MONTANA STATE UNIVERSITY
DEPARTMENT OF MECHANICAL ENGINEERING
         

Mechanical Engineering Laboratory

ETME360 Summer 2014
R. Larson

EXERCISE 5
STRAIN GAGE BEAM LOAD CELL  

Goal:  
Utilize a strain gage-instrumented cantilever beam load cell to determine the breaking strength of polymeric material samples. 
This exercise illustrates the use of resistive strain gages for strain measurement. A LabVIEW program will be written to control the computerized acquisition of force data.

 References:  
1.  A. Wheeler & A Ganji, Introduction to Engineering Experimentation Ch 8.
2.  Any strength of materials book containing theory concerning beam deflection and stresses in beams.
3.  Lecture coverage, ETME360.

Equipment:  
DC power supply 
Linear
Polymer samples
Cantilever beam apparatus with calibration weights
Digital or analog balance (scale
)
PC with Labview Program
National Instruments Compact DAQ with NI9205 modules. Alternatively an NI 6008 or NI 6009 Data Acquisition System may be used.

Discussion:
 
The apparatus (see figure 1) consists of a steel beam 14.5 inches long, 1/4 inches wide, and 3/16 inches thick, fixed at one end. A loop is located at the free end of the beam for use with experiments involving load application such as this one. Four strain gages are mounted on the beam; two on the upper surface and two on the lower surface, located two inches from the fixed end. An additional strain gage is mounted to the base of the unit, in cases where a dummy gage is needed. Each of the gages have
nominal resistance R=350-ohm and gage factor F=2.05.(Our book uses the letter S for gage factor, but F is also common.) These gages are of the foil type and can operate at a current of 20 ma or less. For any particular test apparatus the strain gages have been matched for resistance within + 1 ohm.  

Also included on this apparatus (but not used for this particular experiment) is a micrometer mount to provide known displacement at 12 inches from the fixed end and a dial indicator mount to record displacement at various locations.


 

Figure 1.  Cantilever Beam Apparatus.

For this experiment we will utilize all four strain gages that are mounted on the beam, incorporated into the bridge circuit shown in figure 2. To determine gage location, trace wires from each gage to the associated plug. (Do not rely on any letters/numbers that have been written on the beam for gage ID.)

 
Figure 2. Bridge circuit  

At beginning of lab:


1. Calculate the excitation voltage to be used, in order to limit the current through the strain gages to 20 ma.

2. Analytically determine the SENSITIVITY of the system in volts per pound of load, applied perpendicular to beam axis (i.e. straight down) at the end of the beam. Be sure to use the proper dimensions and circuit details given above.

Record these values for use during remainder of laboratory.


The polymeric material to be used for this exercise consists of fly fishing tippet line, in various sizes. Each size (7x, 6x, 5X etc) is rated by the manufacturer at a specific breaking strength. However, this rating applies to the line rather than the knot that must be used to affix the line to a fly or lure. Depending upon the knot type, the working strength of the line with knot may be significantly less than the rated line strength. For this exercise, an improved clinch knot is recommended: Various investigators claim that this common knot provides breaking strength of between 70% – 95% of the line strength. (You may encounter knot failure during this experiment!)

Procedure:  
1.  PROGRAMMING: 
Create a LabVIEW data acquisition program to record load as a function of the voltage output from the bridge circuit. The program you create should address the following requirements:

You should check & test the system configuration, set gain and verify other DAQ System parameters as demonstrated in other lab experiments.

2.   PROGRAM TESTING, STATIC CALIBRATION:
Connect the hardware components as shown in Figure 2. Run your Labview program as necessary to thoroughly test your system. Then perform a static calibration using the weights provided: Run your program first with no weight, then again with the weight holder, and again with subsequently increasing weight added in order to generate output voltage  vs load values for your apparatus.

 *Note that the program converts voltage to load already: Your static calibration scheme should include the step of reading the load values reported by the program, then backing out the voltage sensed prior to conversion. Other methods are possible. 

*A typical test procedure would involve calculation of system sensitivity, verification by static calibration, and finally a dynamic load test once all the problems were solved. We'll shortcut that process by doing the static calibration test in the same lab period as the dynamic load test: The static calibration numbers should validate the calculated sensitivity value that is used in the Labview program.

3. POLYMERIC MATERIAL CHARACTERIZATION:
Tie each tippet sample to the beam eye as shown in Figure 3, and start your Labview program. Immediately, apply a steady pull on the line until specimen failure occurs, within the time period specified, in order to gather voltage output data for the device. Line pull must be perpendicular to the beam, either straight up or straight down. Repeat for each of 2 types of sample.

Results:
1.   Using an approved common-sense format, plot the results of your static calibration.
2.   Plot the experimental load-vs-time curve for Two polymeric material specimen types (e.g. 5x, 6x, 7x tippet.)
3.   Determine the Sensitivity [ d(eo/ei)/dP ] of the system experimentally, and compare with your analytically determined sensitivity.
4.   Compare your experimental “max load” results with manufacturer’s claims for each tippet specimen.  
5.   Be sure to
include a printed copy of the front panel and diagram from your LabVIEW program.

 

NOTE ON DATA SHEETS for the Strain Gage Applications Exercise:
DO NOT submit the thousands of acquired points in a table as data: Instead, your data sheet should contain all other information (Group Members, Major Equipment, Experiment name and number, etc. etc.) and should provide a reference to the 'Results' figure that shows the data plotted graphically. 

In an industrial application, the actual point file would likely be saved, stored, and made available for future review or manipulation.