Hello, I am a student trying to collect tensile test data by using Labview.
I am using:
NI cDAQ-9178 shassis,
NI 9215 module for voltage collection from load cell,
NI 9237 module for strain collection from three strain gages.
So I just performed several simple tensile tests on carbon composites with 2mm/min rate of pulling.
I am using the DAQ assistant, which is built in the Labview and easy to use.(I know only the newbies use DAQ assistant, but I am a newbie and if it works well, why not use it)
I have several questions concerning the results.
1. When I use the strain gages, it seems like I cannot control the sampling rate.
If I collect the voltage from the load cell only, I can put 'samples to read' and 'rate(Hz)' to whatever I want and get the right sampling rate. For example, if I put both of them as 100, I get 100 sampling data every one second. If i put 10, I get 10 sampling data every one second.
However, When I attach the strain gage to the board, The sampling rate is fixed to every 0.00062 second, which about 1613 Hz sampling rate.
Is this normal? If not, how do I fix this?
2. The Stress-Strain curve of a carbon composite should be straight, or the slop should decrease a little bit in the end.
However, it seems like the slope increases as specimen is taking more and more stress. This means that the specimen is showing hardening, which should not happen in carbon composites.
There may be other factors to this results, and I am considering the Labview DAQ assistant as one of them.
So I am wondering if DAQ assistant becomes inaccurate after certain amount of stresses for strain gage data.
I attached some jpg files and vi file to this message.
If anyone can help me with above two questions, it will be great.
Thank you.
