Introduction:
The main purpose of the experiment was to carry out a tensile test on A36 steel and 6064-T6 aluminum. The experiment aimed at determining yield strength, rupture strength, ultimate tensile strength, the modulus of elasticity, the ductility of the materials under test. In real life engineering applications the tensile test is one of the most important and useful test that can be utilized to determine the mechanical properties of a material. Different materials have different mechanical properties and hence have different engineering uses.
In engineering, it is imperative to determine the mechanical properties of a material as it helps in determining the suitability of the material for different uses. This is attributable to the fact that once a tensile test has been carried out on a material it is possible to determine its load bearing properties as well as the deformations that may occur on the material when a load is applied to it. A tensile test is used to determine the ability of a material to resist changes in a static applied force. The main result of a tensile test is the stress-strain chart, which shows the relationship between stress and strain.
For the tensile test, stress is the ratio between the applied force and the cross sectional area of the specimen under test. It is calculated as:
σ = P / A
Where,
σ = stress
P = the applied force on the specimen under test
A = the cross-sectional area of the specimen under test
On the other hand, strain is the ratio between the change in length in the specimen and the original length in the specimen. It is calculated as:
ε = ΔL / LO
Where,
ε = strain
ΔL = change in length
LO = original length
Procedure:
- Calipers or a micrometer were used to measure the dimensions (width and depth) of the specimen to be tested. From the dimensions measured the cross sectional area of the specimen was determined
- The extensometer was installed on the specimen before mounting it on the machine and two marks were drawn on the specimen based on the position of each leg of the extensometer.
- The distance between the two marks was measured with a pair of calipers and the value recorded as the original gauge length of the specimen and the extensometer was removed
- The testing machine was then run
- The testing computer software was also run with the appropriate testing procedures selected
- The hand control was used to move the upper cross head into the best position
- The specimen was installed on the upper and lower grips of the testing machine
- The extensometer with its safety pin was installed in the machine and the legs were placed on the marks made earlier on the specimen
- The safety pin was removed and the grip on the specimen was confirmed in order to eliminate any slippage
- The test procedure window was utilized to zero the load, strain and position
- The test was started utilizing axial tension force
- The lad value was watched from the computer screen as well as the specimen. Once the load value started to drop, the test was paused.
- The extensometer was removed and OK was hit to complete the test
- The specimen continued being loaded until it failed
- Once the specimen failed it was removed from the grips and the data was saved on the computer
- The two parts of the specimen were put together and the distance between the marks made earlier was determined. This was recorded as final length
- Calipers or a micrometer were used to measure the new dimensions (width and depth) of the specimen. From the dimensions measured the final cross sectional area of the specimen was determined
- The test data was saved and exported to Excel for further analysis
Apparatus:
The following apparatus were utilized in the experiment.
- Steel A36
- Aluminum alloy T6 – 6061
- Universal testing machine – Baldwin type
- Extensometer
- Calipers
Results:
Summary Results:
The tables below shows the summary results for Steel A36 and Aluminum alloy T6 – 6061 based on the data collected from the laboratory.
The chart below shows the stress-strain relationship for aluminum and steel based on the data collected from the laboratory.
Chart 1: Chart showing the stress-strain relationship for Aluminum alloy T6 – 6061
Chart 2: Chart showing the stress-strain relationship for Steel A36
Discussion:
Question 1:
An engineering stress-strain curve is used because it rises and drops when the maximum stress is reached or when the material under stress undergoes deformation. On the other hand, for a true stress-strain curve the curve does not drop when the material undergoes when deformation is reached. The main difference between the two is that for the engineering stress-strain curve the stress is determined by dividing applied force by a constant cross sectional area while for a true stress-strain curve is determined by dividing applied force by a variable cross sectional area.
Question 2:
The value of true stress at fracture is the ratio between the maximum applied force at fracture and the cross sectional area at fracture.
True stress at fracture = Force applied at fracture / cross sectional area at fracture
Question 3:
The reason why a stress-strain curve used rather than a force displacement curve is that the stress-strain curve can easily be used to determine yield strength, rupture strength, ultimate tensile strength, the modulus of elasticity, the ductility of the materials under test
Question 4:
For elastic strain once the applied force is removed the specimen returns to its original shape while for plastic strain if he applied force is removed the specimen remains deformed.
Question 5:
Question 6:
Below is a comparison between theoretical values of yield strength, ultimate strength, and modulus of elasticity with obtained results. The theoretical values have been derived from http://asm.matweb.com/.
The results above are reasonable, as the percentage error is low. The only value that is not acceptable is the yield strength of steel A36.
Question 7:
The sources of error present in the experiment are:
- Parallax errors while reading the
- Equipment errors
- Human errors during calculation
These errors can be prevented by ensuring that the equipment used is calibrated correctly before carrying out the experiment.
Conclusion:
The experiment was carried out successfully and the yield strength, ultimate strength, and modulus of elasticity obtained for Aluminum alloy T6 – 6061 are 43,700 psi, 45,900 psi, and 9,000,000 psi respectively. For steel A36 the yield strength, ultimate strength, and modulus of elasticity were obtained as 59,500 psi, 81,900 psi, and 30,000,000 psi respectively.
Appendix:
References
http://asm.matweb.com/
Sample calculation:
Percentage yield strength for aluminum =
= (43,700 - 40,000) / 40,000
= (3,700 / 40,000) %
= 9%
Raw Data:
