Unit 1.5: Solids Under Stress Flashcards
There are 3 types of solids, name them.
Crystalline, Amorphous and Polymeric
What is a crystalline solid?
These solids have short and long range order; there is a regular pattern of atoms.; the particles are arranged in a lattice
What is an amorphous?
These solids have no long range order and have an irregular pattern of atoms
What is a Polymeric solid?
Polymers consist of very long chains of carbons. The can be both natural (protein, rubber) or synthetic (nylon, polythene)
Give an example of each solid
Crystalline: Salt
Amorphous: Glass, Brick
Polymeric: Rubber, nylon
What solids will extend under tension?
Crystalline and amorphous
State Hooke’s law
Provided, the elastic limit is not exceeded, the extension of a body is proportional to the applied load
State the equation associated with Hooke’s law
F=kx
What graph does Hooke’s law lead to?
Force-Extension graph
On a force-extension graph, what kind of gradient would a stiff material have and a less stiff material have?
If it was a stiff material, it would have a steep gradient and if it was less stiff it would have less of a gradient.
On a force-extension graph, what is the area under the graph equal to?
The Work done by the force on the object
What is Young’s modulus
E=Stress/strain E=σ/ε
What is stress equal to?
Force/area
What is strain equal to?
Extension/original lenght
On a stress and strain graph what is point ‘p’?
The limit of proportionality
What law will a material follow from 0 to point ‘p’
Extension is proportional to load so it follows Hooke’s law
What is point ‘e’ on a stress and strain graph
This is the elastic limit; up to this point, the material will return to original shape and size after force is removed.
What is point ‘Y1’ on a stress and strain graph?
Y1 is the yield point. At this point large extension occurs with little or no stress as planes of atoms start to slip past each other
What is point ‘Y2’ on a stress and strain graph?
In some materials, the material stretches so much that the stress is actually reduced for a while e.g copper.
What is the region between points ‘E’ and ‘X’ on a stress and strain graph called?
The plastic region-the material will not return to its original size once it enters this region.
What is point ‘X’ on a stress and strain graph?
This is known as the breaking point; this signifies the ultimate tensile strength for most material.
What is point ‘X2’ on a stress and strain graph?
Some very ductile materials like copper become copper become narrower and extend rapidly just before breaking
What are edge dislocations?
Edge dislocations are where there is an
extra plane in the crystals, plastic deformation occurs when the
dislocations move due to the large stress
What are ductile fractures?
Ductile fractures (necking) is where the number of edge dislocations increases and causes the elongation of the metal which increases the stress at the neck
Do brittle materials stretch very much?
No. The tend to fracture at a lower stress than expected due a process known as brittle fracture.
What is a brittle fracture?
This occurs when cracks in the surface of the
materials magnify the stress at
that point and cause the material
to break.
How can brittle fractures be avoided?
To avoid brittle fractures, materials
like concrete and pre-stressed glass are always under
compression to stop cracks opening.
What is different about about a stress and strain graph for rubber?
There are 2 curves, stretching and contracting
What is the name given to the curve on a stress and strain graph for rubber?
Elastic hysteresis
On a stress and strain graph for rubber, is there a greater work done in stretching, or contracting?
As the area under the curve = the
work done, more work is done
when stretching than when
contracting. This means the
extra energy used to stretch is
transferred to vibrational energy in
the rubber molecules.
Why will the force exerted on a crystalline material be transmitted equally to each bond?
Because of the long range order
When does plastic behavior occur?
When a solid is extended beyond its elastic limit. The atoms in one plane can slip over atoms in another plane, if the forces are great enough.
Molten metal cools down when smelted. What can happen in terms of ions and what is it called?
Mistakes happen as the metal ions join the crystal. One in every million atomic planes or so, half a plane of atoms is missed out. This is known as edge dislocation
Edge dislocations are the key to what?
Plastic deformation
The bonds around the missed plane of atoms are all under strain. These bonds are places of__?
Weakness
If a small force is put on the side of metal which has an edge dislocation, what happens?
The horizontal bonds are stretched reversibly and the material behaves elastically
If a large force is put on the side of metal which has an edge dislocation, what happens?
The already strained bonds around the edge dislocation are stretched even more. At the yield point of the metal, the bonds snap making the dislocation move.
If a large force is put on the side of a metal and the yield point has already been passed, what happens to the edge dislocation?
The edge dislocation carries on moving, through snapping and reforming of bonds, until it reaches the edge of the crystal
How can metals be strengthened to avoid plastic deformation?
- Having smaller grains-this restricts movement of dislocations
- Introducing foreign atom. This creates a point which inhibits the movement of dislocations
Glass is an example of what solid?
Amorphous
Does glass have a plastic region on a force extension graph?
No, it does not
Why are amorphous solids stiff?
They have no regularity in the way their molecules are locked together in the solid. The lack of crystalline structure makes the dislocation slip impossible for amorphous
Why are glasses brittle?
Because crack can travel through them easily.
How do glasses crack?
A relatively small stress can make a microscopic crack on the surface grow uncontrollably through the solid until it snaps in two
How are sheets of glass cut?
Firstly, a small crack is scored on the surface of the glass with a sharp instrument. Then a modest stress is applied to the glass in such a way as to force the crack open. The result is (usually) a clean fracture along the line of weakness defined by the original crack.
When cutting the glass, does the initial crack have to be deep?
No. A short but narrow crack can result
in a large local increase in stress. So the material at the crack tip will reach its ultimate tensile stress well before the rest of the material does. The crack will therefore be able to grow even though the average stress on the sample is well below the material’s tensile strength.
What is the main factor that dictates the behavior of a polymer?
Cross-links
What are cross-links?
Strong covalent bonds that form between overlapping molecules or even between different parts of the same molecule.
In a polymeric solid, if the molecules don’t have many cross-links what does that mean in terms of ‘flexibility’?
The molecules can slide over each other quite easily. This makes the polymer stretchy like rubber (latex)
How do you increase the number cross-links in a polymer?
Adding impurities such as sulfur
What happens if you increase the number of cross-links in a polymer?
The polymer becomes rigid. The latex is then given a new name-‘vulcanite’
Cross links are strong____
Covalent bonds
Cross links are scarce in natural rubber. What does this mean on a molecular level?
There aren’t enough cross-links to keep the molecules in a tangled, knotted mess.
What is the name given to the bonds formed in the tangled mess of molecules in a polymer?
van der Waals forces
Are ‘van der Walls forces’ weak bonds or strong bonds?
Weak bonds
How are van der Waals bonds formed?
Every time one part of a molecule comes very close to another molecule a van der Waals force can occur – think of it as a weak bond that acts only over very short distances. Many of these are produced between the rubber molecules, that tend
to keep the molecules stuck together.
What do van der Waal bonds explain on a force extention graph?
The initial ‘stiffness’ of the rubber material
The strength of van der Waal decrease rapidly with distance. After the limit of proportionality, what starts to happen between the molecules?
The molecules begin untangling from themselves and from each other. This is why the plastic region on a stress-strain graph has a large extension with a low stress
Once the molecules have been straightened out on a stress-strain graph for polymer, stretching the rubber any further requires a great deal of force. Why is this?
Stretching the rubber any further requires that the strong covalent bonds in between each molecule are lengthened as well as breaking the cross-links. This is much more difficult than unraveling the molecules and so the gradient becomes much steeper.
Looking at the force-extension graph for a rubber band, the extension curve has
little extension at the start. Why?
This is because all those cross-links between the molecules are making it difficult to stretch.
On a force extension graph for a polymer, once the rubber has been fully stretched the cross links have re-attached at new positions. What happens when the tension is decreased?
it is harder for the molecules to (initially) pass over each other, i.e. rubber band doesn’t contract as easily as expected.
Why is there a ‘gap’ in the middle of the hysteresis?
The net effect is that there’s work done internally, a bit like friction. This work done over one extension-contraction cycle is released as heat, and its value is equal to the area between the two curves.
