Solids Under Stress Unit 1.5 Flashcards
Define Extension?
Increase in length of a object when put under tension
Define Tension?
Magnitude of a force that a stretched object exerts on whatever its attached to.
Define elastic?
object returns to original size and shape when tension is removed
Define Hooke’s Law
Provided the Elastic limit is not exceeded extension of a spring is directly proportional to the tension
Define Elastic limit?
point at which deformation ceases o be elastic
Hookes equation
Force = Spring Constant x Extention
Define Stress?
tension per unit cross sectional area
Define strain?
Extension per unit length due to applied stress
Define Young Modulus
Ratio of stress to strain for a material in hooke’s law region.
Equation for strain
strain = increase in length / original length
Equation for Stress?
Stress = Force / AreaEqua
Equation for Young Modulus
E = stress/strain (Pa)
Define Strain Energy?
energy stored in a object when its deformed elastically.
What is the Area under a Force Extension Graph?
Work done
Define Crystalline
consists of crystals; regular arrays of particles (ions)
Define Polycrystalline
consisting of a large number of interlocking crystals
Define a Metal
condensed material in which the atoms have lost more than one electrons to become positive ions which are held together by released delocalised electrons.
Properties of crystals
- Long range order
- regular arrangement (Lattice)
- arranged in a hexagonal shape so max no. of neighbours so potential energy is lowest possible
Properties of Amorphous materials
- No long range order
- if cooled rapidly molecules have no time to make crystal arrangement before they loose ability to move (glass)
Properties of Polymeric materials
- large molecules
- Large number of repeated sections
- formed from monomers; double bond opens giving rise to free bonds which link monomers together
Define Polymer
Material compromising large molecules which consists of many repeating units
Define Monomers
Molecule which can combine with other molecules to form a polymer
Define plastic
When the stress is removed the material is permanently deformed.
Define Ductile
material which can be drawn into a wire
Define elastic limit
stress at which the deformation ceases to be elastic
Define yield point
point on a stress-strain graph at which a large increase in strain occurs for a little/no increase in stress
Define yield stress
stress at the yield point
Learn the graph for stress-strain
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What happens when a increase in stress takes place?
increase in separation of lattice ions.
Define edge dislocations?
an additional part plane of ions in a crystal
How do crystals grow?
Randomly, often have dislocations
if stress exceeds yield stress what happens?
dislocation will move irreversibly to direction where force acts, (Permanent deformation)
What does movement in edge dislocations produce?
large deformations eg: cause the crystal plane to snap at a weak point eg: missing ion
Define Ductile fracture
Breaking that occurs when a ductile material is stressed to breaking point. it involves plastic deformation and necking
for a ductile fracture stress increases …
more edge dislocations generated and migrate causing elongation. area decreases increasing stress at the neck.
Define Ultimate tensile strength (UTS)
Maximum stress a material can withstand before breaking.
In brittle materials how can crack propegation be avoided?
- concrete designed so material is under compression: cracks don’t open up
- pre stressed concrete (concrete with bars)
- prestressed glass (cooling quickly hence tension)
Define Elastic Hysteresis
When a material rubber is put under stress and then relaxed the stress strain graph for increasing and decreasing stress do not join but form a loop
What is the area between the stress strain graph?
Energy dissipated in vibrational energy of rubber molecules
