Prelab

1. Read the lab to familiarize yourself with the procedure

2. Read this page and watch the associated videos detailing testing using the Instron.

3. Print this document and bring it with you to the lab.

4. Bring a USB memory key to the lab.

Problem

A metals foundry and machining plant has approached A.P Contracting about acting as a supplier for their metal fabrication shop. They have provided test samples for characterization and you are tasked with choosing the ideal candidate for use in a product. 

A.P. engineering creates Spacecraft grid-fins which act as aerodynamic surfaces that help in the stabilization of rockets while in the atmosphere. The complex nature of this type of product requires specialized materials. The baseline requirements are as follows:

• • Low ductility and malleability. These are preferred as rigidity and ability to maintain form under high stresses are critical to the design.

  • Yield stress above 260 MPa to sustain drag and turbulent forces during acceleration.

Suggest which sample best fits these criteria, if any, and support your decision with the data from your results/tests.

Procedure

Using the set of digital calipers, determine the gauge length: L0 = 0.7 [ (L2 + L1) / 2 ].

• • 1. • L0 is the gauge length.

       • L1   is the length of the reduced section.

       • L2   is the distance between shoulders.

       • The Gauge length is then 70% of the average length of the two noted sections.

Using the measured thickness and width of the narrow section find the cross-sectional area.

• • 1. Turn on the Instron and allow it to complete the self-test.

    2. Jog the cross-head to allow for a reasonable distance between grips.

    3. Put the sample in the grips and tighten them well.

    4. Press the Track button to update readings.

    5. Calibrate by pressing the Load Cal, and then Enter buttons.

    6. Press the Load Balance, and then Enter buttons.

    7. Reset the Gauge Length by pressing GL Zero.

    8. The screen should show LOAD=0, EXTENSION=0.

    9. Set the speed to 10mm/min.

    10. Press Reset Peak.

    11. Ensure Track is on.

    12. Select Start Logging.

    13. Press the Up or Down button to begin the test.

    14. Observe the change in LOAD and EXTENSION.

    15. After the sample fails, record the peak LOAD and EXTENSION.

    16. Save the data to your USB key.

    17.  Fill in your data on the prelab printout, have the data signed.

Results

The test data will be shared with all groups, and your final reports must include analysis of all samples.

State all measured dimensions, and those you calculate after the fact: 

• Gauge Length

• Cross-Sectional Area

From the provided Load & Extension datasets, and your own measurements create Stress-Strain diagrams showing:

•  All runs on a single plot.

• All runs of the same metal type (identified from the above plot) on a single plot. One for each Aluminum, Brass, Structural Steel, and Mild Steel.

  • Use your judgement in determining similarity between runs. Feel free to ignore any anomalies or outlying runs if appropriate.

• In all you should have 5 graphs, One overall, one for each of the 4 metals.

From the above plots, for each metal, create a table describing the average: 

• Note that the many of the following properties can be found via inspection on the plots and do not require calculation.

• Identify specific values from your data/graph and display them on the plot or in a table for σ and ε values.

• To determine the Young's Modulus you may:

  • graph the linear region and place a trendline showing slope.

  • select two points in the linear region over a reasonably large range and calculate the rise/run.

  • use any other method appropriate.

• Proportional limit (σpl and εpl)

• Young’s modulus of elasticity (E)

• Yield stress using the offset method (σy)

• Ultimate stress and its corresponding strain (σu and εu)

• Fracture stress and its corresponding strain (σf and εf)

• Ductility (% elongation at failure)

Be sure to include a justified response the proposed problem.

Questions

1. Suggest 2 other properties of the chosen grid-fin material (from the problem above) that make it a good candidate for the project outside of those we measured in the lab tests, if none fit the requirements, instead suggest an alternative.

2. Indicate where the Yield Stress, as well as Strain-Hardening and Necking regions, can be found on your Stress-strain diagram.

3. Define "Ductile" vs "Brittle", and how it relates to the experiment. How do you think the Stress-Strain diagram would look for a brittle material?

   • Be sure to explain the difference be a material that is brittle versus ductile in terms of the relevant above properties of materials.