Tensile Test Lab Exercise

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					                                    Tensile Test Lab Exercise

Introduction
        In a standard tensile test, a uniaxial, increasing monotonic load is applied to a
specimen until failure. Prior to specimen testing, the cross-sectional dimensions and the
gage length of the specimen are measured. During the test, two transducers, a load cell and
an extensometer, measure the load applied to the specimen and the elongation of the gauge
length, respectively. A load-elongation plot is generated as the test progresses from these
transducer signals; in some cases, the data may be stored in a computer for subsequent
analysis. Some tensile tests are performed for strength only, in which case only the
maximum load prior to failure is required. Other tensile tests are performed to obtain
strength, Young's modulus (the slope of the linear portion of the stress-strain curve), and
percent elongation at failure.
        Stress-strain data can be generated from the load-elongation data using the following
equations:


                          
                               P
                                                            
                                                                 
                                                                    
                                                                      
                                                                      L f  Li   
                               Ai                                Li      Li

Where  is the tensile stress, P is the applied load, Ai is the initial cross-sectional area,  is
the strain,  is the change in length, and Li and Lf are the initial and final specimen gage
lengths.

Preparatory Exercise
       Use reference material (like the Mechanical Engineering Handbook) to obtain the
following information:
       1. The threads per inch of a 10/24 machine screw.
       2. The major diameter of a 10/24 machine screw.
       3. The minor diameter of a 10/24 machine screw.
       4. The tensile strength of SAE grade 1 bolts (or machine screws).
       5. The tensile strength of SAE grade 8 bolts (or machine screws).
Give the title of the book or the URL of the web page that you used to obtain the
information.

Objectives
       The objectives of this experiment are:
1. To observe and measure the axial loading response of various materials.
2. Determine the desired material properties for the materials.
3. Develop a general understanding of the behavior of the materials.

Laboratory Equipment
1. UI tensile testing machine.
2. Hanson hanging scales/hanging apparatus
Procedure
        Conduct various axial tension tests, one each for a 10/24 pan head machine screw, a
10/24 socket head cap screw, and for a rubberband. This means you will need to observe
the applied load for all cases and the change in length of the gage section (elongation). Your
Lab Assistant will instruct you in the proper experimental procedure for these experiments.
Before taking data you should inspect and understand how the apparatus you are going to
use works. A brief explanation of each is expected in your notebook.

Questions
1. After you plot the stress-strain curve for rubber, can you calculate Young's modulus for
   this material, if not why not?
2. Can you determine when the screws that you tested began to yield? Estimate the
   yield strength if possible.
3. Obtain the maximum strength of the screws and compare it to the published values
   (from the preparatory exercise).



                                        Data Table

                          Load Versus Change in Length Data (Rubberband)
        Linitial =            in      (width =              in, thickness =              in )
        P            Linstantaneous                      P          Linstantaneous         
      (lb.)              (in)           (in)             (lb.)            (in)             (in)