Madrid PAU Tech & Engineering II key terms

Within the elastic range, the stress applied to a material is directly proportional to the strain it produces; the constant of proportionality is the modulus of elasticity, or Young's modulus.

A measure of a material's stiffness, equal to the slope of the linear part of its stress-strain curve. Higher values mean the material deforms less under the same applied stress.

Internal force per unit area that particles of a material exert on each other when loaded. Normal stress acts perpendicular to the section (tension or compression); shear stress acts parallel to it.

The relative deformation of a material, usually expressed as change in length divided by original length. It is dimensionless and, for small loads, proportional to the applied stress.

The maximum stress a material can withstand while still returning to its original shape when the load is removed. Beyond it, permanent plastic deformation begins.

The stress at which a material begins to deform plastically, producing a permanent change of shape. Designers keep working stresses below it, usually with a safety factor.

The ability of a material to undergo large plastic deformation before fracturing, allowing it to be drawn into wires or bent. Mild steel is ductile; cast iron and glass are brittle.

The ratio between a material's failure or yield strength and the maximum stress expected in service. It covers uncertainties in loads, material quality and calculation models.

A metallic material made by melting a base metal with other elements to improve properties. Steel is iron plus carbon; brass is copper plus zinc; bronze is copper plus tin.

Controlled heating and cooling of a metal to change its internal structure and therefore its properties. Tempering, quenching and annealing alter hardness, toughness and internal stresses.

A material formed by combining two or more constituents with different properties, such as fibres in a resin matrix. The components keep their identities and together give improved stiffness-to-weight ratios.

A sketch of a body isolated from its surroundings, showing all external forces, moments and support reactions acting on it. It is the starting point for applying equilibrium equations.

For a planar body at rest, the sum of horizontal forces, the sum of vertical forces and the sum of moments about any point must all be zero. These three equations allow unknown reactions to be found.

The force or moment that a support exerts on a structure to prevent movement. A pin offers two force components, a roller one, and a fixed support two forces plus a moment.

The internal moment at a section of a beam caused by transverse loads. It produces tension on one face and compression on the other, and its maximum value governs the required section modulus.

The internal force acting parallel to the cross-section of a beam at a given point, equal to the algebraic sum of transverse loads to one side of that section.

A geometric property of a cross-section, equal to its second moment of area divided by the distance to the outermost fibre. It converts bending moment into maximum bending stress.

A structure whose support reactions can be determined from the equilibrium equations alone, because the number of unknown reactions equals the number of available equations.

The ratio between the number of teeth on two meshing gears, equal to the inverse ratio of their speeds. It sets how much torque is multiplied and how much speed is reduced.

The factor by which a mechanism multiplies an applied force, equal to output force divided by input force. It is often related to the velocity ratio in an ideal machine.

The ratio of the distance moved by the effort to the distance moved by the load in a machine. It depends only on the geometry of the mechanism, not on friction.

The ratio of useful output work or power to input work or power, usually expressed as a percentage. Losses come from friction, heat and deformation in the mechanism.

The turning effect of a force about an axis, equal to force times perpendicular distance. In rotating machinery it relates to power through the angular velocity.

A mechanism of four rigid links connected by pin joints, in which one link is fixed. It converts continuous rotation into oscillating or complex motion, as in windscreen wipers.

A rotating shaped profile that pushes a follower to produce a specific reciprocating motion. The follower's displacement depends on the cam outline and the type of follower.

Around any closed loop of a circuit, the algebraic sum of voltage rises and drops is zero. It follows from conservation of energy and is used to analyse series and multi-loop circuits.

At any node of a circuit, the sum of currents entering equals the sum of currents leaving. It expresses conservation of electric charge and is applied to nodes in circuit analysis.

For a resistive element at constant temperature, the voltage across it equals the current through it multiplied by its resistance. It is used to calculate currents, voltages and power in DC circuits.

Any linear two-terminal network can be replaced by an equivalent voltage source in series with a resistance. This simplifies the calculation of current through a particular branch.

The electromotive force induced in a circuit equals the rate of change of magnetic flux through it. It explains how generators, transformers and induction motors work.

The direction of an induced current is such that it opposes the change in magnetic flux that produced it. It is a consequence of energy conservation in electromagnetic induction.

An electronic device that performs a Boolean operation on one or more binary inputs to produce a single output. Common types are AND, OR, NOT, NAND, NOR and XOR.

A table listing every possible combination of binary inputs to a logic circuit together with the required output for each combination. It defines the behaviour of combinational logic.

A graphical method for simplifying Boolean expressions by grouping adjacent cells containing ones. Each group corresponds to a product term with fewer variables.

A control system in which the output has no effect on the control action. The actuator acts on a fixed instruction, so disturbances are not corrected.

A control system that measures its output and compares it with a reference value, using the error to adjust the actuator. It can correct disturbances but may become unstable.

Returning a fraction of the output signal to the input of a system so that it can be compared with the reference. Negative feedback reduces error and stabilises the response.

A controller that combines proportional, integral and derivative action on the error signal. The three terms improve speed of response, eliminate steady-state error and damp oscillations.

A programmable logic controller is an industrial computer that reads inputs from sensors, executes a stored ladder-logic program cyclically and drives outputs to actuators in a machine or process.

A device that converts a physical quantity such as temperature, position or light into an electrical signal suitable for measurement or control. Examples include thermistors, LDRs and limit switches.

A component that converts a control signal into physical action, such as a motor, solenoid, pneumatic cylinder or relay. It is the final element of the control chain.

A method of representing a three-dimensional object by parallel projections onto two or more perpendicular planes, giving front, plan and side views without perspective distortion.

A European drawing convention in which the object is placed between the observer and the projection plane, so the view from the left appears on the right of the front view.

The set of rules for indicating sizes on a technical drawing, using dimension lines, extension lines, arrows and figures with a stated unit. It must be complete and free of duplication.

The ratio between a length on a drawing and the corresponding real length, written as 1:n for reduction or n:1 for enlargement. It is stated in the drawing title block.