Solids Under Stress Flashcards
Young Modulus?
Stress / Strain
How strong?
Spring constant?
How stiff?
= F
——-
X
Force extension graph?
Gradient = K
Area under = work done deforming solid
Also 1/2 KX^2
Crystalline?
Regular
Repeating 3D arrangement
Salt
Metals etc
Amorphous?
No regular long range structure of atoms > no crystal planes to slip over each other > only elastic behaviour > breaking stress low if any imperfections
Glass, ceramics
Polymeric?
Large molecules in long chains
Repeating monomers
Eg rubber
Yield point?
Crystal planes slide over each other, small increase in force give big extensions
Ultimate tensile strength?
UTS- greatest force applied before breaking
Fracture?
Material bends, ductile fracture
Grains?
Cooling > crystallisation > grains forms > between are grain boundaries > contain impurity atoms forced out of the grains during crystallisation
Necking?
Narrowing of tension where it breaks
Elastic strain?
Atoms slide and move, but return to original position
Plastic strain?
Forced break bonds between atoms of different planes and planes slip over each other
Bonds reform to prevent movement back again
Dislocations in crystalline structures?
Defects in otherwise perfect arrangement
Edge dislocation - extra 1/2 plane of atoms
Point dislocation - Latrice ion missing, foreign atom present, additional ion
Movement of edge dislocations?
Move through lattice until grain boundary - irreversible - plastic deformation > after process strength increased
Meet up? > tangle up and movement limited > strength increases
Bonds break and remake
Strengthening materials ?
Quenching - smaller freaks boundaries > edge dislocations less free to move
More point dislocations - add foreign impurity atoms > edge dislocations less free to move
Imperfections?
Before > stress lines uniform
Now > stress lines slip around crack > closer together > increase stress > propagates
Support columns?
Compression - top
Tension - bottom
Any cracks propagate if heavy loads > steel fibres added
Strengthening of brittle materials
Strong in compression >
Steel stretched > concrete poured > concrete sets, metal cut, contracts > compresses concrete
Hookes Law?
The tension in a spring or wire or proportional to its extension from its natural length, provided the extension is not too great