OREGON STATE UNIVERSITY
ME 453 – STRUCTURE AND MECHANICS LABORATORY
Lab 1 Experiment – Winter 2012
Status: Where you should be now
At this stage you have prepared for the actual experiment by 1) developing LabVIEW code to collect
data, 2) performing analysis of cantilever beam vertical loading to generate expected results, and 3)
evaluating potential experimental artifacts related to gauge position and spurious loading. You have
also discovered the need for material property data, and have found representative data for the
general material type (polycarbonate).
Remaining Tasks:
1. Conducting a beam bending experiment
2. Measuring material property data
3. Preparing for interrogation on your results and process
Timeline: You have lab session 3 and 4 to complete the bulk of the work. Questions on the
experiment will be handed out at the beginning of lab session 4, and you will have that session to
complete any last-minute work. Responses are due at the beginning of the lab session the following
Tuesday (we will begin a new experiment then).
Task 1: Beam bending experiment
Your goal is to generate plots of strain versus applied load. Applied load is the independent variable,
and various aspects of strain are the dependent variables. Note that we cannot plot “strain” versus
load as “strain” is a tensor quantity, and a traditional x-y plot requires a scalar parameter. Be
prepared to plot any aspect of strain that I ask for, noting that all aspects of strain can be generated
from knowledge of the complete strain tensor. I may ask for the individual strain tensor components
(xx, yy, xy), the principal values, the maximum shear strain value, the orientation of the principal
axes within our coordinate system, etc. Plots of experimental data include 1) data points, 2) an
indication of data variability, 3) trendlines, and 4) theoretical predictions for comparison (Fig. 1).
Figure 1. An example of experimental data plotted and compared with theoretical predictions
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Repeats
To generate a plot of this type, you must conduct independent experimental repeats. For your
experiment, record strain with the beam loaded and the system nulled, then load to three different
levels, recording strain at each load. Unload the beam, and record strain again with no load applied.
Repeat this sequence five (5) times. Your data points will be the average values from the five
repeats, and the variability will be the standard deviation.
Trendlines
There are guidelines to the use of trendlines for experimental data. They are generally not an
arbitrary “best” curve fit applied to the data. In most cases the form of the trendline (e.g. straight
line, parabola, power function, etc.) is matched with the behavior predicted by some underlying
theory, and that is what you should do. The quality of the fit is quantified by an “R-squared” value,
and you should be able to report that number.
Theoretical Bounds
Contrary to popular belief, theory does not predict a single dependent variable value expected for a
given value of the independent variable, it predicts a range of values. This is for the simple reason
that you never know precisely the values of parameters (e.g. modulus of elasticity) that theoretical
predictions require. Even though the concept is simple, dealing with parameter variability is often not
so simple. For this lab project combine information from the multiple material property tests
conducted by the lab groups with information from online data sources to find a reasonable estimate
of low and high values for modulus of elasticity. Use those to calculate lower and upper bounds on
your theoretical predictions.
Task 2: Material Property Data
The course TA’s will take you through the process of conducting a tensile test to determine modulus
of elasticity. You will be provided with load versus displacement data generated during the
experiment, and will have the opportunity to measure sample dimensions. From this information you
will calculate modulus of elasticity (please refer to the ASTM standard referenced by the TA’s). You
will share your property value with the rest of the class, and use all of the repeat measurements to
determine a range of modulus values. We will not be measuring Poisson’s ratio. Find a
representative value from an online source, and be prepared to address the sensitivity of your
theoretical predictions to changes in Poisson’s ratio.
Task 3: Preparing for Interrogation
You will receive a set of questions beginning of the next lab, and have that lab session to tie up
loose ends. Responses are due beginning of the following lab session. If you collect and analyze
data and perform calculations as described in the lab handouts, you will be well-prepared to respond
to the questions. Have your data well-organized, and your calculations procedures in place ahead of
time.
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