L5: Tensile Testing And Material Deformation and L6: hardness, toughness and fracture

Question 1

What is tensile testing designed for?

What is the sample geometry designed for?

Answer 1

Tensile testing- test tensile strength, yield strength, Ym, ductility of materials

Sample geometry- bulletin strength of material tested without regions of high/ non uniform stress being generated

Question 2

Describe the curve generated from a tensile test

Answer 2

Force- extension curve
Elastic region- peice extends but recovers original length when force removed
Plastic region- deforms and does not recover when tension released- neck forms, then curve ends at failure (snapping)

Question 3

Why is a stress- strain curve better than a force- extension one?

Answer 3

Removes effect of test price dimensions, makes values a material property

Question 4

How is the Ym found from a stress- strain graph?
What is it also called?
How does it change with temp?

Answer 4

In the elastic region of the graph- it’s the gradient.
Aka modulus of elasticity/ stiffness
Higher temp, Ym decreases as less energy is required to stretch bonds

Question 5

What are the 2 methods of determining Ym of materials with no linear elastic region- eg polymers

Answer 5

Secant modulus

Tangent modulus

Question 6

What is poisson’s ratio?
What are the equations if the material is isotropic?
What is a rough value of the ratio for most metals?

Answer 6

The negative ratio of lateral to axial strain

v= -Ex /Ez
v=- Ey/Ez

Usually- 0.25- 0.35

Question 7

What is the transition from elastic to plastic deformation called?
How is it and tensile strength determined on a stress- strain graph?

Answer 7

Limit of proportionality/ yielding

Yielding is where the graph starts becoming a curve (stress here is yield stress)

Tensile stress is the maximum of the curve (where neck forms)

Question 8

What materials display a yield point?

Where do plastic deformation and yield strength occur?

Answer 8

Low C steels especially

Plastic deformation at the upper yield point

Yield strength is the level of the lower yield point

Question 9

When does dislocation begin?

Which dislocations are the first to slip?

Answer 9

When the metal starts to yield

Those that are aligned in the correct direction- ie with the shear stress

Question 10

When does yielding occur?

What helps slips to happen more easily?

Answer 10

When the load is large enough to cause dislocations within crystals to slip

Helped by close packing of atoms

Most easily achieved for slip planes at 45degrees to the applied load

Question 11

Describe yielding in polycrystalline materials?

What does the structure look like after deformation?

Answer 11

Some crystals slip more easily than others as they’re randomly orientated

Grains are restricted in shape by their neighbours

A grain cannot deform until it’s neighbour lay can deform so F must be big enough to cause less favoured slip systems to deform.

After- grains tend to elongate in the direction of applied stress

Question 12

Describe the properties of ceramics and the strength test which is best suited to them

Answer 12

Very brittle (little/no plastic deformation)
-dislocation slip almost impossible without fracture
- good in compression, weak in tension
- hard to grip for tensile test (must ensure test piece aligned to avoid bending)
Best to use bend test- places bottom of each end under tension- determines flexural strength

Question 13

What is the glass transition temp Of a thermoplastic polymer

Answer 13

Where it changes from brittle glossy material to a more robbery ductile material

Defamation involves molecule sliding past each other (not dislocation slip)

Question 14

What are the four main methods of hardness testing

Answer 14

Brinell
Vickers
Knoop
Rockwell

Question 15

What do you all methods of hardness testing rely on and what makes it difficult to compare them?

Answer 15

All tests rely on pushing a hard object e.g. diamond (vickers) into the material underknown load and measuring size or depth of the indentation

The type and shape of the hard object used varies with each test method

Question 16

Define hardness

Answer 16

The ability of a material to resist localised plastic deformation (surface indentation or scratching)

Question 17

Give advantages and disadvantages for hardness testing

Answer 17

It is a convenient and largely nondestructive test method to determine if heat treatment or process has been successful

Hardnessnot a material property and its values very depending on the test method used to measure at different test methods and indent materials are used for evaluating different materials

Question 18

Give some examples of hard materials

Answer 18

Many ceramics e.g. WC, diamond, Al2O3

Some alloyed and heat treated steels

Question 19

What types of metals can become brittle and what are the properties of a brittle material

Answer 19

Metals that have become strained hard and all contain impurities that restrict this location movement can become brittle

High hardness values but poor tensile strength

Question 20

Define toughness

Answer 20

The ability of a material to absorb energy during defamation and fracture an indication of how easy it is to break it

Question 21

How was toughness determined from a graph and what materials have high toughness

Answer 21

The energy required to break a material as given by the area under the true stress vs true strain graph

Metals with high strength and ductility are tough brittle materials have low toughness.
The toughness of the material depends on the way in which the load is applied- If the load is applied slowly and increases steadily material so increased toughness

Question 22

What is impact testing used for

Answer 22

To investigate the toughness of materials under sudden loading - materials show reduced toughness

Question 23

What are the two main methods of impact testing and what do they involve

Answer 23

Charpy
Izod
Both methods used are swinging pendulum to measure the amount of energy absorbed by the test piece during fracture

Question 24

What affect in general does change in temperature have on metals and polymers

Answer 24

Increasing temperature leads to reduced Young’s modulus reduced yield strength and higher ductility in many metals and polymers a transition from ductile to brittle behaviour can occur at low temperature

Question 25

What happens to Bcc metals at low temperature and FCC metals

Answer 25

BCC metals become brittle due to the lower rate of dislocation slip however FCC metals tend to remain tough

Question 26

What are the two steps of the fracture process

Answer 26

Crack initiation and propagation

Question 27

What is the difference between ductile materials fracturing and brittle materials fracturing

Answer 27

Ductile- Crack propagation proceeds with significant plastic deformation in front of the crack- Get some warning before failure

Brittle- Little deformation and the crack can spread rapidly- Very little or no warning before failure

Question 28

Why do you cracks appear to tensile strength lower than predicted by theory

Answer 28

Due to microscopic flaws or cracks present in the material as a result of
-crystal defects- Vacancies and dislocations

-Manufacturing defects- porous castings, surface scratchings

Question 29

What are stressed raisers

Answer 29

Defects and flaws

Question 30

What areas in courage cracks due to bad design

Answer 30

Areas of the material experiencing high stresses such as sharp curves or notches made in the material

Question 31

Define fracture toughness

Answer 31

A material property that measures the ability of a material containing a flaw to with stand and applied load

Question 32

How can I designer reduce the spread of a crack

Answer 32

• Small floors in the material
• Ductile materials
• thinner materials are less rigid
• Low rate of load application dislocations have time to slip
• Increasing temperature dislocations can move rapidly
• Small grain size fewer defects

Question 33

What does fracture mechanics enable designers to do

Answer 33

Allow for defects in manufactured products