Crack monitoring strain gauge for fatigue 1

[zerow3]

Hi all, hope I am posting in the right place.

I will be conducting some load control fatigue test on my compact tension sample. I have done these test before and successfully monitored the crack with a CMOD clip gauge. But this time I am running at low load and a paper that I found said that the force of the clip gauge will affect the crack growth and suggested to use back face strain gauge to measure the crack length. I am very new to this back face strain gauge thing and I have no idea how to select the right gauge. I have been looking through micro-measurement website (

http://www.vishaypg.com/micro-measurements/),

and there seems to be two main types of strain gauges, stress analysis strain gauge and transducer gauge. I wonder what is the different between these two types and for my purpose, which type do I need? (Sorry if this is a stupid question, but I had searched online but can’t seems to find the answer)

 

[redpicker]

I'm sorry if this is not applicable; it's been decades since I was involved in this type of testing (think Reagan), but can't you just measure the spring constant of the clip gauge and compensate for it in your load measurement? Measuring crack length with a clip gauge wasn't new when I was doing it, so I am surprised that this compensation is not already worked out.

rp

 

[zerow3]

Hi redpicker, thanks for the reply. I was thinking something similar, but I think it will get quite complicated. Because I will be using load control and probably at high frequency to speed up the test, so it would be quite difficult to get the hydraulic machine to compensate for it automatically. Because of this, I think the back face strain gauge (BFS) method is probably the way to go as it is unlikely to have any effect on the forces.

 

[BrianE22]

Load control and high frequency? The two usually don't work well together due to the acceleration of the load transducer. I'd think about strain control.

 

[redpicker]

zerow3,
I would think, like BrianE22 mentions, you would want displacment control and just read the loads and then just add the additional load from the clip gauge to the recorded loads. The load produced by the transduces is directly related to the displacement, so I would think that would be easy to compensate for. Then, again, it has been decades since I have been directly involved, so maybe there are more problems with this approach.

Good Luck,

rp

 

[BobBartlett]

I use Vishay strain gages all the time for strain measurement. From my read of their website, it seems that the "Transducer Class" is a "home made word" of Micro Measurements this means that (in their words) these gages are:

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"designed specifically for transducer applications with the main objective being optimum performance at lower cost for high-volume usage."
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In case the message was not clear, they offer the following disclaimer on the data sheet for the "transducer" gages.

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"They are not suitable for strain measurement in stress analysis applications. Request our Precision Strain Gages databook, or contact our Applications Engineering Department, for a complete listing of gages for precision strain measurement applications."
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Since it sounds like you only need a few (or even a few gross at most) and you are concerned enough about precision to be worried about the clip gage force, then I would definitely suggest the gages that are intended for stress analysis (i.e. -- not the "Transducer Class").

 

[zerow3]

Thanks for the help Bob, I must have missed the second statement when I was reading their website