Mechanical Behavior Laboratory
 Laboratory - 2 units, one discussion session and one 3-hour laboratory
session per week
Prerequisite: EMS-174 (concurrent enrollment recommended)
Experimental investigations of the mechanical behavior of engineering
materials. Laboratory exercises emphasize the fundamental
relationships between microstructural and mechanical properties.
Contents
- Introduction
- Laboratory Safety
- Laboratory Reports
- Plastic Deformation and the Onset of
Plastic Instability
- Stress Relaxation in Polymers
- The Hall-Petch Relationship
- Superplasticity
- The Ductile-to-Brittle Transition
- Appendices
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1. Introduction
This course consists of four experiments, each dealing with a
different aspect of the mechanical properties of materials. The documents listed below
give specific details, instructor contact information, and other course
requirements.
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2. Laboratory Safety
Laboratory Safety is an essential and integral part of this course.
During your first visit to the laboratory the basic rules and procedures
will be presented and every laboratory session will begin with a brief
review of the safety issues related to the equipment and procedures used
in the experiment being done that day. Please visit the
laboratory safety page of this web site
for additional information on this important subject and to view documents
that cover specific safety policies and procedures.
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3. Laboratory Reports
The formal laboratory report is a well known part of this course and in
many respects is just as important as doing the experiments. In
fact, the whole philosophy behind how we design and teach each experiment
is to help the student write a clear, concise, and informative report, the
type of report they will be writing in their career. Many students
find this effort very difficult, citing difficulty understanding the
reports format and other matters. To help with this we have written
a number of documents offering guidelines, checklists, and suggestions
that will help you write your reports. Please visit the
laboratory reports page to see these
documents.
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4. Plastic Deformation and the Onset of Plastic Instability
In this experiment the
students carefully tensile test a specimen and do a complete analysis of the results using
a spreadsheet. All possible properties are measured and several different analyses of the
stress-strain behavior are done to attempt to understand the reasons that deformation
became localized. This localized deformation, which produces a necked region in the
specimen, causes local stresses to increase, concentrates further deformation in this
region, and eventually leads to the failure of the specimen. The onset of this necking
also marks the maximum load carrying ability of the specimen.
 | Procedure - Notes and the complete procedure for this experiment. |
| Spreadsheet Template - This spreadsheet, the same one used in ENG-45,
should help you get started analyzing the force-elongation data. |
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A view of the tensile testers used in this experiment.
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5. Stress Relaxation in Polymers
Like metals polymers deform elastically and plastically
but unlike metals they are highly anelastic. Anelasticity refers to a behavior where the
recoverable stress is dependent on the deformation rate. For instance, the total stress
will increase rapidly if a specimen is pulled quickly, not as fast if pulled slowly, and
will even decrease if in the middle of the test we stop pulling on the specimen. In this
experiment the students investigate this behavior by measuring the rate and magnitude of
the relaxation of the stress. The stress relaxation behavior of an aluminum alloy is also
evaluated to show that while metals do exhibit anelastic properties it is not as
pronounced as in polymers and it happens much more quickly.
| Procedure - Notes and the complete procedure for this experiment. |
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6. The Hall-Petch Relationship
Hall and Petch showed us how the grain size of a material
influences the yield strength. In this experiment the students systematically investigate
this behavior by annealing brass to obtain desired grain sizes and then tensile test then
to measure the yield strength. By borrowing the results from earlier experiments the
students arrive at a fairly complete mathematical model for the stress-strain behavior of
annealed brass.
| Procedure - Notes and the complete procedure for this experiment. |
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The two micrographs above show the microstructure on the polished and
etched surface of a 70/30 brass tensile sample at the beginning and near
the end of a tensile test. Note the surface roughness and the
different directions of the deformation bands in each grain at the end of
the test.
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7. Superplasticity
Superplasticity is the extraordinary ability of certain materials to
be pulled to 100's, even 1000's, of percent elongation without fracturing. It requires a
proper balance of microstructure, temperature and deformation rate. In this experiment
students perform tensile tests on a commercial superplastic alloy. These tests allow the
students to observe this unusual behavior themselves. They also analyze the results to
determine the mechanism for this behavior.
An
example of the extraordinary tensile elongations possible with
superplastic deformation.
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8. The Ductile-to-Brittle Transition
At room temperature ordinary steel is able to withstand impacts
without fracturing. At around 0°C and below, however, it can be easily broken. This
transition from a tough to a brittle material surprised many ship builders during World
War II and appears to have been a major factor in the sinking of the Titanic. In this
experiment students evaluate the impact resistance of several plain carbon and alloy
steels, a stainless steel, an aluminum alloy and a brass over temperatures ranging from
200°C down to -174°C. They find that only the plain carbon and alloy steels undergo this
ductile-to-brittle transition.
| Procedure - Notes and the complete procedure for this experiment. |
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The fracture surface of this Charpy impact specimen show that the fracture
mode was a mixture of ductile (dull gray) and brittle (shiny, salt and
peppery appearance).
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9. Appendices
The appendices page at this
web site offers a number of documents that you will find useful during and after the laboratory
session. These include tables of materials properties, operating
procedures for the equipment, and documents that will help you get the
most out out your spreadsheet-based assignments and writing the laboratory
reports. The documents you should look are:
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