- To obtain the strain measured from encoder over a period of time for Aluminium specimen of 3mm diameter.
- Compare the results by plotting a graph of strain vs time with the model graph as shown below.
Creep is a form of cold flow of metals which happens when the material is subjected to constant loads for a long duration of time especially at elevated temperatures. This causes material to be strained to as much as 150%. This phenomenon is more profound when the specimen is at elevated temperatures > 0.4 Tm. (melting point). The deformation is permanent and can happen as a result of prolonged exposure to the high mechanical stresses that are still below the yield strength.
To obtain the plot of creep strain growth as a function of time
Rate of deformation is a function of:
- Applied load
- Exposure temperature
- Exposure time
- Material properties
Fig 1: The Creep test experimental setup at Amrita Virtual Lab.
The experimental setup shown above has a 50 kilogram load suspended on an aluminium wire of around 3 millimetre diameter. Inside the protective covering that can be seen on the middle, a circular cylindrical metallic casing with a heat insulator can produce extreme temperature. Varying the temperature can also help us study the thermal effects on creep.
Fig 2: Strain vs time
The diagram above shows strain as a function of time. The portions are split as primary, secondary and tertiary curve. In the beginning, the rate of strain is high and then stabilizes due to work hardening. In the second segment, the strain is minimum and nearly constant which happens as a result of balance between annealing and work hardening. The tertiary segment is similar to the necking phenomenon observed when metals are tension tested in Universal testing machine.
The strain vs. time graphs are plotted with a constant load applied at a constant temperature. Shape of the creep curve will depend on the levels of temperatures and stresses involved. If the temperature is remained constant, the creep curves will shift upward and to the left with increasing applied stresses. If the creep test is carried out at various temperatures but at a constant stress level, the creep rate will increase with increasing temperatures.
The deformation process in creep which occur at elevated temperatures are due to
- Dislocation movement known as slip
- Grain boundary sliding
- Sub-grain formation
At increasing temperature, slip systems are more available. Grain boundary sliding is a type of shear process along the grain boundaries, providing a non-uniform amount of shear displacement. The formation of sub grains normally in the adjacent of the grain boundaries results from lattice distortion. This allows dislocation with opposite signs to form the sub grains.
Application of creep in Engineering
While designing components, it is necessary to select the material which can withstand the operating conditions that the components will be exposed to. Therefore, it is necessary to acquire accurate design parameters such as creep strength from experimentation. The creep strength can be defined as 1) the stress at a given temperature to produce a steady-state creep rate of a fixed amount (normally at 10-11 to 10-8 s-1 or, 2) the stress to produce creep strain at 1 percent of the total creep strain at a given test temperature (usually 1000, 10000, or 100000 hours)