Solids Under Stress (Unit 1)

Question 1

Hooke’s Law?

Answer 1

The tension in a spring or wire is proportional to its extension from its natural length, provided the extension is not too great

Question 2

What is spring constant, how to work out?

Answer 2

• Force per unit extension
  -gradient of force / extension graph

Question 3

What is elastic limit?

Answer 3

point beyond which the wire will not return to its
original length

Question 4

How to work out , work done in deforming a solid (elastic potential energy) in a string?

Answer 4

E= 1/2 fx
E= 1/2 kx^2

Question 5

Young’s Modulus (E) units?

Answer 5

Nm^-2 or Pa

Question 6

What is a crystal?

Answer 6

Solid in which atoms are arranged in a regular array. There is a long range order within crystal structures

Question 7

Crystalline solid?

Answer 7

-long range order, lattice like arrangement

Question 8

Amorphous solid?

Answer 8

atoms arranged quite randomly. There is no long range order in the way atoms are arranged.

Question 9

Polymeric solid?

Answer 9

A solid which is made up of chain-like molecules.

Question 10

Example of polymeric?

Answer 10

rubber, wool, synthetic polymers

Question 11

Stress Strain graph? (METAL)

Answer 11

-Before it bends where proportional (limit of proportionality)
-When it bends (elastic limit) corresponds to the maximum stress that can be applied where the material will return to its original length. Up to this point the material experiences elastic strain.
- Any extension above this point will cause plastic strain
-Y is the yield point – a small increase in the load on the material will cause a large extension. This means that the material is ductile
-At the end of the line the material will break. The corresponding stress is known as the breaking stress.

Question 12

What is plastic strain?

Answer 12

the atoms in the material will begin to move past each other and the material will be permanently deformed

Question 13

How to strengthen metals to avoid dislocations?

Answer 13

By introducing barriers to dislocation movement, such as foreign atoms, other
dislocations, and more grain boundaries

Question 14

What do dislocations do to a metal? (PLASTIC STRAIN)

Answer 14

If the force applied is large enough, the bonds around D are stretched further and will snap, causing the dislocation to move, as long as the force is applied, this process will repeat until the dislocation moves to the edge of the crystal or grain. This will make the crystal longer . (NECKING)

Question 15

What does yield point show on graph and what does it mean?

Answer 15

As metals contain many edge dislocations,this will result in a large increase in strain with only a little stress. This is shown above the yield point on the previous stress-strain graph.

Question 16

Features for a stress strain graph for a brittle material, such as glass?

Answer 16

-straight line with breaking point
- elastic strain , obeying Hooke’s law up to fracture

Question 17

Why is the breaking stress of a brittle material low?

Answer 17

-due to microscopic cracks in material
-tension is applied to a brittle material, the stress at the tip of a crack is high.
-causes the crack to elongate and results in even higher stress at the tip.
- the crack will continue to propagate through the material, leading to failure known as brittle fracture.

Question 18

What is a polycrystalline solid? examples

Answer 18

The latter of a crystalline is strictly a polycrystalline solid. Metals are polycrystalline.

Question 19

Amorphous solid examples?

Answer 19

• glass or brick

Question 20

What is necking

Answer 20

-the fracture of a rod or wire of a ductile material is preceded by local thinning, which increases the stress

Question 21

What causes necking? (ductile fracture)

Answer 21

-increased number of edge dislocations, causing the wire to lengthen.
-the volume of the wire is constant, causing the cross-sectional area to decrease. —-
-Therefore, the stress at that point will increase until the material breaks

Question 22

How can breaking stress be increased for brittle materials?

Answer 22

-reduce the number of cracks on the surface of material
-form the material under compression

Question 23

Examples of reducing number of cracks in brittle material?

Answer 23

-thinner glass fibres have fewer cracks per unit area
-as they have cooled more quickly.
- number of cracks is lower, they will not propagate as readily

Question 24

Examples of forming material under compression?

Answer 24

prestressed concrete
-concrete is poured around steel rods which are under tension. When the tension is released, the concrete will be under compression
-cracks do not open up and propagate

Question 25

What is a brittle material?

Answer 25

Material with no region of plastic flow, which, under tension, fails by brittle fracture.

Question 26

What is elastic Hysterisis?

Answer 26

-When a material such as rubber is put under stress and the stress is then relaxed, the stress-strain graphs for increasing and decreasing stress do not coincide, but form a loop

Question 27

Features of force extension graph for elastic material like rubber?

Answer 27

-Hooke’s law only approximately obeyed
-low Young modulus and the extension due to straightening of chain molecules against thermal opposition

Question 28

Why does elastic hysteresis happen?

Answer 28

-area under the curve = the work done, more work is done when stretching than when contracting.
-The extra energy used to stretch is transferred to vibrational energy in the rubber molecules (called elastic hysterisis)

Question 29

Youngs modulus?

Answer 29

Fl / Ax

Question 30

At the steeper gradient top side of a force extension graph for rubber why is the gradient steeper?

Answer 30

molecules fully stretched