Tensile Properties of Steel

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Tensile Properties of Steel

Mechanics of Materials

Pete Zumpano

2/6/2012

MVCC

Mr. Restive


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Objective:

In order to understand and observe the effects of strain on a material, an experiment

was devised to test the strength of a 0.505 steel bar by putting the bar into tension. The goal

of the experiment was to measure the effects of tension being increased on a steel bar which

creates a strain on that tensile specimen. Fundamental stress formulas were used to model thebehavior of the specimen in tension.

Equipment:

The following was used in the tensile test experiment:

Tinius Olsen Tensile tester Dial caliper Ruler Extensometer/Strain gauge Anvil puncher Hammer

Tinius Olsen Tensile Tester, smallest deviation 50 lbs


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Force Reading Face of tensile tester

The main machine used to test the tensile strength of the steel specimen was a Tinius Olsen

tensile tester. This machine has threaded attachments for which the steel specimen can be

screwed into. The tensile test is hydraulically driven and the amount of force exerted can be

read on a large dial to the right of the machine.


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Extensometer, smallest deviation .0001

Another important piece of equipment is the extensometer, or strain gauge. The strain gauge

has two arms that are set to attach to the specimen two inches apart by using the anvil puncher

which in imprints two holes, two inches apart.

Specimen to be tested

Anvil puncher


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Dial Caliper

Procedure:

To begin the experiment, first a table was created to hold our data while the experiment

was being conducted. The data table had four columns consisting of Load (lbs), Stress (psi),

Deformation (in), and Strain (in/in), however, for conducting the experiment only the Load and

Deformation column were used. Following, the strain gauge was lined up with the holes from

the anvil puncher and then fastened to the steel specimen. Then, the specimen was attached to

the tensile tester and the tester was zeroed. After, a load of 500 lbs was placed on thespecimen to remove any looseness in the machine and the strain gauge was zeroed. At this

point in the experiment, all of the pre-setup is completed and the experiment can initiate.

Following the initial setup, the tensile tester was increased in 500 lbs increments and

the amount of deformation from the strain gauge was measured. This process was continued

until the yield point of the specimen was reached; this data point was noted. The process of

increasing the force on the specimen by 500 lbs was continued until the ultimate load point was

reached and the strain gauge was removed. Finally, the force on the specimen was increased

more until the breaking point of the steel specimen. The force at the break point was recorded

and the final diameter of the specimen was measured at its smallest point.

Results and Discussion

Once the data from the experiment was taken, the majority of the experiment can be

analyzed using one graph; the stress versus strain graph. This graph illustrates all of the

important points of the specimens mechanics including the proportional limit (elastic limit), the

yield strength, and ultimate strength.


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Beginning from when the specimen was loaded with 500 lbs, the stress vs. strain graph

appeared to be increasing linearly. This occurrence implies that in the beginning of the loading

stress is proportional to strain and is related by some constant. The linear relationship observed

is also apparent in the study ofsprings described by Hookes law where force is proportional to

the distance a spring is stretched. For stress versus strain, the relating constant is known as the

modulus of elasticity (E). In, terms of our experiment, the modulus of elasticity is the slope ofthe stress vs. strain graph which can be calculated by finding the change in stress over the

change in strain from point to point of our calculated values of stress and strain. Then, an

average can be taken of these values to find the modulus of elasticity of the specimen. It is

important to note that only the linear part of the stress versus stain graph can be used to find E.

The average modulus of elasticity was calculated to be 30088795.38 psi. The trend line on the

figure 1 represents the fitted modulus of elasticity, however, for the experiment the average

modulus of elasticity for calculations and comparisons. (Refer to figure-1)

Figure-1, from initial loading to near proportional limit

After the linear portion of the graph, the strain in the specimen begins to change

nonlinearly. The boundary between were the curve transition from linear to nonlinear is called

the proportional limit, or elastic limit. Up until this point, if strain on the specimen was reduced,

the specimen would shrink back to its original size following Hookes Law. The proportional

limit was determined to be at 79,000 psi. However, after the proportional limit, the yield

y = 30,723,036.28x + 2,496.00

R = 0.990.000

10000.000

20000.000

30000.000

40000.000

50000.000

60000.000

70000.000

80000.000

0 0.0005 0.001 0.0015 0.002 0.0025

(

psi)

(in/in)

Stress vs. Stain


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strength is soon to be reached which is the point where plastic deformation begins to occur and

subsequent strain will result in permanent deformation. Yield strength can be estimated using

the 0.2% offset method, (explained in further section) and in this experiment was approximated

to be 94,000 psi (Refer to figure-3)

Figure-2, zoomed in where graph becomes nonlinear

In order to find the yield strength, it is common to use a 0.2% offset method to find the

estimated yield strength. To perform this method, 0.2% is added to the strain from the initial

data point of the stress versus strain graph. Then, a parallel line to the linear portion of thestress versus strain graph is draw until it interests the original graph. The 0.2% offset line

intersects the original stress versus strain graph in the nonlinear part. The point at this

intersection is considered to be the yield strength. The stress at this point is referred to as the

yield strength, for after the yield strength, the material begins to be nonlinearly strain and any

addition stress will cause permanent deformation of the specimen.

0.000

20000.000

40000.000

60000.000

80000.000

100000.000

120000.000

0.001 0.011 0.021 0.031 0.041 0.051 0.061

(

psi)

(in/in)

Stress vs. Stain


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Figure-3, where the 0.2% offset line crosses the stress vs. strain curve is the yield

strength

After the yield stress was passed and when stress was further increased, the material

began to deform to the point where the middle portion of the specimen narrowed. At this time,

the material approached its ultimate strength, or the maximum amount of stress it can

withstand which was recorded at approximately 103594.60 psi. Following the ultimate

strength, the specimen narrowed even more at the middle until it broke in half at the breaking

force of 14,750 lbs at a stress of 73639 psi. Several quantitative tests were performed on the

specimen to view its reaction to the stresses applied to it including the percent area reduction,

percent elongation, and toughness. The percent area reduction was found to be 45.43 % and

the percent elongation was calculated to be 18.45%. These values show how much physicalchange the specimen can withstand before breakage. The toughness is measured by

determining the area under the curve of the stress versus strain graph. Toughness refers to how

much energy a material can absorb without breakage. An estimation of toughness can be

determined using . A more accurate representation of toughness is the areaunder the stress versus strain curve. The toughness of the specimen was calculated to be

18621.19 psi by find the area under the curve.

y = 3E+07x - 53014

R = 0.9976

0.000

10000.00020000.000

30000.000

40000.000

50000.000

60000.000

70000.000

80000.000

90000.000

100000.000

110000.000

120000.000

130000.000

140000.000

0 0.005 0.01 0.015 0.02 0.025 0.03 0.035 0.04

(

psi)

(in/in)

Stress vs. Stain 0.2% Offset


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Conclusion:

At the termination of the experiment, the specimen was concluded to be 1018 steelsince the specimens modulus of elasticity was calculated to be 30,088,795.38 psi which

compared well to a known 1018 steels modulus of elasticity of 29,000,000 psi. The

experimental result was just 3.4% off of the actual modulus of elasticity value. Also, both the

percent area reduction and percent elongation were similar to an actual 1018 steel (refer to

appendix). With the three aforementioned figures, there is a strong indication that the

specimen is 1018 steel.

References:

Don, Josen. "Steel-Tube_SAE 1018 Steel Properties." Steel-Tube_Seamless Steel

Tube_Precision Steel Tube_Cold-rolled Steel Tube_Clod-drawing Steel Tube. 23 May

2009. Web. 09 Feb. 2012. .

"Tensile Testing." Instron. Web. 2 Feb. 2012.

.

Appendix:

Definitions:

Stress - is the amount of force per perpendicular amount of area; measured in psi.

Strength - the amount of stress a material can withstand

Yield Strength -the point at which the specimen begins to be permanently deformed


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Ultimate Strength- the maximum amount of load able to be applied to the specimen

Proportional limit- the point of a stress vs. strain curve where the graph becomes nonlinear

Elastic limit- same as proportional limit

Strain- is the amount of deformation of material per original length of that material.

Sample Calculations:

Calculation of Stress for when Force equals 1000 lbs

Calculation of percent elongation:


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Calculation of reduction of area:

Calculation of modulus of elasticity for first two data points of stress and strain:


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Calculation of toughness:

()

Calculation of Toughness from the area under the curve:

Done with excel:

1) Find difference between each strain point

2) Multiply difference (x) by stress at that point

3) Sum all of the stress times x values to gather the total area under the curve

The total area under the curve was found to be 18621.19 psi for toughness.


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Tensile Properties of Steel