Salt Lake Community CollegeDepartment of Engineering

CEEN/MEEN 2145 - Strength of Materials Laboratory(Instructor: Joven V. Calara)

Lab 1: Tensile Testing

Submitted byAbigail Ryder

Date Submitted: June 21, 2012

Term: Summer 2012

Table of Contents

Page

Abstract 3

Introduction and Objectives 4

Materials and Equipment 5

Experimental Procedure 6

Results and Discussions 7

Conclusions 8

Appendix

Figure 1 - Stress-strain curve copper 9

Figure 2 - Stress-strain curve steel 9

Figure 3 - Stress-strain curve aluminum 10

Figure 4 - Stress-strain curve polyethylene 10

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LAB 1 – TENSILE TESTING

ABSTRACT

This lab was conducted to gather data regarding the stress and strain of four different types of materials when applied with a tensile load using an INSTRON machine. The four different types of materials used were: copper, steel, aluminum, and polyethylene. The results indicated that, of these materials, polyethylene has the lowest elastic modulus, and steel the highest. The polyethylene experienced the greatest amount of elongation at over 230% of the original length. Aluminum also withstands the highest measurement of ultimate stress (e.g., maximum stress) of all the materials in this lab, as well as the highest measurement of both engineering and true stress at the break. One approximation that can be made from the data is that a material with a very low elastic modulus may be expected to experience more elongation than other materials with a higher elastic modulus.

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LAB 1 – TENSILE TESTING

Introduction and Objective

The objective of running these tests is to physically demonstrate the parameters of stress and strain in different materials, and how stress and strain change in relationship to each other on a stress-strain graph.

A secondary purpose of the tests is to introduce students to the materials laboratory and the Instron Machine. This machine will be used to apply the load on four different types of materials until they fracture. The data will then be used to infer specific properties of the materials (such as the modulus of elasticity, or Young's Modulus) and provide stress-strain graphs for those four materials.

There are several important regions of a stress-strain graph. Firstly, there is the elastic region, which is the initial, nearly linear region of the graph. The slope of a line (the chord modulus) through two points on this part of the graph is equal to the modulus of elasticity, or Young's Modulus, of the material. An important point on the graph is where the graph is estimated to become non-linear (out of the elastic region) the proportional limit which is also the elastic limit for many materials-- the point past which they will undergo permanent deformation and will no longer "elastically" return to their original shape. Moving to the right, the stress-strain graph will "round a corner" to level off, where the material experiences elongation with little increase in load. This is the "plastic region" and the point between the elastic region and this region is called the "yield point," as this is where the material "yields" and abandons its original shape. The maximum or ultimate load will be found at the highest point of the graph.

(Source graph: http://www.ce.memphis.edu/1101/notes/concrete/section_1_strength%20of%20materials.html)

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Materials and Equipment

The choice in tensile samples reflects their importance to engineering as well as dramatic differences in their respective properties. All of the materials chosen are commonly used in engineering but should reveal extreme differences through testing.

The tensile samples consist of the following materials and dimensions, gage length being marked along the length of the narrow bar of each sample by scratching out black ink which was imposed on the surface, from which to measure elongation:

Sample Material CopperCopper-110

Steel C1010

Aluminum 2024-T351

Polyethylene

Gage Length(in) 3 3 3 3

Width (in.) 0.530 0.531 0.533 0.503

Thickness (in.) 0.124 0.123 0.124 0.127

Uses of material Wiring, sheets, foils

Hardware, sheet stock

Aircraft wings, fuselage, bolts

Milk bottles, plasticware

Approximate shape of samples:

The machine which we will be using to load the materials is the INSTRON Machine (Rev03) pictured here (the software used by the INSTRON is BlueHill software.)

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Experimental Procedure

The basic procedure for this test will be to take samples of essentially identical dimensions-- in this case, flat rectangular bars with narrow midsection-- of four different materials and apply an axial load to each sample, slowly and linearly, until the bar fractures. During the trial it is possible to observe the amount of load via elongation on the live graph in the computer display (during which, elongation and the process of necking and, finally, fracture, may be noticed visually.) The computer will also record the relevant data such as load and elongation which will provide important graphs of the data, up to the fracture event which finishes the test of each sample. The computer will then calculate specific characteristics of the material and provide the final force-elongation plot. The load will be applied at the same rate for all samples except for the polyethylene sample, which will have a faster rate of application due to its ductility.

It is important that careful measurements be taken of the tensile samples before and after running the test so as to accurately calculate engineering stress, as well as to confirm their similarity and cross-sectional areas. These measurements are also important for gathering accurate data regarding the elongation of each sample and true stress, which is different from engineering stress in that it uses the actual, final cross sectional area of the sample for calculation, rather than the initial cross-sectional area (engineering stress.) Graphs will finally be made through calculation from force and elongation into a stress-strain plot for each one of the four materials.

The test specimens have the general shape of a narrow bar with "tabs" at the ends. These "tabs" encourage the break in the material to occur on the narrow part of the bar and not where the machine is "gripping" the sample (which would cause some weakening, perhaps, in that section of the sample.) Before testing the samples it is first appropriate to mark down the "gage length" for each sample. This is a length chosen that is planned to include the fracture and in such a way that it contains all of the extension of the sample. These are then recorded, and re-measured after the test. For these samples, the central three inches were chosen as the initial gage length. The specimens are then mounted in the machine in the slots, carefully pressed against the back wall of the mount while first the bottom grip and then the upper grip are tightened. Then the information about the sample is entered into the Bluehill software and an appropriate load application program is selected before running the test. The length of each test will vary, as each material will break at a different loading point.

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Results and Discussion

Material Alloy des.

Elastic modulus(x10^6)psi

Elastic mod (lit)(x10^6)psi

% Elongation

Max stress(psi)

Engineering stress at break

True stress at break

Copper 110 2.358 17 116.7% 41990 31762 49568

Steel C1010 3.149 29 127.5% 51676 6230 11302

Aluminum 2024-T351

2.358 10 111.3% 69914 63044 70848

Polyethylene

n/a 0.115 0.03 233.3% 4855 1192 19844

(Literary values are from http://en.wikipedia.org/wiki/Young%27s_modulus)

Graphs of each material can be viewed in the Appendix: Copper: Fig 1Steel: Fig 2Aluminum: Fig. 3Polyethylene: Fig. 4

Some notes on each of the samples:

Copper: Sample experienced necking and a consistent taper break.

Steel: Sample indicated some "scalloping" at edges with deformations at the break.

Aluminum: Sample broke near curvature of tab and flared slightly at break.

Polyethylene: Sample had an extraordinarily long necking area (approx. 3.5 inches.)

The comparison of select properties between the four specimens is worth noting. The elastic modulus is highest in the steel sample. This concludes that steel C1010 had less elasticity and a higher melting point than the other samples. Aluminum 2024-T351 had the highest maximum stress, engineering stress, and true stress at break. This is a result of the aluminum sample having the highest tensile strength of the four samples. The polyethylene sample had the highest percent of elongation, which concludes that it had the highest ductility among the samples. This result, of course, should not be surprising. The copper sample displayed a relatively smooth taper at the break point. This could be a result of several factors including ductility. The result is a phenomenon called necking. This is when a large local decrease in the cross-sectional area occurs within a region.

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From left to right: Polyethylene, Aluminum, Steel, Copper, and an untested sample demonstrating the original size and shape.

Conclusions

The Young's modulus found for the materials in the lab does not match particularly well with values from "literature." However that could be explained due to differences in laboratory conditions. Also, the materials varied widely (as expected) in their overall graph shapes, and the modulus of elasticity found for polyethylene was very different from the other moduli (it was much less, which reflects its plastic nature and its ability to stretch a great deal before breaking.)

Also of interest was the engineering stress as well as the true stress at the break. From the chart it can be seen that these in fact differ widely, with the maximum true stress seen in the aluminum sample. It can also be seen from the chart that the elastic modulus (very approximately) inversely affects the amount of stretching undergone by a sample in a tensile load. Roughly speaking, based on these results, the lower the elastic modulus of a sample material, the more elongation it could be likely to undergo when placed in a tensile load.

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Appendix

Fig. 1

9

0 0.05 0.1 0.15 0.2 0.25 0.3 0.350

10000

20000

30000

40000

50000

60000

Steel

Stress-Strain diagram

Strain

Str

ess

(p

si)

0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18 0.20

5000

10000

15000

20000

25000

30000

35000

40000

45000

Copper

Stress-Strain diagram

Strain

Str

ess

(p

si)

Fig. 3

Fig. 4

10

0.0000 0.0500 0.1000 0.1500 0.20000.0000

10000.0000

20000.0000

30000.0000

40000.0000

50000.0000

60000.0000

70000.0000

80000.0000

Aluminum Stress-Strain diagram

Strain

Str

ess

, psi

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 20

1000

2000

3000

4000

5000

6000

Polyethylene

Stress-strain graph

Strain

Str

ess