MECH180 Strengths of Materials
Lab #3
Tensile Test (Multiple Materials)
OBJECTIVE:
A limited investigation of the behavior of some engineering materials when loaded in tension
to fracture to determine the ultimate tensile strength, percent elongation and percent reduction in
area.
DISCUSSION:
Before a member can be properly designed, the conditions of use must be known. Only after
this knowledge is gained can the engineer or designer select a suitable material for his purpose
and arrive at the proper proportions of the member. Certainly it would be poor design to
construct the fuselage of an airplane out of cast iron when lighter metal alloys having a higher
ratio of strength to weight are available, such as aluminum alloys. Similarly, it would be poor
economy to construct a bridge pier out of metal when cheaper concrete masonry would better
serve the purpose. The strength and ductility of a material are important properties which
engineers should be familiar with. An easily obtainable and very important strength value is the
ultimate unit stress, frequently called the ultimate strength. It is defined as being the maximum
unit stress, based on the original area, which is developed in a material before rupture. Another
important property is ductility, defined as that property of a material that allows a load to
develop large plastic deformations before actual rupture. This property, for example, controls the
amount of cold forming to which a material may be subjected. The forming of automobile bodies,
the bending of concrete re-enforcing rebar, and the manufacture of fencing and other wire
products all require ductile materials. Two commonly used quantitative indices of ductility are
the ultimate elongation (expressed as a percentage elongation of the gage length at rupture) and
the reduction of cross-sectional area at the section where rupture occurs (expressed as a
percentage of the original area). If one cannot establish an exact correlation between percent
elongation and percent reduction of area, the material has the ability to stand abuse in fabrication
and service. It is also true that in many instances ductility is necessary to relieve localized and
initial stresses and to act as insurance against destruction and complete collapse due to excessive
loads.
PROCEDURE:
1) Predict maximum load for each sample.
2) Set jaws in universal testing machine. Use proper jaws and shims
3) Set-up universal testing machine as per previous handout and demonstration in class.
4) Use predicted loads to determine the scale to use for the universal testing machine.
5) Test samples to failure.
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MECH180 Strengths of Materials
Lab #3
DATA:
Material
Original
Diameter
Final Dia.
At break
Final Dia.
Original
Away from Gage
break*
length
Final
Gage
length
Maximum
Load
Predicted
Maximum
load
Hot rolled
steel
Cold rolled
steel
Hard
Aluminum
Soft
Aluminum
* Measure the diameter one inch away from break
RESULTS:
Material
Ultimate
Tensile
Strength
Percent
difference
UTS
Percent
Elongation
Percent
Reduction in area
at break
Percent
Reduction in area
away from break
Hot rolled
steel
Cold rolled
steel
Hard
Aluminum
Soft
Aluminum
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MECH180 Strengths of Materials
Lab #3
Concluding Questions:
1. List the materials tested by strength from strongest to weakest.
2. Did any material(s) vary more than 10% from its book value? If so, explain why.
3. List the materials tested by ductility from the least ductile to the most.
a) based on % elongation.
b) based on % reduction in area at the break.
c) based on % reduction in area away from break.
4. Are the results the same for the 3 methods to compare the ductility of different materials? (See
question 3 above). Which material(s), if any, do not repeat in order of ductility by the different
methods. Explain why
5. Compare your results obtained in the table for % elongation and % reduction in area. Is the %
reduction in area at the break, or away from the break, more or comparable to % elongation?
6. Can the break point give an indication of material's ductility? Explain using your sketches for
the data.
7. If a material is stronger than another, could you also say the stronger material is less ductile?
Explain.
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