Extensometer length and Facets size

[[kl...]]

Hello everybody. I am running some analysis with GOM and I noticed that when we use an extensometer of different lengths or when we change the facet size the measurement has some deviations. So I was wondering if there is an appropriate facet size and extensometer length to calculate cracks in tests. Which parameters are adequate? How can I know which result is better? Thank you in advance for your advice.

BR
Kleo

Edited November 13, 2023

[[Th...]]

Hi kleo,

I think there is no "right" or "wrong". Of cause the length, the position (relative to the crack (center)) and also the facet size (is it influenced by the crack, is it stable (enough)) are all variables influencing your measurments and thus results. 
My proposal would be: make the distance and facet size (19x19 standard, with "approx. 3 dots of black per facet") big enough, that facets are not influenced by the crack zone. On the other hand make it short enough, that only the crack opening is measured.
This may vary/ will from crack to crack and also along each crack.
Maybe get in contact with your local AE/supoprt for a more detailed analysis/ help.

Best regards, Thorsten 

[[kl...]]

Hello Thorsten,

Thank you for your response! As I did some tests, I think for these parameters the best solution is to have similar facet sizes for both extensometer and surface component. Regarding the length it is still a little unclear to me but the best case should be when it is big enough to measure crack width.

Thanks for the support,Kleo

[[Th...]]

Hi kleo,
the same facet-size as the surface components is a good measure, as both should fit to the given pattern!
Regarding the width: I would go into a stage with open crack(s), inspect the surface component for major strain and place the extensometers in a diretction and length perpendicular to crack direction and so wide, that the facets are not touching an influence zone. Lets say "stay in in area with major strain< 1%", you could change the legend settings to a nice visualization (p.e. > 1% = bright red) and exactly place your extensometers.