What is Measured?

During the creep test, the following quantities are measured:

• Applied load
• Original gauge length of the specimen
• Extension of the specimen at regular time intervals
• Time elapsed during the experiment

These measurements are used to study the time-dependent deformation of the material.

Why are the Calculations Required?

The recorded measurements help determine:

• Extension of the specimen
• Engineering strain
• Variation of strain with time
• Creep behaviour of the material

These calculations help evaluate the suitability of materials for applications involving sustained loading.

Observation Table

Time (min)	Extension, $\Delta L$ (mm)
0	0.00
1	0.08
2	0.15
3	0.20
4	0.24
5	0.27

Sequential Calculations

1. Measure the Extension

Record the extension of the specimen at each time interval.

2. Calculate the Engineering Strain

$$\varepsilon=\frac{\Delta L}{L}$$

3. Determine the Creep Rate

The creep rate is

$$\dot{\varepsilon}=\frac{d\varepsilon}{dt}$$

4. Plot the Creep Curve

Plot:

• Time on the horizontal axis
• Extension or strain on the vertical axis

The resulting graph represents the creep behaviour of the material.

Solved Numerical Example

Given:

• Original gauge length,

$$L=100\ \text{mm}$$

• Extension after 5 minutes,

$$\Delta L=0.27\ \text{mm}$$

Engineering strain is

$$\varepsilon=\frac{0.27}{100}=0.0027$$

Therefore,

$$\boxed{\varepsilon=2.7\times10^{-3}}$$

Interpretation of Results

• Increasing extension with time indicates creep deformation.
• A gradual decrease in the rate of deformation represents primary creep.
• A nearly constant rate indicates secondary creep.
• Rapid deformation prior to failure corresponds to tertiary creep.

Lead exhibits measurable creep at room temperature because of its relatively low melting point.

Result

The creep test was successfully performed, and the time-dependent deformation of the lead specimen under constant load was observed by measuring the extension at regular time intervals.