Stress redistribution in the very high cycle fatigue range

Table of contents

1. 1. 1. Project data
      2. Report from the year book 2021
      3. Report from the year book 2020

Project data

Titel | Title Numerische und experimentelle Untersuchungen zu den Spannungsumlagerungen von ermüdungsbeanspruchten Betonbauteilen im Very-High-Cycle-Fatigue-Bereich | Numerical and experimental investigations on stress redistribution in concrete structures subjected to fatigue loading in the very high cycle fatigue range Förderer | Funding Deutsche Forschungsgemeinschaft (DFG) Zeitraum | Period 09/2019 – 12/2022 Projektleiter | Project manager Prof. Dr.-Ing. Steffen Marx Bearbeiter | Contributor Dennis Birkner, M. Sc.

Report from the year book 2021

Positive effects from fatigue?

In structures such as wind turbines and railroad bridges, cyclic stresses occur in the cross-section during the service life, leading to successive material fatigue. As a result, the material stiffness decreases, especially in the edge zones, which leads to a redistribution of the stress to the inner part of the cross-section. This effect is investigated in this project utilizing finite element simulations and experimental tests.

For the finite element simulations, an additive strain model for fatigue loaded concrete is being developed further. The concrete strains consist of four components: an elastic strain component as a reversible deformation due to the external loads, a viscous strain component from time-dependent material effects, a plastic strain component that describes irreversible deformations due to material degradation, and a temperature strain component for the deformation due to heating. In addition, a coupling of the elastic modulus of the individual elements to their damage state was implemented. The material model was implemented in ANSYS Mechanical in an iterative calculation sequence. In the calculation, the individual load cycles of the fatigue loading are combined in load collectives with equal load levels and applied quasi-statically to the system. Then, the individual strain components and the updated stiffnesses are calculated for each element and the following load collective is applied. The model parameters are calibrated through experimental investigations on small-scale cylindrical and large-scale beam specimens. Since the stress redistributions in the concrete beams can lead to an increase in the bearable load cycles to more than 10⁷, the beam tests are carried out in a resonance-based testing facility. In this way, the required large stresses can be generated with a comparatively low excitation force and the very-high-cycle-fatigue range can be achieved in less than a week with an excitation frequency of around 18 Hz.

The results of the research project make it possible to take a step towards a more realistic design and thus towards leaner, more economical and more cost-effective designs of fatigue-loaded concrete structures.

Report from the year book 2020

Large concrete beams in resonance

Structures such as wind turbines and railway bridges are subjected to cyclic loads throughout their lifetime, which leads to successive material degradation. The reduced stiffness conditions cause a stress redistribution within the cross-section of the structures. So far, this effect has only been considered in a very simplified manner in the structural design.

This project aims to specifically and systematically investigate stress redistributions occurring in concrete structures subjected to fatigue loads. Within this framework, both finite element simulations and experimental investigations are carried out. An additive strain model for fatigue loaded concrete is refined for the FE simulations. The numerical model can simulate local stiffness changes as well as plastic and visco-elastic deformations. The model parameters are calibrated using experimental investigations enabling a more accurate simulation of the material behaviour subjected to fatigue loads. The experimental investigations are carried out on large-scale specimens since local stiffness changes and stress redistributions can be better determined this way. For this purpose, a resonance testing facility is used in which the concrete beams are excited in their first natural bending frequency using oriented imbalance motors. This makes it possible to reach the required large stresses with a comparatively low excitation force. The test setup is designed to have an excitation frequency of approximately 18 Hz to reach the Very-High-Cycle-Fatigue range with over 10⁷ load cycles in less than one week of test duration. This is essential for the test execution since the stress redistributions lead to a very large number of cycles to failure of the concrete beams. The functionality of the test setup has already been confirmed in previous tests on similar specimens. To determine the material parameters for the numerical model, experiments on cylindrical specimens are carried out additionally to the large-scale investigations.

The results of the project allow a step towards a more accurate design and thus towards more slender, economical and cost-effective designs of fatigue loaded concrete structures.