Solids under Stress Flashcards
What is tension?
The magnitude of the force that a stretched object exerts on whatever its end are attached to.
This is equal to magnitude of the force which is applied to the ends of an object to keep it stretched
What is extension?
The increase in the length of an object when put under tension
When is a material elastic?
If it returns to its original shape and size if the tension is removed
What is the elastic limit?
The point at which the deformation ceases to be elastic
What is Hooke’s law?
Provided the elastic limit is not exceeded, the extension of a spring is directly proportional to the tension
F=kx
What is the spring constant?
The force per unit extension of the spring, the stiffness of the spring
What is stress?
The tension per unit cross-sectional area
What is strain?
The tension per unit length due to the applied stress
What is the Young modulus?
E - the ratio of stress to strain for a material in the Hooke’s law region - usually in the value of around 10-200 GPa
How do you calculate the work done in stretching out a spring?
F=0.5kx^2
Area under a tension-extension graph
Define crystalline
consisting of crystals;regular array of particles (usually ions)
What is a crystalline material?
long range, regular (unit cell repeated)
Solid with a long range order
particles in a regular arrangement called a lattice
Define polycrystalline
consisting of a large number of interlocking crystals
What are polycrystalline metals?
Most frequent crystalline engineering materials
Lattice particles in metals are spherical metal ions arranged hexagonally in the lattice planes
Why are particles arranged hexagonally?
This gives maximum possible number of near neighbours so the potential energy of the hexagonal lattice is the lowest possible
This is the same as the plane below, so each ion is in contact with 12 near neighbours (maximum possible)
Why do crystal planes in a metal have random orientations?
A piece of metal is made up of lots of crystals which all started to form seperately from molten state
What is a metal?
A condensed material (solid or liquid) in which the atoms have lost one or more electrons to become positive ions, which are held together by the released ‘delocalised’ electrons
What are amorphous materials?
short range, irregular
Materials with no long range order, because when they were cooled they didn’t have time to assume a crystal arrangement before they lost the ability to move
What are ceramics?
Not usually formed from molten state
Partly crystalline with an amorphous matrix
Consist of molecules of metals and non-metals, with either covalently or ionically bonded
What is a polymer?
A material comprising large molecules which consist of many repeat units
What is a monomer?
A molecule which can combine with other molecules to form a polymer
Has one or more double bonds
A material is plastic if…
when the stress is removed the material is permanently deformed
What is a ductile material?
A material which can be drawn into a wire
What is the elastic limit?
The stress at which the deformation ceases to be elastic