MEAM.Design - MEAM 247 - P2P1: Uniaxial Loading


Experimental Data


Project 2, Part 1: Uniaxial Loading


1 - Prelab
(due by 11:59 p.m. on Wednesday, 10/19)

1.1 - Read through the Background, Experimental Procedures, and Testing & Analysis sections below.
1.2 - One would expect that the elastic modulus calculated for the same material in tensile and compressive loading would be the same; however, the results are rarely in exact agreement. Discuss possible reasons for discrepancy.
1.3 - Think about something that you've owned or used that has broken (or, if you must, go break something of little value). Can you relate the failure of the object to the basic mechanics of materials concepts (stress, strain, ductility, etc.) discussed here? Please describe.
1.4 - Submit the answers to the above questions in the body of an email to medesign@seas.upenn.edu with the title 247-P2-prelab.
1.5 - Stop by Towne 205, and add your name to a blank line on the sign-up sheet for Instron testing (if you are not signed up by 11:59 p.m. on Wednesday, you will be randomly assigned, which could result in overlaps with other commitments).


2 - Background

When a specimen is loaded uniaxially (along its primary axis), the force acting over the cross-sectional area generates stress and strain within the material. When relatively small loads are applied to the specimen, it will deform elastically, and should return to the original dimensions when the load is removed (this effect is generally observed for both tensile and compressive loading). Further loading of a specimen will induce plastic deformation, and the specimen will not return to the original state when the load is removed. Further loading will then result in stresses which exceed the capacity of the material, and failure will result. See here for more information.


3 - Experimental Procedures

You will be using the Instron and Rockwell testing equipment in room B13 of the LRSM building to gather data on three materials: 1018 cold-rolled steel, Gray-20 cast iron, and 6061-T6 Aluminum. Follow the procedures below to test each material in tension, compression, and hardness.


3.1 - Tensile Testing
During this first phase of testing, we will examine how prepared material specimens behave under extreme tensile loads
3.1.1 - Before loading a material into the Instron machine, you must measure and record both the diameter and the gage length (the length of the straight section) of the specimen. Calipers will be available for you to get accurate measurements of these two parameters.
3.1.2 - Load the material sample into the Instron testing machine.
3.1.3 - To accurately measure strain within the elastic region, you will want use an extensometer clip gauge attached to the sample. This precision instrument has a limited range, and must be removed once past the material's yield point.
3.1.4 - The speed and maximum load level will be programmed into the machine, and the specimen will be stretched automatically.
3.1.5 - Once failure occurs, remove the sample from the machine and record the surface characteristics (color, texture, shape) at the fracture point (you will want these notes for your report). In addition, note the nature of the plastic deformation in each material (i.e. necking, brittle fracture, extensive plastic flow, etc.). Finally, estimate the final cross-sectional area in the region of fracture and place the two halves of the sample together and estimate the final gage length.
3.1.6 - Do not forget to save a copy of the data file from the PC.


3.2 - Compressive Testing
In this second stage of testing, we will observe how material specimens behave under extreme compressive loads
3.2.1 - Before loading a material into the Instron machine, you must measure and record both the diameter and the gage length (usually the overall length) of the specimen. Calipers will be available for you to get accurate measurements of these two parameters.
3.2.2 - Load the material sample into the Instron testing machine and place the plastic shield between you and the specimen (compressive loading can often result in the specimen being ejected from the machine).
3.2.3 - The speed and maximum load level will be programmed into the Instron, and the specimen will be compressed automatically.
3.2.4 - Once failure occurs, remove the sample from the machine and record the specimen characteristics (color, texture, shape) and note the nature of the plastic deformation in each material (i.e. brittle fracture, extensive plastic flow, etc.). Finally, estimate the final cross-sectional area and length of the specimen.
3.2.5 - Do not forget to save a copy of the data file from the PC.


3.3 - Hardness Testing
In contrast to the destructive tests performed above, it is often desirable to obtain estimates of material properties without destroying the specimen. Using measurements of hardness (i.e. - the resistance of a material to deformation), it is possible to estimate the ultimate tensile stress (UTS) while causing only a small indentation on the surface of the sample. There are many methods which have been developed to measure hardness. For the purposes of this lab we will use the Rockwell Scale. The Rockwell Scale is divided into a B scale for softer materials (aluminum, brass, soft steels, etc.) and a C scale for harder materials. These two scales correspond to the type of indenter and the amount of force which is applied: The B scale uses a hardened steel ball with a 100-kg weight, while the C scale uses a diamond cone (called a Brale indenter) with a 150-kg weight.
3.3.1 - Make sure that the Rockwell tester is set up with the proper indenter and weight, then place the specimen in the v-shaped holder.
3.3.2 - Slowly raise the sample up to the indenter by turning the spoked wheel. Once the indenter comes into contact with the specimen, watch the small needle on the display and continue to turn the spoked wheel until this needle is pointing at the dot.
3.3.3 - Zero the Rockwell gauge by turning the bezel until the long needle is pointing at the zero mark.
3.3.4 - Turn the lever on the right away from you (CW), and wait until the needle comes to a stop.
3.3.5 - Flip the lever back toward you (CCW), and read the value from the dial (make sure to read the value corresponding to the proper scale based on the indenter and weight).



4 - Data Analysis

In preparation for reporting your results, it's time to analyze the data that you've collected:

1. Use the measurements and data you collected to generate a bi-drectional (both tension and compression) stress-strain curve for each specimen. Note that the data obtained from the Instron machine is load and displacement, not stress and strain.
The following five aspects should be completed for one of the 6061-T6 Aluminum sample. You needn't do it for all three.
2. Estimate the yield stress, ultimate tensile stress, and elastic modulus from both the tensile and compressive test results.
4. Estimate the ultimate tensile strength of each specimen from the hardness test.
5. Calculate the ductility of the specimen by dividing the change in cross-sectional area by the original cross-sectional area.
6. Estimate the work done to fracture each tensile specimen (which can be computed as the area under the load-deflection curve, or the initial volume of the gage section times the area under the stress-strain curve up to fracture). You may want to use Matlab to conduct this analysis.
7. Estimate the toughness of the tensile specimen, which is defined as the integral of the stress curve from zero to fracture strain (i.e. - the area under the stress-strain curve to fracture).



5 - Report

You must prepare a formal engineering-style lab report to document your work and results on this project. This is an individual task, and your final lab report must conform to the guidelines posted here. Your report must include the following technical features:

1. A well-formatted bi-directional stress/strain plot for each of the materials that you tested in the LRSM, along with a brief discussion of the three materials and the properties that you've determined. You should compare and contrast the materials on the basis of fracture surface properties, plastic deformation, and other observed characteristics. Try to ascertain how the stress-strain curves might be used to predict the type of plastic deformation and fracture that will occur in a material.
2. A table comparing the calculated material properties against published values for the Aluminum sample, including the elastic modulus, tensile yield stress, compressive yield stress, UTS from tensile testing, UTS estimate from hardness testing, ultimate compressive stress, hardness, ductility, and toughness. Cite the source(s) where you obtained the published values, and include a discussion to explain the possible origins of any discrepancies.
3. Lastly, find a place where 6061-T6 Aluminum is being used in an industrial or commercial product. Investigate why it was chosen, and make an argument for the replacement of the material with another. You should address trade-offs which would need to be made (cost, manufacturability, aesthetics, etc.), and be sure to cite all sources.



6 - Submission

Deliver a hardcopy of your report to class on Monday, October 31st. At this point, it will be passed on to the TCP, where it will be reviewed by two fellows who will provide comments on your technical writing. Your draft will be returned to you with comments', and you will have until TBD to submit a final version (along with the draft) for final evaluation.