Current Projects

Determination of elastic-plastic stresses with the simplified yield zone theory under consideration of effects according to theory II. Order

Brief description
The simplified yield zone theory (VFZT) is used for the simplified estimation of state variables of elastically-plastically loaded structures, especially under cyclic loading. These are usually obtained using incremental elastic-plastic analyses for a finite element model of the structure over many loading cycles. The VFZT, on the other hand, allows the approximate determination of the structure behavior in the single-cycle condition in a simplified way. With only a few linear elastic analyses, an estimate of the accumulated strain, the strain amplitude, the deformation state, etc. can be obtained.

So far, however, the VFZT is only available for calculations according to I. order theory, in which the deformations must be so small that it is sufficient to formulate the equilibrium conditions on the undeformed system. However, it is obvious that for structures that can develop a ratcheting mechanism, second-order geometric effects also occur as a result of the progressive deformation, which make it necessary to formulate the equilibrium conditions on the deformed system (second-order theory). Within the framework of the project, a VFZT of the second order is therefore to be developed.

As an example, a pipe bend under in-plane bending is mentioned (in the adjacent figure simplified modelled as a half torus shell), in which downward forces ensure that the maximum stress arises through circumferential bending near the crown. If an internal pressure exists at the same time, its stiffening effect ensures a reduction of the bending stresses according to theory of the second order.

Author
B. Vollrath, M. Sc.
Prof. Dr.-Ing. H. Hübel

Funding Institution
DFG, Project HU 1734/5-1

Keywords
fatigue, ratcheting, simplified elastic-plastic analysis

Thermal effect of fly ash on reinforced concrete silos

Brief description
Silos in coal-fired power plants are used for the intermediate storage of fly ash from the electrostatic precipitator. In addition to the quasi-static effects due to wind, plant technology, payloads, bulk material, among others, high temperature effects in particular must be taken into account. When the fly ash enters the silo, it is not uncommon for its temperature to be more than 80 °C. Depending on the stiffness of the structure, this results in design-determining constraint stresses. Investigations show that the temperature distribution in the bulk material varies strongly in time and location. In order to record the behaviour of the structure in interaction between temperature effect - bulk material load more precisely, filling and emptying processes are modelled numerically.

Author
Dipl.-Ing.(FH) M. Simon

Funding Institution
Internal research

Keywords
Numerical simulation, temperature profile, silo structures, fly ash