Tensile Testing: Load-Extension Graphs

Question 1

What is tensile testing?

Answer 1

To measure the properties of the material, tensile testing stretches the material until it breaks.

Question 2

Explain the tensile testing process (5 points)

Answer 2

1. A standard-sized test specimen is clamped at both ends.
2. the pulling force is increased slowly.
3. Measurements are continually taken of:
   The pulling force (load) applied
   The increase in length (extension) of the piece
4. Measurements are plotted on a load-extension or stress-strain graph.
5. Properties of the material are calculated from the graph.

Question 3

Name the four stages in deformation

Answer 3

Original - elastic - plastic - fracture

Question 4

When force is first applied, what happens?
What does this mean?
What is this called?

Answer 4

When force is first applied, the material stretches elastically.
This means the material would return to its original shape if the force was removed.
This is elastic extension.

Question 5

Explain what plastic extension is.

What does it mean to deform plastically?

Answer 5

With more force applied, the material deforms plastically.

This is plastic extension.

Question 6

What does it mean to deform plastically?

Answer 6

This means the material has deformed permanently and will no longer return to its original shape if the force was removed.

Question 7

Explain what ‘Necking’ is.

Answer 7

‘Necking’ occurs during plastic deformation. This is where the diameter of the test specimen narrows due to plastic deformation.

Question 8

When is elastic extension large or small?

Answer 8

Elastic extension is large in elastic materials and small in stiff materials.

Question 9

When is plastic extension large or small?

Answer 9

Plastic extension is large in ductile materials and small in brittle materials.

Question 10

With more force applied, what will happen?

Answer 10

With more force applied, the piece eventually fractures.

Question 11

Why is less force required to complete the fracture?

Answer 11

Due to necking and as the fracture starts to occur, less force is required to complete the fracture.

Question 12

What types of materials fracture differently?

Answer 12

Brittle and ductile materials fracture differently.

Question 13

What is generated by the tensile testing machine?

Answer 13

Measurements of load and extension are generated by the tensile testing machine.

Question 14

What is load?

Answer 14

Load is the pulling force (measured in kilonewtons)

Question 15

What is extension?

Answer 15

Extension is the increase in length (measured in millimetres)

Question 16

Formula for stress

Answer 16

Stress =
load
————-
cross section area (CSA) {pi(radius)squared}

answer in kN/mm^2

Question 17

Formula for strain

Answer 17

strain =
extension
—————–
original length/gauge

No units

Question 18

How do you calculate the Ultimate Tensile Strength?

Answer 18

Ultimate Tensile Strength:
You must find the C.S.A. (cross-section area) of the object using the formula π r².
The highest point on the extension-load graph over the C.S.A. is the formula for the U.T.S.
Fill in the blanks in the formula and solve.
The units are kN/mm².

Question 19

How do you calculate the 0.1% proof stress?

Answer 19

You must get 0.1% of the gauge length (e.g. 0.1% of 60mm = 0.06mm.
Draw a line parallel to the straight portion of the graph beginning at 0.06mm.
Note the load value where the proof line intersects the load-extension curve = 77kN.
Convert the load value to a stress value by dividing by the C.S.A.
You have gotten the proof stress

Question 20

How do you calculate the Youngs Modulus?

Answer 20

Choose a point in the elastic region of the graph (e.g. 64kN, 0.8mm).
Put the load value over the C.S.A. and divide to get the stress.
Put the extension value over the original gauge length and divide to get the strain.
Put your two new numbers over each other with stress above the strain.