Module 1. Material testing

Question 1

Name two types of tests that can be carried out on materials?

Answer 1

1. Destructive testing
2. Non Destructive

Question 2

What can destructive tests tell you?

Answer 2

they are used to determine a specific property of the material/component being tested e.g. UTS

* usually require a specifically shaped piece to be made and this is damaged/destroyed during testing.

Question 3

Name some destructive tests?

Answer 3

1. tensile tests
2. impact tests
3. fatigue tests

Question 4

Name some none destructive tests?

Answer 4

1. Hardness tests done on components
2. Dye pentrants
3. x-rays
4. ultrasound

Question 5

Describe a Tensile test?

Answer 5

A test piece is extended, usually to fracture, with an increasing tensile load, to determine the resistance of the material to load.

Question 6

Describe a Tensile test machine

Answer 6

The test piece is mounted into grips which are pulled apart hydraulically at a constant rate. The applied load is measured by a load cell and the extension by an extensionsometer.

Stress and strain are then plotted and curves can be compared of different materials.

Question 7

What are the main regions of a tensile test curve (stress/strain diagram)

Answer 7

1. elastic region- no bonds are broken if load is released it returns to original position. (strain is small)
2. elastic limit- end of linear relationship, deformation begins at this point. (strain increases).
3. Plastic region- material deforms plastically (does not return to original shape).
4. necking region- just after maximum stress, localised decrease in cross sectional area.
5. point of fracture (end of graph)

Question 8

what are the common calculations that can be taken from a stress/strain curve?

Answer 8

1. Yield stress.
2. Proof stress.
3. Ultimate Tensile Strength.

Question 9

What is yield stress?

Answer 9

Yield stress is the stress at the yield point or proportional/elastic limit. It is determined by reading the stress at the point where the gradient ceases to be linear. On this graph the yield strenght is 40 MPa

Question 10

What is proof stress?

Answer 10

Proof stress is used when the elastic limit is gradual and hard to locate. It is the stress at a defined plastic strain, often 0.2%. A line is drawn parallel to the curve, depicting the elastic region, to a point on the strain axis. The proof stress is where this line interacts the curve (here 0.2% = 48MPa)

Question 11

What is Ultimate Tensile Strength (UTS)?

Answer 11

Ulimate Tensile Strength is the maximum normal stress which the material withstands and occcurs just prior to necking. It is given by the maximum stress on the curve. On this curve, the UTS is 50MPa.

Question 12

What is Young’s Modulus (E)?

Answer 12

Young’s modulus (E) is given by the gradient of the curve in the elastic region. It is a measure of the stiffness of the material. For this curve the gradient is 40MPa/0.002=20GPa

Question 13

Stress strain diagram for a ferrous meta

Answer 13

Question 14

Stress strain diagram for non ferrous metal (eg copper)

Answer 14

Question 15

Stress strain diagram for a ceramic

Answer 15

Question 16

Stress strain for a composite material

Answer 16

Question 17

Name a type of impact test?

Answer 17

Charpy Impact test- The standard test measuring impact energy is the charpy test. An arm is swung down in a pendulum motion to impact with the test material. The energy required to fracture is recorded.

Question 18

What does a charpy test piece look like?

Answer 18

The charpy impact speciums are produced as a bar containing a notch. The height the pendulum swings to after the failure is a direct measure of the amount of energy absorbed during fracture.

Question 19

What information can a charpy test give you?

Answer 19

The impact energy can give an indication of the toughness of a material. A higher toughness material will absorb more energy.

Sample a) shows a ductile material b) a britlle material.

Question 20

Hardness testing- What are the two main types?

Answer 20

1. Scratch testing -used for brittle materials such as ceramics.
2. Indentation testing- can only be used on materials that undergo plastic deformation such as metals and thermoplastic polymers.

Question 21

Describe scratch testing

Answer 21

The hardness of a material can be determined based on Moh’s scale of hardness, which ranks a material based on a list of syandard materilas with known hardness. The hardness of the material is ranked on the scale between the material it just scratches and the material that it fails to scratch.

Question 22

Describe indentation testing?

Answer 22

A number of different tersts available - the most common are the Brinell test, Vickers Diamond test and the rockwell test.

All three methods involve the indentation of the material and hardness is calculated be measuring the force applied.