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GLOSSARY

The glossary provides a list of terms used in this course with their definitions. The different terms can be opened by clicking on the term.

Anisotropic material

Material with mechanical properties depending on the direction.

Airframe

Structure that takes up all forces during operation of the vehicle.

Brittleness

Property of material that allows little bending or deformation without shattering.

Composites

Engineering materials containing two or more distinct and structurally complementary substances with different physical or chemical properties, having structural or functional properties not present in the individual substances.

Contraction/Expansion

Reaction produced in material as the result of heating or cooling.

Corrosion

In general: degradation of engineering materials due to chemical reaction with its environment.

Metal specific; electrochemical oxidation in reaction with an oxidant such as oxygen.

Damage tolerance

The ability of the structure to sustain anticipated loads in the presence of fatigue, corrosion or accidental damage until such damage is detected through inspections or malfunctions and is repaired.

Density

Weight of a unit volume of material

Ductility

Property of metal that allows it to be permanently drawn, bent, or twisted into various shapes without breaking.

Durability

The ability of the structure to sustain degradation from sources as fatigue, corrosion, accidental damage and environmental deterioration to the extent that they can be controlled by economically acceptable maintenance and inspection programs.

Elasticity

Property enables material to return to its original shape when the force which causes the change of shape is removed.

Engineering stress

Load divided by the original cross-section of the material or component.

Fail safe

Fail-safe is the attribute of the structure that permits it to retain required residual strength for a period of un-repaired use after failure or partial failure of a principal structural element.

Fatigue

Damage phenomenon induced by multiple load cycles below ultimate strength of material or structure causing permanent deterioration of material or structure resulting in a reduction in load bearing capability.

Fiber metal laminate

Composite material combining metal sheets and fiber reinforced composites.

Fracture toughness

Measure of energy needed to fail a specimen containing a crack.

Galvanic corrosion

Electrochemical process in which one metal corrodes when in electrical contact with a different type of metal or material and both metals are immersed in a substance containing electrolyte.

Geometrical tolerances

Maximum variation allowed in form or positioning.

Glass transition temperature

The temperature at which polymers exhibit a transition from a more glassy (hard/brittle) state to a rubbery (elastic/flexible) state.

Hardness

Ability to resist abrasion, penetration, cutting or permanent distortion.

Impact Toughness

Measure of energy needed to fracture a notched specimen.

Isotropic material

Material having identical mechanical properties in all directions.

Modulus of Elasticity or Young’s modulus \(\>\>E\)

Measure for material resistance against imposed Elastic deformation. The value of this Young’s modulus is a characteristic value for a material; a high value indicates a stiff material, a low value a flexible material

Normal stress

Stress that acts perpendicular to the surface of the material

\[ \sigma = \frac{F}{A}\] \[ \sigma = E \epsilon \]

Normal strain

Normalized change in axial length of an object.

\[ \epsilon = \frac{\Delta L}{L} \]

Orthotropic material

Material with properties that differ in all three directions.

Poisson’s ratio

The ratio between the transverse strain and the axial strain in a material.

Primary structural element

Critical load bearing structure of an aircraft or spacecraft that in case of severe damage will fail the entire aircraft/spacecraft.

Quasi-isotropic material

Approximation of isotropic material by placing multiple anisotropic layers in different directions.

Rule of mixtures

A simple first estimate of properties of a composite, based on the properties of its constituents: \[ \rho_{FRP}=\rho_F\frac{V_F}{V_{FRP}} + \rho_M\frac{V_M}{V_{FRP}} \]

RTM

Resin Transfer Moulding

Safe life

Safe-life of a structure is the number of flights, landings, or flight hours, during which there is a low probability that the strength will degrade below its design strength.

Saint Venant’s principle

Disturbances in the stress field remain limited to the direct neighbourhood of the location of disturbance.

Secondary structural element

Structural elements of an aircraft or spacecraft that carry only air and inertial loads generated on or in the secondary structure

Shear modulus \( \> \> G \)

Measure for material resistance against shear. It is related to the modulus of Elasticity: \[ G = \frac{E}{2(1 + \nu)} \]

Shear strain

Ratio of deformation of a material due to shear. It it the angle a square block deforms at when a parallel force is applied.

\[ \gamma = \tan{\theta} \]

Shear stress

Stress that acts parallel along the surface of a material.

\[ \tau = \frac{F}{A} \> \> \> \tau = G \gamma \]

Specific property

Material property divided by its density.

Stress concentration factor

Parameter describing the relation between the peak stress at the (blunt) notch root and the nominal (netto) stress in the cross-section.

Stress intensity factor

Parameter describing the severity (intensity) of stresses near the crack tip.

Toughness

Resistance against fracture. Represented by the area under the stres-strain curve. Expressed in J/m3

True stress

Load divided by the actual cross-section of the material.

VARTM

Vacuum Assisted Resin Transfer Moulding

VI

Vacuum Infusion