Solids under stress Flashcards
Hookes law
The tension in a spring or wire is proportional to its extension from its natural length within the limit of proportionality
Equation for hookes law
F=k x X
Energy stored in stretched springs
E=1/2 x F x X
or
E=1/2 x K x X^2
Spring constant if springs are in series
1/ktotal=1/k1+1/k2
Spring constant for parallel
extension is halfed and k is doubled
Tensile stress equation
force(n)/area(m^2) , area is pir^2
Tensile strain equation
extension/length
Stress definition
force per unit cross sectional area
Strain definition
extension per unit length
Equation for young modulus
Stress /strain
or
F x L/A x X
Elastic
Describes material that regains its shape after stresses are removed
Ductile
This sort of material can be easily stretched or drawn into wire
Malleable
Material can be hammered or pressed into shape without breaking or cracking
Brittle
Material that would snsap without any yield
Stiff
Small strains for large stresses, not stretchy or bendy
Plastic
Material that undergoes permanent deformation under large stress rather than cracking
Strong
a large stress is needed to break it
Hard
Resists indentation on impact
Crystalline solids
Crystal lattice consisting of regularly repeating unit cells
exhibit long range order and symmetry e.g Metals Diamonds Graphite and Salt
Poly crystalline
Structure is split up into many small crystallites or grains which are randomly arranged thus forming different grain boundaries.
Amorphous Solids
Solid structure in which the atoms have no long -range order or symmetry e.g Glasses,Ice, Ceramics
Polymeric Solids
Materials comprised of long molecular chains usually containing carbon, e.g Rubber, Cellulose, polyethylene
Elastic strain
When a force is applied to a material the atoms are stretched apart by a very small distance, for small strain no bonds are broken
Plastic deformation
When the force is removed and the atoms are pulled back into their original positions so that the material regains its original shape.
Plastic Strain
Imperfections within a lattice known as edge dislocations causing bonds to break.
Work hardening
When lots of dislocations meet within a material then the can impeded each other to stop dislocations propagation. This causes material to become stronger and requires larger stresses to create dislocation.
Grain boundaries
Poly crystalline materials have grain boundaries which can form barriers to edge dislocation movement.
Necking
The thinning of the cross sectional area of a material at the weakest point. This usually occurs when the material continues to be stretched after it has reached it ultimate tensile stress.
Ductile fracture
Product of large stress and necking
Why are steel rods used in bridges
Steel rods are placed under tension and stretched
The wet concrete mix is placed around the steel and allowed to set causing it to bond to the steel
the tension is removed from the steel rods allowing it to contract
the concrete is now under permanent compressive forces
Creep
Gradual deformity under a constant load
Cold working
Repeated bending, stretching of alloy, dislocations become tangled and therefore causing each other to stop moving, therefore inhibiting plastic deformation.
Hysteresis
Area under the curve = the work done , more work is done when stretching then when contracting . The extra energy used to stretch is transferred to vibrational energy in the rubber molecules . This is elastic hysteresis
