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Glossary

Asymmetrical	Not symmetrical about a given axis. See symmetrical.
Angle of attack	The angle between the direction of the free stream and the chord line of the wing section (degrees of angle).
Axial Strain	The strain in the direction that the load is applied, or on the same axis as the applied load.
Block and tackle	A combination of two or more pulleys and a rope
Break Elongation	The elongation of the specimen to the break point.
Breaking Load	Load which causes fracture in a tensile, compression, flexure or torsion test. In tensile tests of textiles and yarns, breaking load also is called breaking strength
Breaking Strength	Tensile load or force required to rupture textiles (e.g., fibres, yarn) or leather. It is analogous to breaking load in a tension test. Ordinarily, breaking strength is reported as kN.
Chord	The length of the chord line (metres).
Chord line	The straight line drawn between the leading and trailing edge of a wing section.
Coefficient of Elasticity	An alternate term for modulus of elasticity.
Compression Test	Method for determining behaviour of materials under axial loading in compression. A standard sample of material is placed under load tending to squash the material. Applied force and the resulting reduction in length are recorded, and often plotted as a load - reduction graph which can be converted to a compressive stress – strain graph for analysis. Commonly used in testing cement and concrete products.
Drag	The force that reduces forward motion of an aircraft due to the resistance to air flowing across its surfaces. Measured perpendicular to the free stream in Newtons
Ductility	Extent to which a material can sustain plastic deformation without rupture. Percentage elongation and percentage reduction of area are common indices of ductility.
Efficiency	The ratio between the work input and work output, mechanical advantage divided by velocity ratio
Effort	The force used to move a load
Effort arm	The distance from the effort position to the fulcrum point in a lever system
Elastic Limit	The greatest stress that can be applied to a material without causing permanent deformation. For metals and other materials that have a significant straight-line portion in their stress/strain diagram, elastic limit is approximately equal to the proportional limit. For materials that do not exhibit a significant proportional limit, elastic limit is an arbitrary approximation (the apparent elastic limit) found from a proof test.
Elasticity	Ability of a material to return to its original shape when the load-causing deformation is removed.
Engineering Stress	Load applied to a specimen in a tension or compression test divided by the cross-sectional area of the specimen. The change in cross-sectional area that occurs with increases and decreases in applied load, is disregarded in computing engineering stress. It is also called conventional stress.
Extensometer	An instrument for measuring changes in linear dimensions. Also called a strain gauge.
Force	An entity that tries to move a body
Free stream	The direction of flow of undisturbed air in front of a wing section.
Friction	The resistance encountered when one body slides, or tends to slide, over another body
Fulcrum	A fixed point about which a lever may rotate in a lever system kinetic energy energy possessed by a body in motion
Hooke's Law	Stress is directly proportional to strain. Hooke's law assumes perfectly elastic behaviour. It does not take into account plastic or dynamic loss properties.
Lever	A rigid bar which rotates about a fulcrum to raise a load load a mass that needs to be raised
Lift	The net upward force created by an airfoil section in an airstream (Newton).
Load arm	The distance from the load to the fulcrum point in a lever system
Manometer	A "U" tube shaped device used to measure differences in pressure to atmospheric pressure.
Mechanical advantage	The ratio between the load moved and the effort applied
Modulus	Alternate term for modulus of elasticity, often used in connection with rubber.
Modulus of Elasticity	Rate of change of strain as a function of stress. The slope of the straight line portion of a stress-strain diagram. Tangent modulus of elasticity is the slope of the stress-strain diagram at any point. Secant modulus of elasticity is stress divided by strain at any given value of stress or strain. It also is called stress-strain ratio. Moduli of elasticity in tension and compression are approximately equal and are known as Young's modulus.
Modulus of Toughness	The work done on a unit volume of material as a simple tensile force is gradually increased from zero to the value causing rupture is defined as the Modulus of Toughness. This may be calculated as the entire area under the stress-strain curve from the origin to rupture. Toughness of a material is its ability to absorb energy in the plastic range of the material.
Necking	Localised reduction of cross-sectional area of a specimen under tensile load. It is disregarded in calculating engineering stress but is taken into account in determining true stress.
Offset Yield Strength	Arbitrary approximation of elastic limit. It is the stress that corresponds to the point of intersection of a stress-strain diagram and a line parallel to the straight-line portion of the diagram. Offset refers to the distance between the origin of the stress-strain diagram, and the point of intersection of the parallel line and the 0 stress axis. Offset is expressed in terms of strain (often 0.2%). Often referred to as Proof Stress
Operating Stress	Stress imposed on a part in service.
Percent Elongation	Measure of the ductility of a material determined in a tensile test. It is the increase in gauge length (measured after rupture) divided by the original gauge length represented as a percentage. Higher elongation indicates higher ductility.
Plastic Deformation	Deformation that remains after the load causing it is removed. It is the permanent part of the deformation beyond the elastic limit of a material. It also is called plastic strain and plastic flow.
Plasticity	Tendency of a material to remain deformed, after reduction of the deforming stress, to a value equal to or less than its yield strength.
Poisson's Ratio	Ratio of lateral strain to axial strain in an axial loaded specimen. It is the constant that relates modulus of rigidity to Young's modulus in the equation: where E is Young's modulus; G, modulus of rigidity; and r, Poisson's ratio. The formula is valid only within the elastic limit of a material. Important for materials that are to be pressed into complex shapes such as mudguards for cars.
Potential energy	Energy resulting from the position of a body
Power	The rate of doing work
Proof Stress	Stress that will cause a specified permanent deformation.
Proportional Limit	The highest stress at which stress is directly proportional to strain. It is the highest stress in a stress-strain diagram where the graph is a straight line. Proportional limit is equal to elastic limit for many metals.
Pulley	A wheel that is used to change the line of application of a force
Reduction of Area	A measure of the ductility of metals obtained from a tensile test. It is the difference between the original cross sectional area of a specimen and the area of its smallest cross section after testing. It is usually expressed as % decrease in original cross section. The smallest cross section can be measured at or after fracture. For metals, it is usually measured after fracture, and for plastics and elastomers, it is measured at fracture.
Rupture Strength	Nominal stress developed in a material at rupture. It is not necessarily equal to ultimate strength. And, since necking is not taken into account in determining rupture strength, it seldom indicates true stress at rupture.
Shear Test	A method for determining the behaviour of materials under shear loading. A standard sample of material is placed between a punch and a die. The load tends to shear a section out of the material. Applied load (force) and the resulting deformation are recorded, and often plotted as a load - elongation graph which can be converted to a shear stress – strain graph for analysis. Data from this test is used to determine elastic limit, ultimate shear strength, yield strength and other shear properties.
Strain	Change per unit length in a linear dimension of a part or specimen under load. It is usually expressed as a percentage. Strain, as used with most mechanical tests, is based on the original gauge length of the specimen.
Strain Energy	Measure of energy absorption characteristics of a material under load up to fracture. It is equal to the area under the stress-strain curve, and is a measure of the toughness of a material.
Stress	The internal reaction within a specimen to an externally applied load. Calculated by dividing the applied load by the area through which it acts. As used with most mechanical tests, stress is based on original cross-sectional area without taking into account changes in cross sectional area due to the applied load. This is sometimes called conventional or engineering stress. True stress is equal to the load divided by the instantaneous cross-sectional area through which it acts.
Stress-Strain Diagram	Graph of stress as a function of strain. It can be constructed from data obtained in any mechanical test where load is applied to a material, and continuous measurements of stress and strain are made simultaneously. It is constructed for compression, tension and torsion tests. The applied load is converted to applied stress, and the resulting elongation is converted to strain. The two sets of figures are plotted on a graph. More useful than a load-elongation graph as it allows the direct comparison of many different materials and specimens.
Sub-sonic	A speed lower than the speed of sound (<331.7 m/s in dry air at O°C).
Super-sonic	A speed greater than the speed of sound.
Symmetrical	The same shape on either side of a plane or line.
Tensile Impact Test	Method for determining energy required to fracture a specimen under shock tensile loading. (Also known as Tension Impact Test)
Tensile Test	A method for determining behaviour of materials under axial loading in tension. A standard sample of material is placed under load tending to stretch the material. Applied load (force) and the resulting increase in length are recorded, and often plotted as a load - elongation graph which can be converted to a tensile stress – strain graph for analysis. Commonly used in testing metal and polymer products. Data from this test is used to determine elastic limit, elongation, modulus of elasticity, proportional limit, reduction in area, ultimate tensile strength, yield point, yield strength and other tensile properties.
Thickness	The thickness of a wing section measured perpendicular to the chord line at a given point (metres).
Thickness ratio	The ratio of maximum thickness to the chord, expressed as a percentage.
Toughness	Toughness is the resistance of a material to fracture or break. It is usually measured in units of energy (Joules).
True Strain	Instantaneous % change in length of a specimen in a mechanical test. It is equal to the natural logarithm of the ratio of length at any instant to original length.
True Stress	Applied load divided by actual area of the cross section through which the load operates. It takes into account the change in cross section that occurs with changing load.
Turbulent	Disturbed airflow.
Ultimate Strength	Highest engineering stress developed in a material before rupture. Normally, changes in area due to changing load and necking are disregarded in determining ultimate strength.
Ultimate Compressive Strength	Ultimate strength of a material subjected to compressive loading. It is the maximum stress developed in a material in a compression test.
Ultimate Shear Strength	Ultimate strength of a material subjected to shear loading. It is the maximum stress developed in a material in a shear test.
Ultimate Tensile Strength	Ultimate strength of a material subjected to tensile loading. It is the maximum stress developed in a material in a tensile test.
Velocity ratio	The ratio between the distance the effort moves and the distance the load moves
Yield Point	Stress at which strain increases without accompanying increase in stress. Only a few materials (notably steel) have a yield point, and generally only under tension loading.
Yield Strength	Indication of maximum stress that can be developed in a material without causing plastic deformation. It is the stress at which a material exhibits a specified permanent deformation and is a practical approximation of elastic limit.
Young's Modulus	Alternate term for modulus of elasticity in tension or compression.
Work	Occurs when a force moves a body