Bad readings for a strain gauge on a pipe

[oelindoe]

Hi guys.
Im performing strain gauge readings for a heavy walled 4" pipe now. There is one gauge for axial strain and one for hoop strain. The gauges are mounted on the outside in a non-pressurized area. The gauges are set in a half bridge with a dummy gauge as a temperature compensator.

When the pipe is pressurized the readings for the axial strain is way to small, while the hoop strain is slightly less than in theory. In order to verify proper function of the axial strain gauge, the pipe is streched by using an external axial force and the reading is then correct.

What am i missing here ?.

1. Are the hoop strain effecting the strain gauge?. I thought the strain gauge had a small sectional area thus avoiding poisson effects.

2. Are the hoop strain giving large contraction (poisson effect) of the axial strain in the pipe thus equalizing the axial strains generated by the end cap forces ?

Any thoughts ??

Brgds OddE

 

[arto]

is the T.C. gage on a piece of metal resting on the pipe
(i.e., the same temp as the pipe, not shop air temp)?

Usually the "fat end loops" take care of transverse strain.

How long is the piece? you may have end effects - check the "beam on Elastic Foundation" analysis(i.e., Roark Table 30 in the 5th ed.)

 

[oelindoe]

Looks like the hoop expansion of the pipe gives a lot of contraction in axial direction thus messing up the strain readings. I need to add an internal pressure driven function in the software to compensate. So the resulting reading will be a kvasi measured/theoretical value