EXPERIMENT 4

**TESTING OF MATERIALS IN TENSION**

__Object:__ The object of this experiment is to measure
the tensile properties of two polymeric materials, steel and aluminum at a
constant strain rate on the Tension testing machine.

Background:

For structural applications of
materials such as bridges, pressure vessels, ships, and automobiles, the
tensile properties of the metal material set the criteria for a safe design.

Polymeric materials are being used more
and more in structural applications, particularly in automobiles and pressure
vessels. New applications emerge as
designers become aware of the differences in the properties of metals and
polymers and take full advantage of them.
The analyses of structures using metals or plastics require that the
data be available.

__Stress-Strain__

The tensile properties of a material
are obtained by pulling a specimen of known geometry apart at a fixed rate of
straining until it breaks or stretches to the machines limit. It is useful to define the load per unit area
(stress) as a parameter rather than load to avoid the confusion that would
arise from the fact that the load and the change in length are dependent on the
cross-sectional area and original length of the specimen.

The stress, however, changes during the
test for two reasons: the load increases
and the cross-sectional area decreases as the specimen gets longer.

Therefore, the
stress can be calculated by two formulae which are distinguished as engineering
stress and true stress, respectively.

(1) s= P/A_{o}=
Engineering Stress (lbs/in^{2} or psi)

P = load (lbs)

A_{o}= original cross-sectional area
(in^{2})

(2) s_{T}= P/A_{i} = True Stress

A_{i}
= instantaneous cross-sectional area (in^{2})

Likewise, the elongation is normalized
per unit length of specimen and is called strain. The strain may be based on the original
length or the instantaneous length such that

(3) e=(l_{f}
- l_{o})/
l_{o} = Dl / l_{o} = Engineering
Strain

where

l_{f}= final
gage length (in)

l_{o}=
original gage length (in)

(4)
e_{T}= ln ( l_{i}_{
}/
l_{o} ) = ln (1 +e) = True
Strain

where

l_{i} = instantaneous gage
length (in)

ln = natural logarithm

For a small elongation the engineering
strain is very close to the true strain when l=1.2 l_{o}, then e= 0.2 and e_{T}= ln 1.2 = 0.182. The engineering stress is related to the
true stress by

(5)
s_{T}= s(1 + e)

Hence the true
stress would be 20% higher in the case above where the specimen is 20% longer
than the original length.

As the relative elongation increases,
the true strain will become significantly less than the engineering strain
while the true stress becomes much greater than the engineering stress. When l= 4.0 l_{o
}then
e = 3.0 but the true strain =ln 4.0 = 1.39. Thus,
the true strain is less than 1/2 of the engineering strain. The true stress (s_{T}) = s(1+ 3.0) = 4s, or the true stress is 4 times the engineering stress.

__Tensile Test
Nomenclature__

The tensile test data are characterized
by terminology shown in Figure 4-1.

Figure 4-1: Engineering Stress-Strain Curve

The material test curves have a region
where the deformation caused by the stress is elastic, or not permanent. This means when the stress is removed the
specimen returns to its original length. At stresses greater than a certain
value a portion of the strain becomes permanent, or plastic. The stress required to
cause a 0.2% plastic strain, or off-set, is called the yield stress.

Ductility is measured as % elongation,
representing the ability to deform in the plastic range

(6)
_{}

__Equipment__

Tensile Testing Machines: table-supported
(1000 lb. capacity) and floor-mounted (10,000 lb. capacity)

Dial Calipers

Ruler

__Procedure__

Operating instructions for the various Tensile
Test Machines are contained in Appendices G and H which should be read
thoroughly before operating the machines.

The Instron
machine (1,000lb) contains a chart which moves as a function of time The load is
measured by a load cell and is read across the chart depending on the scale,
0-100 lb, 0-500 lb., etc. The speed of
the chart can be set to run at a fixed speed (inches/minute) or proportional
time (ratio to the extension rate 1:1, 2:1, etc.). The crosshead of the Instron
is set to move at fixed rates either up or down between two limit
switches. The cross-head rate of motion
is controlled by the machines microprocessor (See Appendix G).

The United machine (20,000lb) is
controlled and records the data by means of its computer controller.

You will be provided with one specimen
each of high density and low density polyethylene, steel, and aluminum. The metal samples are tested on the 10,000
lb. machine, while the polyethylene samples are tested on the 1,000 lb.
machine.

Use a strain
rate of 2 inches per minute for the plastomers, and
0.1 inches per minute for the metals.
The chart length represents the sum of the plastic and elastic
deformations.

1. Measure the thickness of the
specimens. Make identifying
marks (1, 2, 3, etc.) on each specimen.

2. The original crosshead distance will be
2.00 to 3.00 inches (gage length). Check
this with a metal ruler, measuring the distance from the top of the lower
clamps to the bottom of the upper ones for the polymers and measuring the
length of the reduced area for the metal samples.

3. Follow the General Operating Instructions
in Appendix G or H for Zeroing, Balancing, and Calibration as needed. (Ask your
instructor if the machined needs to be zeroed balanced or calibrated before
making any changes, the machine may already
be set up for your use).

4. Install the specimen as directed in Appendix
G or H. For the manual lock clamps, tighten the clamps very securely.

5. The order and settings for this experiment
are:

__Sample__ __Range Setting__

A. Low
Density PE (200 lbs. full scale )

B. High
Density PE (500 lbs. full scale )

C.
Steel (5,000 lbs. full scale)

D. Aluminum (5,000 lbs. full scale)

6. Be sure to mark
gage length with an ink pen after installing the specimen in the clamps. Mark as close to the clamp edge as
possible.

8. Tensile Test:

1,000 lb Instron Machine

a. Turn on CHART .

b. Press UP
button to start movement (1000 lb. machine)

c. **One
person should be in charge of checking the graph pen to stop the test immediately if the machine maximum
load (1,000lbs)
is reached.**

d. All members should pay close attention
to the changes occurring in the

specimen as it elongates.

e.
Record the chart and head speeds
and load setting.

20,000 lb
United Machine

a.
After loading the specimen in the
machine refer to appendix H for the test procedure.

b.
Record the test speeds and load
settings in your notebook.

__DATA NEEDED__:

Original
specimen: length, width, thickness, gage
length

At
fracture: length (from graph)

After
fracture: gage length (from graph)

Head and Chart
Speeds

Full Scale
Load (from RANGE setting)

Measure the
overall length and gage length to __+__.1 inch.

Measure the
width to __+__.02 inch (Use a dial caliper).

Measure the
thickness to __+__.002 inches (use a dial caliper).

Measure the
original thickness and width in several places and average.

Measure the
final width and thickness in several locations to find the minimum

cross-sectional area.

__Glossary of
Terms__

Understanding the
following terms will aid in understanding this experiment:

**Ductility**.
The ability of a material to be permanently deformed
without breaking when a force is applied.

**Elastic deformation**.
Deformation of the material that is recovered when the
applied load is removed. This
temporary deformation is associated with the stretching of atomic bonds.

**% Elongation**.
The total percent increase in the length of a specimen during a tensile
test.

**Engineering strain**.
Increase in sample length at a given load divided by the original
(stress-free) length.

**Engineering stress**.
The applied load, or force, divided by the original cross-sectional area
of the material.

**Engineering stress-strain curve**. A plot of the Engineering
stress versus the Engineering strain.

**Hooke's**** law**. The linear relationship
between stress and strain in the elastic portion of the stress-strain curve.

**Modulus of elasticity**. Young's modulus, or
the slope of the stress-strain curve in the elastic region.

**Necking**. Local
deformation of a tensile specimen.
Necking begins at the tensile point.

**Offset yield strength**.
A yield strength obtained graphically that
describes the stress that gives no more than a specified amount of plastic
deformation.

**Plastic deformation**.
Permanent deformation of the material when a load is applied, then
removed.

**% Reduction in area**.
The total percent decrease in the cross-sectional area of a specimen
during the tensile test.

**Tensile strength**.
The maximum engineering stress experienced by a material during a
tensile test (ultimate tensile strength).

**Tensile test**.
Measures the response of a material to a slowly
applied uniaxial force. The yield strength, tensile strength, modulus
of elasticity, and ductility are obtained.

**True strain**.
The actual strain produced when a load is applied to a material.

**True stress**.
The load divided by the actual area at that load in a tensile test.

**Yield strength**.
The stress applied to a material that just causes permanent plastic
deformation.

__Write Up__

Prepare a memo report on the results of
the tests. The report should contain
engineering stress-engineering strain diagrams, and true stress-true strain
diagrams from tensile tests for each material.
**Three of these should be graphed
using a computer.** A spreadsheet
program, such as Excel, should be used to manipulate the data in order to
produce the stress-strain diagrams. Instructions for graphing with Excel can be
found in the Appendices of this manual. PC clones are available in the
materials lab as well as several work rooms around campus. Label (by hand)
diagrams to show Young's Modulus, Yield Stress, Ultimate Tensile Strength, and
Total Strain (Include values). Include these values in your report and discuss
them with respect to published values. **Plot
the fourth diagram by hand on a high quality (like K & E) graph paper**
with about 20 divisions to the inch. Label this diagram in the same manner as
instructed above. Discuss your four plots. Discuss errors in this experiment
and their sources.

References

McClinock,
Mechanical Behavior of Materials

Dieter, Mechanical Metallurgy

Nielsen, Mechanical Properties of
Polymers

Schmitz, Testing of Polymers

Van Vlack,
Elements of Materials Science and Engineering, Chapter 1 and 6