ESE GS&EA key terms

The greatest stress a material can carry and still return to its original shape when unloaded. Beyond it, permanent deformation begins, so design loads are normally kept well below this point.

The stress at which a material shows a small specified permanent strain, typically 0.2 percent offset. It marks the practical boundary between elastic behaviour and general plastic flow in ductile metals.

The ratio of tensile stress to tensile strain while a material stays elastic. It measures stiffness: a high value means little stretch under load, independent of the material's strength.

The negative ratio of lateral strain to axial strain under uniaxial loading. Typical metals sit near 0.3; the value approaches 0.5 for rubber-like materials that conserve volume when stretched.

The ratio of applied hydrostatic pressure to the resulting fractional volume decrease. It expresses resistance to uniform compression and is used when analysing fluids, pressure vessels and deeply buried structures.

The ratio of shear stress to shear strain in the elastic range. It describes how a material resists shape change at constant volume and governs torsional stiffness of shafts.

A geometric property of a cross-section, equal to the sum of area elements multiplied by the square of their distance from an axis. It quantifies resistance to bending about that axis.

The moment of inertia of a cross-section divided by the distance from the neutral axis to the extreme fibre. Bending stress equals bending moment divided by this quantity.

The line within a bent beam cross-section where longitudinal stress is zero and length does not change. It passes through the centroid for elastic bending of a homogeneous section.

Sudden lateral failure of a slender member under compressive load, occurring below the material's crushing strength. It depends on length, end conditions and flexural stiffness rather than strength alone.

The ratio of a structure's or material's failure-level capacity to the load or stress actually expected in service. It covers uncertainty in loads, material properties, workmanship and analysis.

Progressive damage and eventual fracture caused by repeated or fluctuating stress well below the static strength. Cracks usually start at stress concentrations and grow with each load cycle.

Localised increase in stress near holes, notches, sharp corners or sudden section changes. The peak stress can greatly exceed the nominal stress and often initiates fatigue or brittle fracture.

Slow, time-dependent permanent deformation of a material held under sustained stress, significant at elevated temperatures. It limits the service life of turbine blades, boilers and other hot components.

The rate at which heat passes through a unit thickness of material for a unit temperature difference across a unit area. Metals are high; insulation and still air are low.

The heat energy required to raise the temperature of one unit mass of a substance by one degree. It determines how much a component warms for a given heat input.

The heat absorbed or released when a substance changes phase at constant temperature, such as melting or boiling. It produces no temperature change during the transition.

A measure of the disorder or energy dispersal in a system. In any real spontaneous process the total entropy of system plus surroundings increases, setting the direction of change.

The maximum possible efficiency of a heat engine operating between a hot reservoir and a cold reservoir, equal to one minus the ratio of their absolute temperatures. No real cycle can exceed it.

For a refrigerator or heat pump, the useful heat moved divided by the work input. It can exceed one because it counts transferred heat rather than energy generated.

For steady, incompressible, frictionless flow along a streamline, the sum of pressure energy, kinetic energy and potential energy per unit volume stays constant. It links higher speed to lower pressure.

A dimensionless ratio of inertial forces to viscous forces in a flow. Low values indicate laminar flow, high values turbulent flow, with transition depending on geometry.

A fluid's internal resistance to flowing or being sheared. Dynamic viscosity relates shear stress to velocity gradient; kinematic viscosity is dynamic viscosity divided by density.

The reduction in fluid pressure that occurs where a pipe narrows and the flow speeds up, as predicted by energy conservation along a streamline. It underlies carburettors and flow meters.

Pressure applied to an enclosed fluid is transmitted undiminished to every part of the fluid and to the container walls. It explains hydraulic presses and braking systems.

A changing magnetic flux through a circuit induces an electromotive force proportional to the rate of change of that flux. It is the basis of generators and transformers.

An induced current flows in a direction whose magnetic effect opposes the change that produced it. The minus sign in Faraday's law follows from this conservation-of-energy requirement.

A device with two coupled windings on a common magnetic core that changes alternating voltage in proportion to the turns ratio, ideally without changing power.

The ratio of real power to apparent power in an alternating-current circuit, equal to the cosine of the phase angle between voltage and current. Low values mean inefficient use of current capacity.

A logarithmic scale of hydrogen-ion activity in a solution, defined as the negative base-ten logarithm of that activity. Values below 7 are acidic, above 7 basic at standard conditions.

A solution that resists pH change when small amounts of acid or base are added, usually a weak acid with its conjugate base. It works by neutralising added ions.

Gradual deterioration of a metal through electrochemical reaction with its environment, such as rusting of iron in moist air. It requires an anode, cathode and conducting electrolyte.

The concentration of dissolved calcium and magnesium salts in water, expressed as an equivalent of calcium carbonate. It causes scale in boilers and reduces soap lathering.

The amount of dissolved oxygen consumed by microorganisms while decomposing organic matter in a water sample over a set period, usually five days. It indicates organic pollution load.

The oxygen equivalent of organic matter in water that can be oxidised by a strong chemical oxidant under controlled conditions. It is measured faster than BOD and includes non-biodegradable matter.

A measure of the spread of a data set around its mean, equal to the square root of the variance. About 68 percent of normal values lie within one standard deviation of the mean.

A symmetric bell-shaped probability distribution defined by its mean and standard deviation. Many measurement errors and natural variations approximate it, enabling confidence intervals and tolerance limits.

The standard deviation divided by the mean, often expressed as a percentage. It compares relative variability between data sets whose units or magnitudes differ.