Creep Test Laboratory Report (Essay Sample)

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Creep Test Laboratory Report
Nomenclature
A = material parameter
S = spacing between the two turns
n = stress exponent
R = gas constant
T = absolute temperature (K)
��̇ = strain rate
ρ = density
Ï„ = shear stress
D = coil diameter
Q = creep activation energy
d = wire diameter
σ = applied stress
N = turn number
ϵ = shear strain
t = time
Objective
The purpose of this laboratory experiment was to investigate the creep of a given material specimen and the various factors that affect it. This was facilitated by assessing the change in length of the given specimen and then plotting the obtained values on a graph paper. The trend of the resultant curve was then compared against some known theoretical values to validate the credibility of the results of the experiment.
Theory
According to Rayner and Jones, creep refers to a situation whereby materials deform permanently after being subjected to constant mechanical stresses (258). The rate at which creep occurs depends on time, and is differentiated into three major types. The first type of creep is known as transient creep and is characterized by an increase in strain with a corresponding increase in time. Equation 1 below represents the relationship between strain and time associated with this type of creep.


The other type of creep is known as secondary creep or steady-state creep and is characterized by strain rate that is constant as time changes linearly. As indicated in equation 2, to obtain the corresponding equation for the secondary creep, natural logarithms on both sides of the equation 1 are taken.


The third type of creep is referred to as tertiary creep and is characterized by strain increasing nonlinearly as the temperature increases. Generally, the temperature at which creep begins depends on the alloy composition of a material. Fig. 1 below depicts the nature of the curve in a typical creep graph.


Fig. 1. Creep diagram (Rayner and Jones 258)
As observed from the graph above, creep rate is very high at first and it then begins to decrease gradually in the region demarcated as transient creep. This stage is followed by the secondary creep, which is characterized by a minimal creep rate, that is, a strain that increases slowly with the increase in time (Haddad 855). In the last stage, which is marked as tertially creep, strain increases rapidly with increase in time, resulting in failure of the material subjected to creep.
Creep of materials is primarily attributed to the diffusion or the motion of atoms due to dislocation. The science of engineering material recognizes two major types of diffusion associated with the creep: the Nabarri-Herring creep and the Coble creep (Kassner 213). In the case of the Nabarri-Herring creep, diffusion of the atoms is mainly within the grains when the activating temperature is about half of the melting point. During this type of diffusion, the shape of the grain is critical since the rate at which creeping occurs is equivalent to 1 over the grain-size. In the case of the Coble creep, diffusion occurs when the temperature is extremely below the melting point. In this case, diffusion occurs along the grain boundaries.
One of the simplest ways through which the creep of a material can be determined is by applying a load to a wire and then measuring the position of a particular point on the wire over time (Daniels 94). In most cases, to eliminate the amount of errors that can occur due to the large amount of extraneous variables when the wire is hung, as well as the orientation of the load, two points should be marked on the wire. The change in position of the two points is then measured over a given period to access the increase in length of the wire.
The difference between the positions of the two points over a given time interval is then used to calculate the strain rate. By employing equation 2 above, the rate at which strain is changing can be plotted and then the various stages of the creep observed depending on the shape of the curve. Through this experiment, it is also possible to access the stress exponent and the activation energy, in which case the time and temperature have to be constant. In this case, the first consideration is to take into account the stress in each turn of the coil, a scenario which is represented by equation 3.


The drawback of the equation 3 above is that it does not take into account the effect of multiple coils. Therefore, equation 4 below becomes preferred in determining the shear stress of the wire.


Considering that shear strain is a localized process, equation 5 can also be used to determine its magnitude.


This equation is then differentiated with respect to time, resulting in equation 6 as indicated below.


By factoring out S/t in equation 6, the resultant will be the same to equation 7, which infers that strain is directly proportional to the coil spacing divided by time.


Equation 7 can be simplified further by taking the natural log on both sides and treating the term, Cd/d2, as a constant. Adding equations 2 and 4 to equation 7 and simplifying the quotient generates equation 8, which can again be simplified further to give equation 9 that is used to determine the activation energy.




Laboratory Procedure
A tin alloy solder was wrapped around a post about twenty times to create a coil.
On one end, the coil was attached to the top of the post. A slider was then fixed to the post. The purpose of the slider was to compress the coil together when the experiment was not in progress. The whole assembly comprising the post and the slider was then put inside another tube.
The solder that had been wound on the post was then heated by pointing a hair dryer to the tube. During the heating, precaution was taken to prevent heat loss by applying insulating foam in the mouth of the tube.
Using a meter stick placed on the outside of the tube, the distance between the coils was then measured. For proper recording of the distances between the coils, a camera was placed on a tripod stand strategically focusing on the coil and the yardstick.
In each measurement, the slider was lowered at the same pace that the timer was started and the motion captured on the camera. In the course of the experiment, the solder was permitted to fall for a single minute before another picture was taken.
The above experiment was repeated several times by increasing the temperature at an interval of 20 degrees Celsius.
The picture captured in the course of the experiment was then used to measure the separation of the turns of the coil to compute the strain rate, the stress exponent, and the activation energy.
Experimental Data and Analysis
Initially, the experiment was conducted at a room temperature, which was found to be about 20.5 degrees Celsius. Fig. 2 and fig. 3 depict the pictures captured at the start and end of the experiment under the initial temperature and the corresponding values of the yardstick at each coil recorded in table 1.

Fig. 2. The start of the experiment at room temperature

Fig. 3. End of the experiment at room temperature
Table 1. Coil spacing at room temperature
CoilInitial position
(mm)Final position
(mm)coilInitial position
(mm)Final
Position (mm)CoilInitial position(mm)Final position
(mm)coilInitial position
(mm)Final position
(mm)1648.848.21146.445.6644.544143.543.01548.247.61046.045.3544.243.81447.747.0945.645.0444.043.61347.246.6845.344.6343.943.31246.846.0745.044.3243.743.0
The second experiment was conducted by raising the temperature of the dryer to 50 degrees Celsius. The corresponding results are illustrated in fig. 4 and fig. 5 in form of pictures, while table 2 has been used to record the respective numerical data of the experiment.
Fig. 4. Start of the experiment at 50 degrees Celsius


Fig. 5. End of the experiment at 50 degrees Celsius
Table 3. Coil spacing at 50 degrees Celsius
CoilInitial position
(mm)Final position
(mm)CoilInitial position
(mm)Final Position
(mm)CoilInitial position
(mm)Final position
(mm)CoilInitial position
(mm)Final position
(mm)1648.248.5114645.5644.443.6143.441.21547.747.71045.644.0544.243.31447.347.094444.6444.042.81346.846.5844.544.3343.842.61246.446.0744.643.9243.743.0
The final stage of the creep experiment involved heating the hair dryer to a temperature of 70 degrees Celsius. The corresponding pictures for this experiment are recorded in fig. 6 and fig. 7, while the associated data has been recorded in table 3.


Fig. 6. Start of the experiment at 70 degrees Celsius

Fig. 7. End of the experiment at 70 degrees Celsius
Table 3.Coil spacing for 70 degrees Celsius
CoilInitial position
(mm)Final position
(mm)CoilInitial position
(mm)Final Position
(mm)CoilInitial position
(mm)Final position
(mm)CoilInitial position
(mm)Final position (mm)1648.248.51145.045.5644.443.6143.442.31547.747.71045.644.0544.243.31447.347.0945.344.6443.042.01345.46.5845.044.3343.842.61246.445.0744.643.9243.642.5