A steel pressure vessel is tested at 60 bar pressure. Strain gauges are bonded at the front and rear domes and the middle of cylinders. The data obtained from the strain gauges show oscillations of large magnitudes. The stress levels (obtained from the strain conversion) show fluctuations say from 300 MPa to 700 MPa, whereas the average is 500 MPa. Now the question is that do these fluctuations show noise in the data or represent some damage in the structure. (The stress levels are below the material Yield limit).
give more info about the nature of the test and the apparatus applying the force, For hydraulic jacks or even servos it is natural to have fluctuations because of the equilibrium cycle loops to maintain constant load at each increment
Basically it is a hydrostatic pressure test. Pressurized water is supplied by pumps. No hydraulic jacks are used.
Probably the pressurized water is not increased in a sufficiently slow fashion that it is rather a dynamic problem, not static. A look at the readings might shed more light on this.
Water flow was not dynamic. Couldn't it be some flaw in the strain gauges i.e debonding. However all the gauges reproduced similar trends. Usually such fluctuations in pressure/load vs time are also attributed to the material damage.
if the strain gauges debonded, they would give erratic results, not necessarily fluctuations. How did you decide that the water flow is not dynamic? Are there any other kind of supporting measurements? load cells? displacements?accurate flow rate that can be used to confirm whether the flow is static or dynamic?
Thanks for your replies. load cells are also used and the flow rate is slower enough. There is no chance of pressure variation inside the vessel as water is incompressible. The test data is compared with FE results and the anomaly is only in the experimental one. The data acquisition is at a reasonable frequency. Further, the material may damage even if if it is below its yield if there is any small flaw or crack. Now the question is that the fluctuations represent noise or structural damage?.
i would say try repeating the test with different stress amplitude or different container and see if you get similar trends of results. These fluctuations of this high magnitude can only be of dynamic nature.
The strain levels in FE model are uniform as the design pressure load is quasi-static in steady state. I want to know how high frequency peaks will dictate that it is noise. If the test total time is 60 seconds, there are 10 fluctuations. We are planning to carry out NDE of the vessel and then test at lower pressure to observe the performance of the vessel.
What is the need for justification of the last two points. The damage in materials is represented by drops in load/stress vs time histories, which is an established fact. In this regard, you can read my latest publication titled :Evolution and interaction of damage modes in fabric-reinforced composites under dynamic flexural loading
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