Choosing strained area to put strain gauges on a part being tested

Sergio

Hi, I have the luck that one the part (a compressor cilinder in ductil cast iron) that has been designed and analyzed, will be tested, and we will put some strain gauges, the aim is to check if exist some permanent deformation after test (a simple hidraulic test). What variable should I postprocess (in a lineal or plastic analysis) to select the best place to put the strain gauges?

JohnM

Linear gages measure linear strain, rosettes will provide biaxial results. You should be able to look at linear strain, but you want to extract that result across the actual gage length. You can literally glue small soft elements and look at the deltaL/L.

For gage placement, I would avoid trying to measure at the peak strain location in your model. Place gages a little away from the peak, where the gradients are lower. This will make gage placement less sensitive to positioning error. Once you match your result to test, you can confidently report the "true" peak as what your model shows.

Sergio

@JohnM , thanks for the advices. Which component (Exx, Eyy...) of strain should I look? I will definitely model the strain gage and attach to some flat area as you say. Attached some pics of the part, the pressure is applied on the blue surfaces, simetry conditions on vertical planes, and the uper/lower surfaces of the blind flanges were fixed (we use an hidraulic press to keep compressed the assembly during the test, really don´t know if is completly fixed or not).

JohnM

The strain direction has the same issue as when you look at stresses - they often don't align with a global axis. You can look at principal strain (I would look at max prin strain for proof/burst limits) but to get the direction you will have to infer from the deformation. In ANSYS, we could get the principal strain directions but I don't recall if that is possible in Calculix or Mecway. When deciding where to lay gages, if the direction is very unclear, use rosettes. Also remember that it is alot easier for you to read a strain in a fillet than it is for a tech to lay a gage in a tight corner - remember to be practical!

disla

Hi Sergio,

The Strain Energy density can help you to locate the more interesting areas

For an isotropic material the strain energy density function is directly and uniquely related with the principal stretches.