Assignment Task: Lab: A Laboratory Report based on: Tensile Testing of Polymers.
LAB: Tensile Testing Of Engineering Polymers
Laboratory Learning Outcomes: The learning outcomes of this coursework, as a key component of the Structural Mechanics course, arc the following:
To expose the students to practical/laboratory skills required in the testing of an engineering material - in this case - polymers.
To analyze the parameters needed in identifying the mechanical behaviour of chosen engineering materials like nominal and true stresses, nominal and true strains, etc.
To extend the understanding of mechanical behaviour to identifying constitutive models needed to predict the behaviour of the chosen engineering materials.
Questions: Post-Test Analysis
The analysis of experimental test data is the most significant activity an engineer can engage in after each test. This is where you show your understanding of the relationship between the experimental data and the mechanics of materials concepts developed in the lectures. The following set of activities or questions are targeted at helping you obtain reliable mechanical behaviour information from your experimental data.
1. Using either Excel or MATLAB, plot a graph of Force, F versus Displacement, ΔL using the raw data obtained from your experiment for all tested materials and test speeds. Briefly comment on the nature of the graph for all tested materials.
2. Using the equations for nominal stress and nominal strain below, plot again the graph showing nominal stress versus nominal strain for the tested materials and test speeds. Nominal or Engineering Stress, σN = F/Aα; Nominal or Engineering Strain, εN = ΔL/Lα.
In the above equation, Aα = initial cross section area (Aα = Width*Thickness) and Lα = initial gauge length of the specimen, as measured at the start of the experiment. Briefly comment on the nature of your graphs.
3. Convert the nominal stress and nominal strain parameters above into true stress and true strain parameters and plot true stress-strain graph for all tested materials and test speeds. Use the following equations:
True Stress, σT = σN(1+εN) and True Strain, εT = ln(1+εN)
Briefly comment on the nature of your graphs.
4. To show the difference between nominal and true stress versus strain plots per given polymer, combine the plots of nominal and true stress versus strain plots for all tested materials and test speeds. Briefly comment on the nature of your graphs.
5. Using the plot of true stress versus true strain, determine the Secant Modulus of all tested materials. Choose a limiting strain for calculation of the Secant Modulus.
a) Based on your calculated values of secant modulus for the materials, is there agreement between your experimentally determined values and official (engineering handbook) values of modulus of the chosen test materials?
b) If there are differences between your values and official values, calculate the percentage error. Briefly comment on the possible source of error.
c) Draw a bar chart showing variation of Secant Modulus with test speed for all test materials.
6. Include images of the tested specimens and briefly comment on the shape of the specimen before and after testing.
a) Classify the images to ductile and brittle failure categories.
b) Explain the meaning of brittle and ductile failure and how can you conclude on this from a stress-strain graph.
c) Based on images of tested specimens, comment on the shape of the specimen after testing.
d) What is necking? Do you see evidence for necking in some of the specimens?
c) What is responsible for necking in those specimens (where necking exists)?
7. What is Yield? Comment on the yield region for all the tested materials? Is there a relationship between yielding and necking observed in the test specimens?
8. Based on the true stress versus true strain graphs, build a table of the yield stress for all the test specimens tested and all tested materials. To see the variation of yield stress with test material and speeds, draw a bar chart showing on the y-axis the Yield Stress (for all speeds) and on the x-axis the test material.
Briefly comment on the nature of your graphs
What factors are responsible for one of the materials to have the highest yield stress in comparison to others?
9. A common mechanical behaviour of polymers is strain softening? Basing your discussions on the true stress versus true strain plots answer the following questions
a) What is strain softening? Briefly comment on why strain softening happens in materials.
b) Draw a bar chart showing the Y-axis maximum size of strain softening and x-axis the different test materials tested. Show the charts for the two test speeds.
c) Is there any stress softening observed in the materials?
10. Another mechanical behaviour that is evident in the true stress versus strain values is called strain hardening.
a) Define strain hardening.
b) Draw a bar chart of maximum strain hardening observed after yield in the materials by putting strain hardening magnitude in y-axis and test material in x-axis, for the two test speeds.
c) For the same true strain levels e.g. 0.25 and 0.35, which of the materials shows the biggest strain hardening?
d) What do you think is responsible for the significant strain hardening seen in some of the materials?
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Attachment:- Engineering Mechanics Principles.rar