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ABSTRACT

The
tensile strength of an example of AISI 4340 steel was measured using an Intron
Model 4400 Series load frame. This test was carried out in order to determine
some basic properties of the 4340 steel. It was determined that the steel had a
Young’s modulus of 245 GPa, an ultimate strength of 723 MPa, and a fracture
strength of 562 MPa. These values yielded a relatively low percent error
compared to known values of the 4340 steel. This proves the validity of the
test instruments as well as the data evaluation.

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1.

Introduction

1.1
Tension Testing

Tensions
tests are carried out on samples of materials to determine basic properties
that come about by applying tension on a material. This is important to
understand because it determines the strength of a material as well as how it
uniquely performs under stress.  Some of
the properties that can be determined by these tests are the Young’s modulus,
fracture strength, and the ultimate strength. These three properties are
crucial to understanding a material and knowing its appropriate use.

In
order to carry out a tension test one needs to use an instrument capable of
applying a specifically measured tension force to two opposite ends of a
material. Additionally, instruments need to be available which accurately
measure the change in length of the material. Knowing the force applied and the
change in length at a given time provided all the data necessary in order to
measure key properties of a material.

1.2
Young’s Modulus

The Young’s modulus is a
linear relationship between stress and strain that can be seen at the beginning
of a tensile load of a ductile material. It defines a region of loading for a
material where as long as the load does not surpass the yield strength, the
material will return to the shape it had prior to the loading with no permanent
deformation. Once the load surpasses the yield strength the material will have
permanently deformed, and the linear relationship will no longer exist. The
stress involved in the Young’s modulus is defined by the following formula which
is a relationship between force and area:

Furthermore,
the Young’s modulus is a simple was of defining the stiffness of a material, or
a materials resistance to deformation as a material with a higher Young’s
modulus will deform less with a strain than would a material with a lower
Young’s modulus. The strain used in determining the Young’s modulus is the
relation between deformation and the original length of the test object defined
by the following relationship:

Using the definitions of
stress and strain the Young’s modulus can be understood with the following
formula:

 

Where
E is the Young’s modulus measured in GPa, ? is the stress measured in MPA, and ? is the strain, measured in mm/mm.  

 

 

1.2
Ultimate and Fracture Strength

Once a material has
surpassed its yield strength it has moved past the region defined by the
Young’s modulus. It is in this region that a material will deform until
eventually it fractures. Instead of a linear relationship, this region is
defined usually by a parabola, peaking at the tensile or ultimate strength. The
ultimate strength is the largest load a material can handle before the strain
begins increasing with a. decreasing stress. This ultimately leads to a
fracture of the material.

The region following the
linear Young’s modulus can appear to have many shapes. A brittle material will
fail quickly and tends to have the fracture strength and the ultimate strength
very close to each other. On the other hand, a ductile material will still have
a relatively large region between the ultimate strength and the fracture
strength.

2.

Experimental Procedures

2.1
Materials and Equipment

A Instron 4400 series
load frame was used on a sample of AISI 4340 steel to perform the tensile
testing in this laboratory. AISI 4340 steel is a medium carbon known for
strength and toughness. It is typically used for structural manufactured parts
such as gears and sprockets 2. An extensometer will also be used
in order to accurately measure the change in distance of the material
deformation.

2.2
Experimental Preparations

For this experiment it
important to take some precautions as with any experiment that involves
failures under loads. Safety glasses and clothing were worn at all times in the
case of extreme failure of the material.

2.2
Testing Procedure

First, the initial
measurements of the material were recorded. This meant the initial dimeter of
the rod as well as the initial length of the material. Then, the rod was loaded
into the load frame and the extensometer was attached. Once everything is
loaded as it would be for the test the instruments were zeroed and the testing
began. The load frame applied an increasing load to the rod until failure of
the rod was achieved. Once the test material fails it is unloaded from the load
frame and premeasured for the fractured diameter.

3.

Results

3.1
Tension Test

Using equation (1) to
measure the stress on the rod combined with the strain data given from the
extensometer the graph in Figure 2 was constructed to demonstrate the stress
versus strain relationship of the material. The material failed under a fracture
strength of 562 MPA while achieving its largest load at 723 MPA. The linear
relationship of the Young’s modulus was clearly defined in the beginning of the
graph, leading to the calculation of the Young’s modulus to be 245 GPa. The
stress versus strain relationship followed a predictable and well-defined
pattern. The material had a linear stress versus strain relationship until the
yield load was reached. After this point the graph followed a parabolic curve
where it achieved the ultimate strength at the maximum of the curve. At the end
of the curve where the parabola cuts off is where the material completely
failed, giving us the fracture strength of the material. Upon comparison with
known values for 4340 steel the results appeared relatively accurate. While the
Young’s modulus was off by approximately 14% the ultimate strength was off the
known value by 3% 1.

4.

Discussion

Comparatively, the known
values for 4340 steel and the results of the laboratory were very close. 14%
for the Young’s modulus is high but not high enough to question the validity of
the results. This difference could be due to the fact that the best fit line
had outliers in the data and the average skewed the result slightly. Additionally,
the ultimate strength only being off by 3% is a very accurate result which
greatly confirms the validity of the laboratory. Furthermore, the graph in
Figure 2 that the data produced was a highly accurate and very predictable
curve. The behavior of the material based on the data was a perfect match to
the ductile material we knew the material to be.

 

5.

Conclusion

1. The yield strength of this sample of 4340 steel was
562 MPa, the Young’s modulus was 245 GPa, and the ultimate strength of the
material was 723 MPa.

2. AISI 4340 steel is a ductile metal.

6.

Acknowledgements

Thank you to the
Mechanical Testing Instructional laboratory (MTIL) at the University of
Illinois Urbana-Champaign for providing the materials, equipment, and expertise
necessary to compile the data necessary for this report.

7.

References

1. AZoM. (2013, July 11).

AISI 4340 Alloy Steel (UNS G43400). Retrieved January 30, 2018, from https://www.asom.com/article.aspx?ArticleID=6772

2. 4340 (E4340) Alloy Steel.

(n.d.). Retrieved January 30, 2018, from
http://www.benedict-miller.com/content.cfm/AQ-Steel/4340-Steel/category_id/102/page_id/112

 

 

3. J.S. Popovics, L.J. Struble, P. Mondal and D.A. Lange, CEE300/TAM324 Behavior of Materials
University of Illinois at Urbana-Champaign : College of Engineering, Spring
2018.

 

Appendices

A.1
Tables and Figures

Table 1- Tensile mechanical
properties

Quantity

Symbol

Units

Lab

Expected

Percent Difference

Young’s Modulus

E

GPa

245

210

14.3%

Ultimate Strength

?u

MPa

723

745

3.04%

Fracture Strength

?f

MPa

562

 

Figure 1: Nominal tension specimen
dimensions (mm).

Figure 2. Graph of Engineering Stress
versus strain for 4340 Steel.

 

E.3
Sample Calculations

E.3.1
Tensile Strength

One of the most important measurements to determine is
the stress on the material. Engineering stress is found by the simple formula:

Where ?
is the stress, F is the load force and A is the cross-sectional area of the
test material. Using this relation, we can measure the stress of the material:

E.3.2
Young’s Modulus

A key measurement to be made is the Young’s modulus,
the measurement of the stiffness of a material. The young’s modulus defines a
materials ability to return to its original shape and strength after a certain
load has been applied. The Young’s modulus can be found with the following
formula:

with the E representing the Young’s modulus, the ? representing the stress, and the ? representing the strain. Using the data given form
the laboratory we can calculate the Young’s modulus.

 

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