41 terms

Topic 4: Materials

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tensile force
force that puts something in tension i.e. tends to pull it apart
tensile stress
tensile force per unit area
tensile strength
the tensile stress at which a material breaks
tensile strain
extension per unit length
Young Modulus, E
a measure of the ability of a material to withstand changes in length when under lengthwise tension or compression
shear stress
The component of stress coplanar with a material cross section. Shear stress arises from the force vector component parallel to the cross section.
hysteresis
retardation of an effect when the forces acting upon a body are changed (as if from viscosity or internal friction) - how rubber is able to dissipate energy
ductility
a solid material's ability to deform under tensile stress; this is often characterized by the material's ability to be stretched into a wire.
brittle
materials which break or crack with little deformation
tough materials
able to withstand impact forces wihtout breaking and require a large force to produce a small plastic deformation
composite materials
combinations of more than one material
hard materials
resist plastic deformation - usually by denting
malleability
materials which show large plastic deformation before cracking or braeking - may be flattened into thin sheets by hammering or rolling
density
measure of the mass per unit volume of a substance

depends on the mass of the particles from which the substance is made and how closely those particles are packed
fluid
any substance that can flow (any liquid or gas and some solids)
upthrust
upwards force exerted on an object when it is submerged in a fluid
laminar flow
the fluid travels smoothly or in regular paths - generally occurs at lower speeds and for more streamlined objects
turbulent flow
the fluid undergoes irregular fluctuations, or mixing - occurs at higher speeds
streamlines
lines of laminar flow
eddies
the swirling of a fluid and the reverse current created when the fluid is in a turbulent flow regime
aerodynamics
the way air moves around things
viscous drage
the frictional force between a solid and a fluid
terminal velocity
the constant speed that a freely falling object eventually reaches when the resistance of the medium through which it is falling prevents further acceleration
Hooke's Law
The force F exerted by a spring is proportional to its extension ∆x
plastic deformation
a permanent deformation or change in shape of a solid body without fracture under the action of a sustained force
elastic deformation
A temporary shape change that is self-reversing after the force is removed, so that the object returns to its original shape
necking
a mode of tensile deformation where relatively large amounts of strain localize disproportionately in a small region of the material - as the stress increases, the sample begins to narrow at one point
compressive stress
compressive force per unit area
compressive force
the application of power, pressure, or exertion against an object that causes it to become squeezed, squashed, or compacted
stiffness
ability of a material to resist a tensile force
compressive strength
the compressive stress at which a material breaks
compressive strain
extension per unit length (original length)
duralumin alloy
a tough material which is stiff
ceramic
brittle material and may be very strong - can resist high temperatures
copper
ductile material, large plastic region, high electrical conductivity
rubber
initially reluctant to stretch, but then extends easily until it reaches a point where it is hard to stretch

highly elastic
yield point
material starts to stretch without any extra load suddenly - the stress at which a large amount of plastic deformation takes place with a constant or reduced load
limit of proportionality
material stops obeying Hooke's law, but would return to its original shape if the stress was removed
elastic limit
material starts of act plastically i.e. would not return to its original shape when the stress is removed
stiff
difficult to stretch or compress

large Young's modulus - for a given stress, a stiff material will have a lower strain (i.e. a smaller extension) than a less stiff material
strong
the stronger the material, the higher the breaking stress