where the local pressure falls below its vapour pressure, followed by their collapse when the pressure recovers. It causes noise, loss of performance and erosion in pumps, propellers and valves.
Pump impeller simulation - CNC machining
- Cutting a part from solid stock on computer numerically controlled mills, lathes or multi-axis machines, programmed from the CAD model. It suits metals and engineering plastics from one-off prototypes to production runs, and routinely holds tolerances of a few hundredths of a millimetre.
Production - Coefficient of thermal expansion
- The strain a material undergoes per degree of temperature change, written α and often given in units of 10⁻⁶/K. Joined materials with different coefficients are a common source of thermal stress and distortion.
Thermal stress explained - Computational fluid dynamics (CFD)
- The numerical solution of the equations that govern fluid flow and heat transfer, giving velocity, pressure and temperature throughout a fluid region. It is used to predict pressure drop, flow distribution, forces and cooling before hardware exists.
What is CFD? - Conduction
- Heat transfer through a solid, or through a fluid at rest, driven by a temperature gradient. It follows Fourier’s law, q = −k∇T, where k is the thermal conductivity.
Thermal simulation - Conjugate heat transfer
- Simulation of conduction in solids and convection in the surrounding fluid as one coupled problem, with temperature and heat flux matched at their shared surfaces. It predicts component temperatures without assuming a heat transfer coefficient.
Conjugate heat transfer explained - Contact
- The interaction between separate parts that can touch, slide or separate under load. Apart from bonded contact, it makes an analysis non-linear, because the load path changes as surfaces open, close or slip.
Non-linear analysis - Convection
- Heat transfer between a surface and a moving fluid, combining conduction at the wall with transport by the flow. It is forced when a fan, pump or motion drives the flow, and natural when buoyancy does.
Thermal simulation - Core loss
- Power dissipated as heat in the magnetic core of a transformer, motor or inductor, through hysteresis and eddy currents. It rises with frequency and with flux density.
Transformer simulation - Creep
- Slow, time-dependent deformation under sustained stress. It is significant in metals at high temperature, commonly above about 0.4 of their absolute melting temperature, and in many plastics at room temperature.
- Damping
- The dissipation of vibrational energy through the material, friction in joints or added treatments. It limits the amplitude at resonance and is often expressed as a damping ratio, the fraction of critical damping.
Vibration and modal analysis - Datum
- A theoretically exact point, axis or plane on an engineering drawing from which features are located and toleranced. Datums are usually tied to the real surfaces that locate the part in its assembly or fixture, so the tolerances reflect how the part is used.
Manufacturing drawings and tolerances - Defeaturing
- Removing geometric detail that does not affect the result, such as logos, cosmetic threads and small holes away from the load path, before simulation. Features in the path of load, heat or flow are kept.
Preparing CAD for simulation - Degrees of freedom
- The independent quantities solved for at each node of a model, such as three displacements at a solid element node, or displacements and rotations at a shell or beam node. Model size is commonly measured by the total number of degrees of freedom.
What is FEA? - Design for manufacture (DFM)
- Designing parts so they can be made reliably and economically by the intended process, matching geometry, tolerances and materials to what that process can achieve.
Design for manufacture - Draft angle
- A small taper on faces that run parallel to the direction in which a part leaves its mould or die, so that it releases cleanly. Moulded plastics commonly use around 1° to 2°, and more on textured faces.
Design for manufacture - Eddy currents
- Loops of current induced in a conductor by a changing magnetic field. They cause heating and losses, which laminated cores reduce, and they are used deliberately in induction heating and eddy current braking.
Electromagnetic simulation - Elastic modulus
- The ratio of stress to strain in the linear elastic range, also called Young’s modulus and written E. It is around 200 GPa for steels and 70 GPa for aluminium alloys.
Stress analysis - Element
- One of the small, simple shapes, such as tetrahedra, hexahedra, shells or beams, into which a model is divided for finite element analysis. Elements connect at nodes, and the solution within each one is interpolated from the values at its nodes.
What is FEA? - Emissivity
- The ratio of thermal radiation emitted by a surface to that emitted by a perfect black body at the same temperature, between 0 and 1. Polished aluminium is often below 0.1, while black anodised and painted surfaces are commonly 0.8 or higher.
Thermal simulation - Endurance limit
- The stress amplitude below which a material can, in principle, withstand an unlimited number of load cycles without fatigue failure. Many steels show one; aluminium alloys do not, so a fatigue strength at a stated number of cycles is used instead.
Fatigue analysis explained - Explicit dynamics
- A transient solution method that advances in very small time steps without solving a global system of equations at each step. It is widely used for short, violent events with rapidly changing contact, such as crash and high-speed impact.
- Factor of safety
- The ratio of a part’s capacity, such as its yield strength or failure load, to the demand placed on it in service. A factor of 2 against yield means the working stress is half the yield strength.
Factor of safety in design - Fan curve
- The relationship between the volume flow a fan delivers and the static pressure it develops. The operating point is where the fan curve crosses the pressure-drop curve of the system it serves.
Electronics thermal management - Fatigue
- Progressive cracking under repeated or fluctuating load, at stresses that can be well below the yield strength. Cracks usually start at stress concentrations, surface defects or welds.
Fatigue analysis - Fatigue notch factor (Kf)
- The ratio of the fatigue strength of a smooth specimen to that of a notched one. It is usually lower than the stress concentration factor Kt, because materials are not fully sensitive to notches.
Stress concentration factors - Finite element analysis (FEA)
- A numerical method that divides a structure into small elements and solves for displacement, stress, temperature or other quantities across it. It predicts how a design responds to loads before it is built.
What is FEA? - Flow separation
- Detachment of the boundary layer from a surface when the flow meets a rising pressure, forming a recirculating wake. It increases drag and pressure loss, and on wings and blades it reduces lift.
CFD and fluid dynamics - Fluid-structure interaction (FSI)
- Coupled simulation in which fluid pressure deforms a structure and the deformation changes the flow in return. Examples include flexible valve components, seals and slender structures in wind or water.
- Forced convection
- Convection driven by a fan, pump, blower or the motion of a vehicle. Heat transfer coefficients are typically several times to an order of magnitude higher than for natural convection in the same fluid.
Electronics thermal management - General arrangement (GA) drawing
- A drawing of a complete assembly or installation that shows its main components, their relative positions, overall dimensions and interfaces with surrounding equipment. Detail drawings of the individual parts sit beneath it.
Engineering Pack - Geometric dimensioning and tolerancing (GD&T)
- A symbolic language on engineering drawings that defines the permitted variation in form, orientation, location and runout relative to datums. It states functional requirements more precisely than plus-and-minus tolerances alone.
Manufacturing drawings and tolerances - Harmonic response
- The steady-state response of a structure to loads that vary sinusoidally, calculated across a range of frequencies. It shows the amplitudes and stresses at and near resonance.
Vibration and modal analysis - Heat sink
- A component, usually finned aluminium or copper, that spreads heat from a device and passes it to the surrounding air or coolant. Its performance depends on fin geometry, airflow and the thermal interface beneath it.
Heat sink simulation - Heat transfer coefficient
- The coefficient h in q = h(Ts − T∞), relating the heat flux at a surface to the difference between surface and fluid temperatures, in W/(m² K). It depends strongly on the flow, which is why conjugate heat transfer calculates it rather than assuming it.
Conjugate heat transfer explained - Hyperelastic material
- A material model for rubbers and elastomers, which stretch elastically to large strains with a non-linear stress-strain curve. Its parameters are fitted to test data, commonly from tension, compression or shear tests.
Non-linear analysis - Injection moulding
- A process in which molten polymer is forced under pressure into a closed tool, cooled until solid and ejected. Tooling is a large up-front cost, so it suits higher volumes, and parts are designed with even walls, draft angles and as few undercuts as possible.
Production - Joule heating
- Heat generated when electric current flows through a conductor, equal to I²R. Coupled electromagnetic and thermal simulation uses it to predict the temperatures of conductors, coils and busbars.
Multiphysics simulation - Laminar flow
- Smooth flow in ordered layers, where viscous forces dominate and mixing across the flow happens only by diffusion. In circular pipes it is typically found below a Reynolds number of about 2,300.
CFD and fluid dynamics - Linear static analysis
- An analysis that assumes small deformations, linear elastic materials and unchanging contact, under loads that do not vary with time. Its results scale in direct proportion to the load.
Linear vs non-linear FEA - Load case
- One defined combination of loads and boundary conditions representing a particular operating, handling or fault condition. A study usually includes several load cases to find the one that governs.
Stress analysis - Lorentz force
- The force on a current-carrying conductor in a magnetic field, F = I L × B for a straight conductor. It produces the torque in electric motors and the force in voice-coil actuators.
Electric motor simulation - Magnetic flux density
- The strength of the magnetic field within a material, written B and measured in teslas. Its distribution shows where a core approaches saturation and where force and losses are concentrated.
Electromagnetic simulation - Magnetic permeability
- The ratio of magnetic flux density to magnetic field strength in a material, μ = B ÷ H. Ferromagnetic materials have a high, non-linear permeability that falls as they saturate.
Electromagnetic simulation - Magnetic saturation
- The condition in which further increases in magnetising current produce little extra flux density. Electrical steels commonly begin to saturate at around 1.5 to 2 T, which limits the force or torque a design can deliver.
Solenoid simulation - Margin of safety
- The factor of safety minus one, often calculated with a required design factor included. A positive margin means the requirement is met with capacity to spare.
Factor of safety in design - Mesh
- The network of elements and nodes that divides geometry into pieces small enough to solve. Its density and element type determine how finely stresses, temperatures and flow features are resolved.
What is FEA? - Modal analysis
- An analysis that calculates a structure’s natural frequencies and mode shapes. It shows whether any natural frequency lies close to a frequency of excitation in service.
Modal analysis explained - Mode shape
- The pattern of deformation in which a structure vibrates at one of its natural frequencies. Mode shapes describe relative motion, so their amplitude has no absolute meaning on its own.
Vibration and modal analysis - Multiphase flow
- Flow containing more than one phase or immiscible fluid, such as gas bubbles in a liquid, solid particles in air, or oil and water. Each phase can have its own velocity and properties.
Multiphase flow simulation - Multiphysics simulation
- Simulation of interacting physical effects, solved together or in sequence, such as current flow that heats a conductor which then expands and is stressed.
Multiphysics simulation - Natural convection
- Fluid motion and heat transfer driven by buoyancy, as warmed fluid becomes less dense and rises. Orientation and the free space around hot surfaces have a strong effect on it.
Heat sink simulation - Natural frequency
- A frequency at which a structure vibrates freely once disturbed, set by its stiffness and mass distribution. For a single mass on a spring, ωn = √(k ÷ m) in radians per second.
Modal analysis explained - No-slip condition
- The assumption that fluid in contact with a solid wall moves with the wall, so its velocity relative to the wall is zero. It is the origin of the boundary layer.
CFD and fluid dynamics - Non-linear analysis
- An analysis in which stiffness changes during loading because of large deformation, plastic or hyperelastic material behaviour, or changing contact. Results no longer scale in proportion to load, so the load is applied in increments.
Non-linear analysis - Notch sensitivity
- A measure, q = (Kf − 1) ÷ (Kt − 1), of how fully a material responds to a notch in fatigue, from 0 for no effect to 1 for the full theoretical effect. Higher-strength steels tend to be more notch-sensitive.
Stress concentration factors - Pilot build
- A small batch of a product made with production-intent parts, materials and processes before full production. It shows whether the design can be assembled and made consistently by the production route, and provides units for testing against the specification.
Prototype & Test - Plasticity
- Permanent deformation that remains after the load is removed, once stress has exceeded the yield strength. Plasticity models describe yielding and the strain hardening that follows it.
Non-linear analysis - Poisson’s ratio
- The ratio of lateral contraction to axial extension in a material under uniaxial load, written ν. It is about 0.3 for most metals and approaches 0.5 for rubber.
- Preload
- The tension induced in a bolt when it is tightened, clamping the joint together. Adequate preload keeps a joint from separating or slipping and reduces the fluctuating load the bolt carries.
Stress analysis - Pressure drop
- The loss of pressure as fluid flows through a pipe, duct, valve, filter or heat exchanger, caused by wall friction and by changes in flow area or direction. It sets the pump or fan duty needed to deliver the flow.
CFD and fluid dynamics - Principal stresses
- The normal stresses on the three mutually perpendicular planes at a point on which there is no shear stress. The maximum principal stress is often used to assess brittle materials.
Stress analysis - Rayleigh number
- A dimensionless number for natural convection, Ra = gβΔTL³ ÷ (να), combining buoyancy, viscosity and thermal diffusivity. It indicates whether buoyant flow is laminar or turbulent.
Thermal simulation - Residual stress
- Stress that remains in a part with no external load applied, left by welding, machining, heat treatment, forming or moulding. It adds to service stresses and can cause distortion when material is removed.
- Resonance
- The large response of a structure excited at or near one of its natural frequencies. Its amplitude is limited mainly by damping.
Vibration and modal analysis - Reverse engineering
- Recreating the CAD model and drawings of an existing part from the part itself, using measurement and often 3D scanning, where the original design data is missing, incomplete or out of date. It is common for legacy equipment, spare parts and products being redesigned.
Offshore wellhead decommissioning - Reynolds number
- A dimensionless ratio of inertial to viscous forces in a flow, Re = ρvL ÷ μ. It indicates whether flow is laminar or turbulent, and allows flows at different scales to be compared.
What is CFD? - S-N curve
- A plot of stress amplitude or range against the number of cycles to failure, usually on logarithmic axes. It is the basis of stress-life fatigue assessment.
Fatigue analysis explained - Shell element
- An element that represents a thin structure by its midsurface and a thickness, carrying both membrane and bending loads. It is efficient for sheet metal, thin-walled mouldings and panels.
Preparing CAD for simulation - Steady-state analysis
- An analysis of conditions that do not change with time, such as a constant load or the temperatures reached after warm-up. It is used when the settled condition matters more than how the system reaches it.
Engineering simulation - Stiffness
- The force needed to produce a unit displacement, k = F ÷ δ. It depends on both the material’s modulus and the geometry, and it controls deflection and natural frequencies.
Stress analysis - Strain
- Deformation expressed as change in length divided by original length, ε = ΔL ÷ L. It is dimensionless and often quoted as a percentage or in microstrain.
Stress analysis - Strain rate
- The rate at which strain changes with time, in s⁻¹. Many materials become stronger, and some less ductile, at the high strain rates found in drops and impacts.
Drop test simulation: a practical guide - Stress
- Internal force per unit area, σ = F ÷ A, in pascals and usually quoted in MPa for engineering parts. Normal stress acts perpendicular to a plane and shear stress acts along it.
Stress analysis - Stress concentration
- A local rise in stress at a geometric discontinuity such as a hole, fillet, notch or keyway. Its severity is given by the stress concentration factor, Kt = σmax ÷ σnom.
Stress concentration factors - Thermal conductivity
- A material’s ability to conduct heat, written k and measured in W/(m K). Typical values are around 390 for copper, roughly 100 to 220 for aluminium alloys, around 50 for carbon steel, about 15 for stainless steel and 0.1 to 0.5 for unfilled plastics.
Thermal simulation - Thermal contact resistance
- Resistance to heat flow across the interface between two solids, caused by microscopic gaps where the surfaces do not fully touch. Thermal interface materials fill those gaps to reduce it.
Electronics thermal management - Thermal radiation
- Heat transfer by electromagnetic waves between surfaces, needing no medium and rising with the fourth power of absolute temperature. It matters most in sealed or passively cooled products and at high temperatures.
Thermal simulation - Thermal stress
- Stress caused when thermal expansion or contraction is restrained, by temperature gradients or by joined materials that expand at different rates. For a fully restrained bar, σ = EαΔT.
Thermal stress explained - Tolerance
- The permitted variation in a dimension or geometric property. Tighter tolerances raise manufacturing cost, so they are best reserved for features whose function depends on them.
Manufacturing drawings and tolerances - Tolerance stack-up
- The build-up of individual part tolerances along a chain of dimensions in an assembly, which sets the range a gap, fit or alignment can take. A worst-case stack adds the tolerances directly; a statistical stack, such as root sum square, assumes the parts rarely all sit at their limits at once.
Manufacturing drawings and tolerances - Transient analysis
- An analysis that follows how a response changes over time, such as warm-up, a pressure surge or an impact. It is needed when inertia, thermal mass or time-varying loads affect the answer.
Engineering simulation - Turbulence
- Irregular, chaotic fluid motion with eddies across a wide range of sizes. It greatly increases mixing, wall friction and heat transfer compared with laminar flow.
CFD and fluid dynamics - Turbulence model
- A set of equations that represents the average effect of turbulent eddies on the mean flow, instead of resolving every eddy. Common families include the k-ε and k-ω SST models, and the choice affects predicted separation and heat transfer.
CFD and fluid dynamics - Ultimate tensile strength
- The maximum engineering stress a material reaches in a tensile test, which in ductile metals coincides with the onset of necking. It is the usual reference strength for brittle materials.
Stress analysis - Undercut
- A feature on a moulded or cast part that prevents it from being withdrawn straight out of the tool along the opening direction. Undercuts need side actions, collapsible cores or a redesign, each of which adds tooling cost.
Design for manufacture - Von Mises stress
- An equivalent stress that combines the stress components at a point into one value that can be compared with the uniaxial yield strength of a ductile material. Yielding is predicted when it reaches the yield strength.
Von Mises stress explained - Wall function
- In CFD, an approximation that bridges the region between a wall and the first cell using known boundary layer profiles, so that the thin viscous layer next to the wall does not have to be resolved. Its suitability depends on y+.
CFD and fluid dynamics - y+
- A dimensionless distance from a wall to the first cell centre, scaled by the friction velocity and the fluid’s kinematic viscosity. Values around 1 place the first cell in the viscous sublayer, while wall functions are commonly used with values of roughly 30 to 300.
CFD and fluid dynamics - Yield strength
- The stress at which a material begins to deform plastically. For metals without a distinct yield point, it is taken as the stress that leaves 0.2 % permanent strain, known as the 0.2 % proof strength.
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