Mechanical Behavior Laboratory

Experimental investigations of the mechanical behavior of engineering materials emphasizing the fundamental relationships between microstructure and mechanical properties.


1. 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.
Instron 4204
A view of the tensile testers used in this experiment.
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2. 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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3. 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.

The two micrographs to the right show the microstructure at 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.

  • Procedure - Notes and the complete procedure for this experiment.
Brass, polished
Brass, polished and etched
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4. 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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5. 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.

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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6. Griffith Flaws in Brittle Materials
This experiment is based on Ernsberger's work in the 1960's where he demonstrates the existence of pre-existing Griffith flaws in glass, flaws which are too small to see using high-resolution microscopes. The procedure employed an ion-exchange process using KNO3 and NaNO3 to introduce stresses in the surface of glass microscope slides. (Caution! Hydrofluoric acid is used in this experiment!)

  • Procedure - Notes and the complete procedure for this experiment.

Superficial fractures caused by stresses resulting from ion substitution. The glass on the left is untreated.
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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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