Recent Question/Assignment

SEM111 Materials Assignment 1 Trimester 1 2016
Lecturer: Akif Kaynak
Stress/Strain Practical:

The aim of this practical is to carry out tensile tests on selected metal and polymer samples with a view to understanding stress versus strain behaviour, Hooke’s Law, Young’s modulus, ultimate stress, yield, ductile and brittle behaviour and structure-property relationships . You will compare your tensile data with that published in literature.
Hooke’s Law,
Young’s Modulus,
Ultimate Stress,
Yield stress,
Breaking stress,
Breaking strain,
Ductile Material,
Brittle Material
Experimental Procedures:
Using the information below and your observations during the prac session explain experimental procedures in your own words. You may use some of the information given below as well as what you have observed during the laboratory session.
Include the graphical output we obtained during the experiments.
The following information is on the PASCO apparatus for stress strain measurements:
The PASCO AP-8214A Stress/Strain Apparatus illustrates the relationship between stress and strain for various materials. The apparatus stretches a test coupon (and breaks it in some cases) while measuring the amount of stretch and force experienced by the test coupon. Data acquisition software can be used to generate a plot of force versus displacement and also a plot of stress versus strain.
The Stress/Strain Apparatus requires a ScienceWorkshop or PASPORT interface, DataStudio software, a Rotary
Motion Sensor (RMS), and a Force Sensor. Included with the apparatus are four types of metal test coupons and four types of plastic test coupons. Also included are a tee handle with a socket that fits the hex nuts on the coupon clamps, a bar for calibrating the apparatus, and spare hex nuts for the coupon clamps.
AP-8221 Stress/Strain Apparatus
Analysis of your results
Extract the Young’s modulus from the graphs. Young’s modulus for each sample is calculated during the practical by using the straight line fitting within the software.
Read Ultimate stress (tensile strength), yield stress, breaking stress, breaking strain and percentage elongation at break (if the sample is broken).
For metal samples if there is no clear yield point determine the 0.2% offset yield stress.
Tabulate Young’s modulus, Ultimate stress (tensile strength), Yield stress , 0.2% offset yield stress, breaking stress, breaking strain, percentage elongation at break for all the samples you tested.
Include values for the above from literature for comparison on the same table
Comment on the results and possible sources of error.
Comment on the ductile versus brittle behavior from your results. Ie which samples exhibited ductility and which ones failed in a brittle manner.
Comment on the strength of materials, compare and contrast them. Compare metals and polymers with respect to their general tensile behaviour.
Give reasons for the observed behavior.
In this section you will perform some calculations on a problem to test your learning in this prac:
Material samples, called specimens, often are tested using a tensile testing machine. A specimen is prepared in an ASTM (American Standards for Testing and Materials) standard shape, often like the one shown below and loaded into the machine. The machine applies a uniaxial load to the specimen at a slow, constant rate. The increasing load and associated deformation are measured at many points until the specimen fractures. Engineers use this data to calculate properties like the yield stress, ultimate stress, and failure stress. They also might use this data to construct a stress-strain diagram for the material.

Suppose you need to design a tension test machine capable of testing specimens that have nominal ultimate stresses as high as su = 650 MPa. How much force must the machine be capable of generating? (express your answer in kN) Assume the testing specimen has the ASTM shape shown.

If the maximum nominal strain is ?f = 0.5 just before the test specimen fractures and the test machine operates by moving only one grip, how far must that grip be designed to travel? The total length of the deforming part of the specimen (gauge length) is 50 mm. In other words, what is the value of ?l?
You have built the majority of the tension testing machine, but much of the instrumentation is still being assembled. To test the machine, you perform a test on a steel specimen with known properties. The machine provides you with the given load data, and you manually record the lengths between the marks on the specimen at each point using an extensometer to obtain the table of data shown below.
L (mm) 50.03 50.06 50.89 52.03 53.34 54.92 57.12 62.13 64.51 66.91 68.59 69.04
P (kN) 15.61 31.54 35.01 39.28 43.77 48.04 52.36 55.03 52.49 48.09 43.64 41.90
Use these results to plot the stress versus strain curve for the above sample and determine:
Why is stress versus strain diagram is preferable to force versus extension diagram?
Plot the above data using computer
Yield stress
Yield strain
Ultimate stress (tensile strength)
Young's modulus
Modulus of resilience
Using the information in the following graph of a he sample with a gauge length of 50 mm and diameter 10 mm.
if the sample was strained to a value of 0.0035 and released what would be the elastic recovery?
Given that the Poisson’s ratio of this material is 0.29, if we applied a stress of 300 MPa, what would be change in its diameter?
Using the graph below evaluate the approximate Modulus of Resilience of this material.

Your prac must be entirely your own work, otherwise no marks will be given.
The report must be type written including the calculations and the graphs.
Include the graphical outputs from the prac session
Include the plot from question 5C (should be computer plotted)
All the calculations should be worked out individually, and work copied from classmates will be marked as zero.
July 2016, Akif Kaynak.