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fatigue test



the gage length of the specimen is 75mm length and 8mm i included how my machine looks like and the specimen.the specimen will be subjected to constant bending stress through out the gage length i.e.,75 mm i can acquire strain signal at any point in the gage question is how to acquire the strain signal?

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Message 11 of 14
i have given the details of my machine and specimen .will any one help me
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Message 12 of 14

You should have a little patience.  Many of us are in different parts of the world with different time zones.


Your specimen will be subjected to a constant bending moment, not constant stress.  The stress at any point in the specimen will depend on the cross sectional diameter at that point and the distance from the center.  The strain will typically be proportional to the stress.


I don't see any practical way to physically measure the strain at any point in the specimen.  Even if you do, it won't tell you what the maximum strain is, which is probably what you are interested in.  Using strain gauges would interfere with the experiment.


One possibility might be using a camera to look at the side profile of the specimen.  The specimen will have a small arc shape.  You could process the image to find the centerline of the specimen, then calculate the strain based on the curvature of the arc.  I just don't know if there would be enough curvature to get reasonable measurements.


If you know the properties of the material the specimen is made from, you should be able calculate the stress and strain at the midpoint of the specimen.


I would really expect somebody doing an experiment like this to know most of this.



Bruce Ammons
Ammons Engineering
Message 13 of 14

Dear Sir


Is it possible to do fatigue analysis through signal express/sound & vibration assistant?


I'm installed 12 strain gage to a small metal structure and using NI cDAQ with NI9235 & NI9236 C series module.


Thank you

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