Background and Aim
Creep is defined as the ‘’ time-dependent and permanent deformation of materials when subjected to a constant load or stress’’ (Callister 2011, p.265). The aim of this experiment is to find the creep rate of a rubber in tension. Rubbers are classified as elastomers, elastomers are polymers which have a high degree of crosslinking and it is this crosslinking that allows elastomers to return to their original shape after deformation. Elastomers exhibit viscoelastic behaviour; viscoelastic materials have both viscous and elastic characteristics when undergoing deformation. When a load is applied to a viscoelastic material the deformation is time dependent under constant ...view middle of the document...
This was then repeated 3 times.
* A voltage reading was taken for the corresponding distance using a voltmeter accurate to ±0.001V. The measurements were recorded three times.
A mass of 0.5kg was then placed on the experimental set up shown below and the voltage was recorded after 5 second time intervals for the first 30 seconds and then after every 30seconds until 275 seconds. The time was measured using a stopwatch from an iPhone with an accuracy of ±0.01 seconds. The procedure was repeated three times.
* The above procedure was then repeated with a mass of 1kg and the voltage was recorded using the same time intervals detailed above. The test was repeated twice.
Results and Analysis
In the experimental setup the strain of the rubber was measured using the LDVT, the readings given from this were voltage readings; however displacement readings are required in order to find out the strain. Therefore a calibration method must be deployed to transform the voltage into a displacement or length. To achieve this, voltage readings were taken as a gauge attached to the LDVT was compressed. The results are detailed in table 1.
Compression Displacement of the Gauge/ (mm) | Voltage/ (V) |
| Test 1 | Test 2 | Test 3 | Average |
0 | 1.029 | 1.030 | 1.030 | 1.030 |
5 | 0.930 | 0.931 | 0.931 | 0.931 |
10 | 0.851 | 0.851 | 0.851 | 0.851 |
15 | 3.425 | 3.431 | 3.431 | 3.429 |
20 | 7.410 | 7.410 | 7.420 | 7.413 |
25 | 11.870 | 11.890 | 11.890 | 11.883 |
Table 1: table showing the voltage associated with certain lengths as the gauge was compressed
Once this information was obtained a graph of displacement against voltage was plotted in order to determine the relationship between the two. The graph is shown in figure 3 below.
Figure 3: relationship between voltage and displacement. The important part of the graph is the linear region as it shows a proportional relationship between voltage and displacement.
The graph in figure 3 shows a linear relationship after about a distance of 15mm therefore it is this area where the information can be used to calibrate the LDVT data. The linear region is then plotted below in figure 4.
Figure 4: Graph showing the linear relationship between voltage and displacement, the equation of the line and the coefficient of determination are also displayed on the graph. The line equation can be used to convert voltages to distance and the high value signifies the line is a good fit.
The line equation in figure 4 can be used to convert the voltage readings taken as a mass is loaded onto the experimental setup in figure 1 to length and hence strain values. Since there is a linear relationship between voltage and displacement/length and the equation is: