The first instability of circular tubes compressed into the plastic range is
axisymmetric wrinkling, which is stable. Compressed further the wrinkle amplitude
grows, leading to a limit load instability followed by collapse. The two instabilities can
be separated by strain levels of a few percent. This work investigates whether a tube that
develops small amplitude wrinkles can be subsequently collapsed by persistent cycling.
The problem was first investigated experimentally using SAF 2507 super-duplex steel
tubes with D/t of 28.5. The tubes are first compressed to strain levels high enough for
mild wrinkles to form and then cycled axially under stress control about a compressive
mean stress. This type of cycling usually results in accumulation of compressive strain;
here it is accompanied by growth of the amplitude of the initial wrinkles. The tube
average strain initially grows nearly linearly with the number of cycles, but as a critical
value of wrinkle amplitude is approached, wrinkling localizes, the rate of ratcheting
grows exponentially and the tube collapses.
Similar experiments were then performed for tubes involving axial cycling under
internal pressure and the combined loads cause simultaneous ratcheting in the hoop and
axial directions as well as a gradual growth of the wrinkles. The rate of ratcheting and the
number of cycles to collapse depend on the initial compressive pre-strain, the internal
pressure, and the stress cycle parameters all of which were varied sufficiently to generate
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a sufficient data base. Interestingly, in both the pressurized and unpressurized cases
collapse was found to occur when the accumulated average strain reaches the value at
which the tube localizes under monotonic compression.
A custom shell model of the tube with initial axisymmetric imperfections, coupled
to the Dafalias-Popov two-surface nonlinear kinematic hardening model, are presented
and used to simulate the experiments performed. It is demonstrated that when suitably
calibrated this modeling framework reproduces the prevalent ratcheting deformations and
the evolution of wrinkling including the conditions at collapse accurately for all
experiments. The calibrated model is then used to evaluate the ratcheting behavior of
pipes under thermal-pressure cyclic loading histories experienced by axially restrained
pipelines. / text
| Identifer | oai:union.ndltd.org:UTEXAS/oai:repositories.lib.utexas.edu:2152/ETD-UT-2011-12-4442 |
| Date | 01 February 2012 |
| Creators | Jiao, Rong |
| Source Sets | University of Texas |
| Language | English |
| Detected Language | English |
| Type | thesis |
| Format | application/pdf |
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